Note: Descriptions are shown in the official language in which they were submitted.
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SIMULTANEOUS MULTI-BEAM PLANAR ARRAY IR (PAIR) SPECTROSCOPY
TECHNICAL FIELD OF THE INVENTION
[003] This invention relates generally to an apparatus and method for
simultaneously
determining an IR spectrum of multiple sample materials. More particularly,
the disclosed
invention relates to spatial multiplexing of spectroscopically determined IR
spectra of
multiple samples using an apparatus and method that operate in real-time with
simultaneous
background compensation, and which do not require the use of any moving parts.
Still
further, the apparatus and method of the disclosed invention do not require
extensive
mathematical transformation of the detected spectral information to analyze
the composition
of the sample material.
[004] The disclosed invention has industrial applicability to, for example, a
real-time
method to monitor manufacturing processes. Such processes include, but are not
limited to
measurement of thickness, chemical structure, and orientation of coatings on
surfaces (solid,
liquid, chemically bound, physically adsorbed). These measurements include,
but are not
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limited to those made on biological materials, polymers, superconductors,
semiconductors,
metals, dielectrics, and minerals. Further applicability is found to a real-
time apparatus and
method to measure and detect a chemical species present in a chemical reaction
involving
various processing of materials in any of a gaseous, liquid, or solid state.
In addition, the
apparatus and method of this invention provides for self compensation, to
account for sensor
or optical path changes over time, or changes in environmental conditions,
which may affect
the measurements obtained.
BACKGROUND OF THE INVENTION
[005] As industry continues on its path of cost reductions in core
technologies, more
emphasis will be placed on the optimization of processes and performance. This
retrenchment will necessitate the development and introduction of a whole new
class of
sophisticated instrumentation that is portable, rugged, reliable, and capable
of operation over
long periods of time in an aggressive industrial or other non-laboratory
environment.
[006] Spectrometric techniques are often used in analysis of materials.
Classically,
spectroscopy is the measurement of the selective absorption, emission, or
scattering of light
(energy) of specific colors by matter. Visible white light can be separated
into its component
colors, or spectrum, by a prism, for example. The principal purpose of a
spectroscopic
measurement is usually to identify the chemical composition of an unknown
material, or to
elucidate details of the structure, motion, or environmental characteristics
(e.g., internal
temperature, pressure, magnetic field strength, etc.) of a "known" material or
object.
Spectroscopy's widespread technical importance to many areas of science and
industry can be
traced back to nineteenth-century successes, such as characterizing natural
and synthetic
dyes, and determining the elemental compositions of stars.
[007] Modern applications of spectroscopy have generalized the meaning of
"light"
to include the entire range or spectrum of electromagnetic radiation, which
extends from
gamma- and x-rays, through ultraviolet, visible, and infrared light, to
microwaves and radio
waves. All these various forms (or wavelength ranges) of electromagnetic
radiation have their
own characteristic methods of measurement. These different methods give rise
to various
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types of spectroscopic apparatus and techniques that are outwardly very
different from each
other, and which often rely upon difference physical phenomena to make
measurements of
material characteristics. Further, the various experts and other researchers
in these diverse
fields, more often than not, do not cross the technical boundaries between
these areas of
specialization, as different and somewhat compartmentalized knowledge bases
and "rules of
thumb" are used.
[008] The use of infrared (IR) is one of numerous spectroscopic techniques for
analyzing the chemistry of materials. In all cases, spectroscopic analysis
implies a
measurement of a very specific wavelength of light energy, either in terms of
the amount
absorbed or reflected by the sample in question, or the amount emitted from
the sample when
suitably energized.
[009] In the case of IR, an absorption form of spectrometric analysis is
relied upon.
IR radiation does not have enough energy to induce transitions between
different electronic
states, i.e., between molecular orbitals, as seen with ultraviolet (UV), for
example. Unlike
atomic absorption, IR spectroscopy examines vibrational transitions within a
single electronic
state of a molecule, and is not concerned with specific atomic elements, such
as Pb, Cu, etc.
Such vibrations fall into one of three main categories, i.e., stretching,
which results from a
change in inter-atomic distance along the bond axis; bending, which results
from a change in
the angle between two bonds; and torsional coupling, which relates to a change
in angle and
separation distance between two groups of atoms. Almost all materials absorb
IR radiation,
except homonuclear diatomic molecules, e.g., Oz, Hz, N2, Clz, F2, or noble
gases.
[010] IR typically covers the range of the electromagnetic spectrum between
0.78
and 1000 ~,m. Within the context of IR spectroscopy, temporal frequencies are
measured in
"wavenumbers" (in units of cm'), which are calculated by taking the reciprocal
of the
wavelength (in centimeters) of the radiation. Although not precisely defined,
the IR range is
sometimes further delineated by three regions having the wavelength and
corresponding
wavenumber ranges indicated:
"near-IR": 0.78-2.5 ~m 12800-4000 cm';
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"mid-IR" 2.5-50 ~m 4000-200 cm'; and
"far-IR" 50-1000 ~m 200-10 cm'
[011] For a molecule to absorb IR, the vibrations or rotations within the
molecule
must cause a net change in the dipole moment of the molecule. The alternating
electric field
of the incident IR radiation interacts with fluctuations in the dipole moment
of the molecule
and, if the frequency of the radiation matches the vibrational frequency of
the molecule, then
radiation will be absorbed, causing a reduction in the IR band intensity due
to the molecular
vibration.
[012] An electronic state of a molecular functional group may have many
associated
vibrational states, each at a different energy level. Consequently, IR
spectroscopy is
concerned with the groupings of atoms in specific chemical combinations to
form what are
known as "functional groups", or molecular species. These various functional
groups help to
determine a material's properties or expected behavior by the absorption
characteristics of
associated types of chemical bonds. These chemical bonds undergo a change in
dipole
moment during a vibration. Examples of such functional groups and their
respective energy
bands include, for example, hydroxyl (O-H) (3610-3640 cm'), amines (N-H) (3300
- 3500
cm'), aromatic rings (C-H) (3000-3100 cm'), alkenes (C-H) (3020-3080 cm'),
alkanes (C-
H) (2850-2960 cm'), nitrites (C=N) (2210-2260 crri'), carbonyl (C=O) (1650-
1750 cm'), or
amines (C-N) (1180-1360 cm'). The IR absorption bands associated with each of
these
functional groups act as a type of "fingerprint" which is very useful in
composition analysis,
particularly for identification of organic and organometallic molecules.
[013] By knowing which wavelengths are absorbed by each functional group of
interest, an appropriate wavelength can be directed at the sample being
analyzed, and then the
amount of energy absorbed by the sample can be measured. The intensity of the
absorption is
related to the concentration of the component. The more energy that is
absorbed, the more of
that particular functional group exists in the sample. Results can therefore
be numerically
quantified. Further, the absence of an absorption band in a sample can often
provide equally
useful information.
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[014] Intensity and frequency of sample absorption are depicted in a two-
dimensional plot called a spectrum. Intensity is generally reported in terms
of absorbance,
the amount of light absorbed by a sample, or percent transmittance, the amount
of light that
passes through it. In IR spectroscopy, frequency is usually reported in terms
of
wavenumbers, as defined above.
[O15] Infrared spectrometers may be built using a light source (e.g., the
sun), a
wavelength discriminating unit or optically dispersive element such as a
prism, for example,
and a detector sensitive to IR. By scanning the optically dispersive element,
spectral
information may be obtained at different wavelengths, by using either a
reflection mode, i.e.,
reflection of the light source off the sample, or a transmission mode, i.e.,
transmitting a
portion of the light source through the sample. However, one drawback to this
approach is
the moving parts associated with the required scanning operation. Such moving
parts
inherently limit the ruggedness and portability, for example, of such a
device.
[016] More recently, a Michelson interferometer has been used to generate a so-
called interferogram in the IR spectrum, which later is subjected to Fourier
transform
processing such as a fast Fourier transform (FFT) to yield the final spectrum.
In the IR range,
such spectrometers are called FTIR interferometers, and the first commercially
available
appeared in the mid 1960's. A representation of an FTIR interferometer is
provided in Fig. 1.
[017] The key components of FTIR interferometer 100 are IR source 110,
interferometer (130, 140, 150), and IR detector 160. FTIR interferometer 100
provides a
means for the spectrometer to measure all optical frequencies transmitted
through sample 120
simultaneously, modulating the intensity of individual frequencies of
radiation before
detector 160 picks up the signal. Typically, moving mirror arrangement 150 is
used to obtain
a path length difference between two (initially) identical beams of light.
After traveling a
different distance than a reference beam, the second beam and the reference
beam are
recombined, and an interference pattern results. IR detector 160 is used to
detect this
interference pattern.
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[018] The detected interference pattern, or interferogram, is a plot of
intensity versus
mirror position. The interferogram is a summation of all the wavelengths
emitted by the
sample and, for all practical purposes, the interferogram cannot be
interpreted in its original
form. Using the mathematical process of Fourier Transform (FT), a computer or
dedicated
processor converts the interferogram into a spectrum that is characteristic of
the light either
absorbed or transmitted through sample 120.
[019] The invention of FT spectroscopy has proven to be one of the most
important
advances in modern instrumentation development in the 20th Century. Optical
spectroscopy
utilizing the interference of light has made fast, sensitive detection of
molecular
vibration/rotation possible due to the large throughput and multiplex
advantages provided by
FT instrumentation. In Nuclear Magnetic Resonance (NMR) and mass spectroscopy
where
high-resolution spectra are required, FT instrumentation has also prevailed as
the state of the
art.
[020] The same technological innovations that have made FT instruments those
of
choice for a generation of spectroscopists, however, have also made them
extremely sensitive
to their operating environment. For these reasons, FT interferometers are
mostly limited to
laboratory conditions which require the use of an optical bench to prevent
vibration, and
which also require stringent environmental controls to control temperature
variations that
adversely affect the interferogram by thermally inducing path length
differences. While this
type of scanning approach has proven to be workable, the signal-to-noise-
ratios (SNR)
obtainable in some situations often require substantial signal averaging of
multiple
interferograms, thus making FTIR systems inherently slower than desired under
some
circumstances, with reduced speed and potentially lower reliability resulting
from the
numerous moving parts of these systems.
[021] In spectroscopy, resolution is a measure of the ability to resolve or
differentiate two peaks in the spectrum, where high resolution corresponds to
a small
wavenumber difference between the peak positions, and low resolution is
associated with a
larger wavenumber difference between the peak positions. Fourier Transform
interferometers are capable of extremely high resolution, on the order of
1/1000'h cm',
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depending on the amount of possible movement of the mirror, or the path length
difference
that can be generated by the particular apparatus. "Low" resolution is
generally considered to
be in the range of 16-32 cm-~, although no bright-line demarcation between
"low" and "high"
resolution exists, as resolution is chosen based on the required measurement
and specific
application. For typical chemical analysis and identification associated with
FTIR, "high"
resolution of 8 cm 1 or better is common. Otherwise, chemical information is
lost if the
resolution is too low, as adjacent peaks identified with a particular chemical
bond or vibration
state may be "smeared" together and rendered indiscernible if a lower
resolution is used.
[022] The need for thermal stability, mechanical vibration isolation, and
stringent
optical alignment has put severe constraints on where and how FT instruments
can be used
and, in particular, has limited the portability of such instruments. If
discussion is limited to
FTIR interferometers, then an examination of the specific technology used in
currently
available instruments reveals where some of the shortcomings can be found.
Table 1
compares the four most commonly used techniques for the operation of an
optical
interferometer, and their limitations.
Table 1. Common FTIR Interferometer Designs and their Limitations
Operating Technologies Limitations
Air-Bearings ~ Requires stable supply of clean, dry air and
a tightly leveled travel plane for the
moving mirror. Low tolerance for
vibration.
Magnetic Coils ~ Requires highly regulated power supplies.
Low tolerance for vibration.
Piezo Stacks ~ Limited travel range. High voltage power
supplies needed to operate the piezo
elements.
Mechanical/Piezo Hybrid ~ Requires large mechanical structures and
complicated feedback system for piezo
element operation.
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[023] FTIR has been applied to a variety of studies in industry, government,
and
academic laboratories, and has resulted in a major improvement upon
conventional methods
of performing analysis on a variety of samples. However, it has become clear
that the
moving mirror mechanism in a traditional interferometer has limited the design
and
construction of a more compact and portable FTIR. One potential solution
attempted by
Stelzle, Tuchtenhagen, and Rabolt ("Novel All-fibre-optic Fourier-transform
Spectrometer
with Thermally Scanned Interferometer"), was to construct an all-fiber-optic
FT
Spectrometer, which had no moving parts, and which was used to perform
infrared
spectroscopy.
[024] In this feasibility study, an attempt was made to build an
interferometer in the
near-IR (10000-5000 cm') range using fiber optics. Two carefully measured and
cleaved
optical fibers were used as the two light channels, or optical paths, with one
fiber kept at
ambient temperature while the other fiber was heated/cooled repeatedly. The
resulting
optical path difference (OPD) between the two fiber channels due to changes in
both the
length and the refractive index of the heated/cooled fiber caused interference
in the combined
channel. The heating/cooling cycle was used to generate an OPD of 3cm, thus
producing an
interferogram with the power spectrum calculated accordingly.
[025] However, the interference of two light beams in the optical fibers under
different thermal and mechanical conditions turned out to be very complex. In
contrast to the
traditional Michelson interferometer, whose only source of optical path length
difference
comes from the geometric path length resulting from the moving mirror, a fiber-
optic
interferometer responds to any mechanical or thermal changes of the operating
environment,
which causes a scrambling or loss of the phase information necessary for
interference to
occur. It was concluded that although the fiber optics concept is a good one,
a more prudent
plan for a no-moving parts IR instrument had to be developed.
[026] In surveying the literature, it became apparent that, without regard to
the band
of interest, e.g., visible, near-IR, or IR, other approaches to the
construction of an FT
interferometer with no-moving parts had also been attempted, as depicted in
Fig. 2. Such
approaches used either a linear array detector or a focal plane array (FPA) to
collect
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interferograms. These designs involved the projection of the center portion of
the
interferogram onto the detector, and then used the "imaged" interferograms to
calculate the
power spectra after Fourier Transform processing. One difficulty of these
conventional
techniques is that the array detector size, its dynamic range, and the limited
range of spectral
response available limited the range of the interferograms that could be
captured by the array
detector.
[027] In addition, even without moving parts, these approaches still rely upon
calculation-intensive Fourier Transform processing to derive the power
spectrum. Hence,
there is still a need for a rugged, non-interferometric, no-moving part
spectrometer in the
mid-IR range.
[028] Aside from, and even prior to Fourier Transform spectroscopy,
spectroscopy
based on dispersion provided a possible implementation. In this approach, an
optically
dispersive element, such as a prism or diffraction grating, is used to
separate the spectral
frequencies present in the incident light radiation. The dispersive element
was then rotated,
in order to allow the various wavelengths present in the incident light to be
detected.
[029] IR spectroscopy based on dispersion became obsolete in most analytical
applications in the late 1960's due to its slow scan rate and lower
sensitivity. It is well known
that the scanning mechanism in a dispersive spectrometer, e.g., a moving
prism, intrinsically
limits both its ruggedness and optical throughput. The need for scanning comes
from the fact
that point detection of photons was the only available method at that time,
and this was
especially true in the IR range of the spectrum. Today, however, array
detectors in the visible
and near-IR range are widely available for area detection of photons. Charge-
coupled-
devices (CCD) capable of >80% quantum efficiency (QE) in the visible range
have been
made and utilized in many applications, such as the visible/near-IR camera
aboard the Hubble
Space telescope. As a result of this progress, CCD-based high performance
spectrograph
systems in the visible and near-infrared range can now be purchased through
commercial
suppliers. These systems provide alternatives to traditional FT
interferometers.
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[030) However, the range of scientific problems which could now benefit from
IR
investigations has increased significantly, and applications involving samples
which may
change their position in the beam (e.g., vibrate or oscillate) while the
spectrum is being
recorded can not be routinely addressed using conventional FTIR instruments.
The scanning
architecture of FTIR instruments and the resulting modulation of the different
optical
frequency components can become modified further by a sample whose position
fluctuates,
and this can render the spectral information useless.
[031) For example, few techniques exist which can provide in-situ structural
information about Langmuir films. Infrared reflectance-absorbance spectroscopy
(IRRAS) is
a non-destructive technique that provides direct structural information about
either the
expanded or condensed phase of a Langmuir monolayer. The technique can also
provide
information about both the hydrocarbon tails and the head groups independently
by
monitoring vibrational modes with frequencies in the 4000 to 400 cm' region.
Because
polarized infrared spectroscopic measurements are sensitive to the orientation
of transition-
dipole moments, )RRAS can be used to determine the orientation of different
sub-
components of an amphiphilic molecule.
[032) Since, in order to fabricate Langmuir-Blodgett (LB) films, Langmuir
monolayers of these amphiphilic polymers must be first formed on a water
surface, where
their thermodynamic state of order is known to have a dramatic effect on the
structure of the
transferred LB films. Hence, it becomes critically important to understand the
structure of
the monolayer in situ on the water surface under conditions for which the
thermodynamics
are well understood. One of these conditions is the continuous compression of
the Langmuir
monolayer film since, in general, the most accurate and reproducible
thermodynamic
measurements have been obtained during this process. However, to date,
pressure-dependent
IRRAS spectra have been collected exclusively in a "step-wise" manner, i.e.,
no IRRAS
spectra have been reported that correspond to a Langmuir monolayer undergoing
a
continuous compression.
[033] While IRRAS using conventional FTIR spectroscopy offers a variety of
instrumental advantages for investigating thin films compared to standard
transmission
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measurements, the technique does suffer from several inherent limitations. The
inherently
weak monolayer absorbance bands result in a relatively poor signal-to-noise
(S/1~ spectrum
and, since environmental fluctuations are difficult to minimize, spectral
compensation for the
water vapor that is present above the Langmuir trough remains a challenge.
[034] Over the last decade the SIN observed in IRRAS experiments on dielectric
substrates has gradually improved due to advances in the instrumentation and
in the optical
interface. There are a variety of ways to minimize the problem of water vapor
compensation,
including strict humidity control and a shuttle transport system that allows a
sample trough to
be repeatedly replaced with a reference trough allowing both a sample and
reference
spectrum to be recorded.
[035] Another way to minimize the problem of water vapor compensation includes
the application of polarization modulation infrared reflectance-absorbance
spectroscopy (PM-
IRRAS). In PM-IRRAS, the polarization of the incident beam undergoes a fast
modulation
between two orthogonal directions via a photoelastic modulator. The detected
signal passes
through a two-channel electronic system and is mathematically processed to
give a
differential reflectivity spectrum. In theory, because of the fast
polarization modulation, the
PM-IRRAS signal is devoid of all polarization-independent signals such as
strong water
vapor absorptions, instrumental drifts and fluctuations.
[036] Despite the previously mentioned limitations, the IRRAS or PM-IRR.AS
technique has been used to investigate a variety of Langmuir monolayers,
including studies of
fatty acid, phospholipid and phospholipid-protein monolayers. The technique
has been used
to provide information on lipid conformation, molecular tilt angle, and the
structure of head
groups, as well as protein secondary structure and orientation. However,
neither IRRHS nor
PM-1RRAS (both of which utilize FTIR) have been able to provide in situ time-
resolved
measurements of Langmuir monolayers in the 1 ms to 1 s time regime, nor have
any of the
known techniques been able to simultaneously provide multiple independent
measurements.
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[037] Hence, the need for a non-scanning instrument with convenient delivery
and
detection of IR radiation could never be stronger. For example, applications
requiring on-line
studies of micro mechanical deformation in polymer thin films during
processing, in situ
structural studies of aging in Light Emitting Diodes (LEDs), and the
monitoring of inorganic
(silicon, SiN, etc.) thin film growth on flexible polymer substrates would all
benefit from an
IR instrument with no moving parts, which as a consequence, will also be
robust and
portable. Such a portable instrument would facilitate materials research by
providing a
powerful new tool for thin film studies, especially those with fluctuating
sampling geometries
or in a remote sample location.
[038] Further advantages for such a non-scanning, real-time instrument in the
IR
range could be found in environmental monitoring, including monitoring near
military or
civilian personnel during potential chemical or biological warfare attacks.
The complex
chemical compositions in such agents show strong IR absorbance, and thus could
be readily
identified.
[039] In spite of the inroads made in spectroscopy by spectrographs in the
visible
and near-infrared range, primarily due to the progress in CCD detectors
mentioned
previously, FT instrumentation still remains dominant in spectroscopy in the
mid to far-
infrared range and, therefore, instruments in this range are still extremely
limited by the
operating environment of the interferometer.
(040] Further, all spectral techniques require the collection of a reference
spectrum
for comparison with that obtained from the sample. In almost all cases, these
two
measurements are done in series, basically doubling the time of measurement.
If this time is
long, as in the case of obtaining spectra of thin films or of molecules in the
gas phase, then
variations in the instrument or sample conditions, for example, due to
temperature or
humidity fluctuations in the instrument or environment, can prevent
compensation of the
instrumental background.
[041] Thus, making background and sample measurements in parallel removes or
compensates for any instrumental fluctuations, reduces the total time of
spectral collection,
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preserves sample integrity in case there is "aging" or degradation with time,
and provides
additional advantages in the portability of such an instrument since "real-
time" background
compensation in aggressive field or non-laboratory environments can be made.
[042] What is needed, then, is a robust, compact, and portable instrument
(with no
moving parts) in the IR range to address specific applications where sample
fluctuations
cause significant deterioration of the signal-to-noise ratio in conventional
FTIR spectra.
[043] What is further needed is a portable and reliable IR spectroscope which
allows
multiple, simultaneous spectral measurements.
[044] Still what is further needed is a real-time, sensitive and relatively
high-
resolution apparatus and method for IR spectroscopic materials analysis, which
does not rely
upon interferometric or a calculation-intensive Fourier Transform approach,
and which is
relatively insensitive to harsh environments, including high vibration and
wide temperature
variations, and which provides the ability to compensate for background
spectral components
and component degradation in real-time.
[045] There is, therefore, also a need for an apparatus and device capable of
collecting multiple independent spectra simultaneously with background
environment
compensation and compensation for the aging of components, including
orthogonally
polarized measurements, which allows time-resolved measurements on Langmuir
monolayers, including time-resolved molecular orientation measurements.
SUMMARY OF THE INVENTION
[046] The present invention solves many of the aforementioned problems of
providing a robust, high-resolution and sensitive apparatus and method for
determining
background-compensated IR spectra of multiple samples, without the use of
moving parts, or
calculation-intensive Fourier Transform interferometric techniques.
[047] The multi-beam planar array infrared (PAIR) spectrograph offers numerous
advantages over conventional FT-IR interferometry for a variety of important
materials
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characterization applications. Some of these include routine IR spectroscopy,
time-resolved
IR spectroscopy, time-resolved spectroscopic imaging, monolayer spectroscopy
and on-line
monitoring of processes in aggressive environments. The PAIR spectrograph may
also be
used to investigate fundamental dynamics associated with thick and thin
polymer films
undergoing an irreversible change.
[048] An extension of the apparatus and method disclosed and claimed in
International Publication WO 03/029769 and U.S. Patent No. 6,784,428 to two or
more
background-ground compensated beam measurement is made possible, at least in
part, by use
of a focal plane array (FPA) detector having a relatively large area. For
example, a 320 x 256
pixel Indium-Antimonide (InSb) FPA detector, or other suitable material, may
he used in the
construction of the PAIR spectrograph. Multiple beams from multiple samples or
multiple
spatial areas of one sample, are preferably dispersed by one or more prisms or
gratings, and
simultaneously focused on the detector.
[049] At least one of these multiple samples could be a background reference
sample, from which the spectrum of the background environment could be
determined. This
allows background compensated data from several samples to be collected
simultaneously in
real-time, or compensated data from several spatial locations in the same
sample could be
collected simultaneously. In addition, IR polarizers selective to certain
electric field
components of the 1R beam can be inserted in the optical path, thus providing
for
simultaneous collection of IR dichroic data.
[050] Multi-beam PAIR spectroscopy can be used for the "real-time" spatial
mapping of films for quality control applications in a processing line.
Compensation for
environmental factors, e.g. spectral compensation for water vapor present in
the optical path,
or for the aging characteristics of the sensor can also be accomplished in
real-time, without
complicated calibration procedures. It can also be used for detection of
chemical toxins and,
if used in conjunction with an analyze or bio-specific reagent, it can be used
to detect
biological agents in the environment, e.g., virus or bacteria. Further,
multiple beam PAIR
using the apparatus and method of the invention can also be used to measure
pollutants in
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water, dielectric film growth in semiconductors, and the development of
orientation in a
polymer film production line.
[051] One aspect of the invention includes an apparatus for determining an IR
spectrum of a plurality of sample materials using IR FPA technology to capture
the IR
spectral information for each of the samples, without utilizing a scanning
mechanism, or any
moving parts, and without the use of computation-intensive signal processing,
e.g., Fourier
Transform.
[052] This aspect includes an apparatus for simultaneously spatially
multiplexing IR
spectral information for each of a plurality of samples, and includes at least
one IR light
source; at least one sample holder which positions the plurality of samples in
an optical path;
an optically dispersive element in the optical path, wherein an emission from
the at least one
IR light source interacts with each of the plurality of samples along the
optical path to form a
corresponding plurality of sample emissions, said plurality of sample
emissions interacting
with the optically dispersive element to form a corresponding plurality of
dispersed sample
light beams, each of said plurality of dispersed sample light beams
corresponding to a
respective one of the plurality of samples; and an IR FPA detector arranged in
the optical
path, said IR FPA detector having multiple pixels arranged in plural rows and
columns,
wherein the IR FPA detector detects the corresponding plurality of dispersed
sample light
beams and provides at least one output which represents the IR spectral
information for each
of the plurality of samples.
[053] In another aspect of the invention, a real-time, non-interferometric
apparatus
using IR absorption phenomena and no moving parts during operation to
simultaneously
perform chemical analysis in a plurality of sample volumes includes a
broadband light
source; at least one sampling accessory for positioning the plurality of
sample volumes so
that at least a portion of light emitted from the broadband light source
interacts with each of
the plurality of sample volumes; adjustable means for optically dispersing the
at least a
portion of light interacted with each of the plurality of sample volumes to
obtain a plurality of
corresponding dispersed sample beams; a two-dimensional IR detector array
having a
plurality of detector elements arranged in rows and columns, optical coupling
means for
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coupling the plurality of corresponding dispersed sample beams onto the two-
dimensional IR
detector array; and processor means for controlling the two-dimensional IR
detector array and
providing non-interferometric chemical analysis of said plurality of samples
based at least
upon an IR absorption spectrum in one or more particular wavelength regions,
wherein each
of the plurality of corresponding dispersed sample beams are projected on
multiple rows in a
different area of the two-dimensional 1R detector array, and corresponding
column detector
elements in each of the multiple rows are added together within each different
area of the
two-dimensional IR detector array to determine an intensity of an IR spectral
component at a
particular wavelength in real time, wherein a signal-to-noise-ratio of a
signal representing the
intensity of the IR spectral component at the particular wavelength is
increased by adding the
corresponding column detector elements in each of the multiple rows.
[054] In a method relating to this aspect of the invention, chemical analysis
of the
plurality of samples is performed by determining an IR absorption spectrum of
each of the
plurality of samples. The method includes projecting at least a portion of an
emission of the
broadband light source onto the plurality of sample volumes; interacting the
at least a portion
of an emission of the broadband light source with the plurality of sample
volumes; providing
a corresponding plurality of sample emissions to an optically dispersive
element; forming a
plurality of corresponding dispersed sample beams; optically coupling the
plurality of
corresponding dispersed sample beams onto the two-dimensional IR detector
array, wherein
each of the plurality of corresponding dispersed sample beams are projected on
multiple rows
in a different area of the two-dimensional IR detector array; non-
interferometrically
processing, within each different area of the two-dimensional IR detector
array, an output
from each detector in a plurality of rows of detectors, wherein each column of
detectors
represents a particular wavelength within each different area; determining the
IR absorption
spectrum of each of the plurality of samples by evaluating a processed output
from said each
detector; and at least partially analyzing a chemical makeup of each of the
plurality of
samples by comparing the processed output to one or more reference standards.
[055] In another aspect of the invention, a method of simultaneously
determining an
IR spectrum of a plurality of sample volumes using a non-interferometric
apparatus capable
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of operating using no moving parts is disclosed. The method includes providing
an IR
source; positioning the plurality of sample volumes in an optical path;
interacting at least a
portion of an emission of the IR source with the plurality of sample volumes
along the optical
path to form a plurality of sample emissions; optically dispersing the
plurality of sample
emissions to form a corresponding plurality of dispersed sample beams;
detecting each of the
plurality of dispersed sample beams on spatially separated areas on a focal
plane array having
rows and columns of pixels thereon; and simultaneously and non-
interferometrically
determining the IR spectrum of each of the plurality of sample emissions by
evaluating a
combined output from each spatially separated area of the focal plane array,
wherein each
column of pixels in one of the spatially separated areas represents a
wavelength contained
within an associated one of the plurality of sample emissions.
[056] In another aspect of the invention, an apparatus is provided for
simultaneously
collecting, processing, and displaying IR spectral information for one or more
samples. The
apparatus includes a plurality of IR light sources; at least one optically
dispersive element; a
plurality of optical paths; an IR FPA; processing means for processing an
output of the IR
focal plane array and determining the IR spectral information; and display
means for
displaying the IR spectral information, wherein each of the plurality of IR
light sources
presents a different angle of incidence with respect to the one or more
samples, wherein each
of the plurality of optical paths directs an associated one of a plurality of
reflected IR beams
to a different spatial area on the IR FPA.
[057] In another aspect of the invention, a method of determining anisotropic
IR
optical constants of a material is provided. The method includes providing a
substrate;
projecting an IR light source onto a surface of the substrate at a non-
perpendicular angle of
incidence; transmitting a first transmitted portion of the 1R light source
through the substrate;
coupling the first transmitted portion of the IR light source through an
optical path and onto a
first area on the FPA; providing a film material on the substrate; projecting
the 1R light
source onto a surface of the film material at the non-perpendicular angle of
incidence;
transmitting a second transmitted portion of the IR light source through the
film material and
the substrate; coupling the second transmitted portion of the IR light source
through the
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optical path onto a second area on the FPA; rotating a mirror in the optical
path to move the
second area on the FPA so as to coincide with the first area on the FPA; and
determining an
angle of refraction within the film material by measuring an angle of rotation
of the mirror.
[058] There are a variety of ways to minimize the problem of water vapor
compensation during measurement of IR sample spectra. In yet another aspect of
the
invention, a Planar Array Infrared Reflection-Absorption Spectroscopy (PA-
IRRAS)
arrangement for measuring an orientation of a thin film on a substrate is
provided, which
includes an IR source; two orthogonally polarized filters which receive an IR
light beam from
the IR source; a PAIR detector; and a processor, wherein two orthogonally
polarized IR
beams emanating from the two orthogonally polarized filters are reflected from
the thin film
and detected by the PAIR detector, wherein a differential reflectivity
spectrum is calculated
by the processor, and wherein the differential reflectivity spectrum is
substantially free of any
polarization-independent signals including water vapor absorptions,
instrumental drifts, and
signal fluctuations. The processor then uses the calculated differential
reflectivity spectrum
to determine a molecular orientation of the thin film.
(059] In a related aspect, a method of determining an orientation of a thin
film on a
substrate is provided, which includes providing an IR source; producing two
orthogonally
polarized light beams from the IR source; reflecting the two orthogonally
polarized light
beams from the thin film, detecting the two reflected orthogonally polarized
light beams with
a PAIR detector; and calculating a differential reflectivity spectrum in the
processor using the
two reflected orthogonally polarized light beams, wherein the differential
reflectivity
spectrum is essentially free of any polarization-independent signals including
isotropic water
vapor absorptions, instrumental drifts, and signal fluctuations.
(060] In all aspects of the invention, either direct lens coupling through an
aperture,
or through mid-IR optical fibers, for example, may be used to collect sample
light emissions
representing the samples. Use of optical fibers may provide desired
flexibility in placement
of the apparatus, and allow remote sensing of, for example, smokestacks, and
also allow
easier implementation of multiple channel detection and chemical analysis.
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[061] The apparatus and method of the present invention do not require moving
parts to determine spectral information. The method and apparatus are,
consequently, well
adapted to relatively harsh environments, such as, for example, high vibration
environments
in a manufacturing plant, or temperature extremes, as might be found in the
field.
[062] At least partially as a result of the no-moving-part construction, the
method
and apparatus may also be used in various industrial applications to measure
and detect the
thickness, either in transmission or reflection mode, the chemical structure
and orientation of
coatings/films (solid, liquid, chemically bound, physically adsorbed) on
liquid surfaces,
including but not limited to water, oil and other solvents, and also to
measure the thickness,
orientation and chemical structure of films electrochemically deposited on
solid substrates,
including but not limited to metals and semiconductors.
[063] The multi-beam PAIR spectrograph offers numerous advantages over
conventional FT-IR interferometry for a variety of important materials
characterization
applications, including routine IR spectroscopy, time-resolved IR
spectroscopy, time-
resolved spectroscopic imaging, monolayer spectroscopy, on-line monitoring of
processes in
aggressive environments, and probing fundamental dynamics associated with
thick and thin
polymer films undergoing an irreversible change.
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BRIEF DESCRIPTION OF THE DRAWINGS
[064] The features and advantages of the invention will be more readily
understood
upon consideration of the following detailed description of the invention,
taken in
conjunction with the accompanying drawings in which:
[065] FIG. 1 provides a representation of a conventional FTIR interferometer;
[066] FIG. 2 provides two different schemes used for conventional
interferometry
based on Fourier Transform, but which do not require moving parts to generate
a difference
in optical path length;
[067] FIG. 3A depicts an aspect of the present invention suitable for non-
interferometric IR spectroscopy of multiple samples in one sample holder is
accomplished
using no moving parts;
[068] FIG. 3B depicts another aspect of the present invention in which non-
interferometric IR spectroscopy of multiple samples is accomplished using
multiple IR
sources and optical paths, and no moving parts;
[069] FIG. 3C shows further optical path details for an arrangement suitable
for the
spatial multiplexing of multiple beams for the apparatus depicted in FIG. 3B;
[070] FIG. 3D shows sampling with polarized light;
[071] FIG. 4 shows another aspect f the invention using a Pellin-Broca prism
as the
optically dispersive element, and which shows IR optical fiber being used to
couple the light
into the apparatus;
[072] FIG. 5 provides a graph of refractive index dispersion of ZnSe, and
optical
refraction for the Pellin-Broca prism of Fig. 4;
[073] FIG. 6 shows a configuration suitable for real-time background
correction;
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[074] FIG. 7 shows an arrangement suitable for measuring multiple angles of
incidence of IR radiation reflected from a surface;
[075] FIG. 8 depicts a stratified three-phase system representative of a thin
film on a
substrate;
[076] FIG. 9 shows an arrangement suitable for reflection/refraction
measurement
used in determining optical constants of a thin film;
(077] FIG. 10 shows an arrangement for conventional Polarization Modulation
Infrared Reflectance-Absorbance Spectroscopy (PM-IRRAS); and
[078] FIG. 11 shows an arrangement of the invention suitable for Planar Array
Infrared Reflectance-Absorbance Spectroscopy (PA-IRRAS).
DESCRIPTION OF VARIOUS AND PREFERRED EMBODIMENTS
[079] A first aspect of the invention will be explained with reference to Fig.
3A.
Apparatus 300 includes an IR light source 310, which may be any common IR
light source,
including, for example, tungsten lamps, Nernst glowers, or glowbars or, in
some applications,
IR radiation from the sun may be used. In a preferred embodiment, the IR
source may be a
IR Emitter with ZnSe window, manufactured by Cal-Sensors, for example.
Ideally, IR
source 310 has a "flat" or uniform intensity across the IR spectrum, or at
least a portion of the
IR spectrum. However, if IR source 310 is not uniform, such non-uniformity may
be
accounted for during the analysis process.
[080] Adjustable aperture 320 is used, at least in part, to establish the
resolution of
the apparatus, i.e., a smaller-sized opening provides higher resolution.
Adjustable aperture
320 may be a circular iris or, in a preferred embodiment, an adjustable
rectangular slit, having
a length dimension, for example, of approximately 1 cm, and an adjustable
width of 0-2 mm.
Such a slit is manufactured by RIIC, as model WH-O1.
(081] Sampling accessory 330 positions one or more sample volumes, which
contain
one or more samples to be analyzed, in the optical path. Sampling accessory
330 may be, in
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a preferred embodiment, a simple sample holder, which merely positions a small
sample
volume of material to be sampled, e.g., polymer film, near IR source 310, or
it may comprise
a more elaborate sampling volume arrangement known and used for sampling
gases, or may
hold a plurality of samples.
[082] Gases, which have a lower density than solids or liquids, may require
such a
relatively more elaborate sampling accessory having a set of mirrors or other
suitable
arrangement (not shown) to provide for multiple passes of the IR source
through the sample
volume. Such multiple passes are useful in ensuring that sufficient optical
density is
achieved for the IR absorption phenomena to be reasonably measured. Multiple
pass
arrangements may also be used, in other embodiments, to monitor smokestack
emissions, or
to monitor hazardous chemical fumes or vapors in laboratory, military, or
industrial
environments.
[083] Sampling accessory 330 could also comprise optics including a telescope
or
microscope arrangement, or coupling to a single optical fiber or bundle of
optical fibers.
[084] Turning now to Fig. 3B, apparatus 300' may include a plurality of
sampling
accessories 330, 331 (or more) that may be used, along with appropriate
optics, to pass a
portion of an emission from IR source 310 through sampling accessory 330, and
a portion of
an emission from second IR source 311 through aperture 321, and sampling
accessory 331.
[085] Optically dispersive element 350 receives portions of an emission from
IR
light sources 310 and 311 that are passed through respective sample volumes,
and reflected
from mirrors 340 and 341. The entire IR spectrum, representative of IR source
310, may not
be passed through the sample volume because of the absorption of one or more
IR
wavelengths in the sample volume within sampling accessory 330. The non-
absorbed IR
wavelengths then interact with optically dispersive element 350 to form a
dispersed light
beam, which separates or spreads, in one direction, the wavelengths present in
the IR light
exiting sampling accessory 330.
[086] Optically dispersive element 350 may be, in one aspect of the invention,
a
ruled diffraction grating having 300 lines (or "grooves") per mm, with a blaze
wavelength of
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4.0 pm, for example. Such a grating is manufactured, for example, by SPEX, as
model 300
g/mm Holographic Grating. Although not shown, there may be two optically
dispersive
elements, appropriately arranged in one or more associated optical paths. The
second
optically dispersive element may have, for example, 50 grooves per mm, and a
blaze
wavelength of 9.Omm, to allow two different spectral regions to simultaneously
be collected
on the FPA, to more efficiently use more of the surface area of the FPA for
signal analysis,
and to allow for simultaneous analysis of multiple signals.
[087] In another aspect, the optically dispersive element may be a prism, as
shown
in Fig. 4. In a further preferred aspect of this embodiment, Pellin-Broca
prism 450 may be
used. In IR wavelengths, the Pellin-Broca prism may be machined from zinc
selenide (ZnSe)
in order to minimize the material absorption in these IR spectral ranges, and
to ensure
adequate optical dispersion as a function of wavelength. Figure 5 provides a
graph of
refractive index dispersion of ZnSe and optical refraction for an exemplary
embodiment of
the Pellin-Broca prism of Fig. 4. Apparatus 400 operates similarly to
apparatus 300 shown in
FIG. 3A, however variations in components are optionally present.
[088] For example, a light coupling means may include IR fiber 410, which may
also include a multi-fiber bundle; off axis parabolic mirror 440; concave
mirror 442; and
convex mirror 444. The light being projected by IR fiber 410 may include light
coming from
the sample volume being illuminated, or the 1R fiber may be used to illuminate
the sample
volume. Focusing optics 360 may be, in this embodiment, a germanium (Ge)
condensing
lens used to properly project the light emanating from prism 450 onto IR
detector 370. The
parabolic-shaped mirrors are preferable when using an IR fiber, in order to
collimate the
cone-shaped fiber output light beam. The Pellin-Broca prism may also be used
with the
optical coupling and IR source 310 in FIG. 3, as well as in the fiber optic
implementation.
Conversely, the ruled diffraction grating may be used with fiber optics,
assuming that
appropriate measures are taken to collimate the conical beam emanating from
the fiber, and
to couple the light into the system, and onto the diffraction grating, when
used as optically
dispersive element 350.
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[089] Although a diffraction grating can provide adequate resolution for many
application, the Pellin-Broca geometry may provide three benefits: (1) optical
dispersion is
only a function of the refractive indexes at different wavelengths, thus
simplifying the optical
design; (2) the two-in-one prism design has a very high angular dispersion
efficiency, and the
approximate 90° beam folding available allows a compact footprint of
the optical system to
be achieved; and (3) a Brewster angle incident configuration may be utilized
in order to
maximize the transmission of light at the ambient/ZnSe interface. The latter
is crucial in the
IR range where reflection loss is a major concern due to the high refractive
index of ZnSe
(~2.4).
[090] Based on a ray-tracing calculation with the refractive index information
shown
in FIG. 5, a 67.5° Pellin-Broca prism made of ZnSe operating in the
"short-side entrance"
geometry at approximately the Brewster angle (6B of ZnSe ~ 67°) will
give angular
dispersion of about 6° between the 3 and 13 wm wavelength beams. The on-
chip spatial
separation between the different wavelengths is determined by the focusing
optics used, the
size of the Pellin-Broca prism, and the f number of the system. A span of
between 500 to
1000 cm' of the spectral range may be focused onto the FPA horizontally (256,
320, etc.
pixels). Given the number of pixels in the FPA along the dispersion direction
of the optical
beam, the maximum resolution is about S cm I. However, using different optical
components, such as a finer grooved grating, for example, a resolution of
better than 5 cm-1 is
readily achievable for this spectrometer.
[091] Besides the Pellin-Broca prism design, special diffractive gratings
optimized
for mid-IR performance, can theoretically provide similar, if not better
throughput and
dispersion than a prism approach. However, the dependence of resolution on
both the groove
number and grating size may put more constraints on the optical design using
gratings.
Therefore, there are trade-offs to be considered when considering the use of
gratings versus
prisms. Relatively low-cost off the-shelf gratings with low groove numbers may
suffice for
many applications, and in situations where higher resolution is required than
can be currently
obtained with prisms.
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CA 02460129 2004-03-31
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t .:.. ~.~.'~s t...S if'..fF 4i'.. ..r~.~ il'mst f..[' :....~i
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fE..:~ )...~. ....~.
[092] In either case of using a prism or a diffraction grating, optically
dispersive
element 350 may be adjustable with respect to an angle of incidence between
its surface and
incident light which is projected onto the surface. Such an angular adjustment
may be used
to control the wavelength range, or spectral bandpass that is presented to IR
detector 370,
discussed below.
[093] Focusing optics 360 couples light from optically dispersive element 350
into
IR detector 370 which has a plurality of detection elements arranged at least
along a
dispersion direction corresponding to the direction of the dispersed light
beam. Typically,
incident light is projected onto more than one row of pixels, and the
projected light from the
optically dispersive element may cover 20 pixels. IR detector 370 detects the
dispersed light
beam from optically dispersive element 350, and provides an output, which is
subsequently
used to determine the IR spectral information of the sample in the sample
volume contained
in sampling accessory 330.
[094] In one aspect of this embodiment, IR detector 370 may be an InSb camera
sensitive in the 3-5 pm Wavelength range, for example, Merlin Mid model,
manufactured by
Indigo Systems. Such a detector includes a 320 x 256 pixel InSb detector, with
30 wm pixel
pitch; a 3.0 - 5.0 micron changeable cold filter; user selectable frame rates
of 15, 30 or 60
frame-per-seconds (fps) (minimum); a liquid nitrogen cooled dewar, having a
minimum hold
time of 4 hours; a noise equivalent temperature difference NEST < 20 mI~elvin;
user
selectable integration times from 10 ~s to 16.6 ms; and corrected non-
uniformity < 0.1 °10.
InSb detectors in this range may also be thermoelectrically cooled to enhance
portability.
[095] This particular InSb camera may be controlled via on camera controls or
via
an RS-232 interface with a vendor supplied Graphical User Interface, or
standard Windows~
terminal communications program, or commercially available interfaces such as
Universal
Serial Bus (USB) or IEEE 1394 standard interface. In addition, this camera
provides an
automatic gain control (AGC) algorithm, adjustable detector gain and bias to
allow viewing
of both high and low brightness scenes, and data outputs which may include
NTSC, S-Video,
and 12 bit corrected digital video. In addition, focusing optics 360 may be
provided along
_25_
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~;;;'.s o ' ,.,;~." [ ,~ ~(",~~ !, :~, H,~~ t";". ; ~ :~,"~~ ij;. Z' ";, Ir
",f~,~ ~ ,.~. , .._y ~ ;~ E,...7 i ~~y ,y,! ; . ii :~~~t .'::;=
iZef. No. 131-289 f UD 02-28PCT - "°, ' . ~:,. :~ ".~ ,.,.. ,.L~..~".
r,.,. r" ~ ,.
with IR detector 370; the above-described InSb detector is commercially
available with a 25
mm mid-IR lens.
[096] In another aspect, IR. detector 370 may be a microbolometer camera, also
manufactured by Indigo Systems as model Merlin Uncooled. This particular
camera includes
a 320 x 240 pixel microbolometer detector having 51 micron pixel pitch in a
7.5 -13.5
micron spectral range. User selectable frame rates of 15, 30 or 60 fps
(minimum) are
available. This device, in contrast to the InSb camera, is thermoelectrically
(TE) stabilized at
313K; has a noise equivalent temperature difference NEST < 100 mKelvin; and
has user
selectable integration times from 1 - 48 ~,s.
[097] This detector array may be controlled in the same manner as for the InSb
array, as discussed above. Similar detector gain controls, and data outputs
are available, as in
the InSb model.
[098] Further, in yet another embodiment, mercury-cadmium-telluride HgCdTe
("MCT") array may be used as IR detector 370, and has improved sensitivity and
bandwidth
in comparison to the InSb and microbolometer devices. Presently, such arrays
are somewhat
difficult to manufacture, and are more expensive than other available IR
detectors. Using an
available MCT FPA having a maximum frame rate of 6000 Hz, a single beam
spectrum may
be collected every 170 ps, and integration times as low as 10 qs are
achievable.
[099] Although both InSb and microbolometer types of detectors may be cooled
thermoelectrically, the sensitivity of the InSb FPA is much higher than that
of the
microbolometer FPA. As a matter of fact, the sensitivity for the InSb FPA
identified above is
better than a liquid nitrogen-cooled MCT detector conunonly used in
traditional FTIR. On
the other hand, the sensitivity of the state-of=the-art microbolometer-based
FPA is still about
one order of magnitude lower than that of liquid nitrogen-cooled MCT detector.
However,
sensitivity at the performance level of a liquid nitrogen-cooled MCT detector
is not always
necessary and, for many applications, it is possible that the lower
sensitivity of the
microbolometer FPA will not cause any significant efficiency problems in the
apparatus. In
addition, the key advantage of using an FPA, when compared to single element
detector, is
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the possibility of vertical binning. By adding the signal from a finite height
of pixels, SNR
can be significantly improved.
[0100] An optical path or light coupling means between the various elements in
apparatus 300 may include, in one aspect, standard IR mirrors 340, 341, 342 of
various
configurations to couple light from IR sources 310, 311 through the sample
volume in
sampling accessories 320, 321 onto or thru optically dispersive element 350,
and onto 1R
detector 370 through focusing optics 360. This configuration could include
multiple
sampling accessories or polarizers, for example. The mirrors may include, for
example, 3-
inch (~7.6 cm) diameter front surface aluminum mirrors, manufactured by
Newport
Corporation. Other mirror coatings available for use in the IR band may be,
for example,
copper, and preferably gold.
[0101] Turning to Fig. 3C, more details of the arrangement shown in Fig. 3B
are
shown. For example, the beam optics may be arranged and adjusted to present
the images
from each of the samples to a different position on optically dispersive
element 350. This
results in, effectively, partitioning IR FPA detector 370 into different
regions for each of the
sample emissions, or for different spatial areas of one sample. Further, such
partitions could
be used with additional IR sources and/or samples. Known methods could be used
to address
selected rows and/or columns of the FPA, for example.
[0102] The emission from either or both IR light sources 310 and 311 may also
be
arranged to interact with a background reference environment arranged along
the optical path
to provide a background reference emission, and the IR FPA detector preferably
detects the
resulting dispersed background reference light beam on a spatially separated
area form the
emissions representing the samples.
[0103] The processor preferably receives an output from the FPA including a
signal
representing the dispersed background reference light beam and, essentially in
real time,
determines compensated IR spectral information for each of the plurality of
samples by
compensating for the background reference environment.
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[0104] In another aspect, and with reference to Fig. 3D, first and second
polarizers
335, 336, orthogonally polarized with respect to each other, are placed in the
optical path to
receive separate IR emissions, which could be, for example, provided by IR
light sources 310
and 311. The resulting polarized beams both pass through the sample held by
sampling
accessory 330, for example, and resulting first and second polarized sample
emissions are
coupled along one or more optical paths to interact with optically dispersive
element 350, and
are projected onto FPA detector 370. Alternatively, a beamsplitter (not
shown), in
conjunction with the polarizers, may be used to obtain two orthogonally
polarized light
beams from one IR source beam.
[0105] The first polarized sample emission may orthogonally polarized with
respect
to the second polarized sample emission. These orthogonally polarized light
beams may be
used to determine a molecular orientation of a polymer film by comparing the
intensities of
each of the polarized beams to each other, or to empirical standards.
[0106] As mentioned previously, the IR FPA detector may preferably detect each
of
the corresponding plurality of dispersed sample light beams on spatially
separated areas of
the IR FPA detector.
[0107] In all aspects of the invention, the IR FPA detector simultaneously
detects the
corresponding plurality of dispersed sample light beams, and the at least one
output of the
FPA determines the 1R spectral information for each of the plurality of
samples at a same
instant in time. Further, the FPA preferably comprises InSb, HgCdTd (MCT), or
a
microbolometer FPA, and preferably detects light having a wavelength at least
in a mid-IR
band.
[0108] In another aspect of the invention, the IR FPA detector comprises an IR
camera. Ab InSb focal plane array (FPA) may be used to detect absorptions in
the 3-5 pm
range, while a microbolometer-based FPA may be utilized for the 7-13 pm range.
Further, a
MCT array, or other InSb or other type of array having a wider or different
spectral response
may be used. Further, the at least one output from the IR FPA detector
includes a plurality of
summed pixel outputs at each of a plurality of wavelengths present in the
dispersed light
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beam. The plurality of summed pixel outputs at one of the plurality of
wavelengths improves
a signal-to-noise-ratio of a signal representing an intensity of said one of
the plurality of
wavelengths.
[0109] In another aspect, the IR FPA detector may be partitioned into multiple
segments each containing a different subset of the multiple pixels. Each of
the corresponding
plurality of dispersed light beams are preferably projected onto an associated
one of the
multiple segments. The "partitioning" of the IR FPA is not necessarily
intended to imply an
actual physical partitioning realized in hardware, per se, but may be
implemented using
known techniques for addressing particular rows and columns of pixels on the
IR FPA using
a relatively simple software control interface between the processor and the
IR FPA.
[0110] In all aspects of this embodiment, the corresponding plurality of
dispersed
sample light beams are preferably projected onto the IR FPA detector such that
a row
direction on the IR FPA detector is essentially aligned with a dispersion
direction of said each
of the corresponding plurality of dispersed sample light beams. Each column of
the focal
plane array, within each of the multiple segments, corresponds to a particular
wavelength of
light contained in the plurality of dispersed sample light beams.
[0111] Further, within at least one of the multiple segments, an output from
one pixel
in each of a plurality of rows may be added together along one column of the
focal plane
array to improve a signal-to-noise-ratio of a signal representing an intensity
of an associated
wavelength of light.
[0112] Further, in another aspect of the invention, dispersed sample light
beams
associated with different spatial sections of one of the plurality of samples
are preferably
projected onto two or more of the multiple segments. Different wavelengths may
be
represented within at least two of the multiple segments, whether imaging
different spatial
sections of one sample, or imaging different samples in the multiple segments.
[0113] In another aspect, at least one of the plurality of samples preferably
includes a
background target containing an analyte. The analyte may be selected to react
to a specific
type of biological agent to produce an IR absorption change in the background
target.
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[0114] For example, the analyte may be a bio-specific reagent reactive to one
or more
biohazardous materials, for example, a virus or bacteria. Further, an audible
or visual alarm,
or both, may be activated when the bio-specific reagent reacts to any
biohazardous materials.
[0115] As discussed with reference to Fig. 3B, the at least one sample holder
or
accessory includes a plurality of sampling accessories, each of said plurality
of sampling
accessories positioning a different sample volume in the optical path. The
apparatus
preferably simultaneously determines IR spectral information for each of the
different sample
volumes. The sample holder is preferably configured to provide an optical path
for each of
the plurality of samples that is suitable for detection of an IR absorption
phenomenon within
the optical path.
[0116] In another aspect, a plurality of optically dispersive elements 350
(not shown)
is preferably provided for forming a plurality of dispersed light beams, each
corresponding to
a different sample. Each of the plurality of dispersed light beams may be
projected onto a
different spatial area on the IR FPA detector 370.
[0117] In one aspect, a display for displaying an IR spectrograph for one or
more of
the plurality of samples, and means for controlling the IR FPA detector and
the display are
preferably provided. The means for controlling the IR FPA detector and the
display
preferably includes at least a processor or a personal computer.
[0118] ,Further, in a transmission mode, IR light source 310, 311 may be
transmitted
through each of the plurality of samples along the optical path. In a
reflectance mode, the
emission from the IR light source may be reflected from each of the plurality
of samples
along the optical path.
[0119] In another aspect of the invention depicted in Fig. 4, the optical path
may
include the use of an optical fiber or optical fiber bundle, particularly
multimode IR optical
fibers, such as, for example, fiber model C1-500 manufactured by Amorphous
Materials, Inc.
Different sample types and sampling geometry may advantageously allow a mid-IR
optical
fiber to be incorporated between the source and dispersing element to deliver
the IR source to
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the sample volume, and to provide an optical path for the IR light after
absorption in the
sample volume to the dispersive element.
[0120] Optical fibers with loss below 1 dB/m in the mid-IR range (including
the 3-5
or 7-13 pm range), are commercially available. These multimode fibers offer
features such
as flexibility and ease-of use as found in their fiber counterparts in the
visible and near-IR
range. The thermal and mechanical properties of these optical materials have
been improved
dramatically over the past decade.
[0121] When combining a FPA detector and a multichannel fiber bundle,
simultaneous measurements of several samples, or the same sample at different
locations,
become possible. This means that the proposed spectrometer can offer multiple
detection
channels with a single instrument, therefore dramatically reduce the cost-of
ownership on a
per channel basis. In the general design scheme shown in FIG. 4, off axis
parabolic mirror
440 is utilized to collect and collimate the signals from either the entrance
aperture or an
output end of IR fiber 410 or fiber bundle. An adjustable aperture 420 may be
used to control
the size of the collimated beam, and subsequent condensing optics 442, 444 are
used to
couple the signal into the prism. The combination of the beam condensing
optics and
aperture size determines the f number of the spectrometer, and therefore the
spectral
resolution.
[0122] Processor 480 may be a special purpose computer adapted specifically
for IR
spectral processing, and may be implemented in so-called "firmware" or
integrated circuits
such as a custom application specific integrated circuit (ASIC), or may be a
common
personal computer (PC). Processor 480 preferably provides control
software/hardware for IR
detector 470.
[0123] In an aspect of the invention using any one of the FPAs discussed
above,
"Talon Ultra" Data Acquisition System, manufactured by Indigo Systems may be
used.
Processor 380 may be implemented as a dedicated IR image acquisition station
which
includes a 500 MHz Pentium~ III PC, 256 MB RAM, 12 GB hard drive, Windows~ NT
4.0
operating system, IR camera digital interface cable (10 ft, or ~3 m), high
speed 16 bit frame
-31-
CA 02460129 2004-03-31
Ref. No. 131-2 t°;;~ sp°°: .,.~~,.: , ; ~ ii""'~!t:r:,
t~°H) ;~ ~ ,, :.,;: ,;~;;;; ~~,.h~ :. a ,:. i ."s ;; - ;
89 / LTD 02- .28PCT ... .""t ;,.:,., . '' ~.:,s: ::~,. .".;~ G,;;..~l.:a: ".
,u.:;E ="~ ~t'~ ~ . ~,~;.E t~:", ,,:,~t ".f,
.n,.,. ",; ,;::;t: t.;:, .~.....: :~,.:: :""
grabber, camera interface software, and image analysis software based on Image
Pro~ 4.0 or
equal. Such an exemplary package provides a full range of utilities for
processing,
measuring, analyzing, and outputting images to capture, study, manipulate, and
store images
and data from the IR camera.
[0124] Display device 390 may be either a standard computer monitor such as a
CRT
or LCD display, or may be a printing device.
[0125] Although this particular exemplary embodiment may use the PC system
memory for data acquisition, a special-purpose, dedicated high-speed memory
may also be
utilized (not shown). For added portability, processor 380 of 480 may be
incorporated into a
laptop or notebook computer, with an integral LCD display.
[0126] In an exemplary embodiment, software running on processor 380 or 480
preferably provides a wide variety of features such as real-time histograms;
real-time digital
filtering; real-time frame averaging, a user definable region-of interest
(ROI); full-featured
data display, reduction, analysis capability; and Visual Basic-compatible
macro language for
automating data collection, analysis, and reporting.
[0127] In this type of application, "real-time" is preferably considered to be
less than
one second, from initialization, through sampling and analysis, and is even
more preferably
considered to be less than 500 ms, and is even more preferable to be less than
20 ms. This
type of response time provides favorable results over the conventional
scanning and
interferometric techniques. Further, "real-time" detection more preferably
means the ability
to continuously monitor a process as it happens, where the time domain between
collected
data sets, or duty cycle is, in general, in the 5-100 p.s range.
[0128] Additional analysis software may operate in processor 380, 480 to
analyze the
IR spectral information, and to determine one or more specific functional
groups found in the
sample volume, e.g., fluorocarbons, hydrocarbons, or complex molecular bonds
or
"signature" functional groups, such as those found in chemical or biological
warfare agents.
Further, an alarm, either audible or visual, or both may also be activated if
a particular
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signature functional group or chemical composition is determined to be in the
sample
volume.
[0129] Although some components of apparatus 300, 400 are adjustable to
facilitate
setup or to provide for optimal data collection, it should be noted that
apparatus 300, 400 are
capable of determining IR spectral information using no moving parts
whatsoever during
operation.
[0130] The non-interferometric apparatus of the first embodiment is operated
to
determine an IR spectrum of a sample in a sample volume by providing an IR
source;
positioning the sample volume in the optical path; passing at least a portion
of an emission of
the IR source through the sample volume and into the optical path; optically
dispersing at
least a portion of an emission of the 1R source to form a dispersed IR light
beam; detecting
the dispersed IR light beam using the plurality of detectors; and non-
interferometrically
determining the IR spectrum of the sample by evaluating an output from the
plurality of
detectors. In a more preferred method, a two-dimensional detector array, such
as a FPA, for
example, is operated, wherein each column of detectors represents a wavelength
contained
within the dispersed IR light beam, and at least two rows of detector elements
are used to
improve a SNR of the detected signal.
[0131] Before the apparatus may reliably be used, IR sources 310, 311 must be
calibrated, or preferably at least the spectral intensity across the band of
interest must be
known, in order to compensate for possible non-uniform source intensity.
[0132] Conventionally, the source calibration process included a serial
process of
collecting the background power spectrum without a sample volume in the
optical; collecting
the sample power spectrum; and then dividing (or forming a ratio of) the
sample power
spectrum by the background power spectrum to determine the sample
intensity/background
intensity, or transmission, for every frequency position reported by the
apparatus.
Customarily, the data is further processed by a logarithmic operation, i.e.,
determining the
absorbance spectrum (ABS), as
ABS ~c -log,o (sample/background).
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[0133] However, with the multi-beam approach of this invention, source and
environment calibration are preferably carried out simultaneously with sample
emission
detection. Processors 380, 480 then compensate the sample measurements
essentially in real-
time, using the source and environment calibration data.
[0134] Once an absorbance spectrum has been determined, the disclosed
apparatus
and method may be used in industrial or environmental process monitoring to
measure a
thickness of a solid or liquid film or coating on another solid or liquid, for
example.
[0135] Based on the general operation procedures describe above, the
absorbance
spectrum of a sample is obtained with the disclosed invention. The quantity of
absorbance
(ABS) can be expressed, in general, as follows:
ABS=AxBxC,
where A is the absorption coefficient of the absorbing functional groups
present in the
sample; B is the path length within the sample (thickness), and C is the
concentration of the
functional groups. This quantitative relation is widely known as "Beer's Law".
[0136] Concentration and thickness measurements can be made using a standard
sample with known concentration C and known thickness B, to calculate the
absorption
coefficient A for any vibrational band shown by that sample. Once A is known
for the
absorption band, one then can use Beer's Law to measure either the
concentration or the
thickness.
[0137] For example, in a film processing line, if the material formulation is
held
constant, then the corresponding C and A values are also constant. In this
case, one can use
the disclosed invention to monitor the film thickness, since the absorbance
level is directly
proportional to B. On the other hand, in a semiconductor chemical vapor
deposition (CVD)
processing chamber, for example, the concentration of the gaseous species can
be measured
with the disclosed invention since A (a known species) and B (a fixed chamber
size) are held
constant, leaving the concentration to be determined as being directly
proportional to the
measured absorbance.
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[0138] Orientation measurements are made in the following way. When non-
polarized IR light is used in 1R measurements, all functional groups with the
matching
vibration frequencies will cause absorption. However, when the incident IR
light is linearly
polarized so that only electromagnetic waves oscillating in a particular
direction are passed,
then only the functional groups having both matching frequencies and a dipole
moment
change in the same direction as the polarized light can absorb the incident
light.
[0139] For randomly oriented samples, all dipole directions are equally
sampled, and
therefore no dependence on the polarization direction would be observed. On
the other hand,
for samples with preferred orientation caused by processing steps, there would
be much
stronger absorbance when the polarization direction matches that of the sample
dipole change
direction. By comparing the absorption spectra with polarized and non-
polarized IR light,
one can deduce to what extent the sample under study is oriented, and in which
direction.
[0140] The polarization of infrared light is often accomplished with the use
of a gold
wire polarizer. This optical device may be composed of, for example, finely
separated gold
wires arranged in parallel on a IR transparent substrate, such as ZnS.
[0141] The quantitative relation between the polarization direction and the
sample
dipole direction is depicted as follows:
~SObserved °0 COS ~~~,
where O is the angle between the sample's dipole moment change direction
during the
vibration, and the polarization direction of the incident IR light. From the
above relation, one
can see that, when O = 90°, there will be no absorption, even if the
vibration frequency
condition is satisfied.
[0142) In another aspect of the invention, a method further includes adjusting
an
optical dispersion of the plurality of sample emissions to control a range of
wavelengths in
the plurality of dispersed sample beams. Typically, an angle of incidence on
either optically
dispersive element 350, e.g., a grating, or prism 450, e.g., a Pellin-Broca
prism, is adjusted to
vary the range of wavelengths presented to the IR FPA 370, 470.
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[0143] The method may further include increasing a signal-to-noise-ratio by co-
adding a plurality of pixel outputs in said each column of pixels in one of
the spatially
separated areas.
(0144] As previously mentioned, in another aspect, the method includes
simultaneously evaluating a reference spectrum of an environmental background;
and
correcting the IR spectrum of each of the plurality of sample to account for
the reference
spectrum of the environmental background. The method may also include
simultaneously
evaluating a spectrum of the IR source; and correcting the IR spectrum of each
of the
plurality of sample to account for the spectrum of the IR source.
[0145] In another aspect, the method may include processing the IR spectrum of
each
of the plurality of sample emissions to identify one or more signature
functional groups in the
plurality of sample volumes; and enabling an alarm if one or more signature
functional
groups, for example, a chemical or biological warfare agent, are found in any
one of the
plurality of sample emissions.
[0146] In this regard, the method may include providing a background target
having a
bio-specific reagent thereon; and reacting the bio-specific reagent with a
sample volume
containing said one or more signature functional groups.
[0147] In another aspect of the invention, a method further includes
maintaining the
broadband light source, the optically dispersive element, and the two-
dimensional IR detector
array relatively motionless at least with respect to each other at least
during said steps of
projecting, interacting, coupling, forming, and optically coupling steps.
[0148] The optical coupling step may include fiber optic coupling of the
sample light
emissions, and/or the projecting step may include fiber optical coupling a
portion of the
emission of the broadband light source into the plurality of sample volumes.
[0149] In yet another aspect of the invention, the method may further include
determining, from the IR absorption spectrum of one or more of the plurality
of samples, at
least one physical attribute of the one or more of the plurality of samples,
wherein at least one
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physical attribute is continuously determined essentially in real-time. The
physical attribute
may include a molecular orientation of one of the plurality of samples, for
example, which is
accomplished, at least in part, by comparing two orthogonally polarized sample
emissions
associated with said one of the plurality of samples. The physical attribute
may also include
measuring a thickness of a film in real-time, in particular, a monolayer
polymer film.
[0150) Further, each of the plurality of IR light sources 310, 311 may have a
different
intensity and, in another aspect of the invention, one or more of the optical
paths may include
a polarizing element.
[0151] In another aspect, the processing means may be used to ascertain a
molecular
orientation of a monolayer, including a polymer monolayer, from IR spectral
information
determined from the different spatial areas on the IR FPA, particularly where
orthogonally
polarized sample emissions are evaluated.
[0152] In another aspect of the invention, the method further includes
computing a
refractive index and an absorption coefficient of the film material. The
substrate may include
a dielectric substrate having known optical properties, as used in
semiconductor processing,
for example. The monolayer film also may be adsorbed on the substrate.
[0153] In this aspect, the method may further include projecting the IR light
source
onto a surface of the substrate at a plurality of non-perpendicular angles of
incidence; and
determining the angle of refraction within the film material by measuring the
angle of
rotation of the mirror for each of the plurality of non-perpendicular angles
of incidence.
[0154] In addition, polarized IR radiation may be projected through the film
material
and the substrate; directionally specific angles of refraction within the film
material may be
determined; and the directionally specific complex indices of refraction of
the film material
may be computed.
[0155] Further in this regard, a molecular orientation of at least one
molecular group
in the film material may be determined.
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[0156] Using the PAIR spectrograph apparatus and method of the invention it is
possible to perform external reflection measurements with real time background
compensation. In one aspect of the invention, a thin Teflon~ barrier may
preferably be
inserted in Langmuir film balance, so that two separate troughs are created,
as seen in Fig. 6.
The smaller trough may be used as a reference trough, while the larger tough
may be used as
a sample trough. The center of a relatively wide collimated infrared beam
(e.g., 4 or 5 cm)
may be reflected at the point where the two troughs are separated, i.e., at
the thin Teflon
barrier.
[0157] Half of the IR beam would then be reflected by the reference subphase,
while
the other half of the beam would be reflected by the monolayer covered
subphase. This
results in separate "sample" (located at the top of the FPA, for example) and
"reference"
(located at the bottom of the FPA) spectral images projected on the FPA pixel
array
simultaneously (see Fig. 3C, for example). In this way, spectra obtained from
the top rows of
pixels would contain information on the Langmuir monolayer, while spectra
obtained from
the bottom rows of pixels will contain data from the substrate or reference
surface, which
could be, for example, water. A ratio of these two spectra at each wavelength
will provide an
absorbance spectrum, with the water vapor completely compensated.
[0158] In another aspect, polarized infrared spectra may readily be obtained
by
measuring the sample, e.g., a film, through polarizing elements using
transmission or
reflection of the IR beam, and then immediately directing the beam through a
dispersive
element. Alternatively, an emission may be split in a beam sputter (not
shown), and then
each split beam could be passed through two different, orthogonally polarized
elements in
respective optical paths, and through the sample or samples, to determine
polarization-
specific information.
[0159] Each polarized beam would then be dispersed by the optically dispersive
element, i.e., the grating or prism. This would result in two orthogonally
polarized beams
being imaged on different rows of the FPA detector simultaneously, resulting
in two separate
,polarized IR spectra. Hence, polarized infrared spectra would then be
available from the
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same place on the film at the same point in time, allowing determination of
time-resolved
dichroic ratios, for example.
[0160] Four spectral images (s-polarized reference, p-polarized reference, s-
polarized
sample and p-polarized sample) may be simultaneously projected onto the FPA.
Orientation
values could then be determined by comparing measured dichroic ratios with
theoretical
dichroic ratios obtained from simulations or handbooks, for example.
[0161] Further, for thin films, e.g., Langmuir films, time-resolved
measurements in
the sub-millisecond time regime may be obtained during the recording of a
pressure area
isotherm for polymers of interest by using signal averaging. The PAIR
instrument's multiple
spectral image capability allows higher intensity sources to be used to
collect both s-polarized
and p-polarized spectra (reflectivity is significantly higher for s-polarized
radiation than for
p-polarized radiation) of monolayers during compression, thereby providing a
continuous
molecular picture of the development of order and orientation at all points
along the isotherm.
[0162] In another aspect of the invention, using the PAIR spectrograph,
external
reflection measurements using multiple angles of incidence may simultaneously
be made. A
diagram of the arrangement is shown in Fig. 7, where small mirrors or optical
fibers may be
used to simultaneously collect multiple IR beams produced by multiple IR light
sources. The
separate infrared beams are directed to different areas of the entrance slit
of an aperture
portion of the instrument. This process ultimately produces multiple spectral
images in
different locations on the FPA, where each image corresponds to a different
angle of
incidence. Accurate molecular orientations of Langmuir monolayers may be
determined
using multiple angles of incidence. This additional capability of the PAIR
instrument
provides more accurate determination of molecular orientations in the
investigation of
Langmuir polymer monolayer characteristics.
[0163] In a further aspect of the invention, molecular orientations of thin
films using
polarized infrared spectra of the thin films may be determined. There are
known techniques
for doing so, however, for IR external reflection measurements from dielectric
substrates, a
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knowledge of the anisotropic IR optical constants (e.g., index of refraction
and extinction
coefficient) is required. This information is often difficult to obtain.
[0164] Historically, there have been attempts to obtain anisotropic refractive
indices
using IR ellipsometry. The use of this technique has been typically limited to
the
determination of optical constants of very simple molecules, whose dimensions
and
orientation could be easily deduced from a model. Other methods use external
reflection,
attenuated total reflection (ATR), or transmission IR spectrometry to obtain
optical constants.
[0165] More accurate IR spectroscopic methods have exploited the
interdependence
of the refractive index and the extinction coefficient via the Kramers-Kronig
relationship
along with the Fresnel equations to obtain the optical constants.
Unfortunately, these
methods often require a number of assumptions that can affect the reliability
of the resulting
complex refractive indices. This point is underscored by the fact that many
researchers have
used isotropic optical constants obtained from literature data, measured in
bulk, that can
further compromise the reliability of orientation measurements. It is clearly
desirable to have
a reliable technique for determining infrared optical constants that uses the
same infrared
spectrometer employed in the thin film analysis.
[0166] With this in mind, the PAIR spectrograph of the present invention may
be
applied to determine the anisotropic optical constants of the thin films. From
an optics
perspective, a monolayer film adsorbed on a dielectric substrate can
conveniently be
considered as a stratified three-phase system, such as that shown
schematically in Fig. 8. The
optical properties of the jth phase are characterized by the complex
refractive index n~ where
n~ is the real refractive index and k~ is the absorption coefficient, i.e., n~
= n~ + a'k~, where
i=~.
[0167] The film thickness is represented by d, which may be as small as one
molecule, i.e., a monolayer. When the values n1, n3 and d are known, a PAIR
spectrograph
can be used to determine n2 and k2, i.e., the optical constants of the
monolayer film. Film
thickness is easily determined with a known visible ellipsometer, while n~
(air) and n3 (a
typical dielectric substrate, for example) have known values.
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[0168] Figure 9 shows reflection and refraction of infrared radiation that is
incident
on a dielectric substrate, where E is the intensity of the reflected radiation
(at a given
frequency); E' is the intensity of the transmitted (refracted) radiation; 0,
is the angle of
reflection, which is equal to the angle of incidence and is easily measured,
and 92 is the angle
of refraction. With the present invention, it is possible to measure E, E' and
02. The angle 9z
may be measured using the following procedure.
[0169] First, a clean dielectric substrate is placed horizontally in the
sample position
(see Fig. 9). An arbitrary IR light source may be transmitted through the
dielectric substrate
at a known angle relative to the surface normal, where it then strikes the FPA
at a specific,
known area, "A" (not shown). The same dielectric substrate, now with an
adsorbed
monolayer film on it, is then placed in the sample position. The plane mirror
is then rotated
until light strikes the same area, "A", of the FPA. By accurately measuring
the amount of
mirror rotation necessary to return the transmitted IR beam back to area "A",
the angle of
refraction can be determined.
[0170] Once the values of E, E', 9,, 62, n1, n3, and d are all known for
several angles
of incidence, the optical constants of the monolayer film (n2 and k2) can be
determined using
the Fresnel equations and a known iterative procedure. Using multiple angles
of incidence 6~
improves the accuracy of these determinations.
[0171] The spectroscopy community and segments of industry alike are also
faced
with the problem of compensating for water vapor in the optical path. There
are a variety of
ways to minimize the problem of water vapor compensation affecting the
measurement of
sample spectra. For example, and with reference to Fig. 10, conventional
polarization
modulation infrared reflectance-absorbance spectroscopy (PM-IRRAS), using FTIR
is used.
[0172] In conventional PM-IRRAS, the initially polarized incident FTIR light
beam
(via a wire grid polarizer, for example) undergoes a fast modulation between
two orthogonal
polarization directions via a photoelastic modulator. The detected signal
passes through a
two-channel electronic system, and is mathematically processed to give a
differential
reflectivity spectrum. Because of the fast polarization modulation, the PM-
IRRAS
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differential reflectivity spectrum signal is essentially devoid of all
polarization-independent
signals, such as strong water vapor absorptions which are isotropic,
instrumental drifts, and
fluctuations in signal strength. However, this approach still relies upon the
moving part and
calculation-intensive Fourier Transform approach, which this invention
specifically disfavors.
[0173] In another aspect of the invention directed, at least in part, to the
above-
identified problem of accounting for the presence of water vapor, and with
reference to Fig.
11, a non-interferometric PAIR arrangement for Planar Array Infrared
Reflectance-
Absorbance Spectroscopy (PA-IRR.AS) for measuring an orientation of a thin
film on a
substrate includes an IR source; two fixed, orthogonal polarizers 335, 336; a
PAIR detector as
previously described and illustrated, including a processor and a display.
[0174] The orthogonally polarized beams are reflected from the thin film, and
detected by the PAIR detector. The processor then calculates a differential
reflectivity
spectrum based upon analysis of the two orthogonally polarized signals
received by the PAIR
detector. The differential spectrum is substantially free of any polarization-
independent
signals including isotropic water vapor absorptions, instrumental drifts, and
signal
fluctuations, because these effects are essentially canceled out by the
differential technique.
The differential reflectivity spectrum may be further used to determine a
molecular
orientation of the thin film. The polarization modulator may be a photoelastic
modulator, and
the FPA could be an InSb FPA, an MCT FPA, or a microbolometer FPA, for
example.
INDUSTRIAL APPLICABILITY
[0175] The application and method of the disclosed invention has wide
applicability
to a variety of industrial and environmental processes, as discussed above,
including
measuring characteristics of thin films, including optical constants.
[0176] Some further applications include a method to measure the thickness,
the
chemical structure and orientation of coatings (solid, liquid, chemically
bound, physically
adsorbed) on solid surfaces, including but not limited to semiconductors,
metals and
dielectrics.
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[0177] For example, in modern materials processing utilized in device
manufacturing,
subtle differences in the processed materials on a molecular level can
determine the success
or failure of a specific procedure. Molecular parameters such as crystalline
order, chain
orientation, and hydrogen bonding strength can have important effects on the
functionality of
the final devices. For example, liquid crystal displays used in notebook
computers rely on
the chain orientation of the polymer coating used on the glass templates to
define the "off
orientation of the liquid crystal molecules, which act as a light modulator.
[0178] The orientation of such polymer chains, however, is produced by a
"buffing"
process during which a piece of velour cloth is used to rub the polymer-coated
glass in a
given direction in order to induce chain orientation. Although it is well
known that the yield
of a flat panel display manufacturing line is critically dependent on a
successful buffing
process, there is no monitoring process used during the various manufacturing
stages that can
assess the chain orientation induced by buffing before final assembly is
completed. Hence
glass templates with bad LC aligning properties are not removed from the
assembly line until
the manufacturing process is completed. The cost of discarding failed fully
assembled
displays is several times higher than that of removing polymer-coated-and-
buffed glass plates
with poor alignment properties. The main difficulty in realizing this more
efficient quality
control process is that, until now, there was no reliable detection method
that can survive the
aggressive operating conditions found in a manufacturing plant
[0179] Process methods such as scanning probe microscopy and x-ray
diffraction, for
example, can be destructive in nature, requiring long data collection times
and removal of
samples from the production line. Consequently, the real-time statistics
needed for a
successful on-line process monitoring method cannot be achieved with
conventional
techniques. The disclosed apparatus and method can non-destructively monitor
processes in
real-time, for example, information about chain orientation of large area
samples can be
obtained in situ after the buffing process is completed.
(0180] Further, because of the multi-beam approach, different sample areas of
the
same sample or different samples can be simultaneously monitored, while
compensating for
background spectra and component aging, essentially in real time.
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[0181] The present inventors have been involved in the study of liquid crystal
alignment using different organic, inorganic and polymer surfaces, and have
shown that the
ordering, orientation, morphology, and topography of the template surface
plays an important
role in the final LC orientation. This information will be readily accessible
to the flat panel
display industry with the use of the portable infrared spectrometer disclosed
here.
[0182] An environmental application of IR spectroscopy in an aqueous
environment,
for example on a lake, river, or on the ocean could be detection and
measurement of oil or
other contaminants on the surface using reflected IR energy to determine the
presence or
absence of specific functional groups.
[0183] In addition, because the IR spectrometer is highly mobile, it may be
used as a
water pollution monitor, capable of operation in the field as discussed above.
The spectral
coverage of the invention will detect the spectral features in the fingerprint
region for most
aromatic pollutants. Since the IR bands (1600-1750 cm I) assignable to water
will not
interfere with the pollutants' signal in this spectral range, bulk analysis of
wastewater in the
field is also possible with this instrument.
[0184] Another application, discussed in connection with one aspect, above,
involves
IR spectroscopy on thin films. Many of the optical, mechanical and aging
properties of
polymers are a direct function of the order, orientation, and morphological
development,
which occurs during processing. Ironically little, if any, understanding
exists on the
structural development of orientation and order at the time when polymers are
formed into
thin films. The ability to structurally characterize the nature of polymer
chain organization
by real-time IR spectroscopic methods would allow the optimization of
processing protocols
providing eventual control of the desired amount of crystallization and
orientation relative to
the direction of micro mechanical deformation.
[0185] In many cases, this is simply manifested by specific IR bands that can
be
attributed to either traps or gauche bonds, and crystalline or amorphous
material. Following
both the intensity and the frequency of IR bands as processing (heating,
stretching, cooling)
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of thin films occurs will allow us to follow the molecular development of
orientation and
crystal morphology as it occurs.
[0186] Although many studies on polyethylene) (PE) films and fibers have been
done, the information provided is usually obtained both before processing, and
after
deformation, heating, etc., has been completed. Providing spectroscopic
information in
different spatial regions and in real-time is possible with the disclosed IR
instrument.
Depending on the spectral range of the focal plane array chosen, it is
possible to investigate
the development of crystallinity using the 1460-1470 cm' (doublet) CHz
scissors vibration,
and the 720-730 cm' (doublet) CHZ rocking vibration, which are characteristic
of the
orthorhombic unit cell. Furthermore, since the transition moments of the CHZ
rocking
components at 730 and 720 cm 1 are parallel to the "a" and "b" axes of the
unit cell ("c" is
along the chain axis) respectively, it should also be possible to determine
the extent of biaxial
orientation which is introduced in the drawing process by following the
relative intensities of
the 730 and 720 cm 1 bands in the polarized IR beam during processing.
[0187] In addition, since both sets of bands (rocking and scissors) are highly
polarized perpendicular to the polymer chain axis, their intensity can also be
used to provide
information on axial orientation related to the direction of mechanical
deformation. Likewise
the CH stretching vibrations located at 2920 cm' (asymmetric CHZ stretch) and
2850 cm'
(symmetric CHZ stretch) are strongly polarized out of the plane of the carbon
backbone and in
the plane of the carbon backbone respectively. Hence these vibrations can also
be used to
determine the extent of "a" and "b" axis orientation in biaxially oriented
films.
[0188] Unlike Raman spectroscopy where the intensities depend on changes in
polarizabilities, making the interpretation of induced orientation less
straightforward, IR
intensities depend on the change in dipole moment (for a particular
vibrational mode), and
hence provide a more direct assessment of chain orientation, provided the
direction of the
orientation of the change in dipole moment is known, relative to the polymer
chain axis. In
the case of PE, these are well known, and PE is an appropriate polymer on
which to conduct
IR spectroscopy.
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[0189] Another application is to measure a series of polyester) thin films.
Although
a number of studies on polyethylene terephthalate) (PET) films pre- and post-
processing
have appeared in the literature, no studies on PET during processing have been
reported. In
addition, little work has appeared on structurally related
poly(ethylenenaphthalate) (PEN).
Since the primary commercial market for PEN is now specialty films, because of
its
improved (relative to PET) thermal and dielectric properties, an understanding
of the effect of
various processing parameters on properties would be both fundamentally
important and
timely.
[0190] In previous studies of PET after stretching, it has been shown that
bands at
973 and 1041 cm', previously assigned to traps and gauche conformations of the
-
OCHZCH20- groups, show a considerable change in intensity (973 cm' also shifts
in
frequency) after the application of stress. This suggests that stress
transforms gauche bonds
into traps, although this evidence alone did not indicate that the overall
sample crystallinity
had increased. This required the use of the 848 cm' CHZ rocking vibration
characteristic of
traps conformers in the crystalline regions which was also followed as a
function of stress
and found to increase as the 973 cm' traps band increased.
[0191] Similar behavior was also observed for the 1386 cm' CHZ wagging mode
which has also been observed to be characteristic of traps bonds in the
crystalline regions of
PET. Since the -OCHzCHzO- groups are common linkages between the aromatic
groups in
both polyester chains, monitoring the intensity and frequency changes of the
973, 1041, 848
and 1386 cm' bands so as to understand the effect of processing parameters on
the
development of orientation, all traps content and crystallinity in both PET
and PEN films. In
addition, changes in crystallization and orientation in PET and PEN can also
verified by
following the CH stretching modes at 2870 and 2850 cm' while orientation alone
can be
followed using the C=O overtone vibration at 3200 crri'.
[0192] Further industrial applications of the disclosed apparatus include a
method to
measure and detect the thickness, either in transmission or reflection, the
chemical structure
and orientation of coatings/films (solid, liquid, chemically bound, physically
adsorbed) on
liquid surfaces, including but not limited to water, oil and other solvents.
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[0193] Although discussion of aspects of the present invention have been
directed to
determining IR spectral information, the method and system of the present
invention is not
limited merely to such a narrow implementation. For example, the present
invention may
also be applicable to the above-discussed industrial and environmental
processes, and may
further be incorporated into a control system in a batch production line to
control one or more
physical attributes, such as a polymer film thickness, or in semiconductor
processing, for
example, while measuring multiple samples simultaneously, and while
compensating for
background emissions.
[0194] It will be obvious that the present invention may be varied in many
ways. For
example, the specific optical components may be varied, as may their
particular location with
respect to the sample volumes or IR sources. Such variations are not to be
regarded as a
departure from the spirit and scope of the invention, and all such
modifications as would be
obvious to one skilled in the art are intended to be included within the scope
of the following
claims. The breadth and scope of the present invention is therefore limited
only by the scope
of the appended claims and their equivalents.
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