Note: Descriptions are shown in the official language in which they were submitted.
CA 02875303 2014-12-17
UNCOOLED MICROBOLOMETER PIXEL AND ARRAY FOR CONFIGURABLE
BROADBAND AND MULTI-FREQUENCY TERAHERTZ DETECTION
TECHNICAL FIELD
The general technical field relates to uncooled microbolometers and, in
particular, to a
configurable microbolometer pixel array suitable for broadband or multi-
frequency
absorption and detection of terahertz radiation.
BACKGROUND
Thermal detectors operate by absorbing energy from incident electromagnetic
radiation and by converting the absorption-generated heat into an electrical
signal
indicative of the amount of absorbed radiation. Perhaps the most prominent
type of
thermal detectors currently available is uncooled microbolometer detectors, or
simply
microbolometers. A microbolometer is typically based on a suspended platform
or
bridge structure having a low thermal mass, which is held above and thermally
insulated from a substrate by a support structure. The platform is provided
with a
thermistor, which is a resistive element whose electrical resistance changes
in
response to temperature variations caused by the absorbed radiation. The
thermistor
may, for example, be composed of a material having a high temperature
coefficient of
resistance (TCR), such as vanadium oxide and amorphous silicon. Because they
do
not require cryogenic cooling, uncooled microbolometers can operate at room
temperature, which makes them well suited for integration within compact and
robust
devices that are often both less expensive and more reliable than those based
on
cooled detectors.
Arrays of uncooled microbolometers can be fabricated on a substrate using
common
integrated-circuit fabrication techniques. Such arrays are often referred to
as "focal
plane arrays" (FPAs), while the individual microbolometers forming the arrays
may be
referred to as "microbolometer pixels", or simply "pixels". In most current
applications,
CA 02875303 2014-12-17
,
2
arrays of uncooled microbolometer pixels are used to sense radiation in the
infrared
region of the electromagnetic spectrum, usually in the mid-wave infrared,
encompassing wavelengths of between about 3 and 5 micrometers (pm), or in the
long-wave infrared, encompassing wavelengths of between about 8 and 14 pm.
These arrays are often integrated in uncooled thermal cameras for sensing
incoming
infrared radiation from a target scene. Each microbolometer pixel absorbs some
infrared radiation resulting in a corresponding change in the pixel
temperature, which
in turn produces a corresponding change in electrical resistance. A two-
dimensional
pixelated thermal image representative of the infrared radiation emitted from
the
scene can be generated by converting the changes in electrical resistance of
each
pixel into an electrical signal that can be displayed on a screen or stored
for later
viewing or processing. By way of example, state-of-the-art arrays of infrared
uncooled
microbolometer arrays now include 1024 by 768 pixel arrays with a 17-pm pixel
pitch.
In the last decade, there has been a growing interest in extending uncooled
microbolometer spectroscopy and sensing applications beyond the traditional
infrared
range, namely in the far-infrared and terahertz (or sub-millimeter) spectral
regions. As
known in the art, these regions of the electromagnetic spectrum have long been
relatively unused for industrial and technological purposes at least partly
due to the
lack of efficient techniques for detection and generation of radiation in this
spectral
range.
Extending the absorption spectrum of uncooled microbolometers beyond 30-pm
wavelength is not straightforward, notably because the materials used to
fabricate the
detectors absorb predominantly in the infrared, and also because the pitch of
terahertz-sensitive pixels is typically larger than that of infrared-sensitive
pixels to
avoid diffraction effects. Additionally, in order to optimize radiation
absorption in the
desired spectral band, conventional infrared microbolometer detectors
generally
include a reflector deposited on the underlying substrate to form a quarter-
wavelength
CA 02875303 2014-12-17
3
optical resonant cavity with the suspended platform. However, forming such a
resonant cavity for detecting electromagnetic radiation at wavelengths longer
than
pm is generally not practical with surface micromachining techniques commonly
used in the fabrication of uncooled microbolometers.
5
Several approaches have been devised in order to improve the spectral response
of
uncooled microbolometers beyond 30 pm. One approach that has been studied and
used in different applications relies on broadband thin-film absorbers, such
as metallic
blacks, organic blacks, and carbon nanotubes. However, fabricating these thin-
film
10 absorbers requires special deposition and processing techniques, which
are generally
not fully compatible with standard microfabrication and packaging processes of
uncooled microbolometers.
Another approach is based on antenna-coupled microbolometer detectors, in
which
the electromagnetic radiation is absorbed by planar antennas designed for
sensing
specific wavelengths determined by the geometry of the antennas. A 50-100 ohm
heat-sensitive thin-film resistor is commonly used as an antenna load to
convert
variations of incident optical power into an electrical signal, usually a
voltage or
current. Although this approach may be promising for some applications, it is
generally not fully compatible with existing microbolometer focal plane array
technology, as fabricating these antenna-coupled microbolometer detectors
involves
electron-beam or deep-ultraviolet lithography and a redesign of the underlying
readout integrated circuit (ROIC).
Accordingly, various challenges exist in the development of uncooled
microbolometer
arrays that are operable in the terahertz and far-infrared regions and that
could
advantageously provide configurable broadband or multi-band absorption
spectra.
I I
CA 2875303 2017-04-10
4
SUMMARY
According to an aspect of the invention, there is provided an uncooled
microbolometer
pixel for detection of electromagnetic radiation. The uncooled microbolometer
pixel
includes:
- a substrate;
- a thermistor assembly comprising a thermistor platform suspended above the
substrate in a spaced relationship therewith, at least one thermistor on the
thermistor platform, and an electrode structure electrically connecting the at
least one thermistor to the substrate; and
- an absorber assembly comprising:
o an absorber platform suspended above the thermistor platform in a
spaced relationship therewith, the absorber platform overlying the at least
one thermistor and the electrode structure;
o an optical absorber provided on the absorber platform, the optical
absorber being in thermal contact with the at least one thermistor and
exposed to the electromagnetic radiation, the optical absorber comprising
a set of elongated resonators determining an absorption spectrum of the
optical absorber; and
o a reflector extending on the thermistor platform over the at least one
thermistor and the electrode structure, the reflector and the optical
absorber together forming a resonant cavity and shielding the at least one
thermistor and the electrode structure from the electromagnetic radiation.
In some embodiments, the optical absorber extends on the thermistor platform
over
the at least one thermistor and the electrode structure, and the reflector is
disposed
on the substrate under the thermistor platform. Also, the at least one
thermistor and
the electrode structure each lies under one or more of the elongated
resonators. In
CA 02875303 2014-12-17
some of these embodiments, the absorber assembly may further include an
additional
optical absorber provided with a set of additional elongated resonators. These
additional optical absorber extend on the thermistor platform under the at
least one
thermistor and the electrode structure such that the at least one thermistor
and the
5 electrode structure each lies over one or more of the additional
elongated resonators.
In some embodiments, the optical absorber shields at least partially the at
least one
thermistor and the electrode structure from the electromagnetic radiation.
Additionally
or alternatively, in other embodiments, the reflector shields at least
partially the at
least one thermistor and the electrode structure from the electromagnetic
radiation.
In some embodiments, the set of elongated resonators is divided in a number of
subsets of elongated resonators, and the elongated resonators from each subset
have different lengths determining different absorption spectra.
In some embodiments, the set of elongated resonators is divided in a first and
a
second subset of elongated resonators extending along first and second
orthogonal
axes and configured to absorb a first and a second component of the
electromagnetic
radiation polarized along the first and second orthogonal axes, respectively.
In some embodiments, the optical absorber is configured to absorb the
electromagnetic radiation in a wavelength range from about 30 to 1000 pm,
encompassing the terahertz region of the electromagnetic spectrum.
According to another aspect of the invention, there is provided a
microbolometer
array including a plurality of arrayed uncooled microbolometer pixels as
described
above.
CA 02875303 2014-12-17
6
In some embodiments, the elongated resonators of different ones of the
uncooled
microbolometer pixels have different lengths determining different absorption
spectra.
In some embodiments, a combination of the absorption spectra of the plurality
of
uncooled microbolometer pixels forms a continuous broadband absorption
spectrum
of the microbolometer array.
In some embodiments, the plurality of uncooled microbolometer pixels is
arranged in
a matrix of rows and columns, and, for either each column or row, the uncooled
microbolometer pixels have substantially identical absorption spectra, and the
elongated resonators of uncooled microbolometer pixels of adjacent columns or
rows
have different lengths determining different but partially overlapping
absorption
spectra.
In some embodiments, the plurality of uncooled microbolometer pixels is
divided in
groups of uncooled microbolometer pixels, and a combination of the absorption
spectra of the uncooled microbolometer pixels of each group forms a respective
continuous absorption band in an absorption spectrum of the array.
Other features and advantages of the embodiments of the present invention will
be
better understood upon reading of preferred embodiments thereof with reference
to
the appended drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig 1 is a schematic perspective view of an uncooled microbolometer pixel, in
accordance with an embodiment.
Fig 2 is a cross-sectional perspective view of the uncooled microbolometer
pixel of
Fig 1, taken along section line 2. Fig 2A is an enlargement of portion 2A of
Fig 2.
CA 02875303 2014-12-17
7
Fig 3 is a partially exploded perspective view of the uncooled microbolometer
pixel of
Fig 1.
Fig 4 depicts theoretical absorption spectra plotted as a function of
wavelength for
different lengths of an elongated resonator consisting of a monopole-like
antenna
suspended above a perfectly reflecting plane.
Fig 5 is a schematic perspective view of an uncooled microbolometer pixel, in
accordance with another embodiment.
Fig 6 is a cross-sectional perspective view of the uncooled microbolometer
pixel of
Fig 5, taken along section line 6. Fig 6A is an enlargement of portion 6A of
Fig 6.
Fig 7 is a partially exploded perspective view of the uncooled microbolometer
pixel of
Fig 5.
Fig 8 is a schematic perspective view of an uncooled microbolometer pixel, in
accordance with another embodiment.
Fig 9 is a cross-sectional perspective view of the uncooled microbolometer
pixel of
Fig 8, taken along section line 9. Fig 9A is an enlargement of portion 9A of
Fig 9.
Fig 10 is a partially exploded perspective view of the uncooled microbolometer
pixel
of Fig 8.
Fig 11A is a schematic top plan view of an uncooled microbolometer pixel
including
three subsets of elongated resonators, in accordance with another embodiment.
The
resonators are all parallel to one another and the resonators from different
subsets
CA 02875303 2014-12-17
8
have different lengths. Fig 11B is a schematic absorption spectrum plotted as
a
function of wavelength of the optical absorber of the uncooled microbolometer
pixel of
Fig 11A.
Fig 12A is a schematic top plan view of an uncooled microbolometer pixel
including
two subsets of elongated resonators, in accordance with another embodiment.
The
resonators from different subsets are orthogonal to one another and have
different
lengths. Fig 12B depicts schematic absorption spectra plotted as a function of
wavelength of the optical absorber of the uncooled microbolometer pixel of Fig
12A,
in the case of incident electromagnetic radiation polarized along the x axis
(solid line)
and along the y axis (dashed line).
Fig 13 depicts theoretical absorption spectra plotted as a function of
wavelength of an
optical absorber including a set of aligned and identical elongated
resonators, in the
case of parallel polarization (solid line) and perpendicular polarization
(dashed line).
Fig 14 is a schematic perspective view of a microbolometer array including a
plurality
of uncooled microbolometer pixels arranged in a two-dimensional matrix of rows
and
columns, in accordance with an embodiment.
Fig 15A is a top plan view of the microbolometer pixel array of Fig 14. The
array is
divided in four subsets of pixels, each of which corresponding to one
particular
column of the array. The subsets are designed such that the elongated
resonators of
the pixels have identical lengths within each subset, but different lengths in
different
subsets. Fig 15B is a schematic absorption spectrum plotted as a function of
wavelength of the microbolometer array of Fig 15A.
Fig 16A is a schematic top plan view of a microbolometer array, in accordance
with
another embodiment. The array is divided in a first and a second group of
pixels, the
CA 02875303 2014-12-17
9
elongated resonators of the pixels in the first group being orthogonal to the
elongated
resonators of the pixels of the second group. Each group is further divided in
a
number of subsets of pixels, the resonators of pixels from different subsets
having
different lengths. Fig 16B depicts schematic absorption spectra plotted as a
function
of wavelength of the microbolometer array of Fig 16A, in the case of incident
electromagnetic radiation linearly polarized along the y axis (thick solid
line) and
along the x axis (thick dashed line).
DETAILED DESCRIPTION
In the following description, similar features in the drawings have been given
similar
reference numerals, and, in order to not unduly encumber the figures, some
elements
may not be indicated on some figures if they were already identified in
preceding
figures. It should also be understood herein that the elements of the drawings
are not
necessarily depicted to scale, since emphasis is placed upon clearly
illustrating the
elements and structures of the present embodiments.
General overview
The present description generally relates to an uncooled microbolometer pixel
for
detection of electromagnetic radiation, and to a microbolometer array
including a
plurality of such uncooled microbolometers pixels.
It is to be noted that, for simplicity, the expression "uncooled
microbolometer pixel"
may in some instances be shortened to "uncooled microbolometer",
"microbolometer
pixel", "microbolometer" or "pixel". Likewise, the "microbolometer array" may
in some
instances be referred to as "microbolometer pixel array", "pixel array" or
simply
"array".
Throughout the present description, the term "microbolometer" is understood to
refer
to an uncooled thermal radiation detector that operates by absorbing incident
CA 02875303 2014-12-17
,
,
electromagnetic radiation and converting the absorbed radiation into heat.
Meanwhile,
the term "uncooled" is intended to refer to a microbolometer that operates at
or near
ambient temperature, without requiring any type of cryogenic cooling system.
5 A typical microbolometer generally includes one or more thermistors. A
thermistor is a
resistive element whose electrical resistance changes in response to
temperature
variations caused by the absorbed radiation. This physical property is used to
measure the energy or power carried by the radiation incident on the
microbolometer,
assuming an appropriate calibration of the response of the thermistor. The
thermistor
10 is generally thermally insulated from an underlying substrate and from its
surroundings to allow the absorbed incident radiation to generate a
temperature
change in the thermistor while remaining mostly unaffected by the temperature
of the
substrate and surroundings. Microbolometers are generally fabricated using
integrated-circuit fabrication techniques and find applications, among other
fields, in
night vision, thermal imaging, remote sensing, spectroscopy, radiation and
explosive
detection, environmental monitoring, and medical diagnosis.
As for most commonly known microbolometer structures, the microbolometer pixel
and array according to embodiments of the invention may be fabricated using
conventional surface micromachining and photolithography techniques. For
example,
in some embodiments, the microbolometer pixel may be fabricated using a
monolithic
integration approach, wherein the substrate of the microbolometer pixel array,
preferably provided with an underlying ROIC, is pre-manufactured using
standard
complementary metal-oxide-semiconductor (CMOS) processes. More particularly,
it
can be an advantage of certain embodiments of the present invention to provide
uncooled microbolometer pixels whose absorption spectrum encompasses
wavelengths longer than the wavelengths of infrared radiation but whose
fabrication is
carried out using techniques similar to those of common use for manufacturing
infrared microbolometers. In such embodiments, the various components of the
CA 02875303 2014-12-17
11
microbolometer pixel may successively be deposited and patterned on the
substrate
using common thin-film deposition techniques paired with selective photoresist
and
sacrificial layer etching processes. However, it will be understood that the
uncooled
microbolometer pixel according to embodiments of the invention may be
fabricated
using other manufacturing techniques, for example bulk micromachining, without
departing from the scope of the invention.
Throughout the present description, the term "electromagnetic radiation" is
intended
to refer to photons in any appropriate wavelength range. In particular, the
terms "light"
and "optical" are meant to refer to electromagnetic radiation in any
appropriate region
of the electromagnetic spectrum, and are not limited to visible light.
Furthermore,
although some embodiments of the microbolometer pixels may be useful in
terahertz
applications, those skilled in the art will recognize that other embodiments
could
additionally or alternatively operate in other regions of the electromagnetic
spectrum,
for example in the millimeter, infrared and visible regions, without departing
from the
scope of the invention.
Broadly described, and as will be discussed further below, embodiments of the
microbolometer pixel include a substrate, a thermistor assembly and an
absorber
assembly. The thermistor assembly includes a thermistor platform suspended
above
the substrate, one or more thermistors on the thermistor platform, and an
electrode
structure electrically connecting the thermistors to the substrate. Meanwhile,
the
absorber assembly includes an optical absorber over the thermistor assembly
and a
reflector provided under the optical absorber. The optical absorber and the
reflector
together form a resonant cavity. The optical absorber is exposed to the
electromagnetic radiation and includes a set of elongated resonators which
determine
an absorption spectrum of the optical absorber.
CA 02875303 2014-12-17
12
In some implementations, by properly selecting the length and orientation of
the
individual elongated resonators, the absorption spectrum of the optical
absorber of
the pixel can be designed to achieve wavelength- and polarization-selective
absorption in specific regions of the electromagnetic spectrum, for example in
the
terahertz region. In the case of an array of microbolometer pixels, the
elongated
resonators of different pixels can differ from one another to achieve
different
absorption spectra. For example, in some embodiments, the absorption spectrum
of
the array as a whole can be configured to achieve broadband absorption of
radiation
in one or more bands in a region of the electromagnetic spectrum by
configuring the
absorption spectra of different pixels to have partially overlapping
absorption peaks.
Uncooled microbolometer pixel
In accordance with an aspect of the invention, there is provided an uncooled
microbolometer pixel 20 for detecting electromagnetic radiation, a first
exemplary
embodiment of which is illustrated in Figs 1 to 3. The microbolometer pixel 20
includes a substrate 22, a thermistor assembly 24, and an absorber assembly 26
provided with an optical absorber 28 and a reflector 30. Each of these
components
will be described in greater detail below.
Substrate
The substrate 22 may be made of silicon (Si), silicon carbide (SiC), gallium
arsenide
(GaAs), germanium (Ge) or any other suitable substrate material that may, but
need
not, support integration of semiconductor devices. The substrate 22 may be
provided
with an electrical ROIC embodied, for example, by one or more CMOS circuitry
layers
formed on or in the substrate 22 according to conventional CMOS processes.
Alternatively, the electrical ROIC may be provided outside of the substrate
22. The
substrate 22 may be a multilayered structure made of several dielectric,
semiconductor and metallic layers including, but not limited to, the reflector
30 (see,
CA 02875303 2014-12-17
13
e.g., Figs 5 to 10), one or more protective dielectric layers, and electrical
contacts for
electrical connection with the ROIC.
Thermistor assembly
In the embodiment of Figs 1 to 3, the thermistor assembly 24 includes a
thermistor
platform 32 suspended above the substrate 22 by a support structure 34, at
least one
thermistor 36 on the thermistor platform 32, and an electrode structure 38
electrically
connecting the at least one thermistor 36 to the substrate 22.
As used herein, the term "platform" generally refers to a substantially planar
and rigid
suspended structure or membrane, typically having greater horizontal
dimensions
than vertical thickness. In this regard, it is noted that throughout the
present
description, the terms "vertical" and variants thereof refer to a direction
perpendicular
to a plane parallel to the conventional plane or surface of the substrate.
Likewise, the
terms "horizontal" and variants thereof are used to refer to directions lying
in a plane
which is perpendicular to the vertical direction as just defined. Both terms
are not
meant to refer to a particular orientation of the microbolometer pixel.
Similarly, terms
such as "above" and "below", "over" and "under", "upper" and "lower", "top"
and
"bottom", and other like terms indicating the position of one element with
respect to
another element are used herein for ease and clarity of description, as
illustrated in
the figures, and should not be considered limitative. It will be understood
that such
spatially relative terms are intended to encompass different orientations of
the
microbolometer pixel in use or operation, in addition to the orientation
exemplified in
the figures.
The thermistor platform 32 preferably provides thermal isolation to each
thermistor 36
by minimizing heat transfer through thermal conduction (except through posts
and
arms of the support structure 34). The thermistor platform 32 may be shaped as
a
substantially rectangular single or multilayer thin film, and be made of an
electrically
CA 02875303 2014-12-17
,
14
insulating, mechanically self-supportive and low-stress material. Suitable
materials for
inclusion in the thermistor platform 32 can include, without limitation,
silicon nitride
and silicon dioxide. In some embodiments, the thermistor platform 32 may have
horizontal dimensions selected between about 10 and 1000 pm, and it may have a
vertical thickness selected in the range of about 0.1 to 1 pm.
The thermistor platform 32 may generally be formed on top of a sacrificial
layer (not
shown), which may be deposited on the substrate 22 during the fabrication
process of
the microbolometer pixel 20, and be subsequently patterned, selectively
etched, for
example in an oxygen plasma.
As illustrated in Figs 2A and 3, in an exemplary embodiment, the thermistor
platform 32 includes three vertically stacked dielectric layers 40a to 40c.
These three
dielectric layers 40a to 40c provide mechanical rigidity and an electrical
separation
between the at least one thermistor 36, the electrode structure 38, and the
reflector 30 (or the optical absorber 28, as in Figs 7 and 10). However, it
will be
understood that depending on the intended application of the microbolometer
pixel 20, the thermistor platform 32 may take a variety of shapes, dimensions
and
configurations without departing from the scope of the invention.
It is to be noted that, in contrast to common uncooled infrared microbolometer
pixels
whose absorption waveband is determined essentially by the infrared absorption
properties of the material making up the thermistor platform (e.g., silicon
nitride), in
embodiments of the invention, the absorption spectrum of the microbolometer
pixel is
primarily defined by that of the optical absorber, as described in greater
detail below.
As a result, the thermistor platform is primarily intended for supporting and
providing
thermal isolation to the one or more thermistors.
CA 02875303 2014-12-17
The term "support structure" as used herein refers broadly to a structure that
holds
and mechanically supports the thermistor platform of the microbolometer pixel
in a
spaced relationship above the substrate.
5 Referring back to Figs 1 to 3, it may be advantageous for the support
structure 34 to
provide enough mechanical rigidity and strength to maintain the thermistor
platform 32 at a height of between about 1 and 10 pm from the substrate 22. It
will
also be understood that, in addition to providing mechanical support and
thermal
isolation, the support structure 34 can also provide a path to the electrode
10 structure 38 for electrically connecting each thermistor 36 to the ROIC
in the
substrate 22, for example by means of an electrically conductive contact pad
72
formed at the surface of the substrate 22 (see Figs 7 and 10).
The support structure 34 generally includes posts 42 and arms 44. As used
herein,
15 the term "post" refers generally to a structural element of the support
structure that
extends mainly vertically along a height thereof from the substrate. In
particular, the
height of each post essentially defines the spacing between the thermistor
platform and the substrate. In contrast, the term "support arm" refers broadly
to a
structural element of the support structure that extends mainly horizontally.
As for the thermistor platform 32, the support structure 34 is preferably made
of a low-
stress and self-supporting material, for example silicon nitride or silicon
dioxide, which
may be provided in the form of one or more thin-film layers, having for
example a
thickness of about 0.1 to 1 pm. The support structure 34 is generally
fabricated
concurrently with the thermistor platform 32, such that the support structure
34 and
thermistor platform 32 may share one or more material layers. In the
illustrated
embodiment, the support structure 34 is generally disposed along an outer
perimeter of the thermistor platform 32. The support structure 34 includes two
posts 42 connected to and projecting substantially vertically from the
substrate 22.
CA 02875303 2014-12-17
16
Each post 42 includes a proximal end connected to the substrate 22 and a
distal end
terminating near the outer perimeter of the thermistor platform 32 and
connected to a
support arm 44.
However, those skilled in the art will understand that the general
configuration and
disposition of the support structure 34 can be varied in other embodiments.
For
example, in order to meet the thermal, mechanical and electrical constraints
of certain
applications, each post 42 of the support structure 34 may have a variety of
lengths,
cross-sectional shapes and dimensions, which are all considered to be within
the
scope of the present invention. Similarly, the support structure 34 need not
be
provided outwardly of the thermistor platform 32 but may be disposed
completely or
partially underneath the thermistor platform 32, as shown in Figs 5 to 7. In
microbolometer pixel arrays, such configurations may provide a higher fill
factor for
optical absorption while simultaneously mitigating diffraction effects.
Furthermore, the
support arms 44 need not be straight, but may also include transverse sections
and
be arranged according to meandering or serpentine configurations. Such
geometric
patterns allow increasing the effective length of the support arms 44 and,
hence, the
thermal isolation they provide to the thermistor platform 32 and to one or
more
thermistors 36 provided thereon.
As used herein, the term "thermistor" generally refers to an uncooled
thermally
sensitive resistor and is meant to encompass any suitable material, structure
or
device having an electrical resistance that changes as a function of its
temperature,
ideally in a predictable and controllable manner.
Referring to Figs 2A and 3, each thermistor 36 may be made of a material
having a
high TCR near room temperature, preferably of at least 0.5% per kelvin,
including but
not limited to a vanadium oxide material and an amorphous silicon material. Of
course, the composition of each thermistor 36 is not limited to those cited
above. Any
CA 02875303 2014-12-17
17
material or combination of materials having a suitable TCR is considered to be
encompassed within the scope of the present invention.
In Figs 2A and 3, the microbolometer pixel 20 includes a single thermistor 36
disposed on the thermistor platform 32 between the first and second dielectric
layers 40a and 40b, and embodied by a thin film element having a substantially
rectangular shape with a width, length and thickness which may be selected
according to a desired electrical resistance of the thermistor 36. Of course,
the
number, size, shape and arrangement of the one or more thermistors 36 may be
varied without departing from the scope of the invention, as exemplified in
the
embodiments of Figs 7 and 10. The thermistors 36 may be deposited onto the
thermistor platform 32 using common deposition techniques such as evaporation,
sputtering, spin coating or any other appropriate thin-film transfer
technique. Likewise,
the size, shape and disposition of each thermistor 36 may be subsequently
delineated
by means of various selective wet and dry etching techniques combined with
photolithographic processes.
Referring still to Figs 2A and 3, the electrode structure 38 provides an
electrical
connection between the thermistor 36 and the substrate 22. For example, in the
illustrated embodiment, the electrode structure 38 may establish electrical
contact
with the thermistor 36 via contact openings 46 defined in the second
dielectric
layer 40b forming the thermistor platform 32 and may extend along the support
arms 44 and posts 42 down to the substrate 22. As mentioned above, the
substrate 22 may include an electrical ROIC electrically connected to the
electrode
structure 38, for example by means of contact openings 46 lithographically
defined at
the bottom of each post 42 during the fabrication process of the
microbolometer
pixel 20.
CA 02875303 2014-12-17
18
The electrode structure 38 may be deposited and delineated using known
microfabrication techniques and may be made of any material having a suitable
electrical conductivity including, without limitation, gold, aluminum,
titanium, copper,
silver, tungsten, chrome and vanadium. In the illustrated embodiment (see Figs
2A
and 3), the third dielectric layer 40c of the thermistor platform 32 is
deposited over the
electrode structure 38. It will be appreciated that the geometry of the
electrode
structure 38 may be adjusted to procure a thermal conductance and an
electrical
resistance that optimize the performance of the microbolometer pixel 20. In
particular,
in embodiments provided with more than one thermistor 36, it may be possible
to
adjust the equivalent resistance of the thermistors 36 by connecting the
thermistors 36 in one of a series, parallel and series-parallel circuit
schemes.
Absorber assembly
Referring to Figs 1 to 3, the absorber assembly 26 includes an optical
absorber 28
over the thermistor assembly 24 and a reflector 30 provided under the optical
absorber 28. The optical absorber 28 and the reflector 30 together form a
resonant
cavity 48. The optical absorber 28 is in thermal contact with the at least one
thermistor 36 and is exposed to the electromagnetic radiation. The optical
absorber 28 also includes a set of elongated resonators 50 determining an
absorption
spectrum of the optical absorber 28.
The term "optical absorber" is intended to refer herein to a material or
structure of the
microbolometer pixel that can, upon exposure to certain wavelengths of
electromagnetic radiation, absorb electromagnetic energy from the incident
electromagnetic radiation and convert the absorbed electromagnetic energy into
thermal energy. In particular, as mentioned above, the term "optical" refers
to the
electromagnetic spectrum in general and is not limited to the visible or to
another
portion of the electromagnetic spectrum.
CA 02875303 2014-12-17
,
,
19
The term "absorption spectrum" is intended to refer herein to a spectrum of
electromagnetic energy over a range of wavelengths whose intensity at each
wavelength corresponds to a measure of the fraction of absorbed
electromagnetic
radiation. A given absorption spectrum may include one or more absorption
bands
within which electromagnetic radiation is predominantly absorbed. Each
absorption
band may in turn exhibit one or more absorption peaks, each peak having a
corresponding peak or resonance wavelength. As will be discussed further
below, in
some embodiments of the invention, the absorption spectrum of the optical
absorber
may exhibit one or more absorption peaks, each of which having a corresponding
peak wavelength that is linearly or nearly linearly related to the length of a
number of
the elongated resonators, at least within a certain range of resonator
lengths.
Referring still to Figs 1 to 3, in some embodiments, the optical absorber 28
may be
optimized for detecting radiation in the terahertz region of the
electromagnetic
spectrum, while being provided advantageously on a microbolometer pixel 20
having
a substrate 22, a thermistor platform 32, a support structure 34 and
thermistors 36
that are similar to those found in conventional infrared microbolometer
detectors. As
used herein the term "terahertz radiation" refers to electromagnetic radiation
having
wavelengths in a range between about 30 pm and 1000 pm, corresponding to
frequencies ranging from approximately 10 THz to 0.3 THz, respectively.
However,
while particularly useful for terahertz applications, those skilled in the art
will
appreciate that embodiments of the invention could additionally or
alternatively be
used in other regions of the electromagnetic spectrum, for example in the
infrared and
visible regions, without departing from the scope of the invention. It will be
understood
that as the wavelength of the electromagnetic radiation detected by the
microbolometer pixel decreases, the characteristic size of the geometrical
parameters
of the optical absorber decreases accordingly. In particular, the design of
the optical
absorber is generally limited mainly by the minimum critical dimension that
can be
achieved by the fabrication process of the microbolometer pixel.
CA 02875303 2014-12-17
As used herein, the term "thermal contact" generally means that heat
conduction
occurs directly or indirectly between two components, that is, the two
components
may be in direct contact with each other or may have a sufficiently thermally
5 conducting material provided between them. More specifically, the term
"thermal
contact" is intended to refer to the fact that when the optical absorber is
heated upon
absorption of electromagnetic radiation, the heat generated thereby is
conducted,
radiated or otherwise transmitted to the one or more thermistors. It will also
be
understood that the term "over" in specifying the spatial relationship of the
optical
10 absorber relative to the thermistor assembly denotes that the optical
absorber is
either in direct contact with or separated by one or more intervening elements
from
the upper surface of the thermistor assembly.
The reflector 30 may be embodied by a thin film providing a radiation
reflecting
15 surface. The reflector 30 may be made of aluminum or of another highly
reflective
metal and be deposited during the fabrication process of the uncooled
microbolometer pixel 20. Those skilled in the art will recognize that the
reflector 30
can form a resonant cavity 48 with the optical absorber 28 disposed thereover,
which
serves to enhance absorption of electromagnetic radiation incident on the
optical
20 absorber 28 by optimizing the overall impedance of the absorber assembly
26.
The reflector 30 can provide additional absorption by reflecting back toward
the
optical absorber 28 the electromagnetic radiation which the optical absorber
28 is
configured to absorb but which has not been absorbed on its first passage
therethrough.
Throughout the present description, the term "elongated resonator" or simply
"resonator" is intended to refer to a discrete, individual absorbing element
of the
optical absorber that is configured to achieve resonance in a certain
absorption
CA 02875303 2014-12-17
21
waveband. In other words, each elongated resonator of the optical absorber is
designed in a manner such that electromagnetic radiation within one specific
wavelength or frequency band is selectively absorbed while electromagnetic
radiation
of wavelength or frequency out of the band is mostly unabsorbed by the
resonator.
The term "elongated" as used herein refers to a resonator, in which one of the
three
dimensions, referred to as the "length" or "longitudinal dimension" of the
resonator, is
many times greater than either of the other two dimensions, referred to as the
"transverse dimensions" of the resonator. The ratio between the length and any
of the
transverse dimensions is known as the "aspect ratio" of the resonator. In an
exemplary non-limitative embodiment, a resonator can be deemed to be elongated
when its aspect ratio is greater than about ten. For example, and without
limitation, in
terahertz applications, the length of the resonators can be from about 10 pm
to about
350 pm, while the transverse dimensions of the resonators can be from about 2
pm to
about 6 pm. In some embodiments, the transverse dimensions of the resonators
may
be selected so as to remain significantly smaller than the smallest wavelength
to be
detected in a particular application. This is to prevent or at least reduce
undesirable
radiation absorption caused by the excitation of a transverse resonator mode
oscillating at shorter wavelengths. In other embodiments, the smallest
transverse
dimensions of the resonators may be selected based on the resolution limits of
photolithography techniques.
It is to be noted that the term "elongated" does not refer to a particular
transverse
cross-sectional shape (i.e., the cross-section extending perpendicularly to
the length
of the resonator), so that the resonator may be, without limitation, a
circular rod, a
square or rectangular rod, a tube, a needle, a prolate ellipsoid, or have any
other
shape as desired. For example, in some embodiments, the elongated resonators
consist of monopole-like or dipole-like antennas with large aspect ratios. In
some
implementations, a monopole-like antenna can consist of a single rod- or wire-
shaped
CA 02875303 2014-12-17
22
conductor (see, e.g., Fig 1), while a dipole-like antenna can consist of a
pair of rod- or
wire-shaped conductors coaxially arranged end-to-end, with a load resistor
element
connecting the two adjacent ends and provided with a value that optimizes
performance (see, e.g., Fig 11A). It is also to be noted that for the optical
absorber
according to embodiments of the invention, the resonators are generally
arranged
horizontally, that is, within a plane which is generally parallel to the
conventional
plane or surface of the substrate, as defined above.
Referring still to Figs 1 to 3, the elongated resonators 50 may be made of a
metal
(e.g., aluminum, gold silver, copper or any other suitable low-resistivity
metal), a
metal alloy or any other appropriate electrically conductive material or
combination
thereof. In this regard, the absorption spectrum of each elongated resonator
50 will
generally depend, at least to some extent, on its material composition,
particularly its
electrical conductivity, which can be frequency-dependent and have an
influence on
the profile of the absorption spectrum. The elongated resonators 50 can be
fabricated
on the microbolometer pixel 20 by any appropriate etching, patterning or
deposition
processes, or combinations thereof, including micro-electro-mechanical system
(MEMS) surface micromachining techniques. Those skilled in the art will
recognize
that by virtue of their inherent electrical conductivity, the elongated
resonators 50 can
absorb electromagnetic radiation according to their respective absorption
spectra and
transfer the absorbed electromagnetic energy as thermal energy through
radiation
and thermal conduction. It is also to be noted that in some implementations,
the set of
elongated resonators may be embodied by a set of elongated slots formed in a
layer
or sheet of electrically conducting material. In such implementations, the
elongated
slots are used to sense incident electromagnetic radiation having electric and
magnetic fields respectively polarized perpendicular and parallel to the
length of the
elongated slots.
CA 02875303 2014-12-17
23
In some implementations, an advantage of using elongated resonators is that
the
spectral position of their absorption peak is governed mainly by their length.
More
particularly, a given elongated resonator has a resonant wavelength which
varies,
over a certain range of wavelengths, substantially linearly with respect to
the
resonator length. However, the profile of the absorption spectrum (e.g., its
shape,
width or amplitude) remains essentially independent of the resonator length.
Thus, in
such implementations, providing an elongated resonator of a greater or shorter
length
can essentially result in a translation of the absorption spectrum toward
longer or
shorter wavelengths, respectively. This simple linear relationship between
resonant
wavelength and resonant length may be used advantageously in some embodiments
to tailor the absorption spectrum of the microbolometer pixel in a less
complex and
more straightforward manner.
Those skilled in the art will recognize that several types of elongated
resonators can
exhibit a substantially linear variation of resonance wavelength with respect
to
resonator length. In a non-limitative embodiment, a linear relationship
between
resonance wavelength and resonator length can be achieved with elongated
resonators that consist of high-aspect-ratio monopole-like or dipole-like
antennas. For
example, Fig 4 is a theoretical absorption spectra plotted as a function of
wavelength
of an elongated resonator embodied as a monopole-like antenna suspended above
a
perfectly reflecting plane and having different lengths. It can be seen from
the
absorption spectra that as the resonator length increases from 50 pm to 70 pm,
the
resonance wavelength increases approximately linearly from about 135 pm to 180
pm
(corresponding to a decrease in resonance frequency from about 2.20 to 1.65
THz)
while the spectral profile remains substantially unaffected.
Referring back to Figs 1 to 3, in some implementations of the microbolometer
pixel 20, it may be desirable that the absorption spectrum of the optical
absorber 28
be as close as possible to a direct superposition of the absorption spectra of
the
CA 02875303 2014-12-17
24
elongated resonators 50. In some embodiments, this can be achieved when the
set of
elongated resonators 50 is arranged so that there is little or no mutual
interaction
therebetween such as, for example, capacitive and inductive coupling. In this
scenario, the elongated resonators 50 absorb the incident electromagnetic
radiation
mostly independently from one another, and the resulting absorption spectrum
of the
optical absorber 28 is more or less determined by the absorption spectra of
the
individual elongated resonators 50, with minimal or negligible coupling
therebetween.
Those skilled in the art will appreciate that an absence or near absence of
capacitive
and inductive coupling between the elongated resonators 50 can be achieved by
ensuring that their separation remains sufficiently large. In such a case, the
elongated
resonators 50 operate not as a collection of mutually interacting and coupled
absorbing elements, but rather as a set of independent and substantially
electromagnetically isolated absorbing elements. In other words, in some
embodiments of the invention, the absorption spectrum of the whole optical
absorber 28 remains representative of the absorption spectra of the discrete,
individual elongated resonators 50. It will thus be recognized that the
optical
absorber 28 as described herein differs from planar two-dimensional meshes of
capacitive and inductive grids of closely-spaced metallic elements whose
frequency
selective properties stem from the mutual interactions and collective
resonance of the
metallic elements. The response of these grid-based structures generally
exhibits a
non-linear behavior with frequency, which can render their use in the design
of
frequency tunable and broadband microbolometer focal plane array more complex.
It
will also be understood that while, in some embodiments, it may be
advantageous in
terms of sensitivity to provide a large number of elongated resonators 50 on
the
optical absorber 28, a sufficient inter-resonator spacing should be maintained
to
reduce mutual interactions between adjacent resonators 50.
CA 02875303 2014-12-17
,
,
Referring still to Figs 1 to 3, in some embodiments, it could therefore be
envisioned to
design the absorption spectrum of the microbolometer pixel 20 as a whole by
proper
selection of the length of the elongated resonators 50. However, the
absorption
spectrum of the microbolometer pixel 20 generally depends not only on the
5 absorption spectrum of the optical absorber 28, but also, typically to a
lesser extent
and often adversely, on the absorption spectra of other components of the
microbolometer pixel 20 also exposed to the electromagnetic radiation.
On the one hand, some of the pixel components can actually enhance the
absorption
10 spectrum of the optical absorber 28. This is the case for the reflector
30, which, as
mentioned above, forms a resonant cavity 48 with the optical absorber 28. In
particular, the absorption spectrum of the cavity 48 is generally related to
both the
absorption spectrum of the elongated resonators 50 forming the optical
absorber 28
and the height of the resonant cavity 48, that is, the vertical spacing
between the
15 optical absorber 28 and the reflector 30. It is to be noted that the
absorption spectrum
of the resonant cavity 48 will generally preserve the linear relationship
between
resonance wavelength and resonator length. On the other hand, other pixel
components, such as the thermistors 36, the electrode structure 38 and the
thermistor
platform 32, may degrade the absorption spectrum of the microbolometer pixel
20
20 compared to that of the optical absorber 28, for example by producing
peak shifting,
lower maximum absorption and unwanted diffraction peaks. These components are
therefore generally considered to have a detrimental or undesirable effect on
the
absorption spectrum of the microbolometer pixel 20 as they tend to make it
more
difficult to benefit from the linear dependence of the resonant wavelength on
25 resonator length.
Therefore, in order to mitigate the impact of radiation absorption by the
thermistor
assembly on the absorption spectrum of the microbolometer pixel as a whole, it
may
be desirable that the absorber assembly, through either or both of the optical
CA 02875303 2014-12-17
26
absorber and the reflector, shields the thermistor assembly, and particularly
the
thermistors and the electrode structure, from the incident electromagnetic
radiation.
Three exemplary, non-limitative implementations of the absorber assembly that
can
provide such an electromagnetic shielding will now be discussed.
First, in the embodiment of Figs 1 to 3, the absorber assembly 26 includes an
absorber platform 52 suspended above the thermistor platform 32 in a spaced
relationship therewith. The optical absorber 28 is provided on the absorber
platform 52 and the reflector 30 extends on the thermistor platform 32 over
the at
least one thermistor 36 and the electrode structure 38 so that the reflecting
surface of
the reflector 30 faces the underside of the absorber platform 52. It is seen
that in this
configuration, both the optical absorber 28 and the reflector 30 contribute to
shielding
the thermistor assembly 24 from the electromagnetic radiation and, thus, to
ensuring
that the peak position or positions of the absorption spectrum of the
microbolometer
pixel 20 are mainly determined by the absorption spectrum of the optical
absorber 28.
In Figs 1 to 3, the absorber platform 52 is maintained above the thermistor
platform 32, for example at a height between about 1 to 10 pm, by a suitable
support
structure 54. In this embodiment, the support structure 54 includes two
pillars
projecting upwardly from a central region of the thermistor platform 32. Of
course, the
configuration and disposition of the support structure 54 supporting the
absorber
platform 52 can be varied in other embodiments depending on the pixel size,
the
thermal conductance of the support structure 54 and its mechanical stability.
For
example, in some embodiments, it may be desirable that the reflector 30
provides a
continuous reflecting surface that covers most of or the entire area of the
thermistor
platform 32 underneath the absorber platform 52 to maximize the back
reflection level
toward the overlying optical absorber 28. It may also be advantageous that the
surface area of the absorber platform 52 be made larger than that of the
thermistor
platform 32 to provide better shielding of the thermistor platform 32 from the
CA 02875303 2014-12-17
27
electromagnetic radiation. In some cases where the size of the thermistor
platform 32
is significantly smaller than the absorber platform 52, it may be envisioned
that the
reflector 30 be provided on the substrate 22 rather than on the thermistor
platform 32.
In such a case, it will be understood that the surface area of the reflector
30 on the
substrate 22 would correspond substantially to that of the absorber platform
52.
The absorber platform 52 and its support structure 54 can be made of an
electrically
insulating, mechanically self-supportive, low-stress and high strength
material, for
example silicon nitride or silicon dioxide. The absorber platform 52 and its
support
structure 54 also preferably provide a thermal conductance path between the
optical
absorber 28 on the absorber platform 52 and the one or more thermistors 36 on
the
thermistor platform 32 so that when the optical absorber 28 is heated upon
absorption
of electromagnetic radiation, the heat generated thereby is quickly and
efficiently
transferred to the one or more thermistors 36. Depending on the thermal
requirements of a particular application, the thermal conductance can be
enhanced
by increasing the number of posts of the support structure 54 or by coating
the posts
of the support structure with a metallic layer.
Turning now to Figs 5 to 7, in a second exemplary implementation, the optical
absorber 28 extends over and is in contact with the thermistor platform 32
such that
the thermistors 36 and the electrode structure 38 (or at least a substantial
portion
thereof) each lies under and in vertical alignment with one or more of the
elongated
resonators 50. Meanwhile, the reflector 30 is disposed on the substrate 22
under the
thermistor platform 32. Preferably, the reflector 30 extends over most of or
the entire
area of the substrate 22 underneath the thermistor platform 32 to maximize the
back
reflection level toward the overlying optical absorber 28. In this embodiment,
it will be
understood that the optical absorber 28 acts as a physical barrier that
prevents or at
least impedes the electromagnetic radiation from reaching the thermistors 36
and the
electrode structure 38, and that contributes to ensuring that the peak
position or
CA 02875303 2014-12-17
28
positions of the absorption spectrum of the microbolometer pixel 20 are mainly
determined by the corresponding peak position or positions of the absorption
spectrum of the optical absorber 28.
Referring to Figs 8 to 10, there is illustrated a third exemplary
implementation of the
absorber assembly 26. The configuration of Figs 8 to 10 shares some
similarities with
the configuration of Figs 5 to 7, in that the optical absorber 28 extends over
and in
contact with the thermistor platform 32 and the reflector 30 is disposed on
the
substrate 22 under the thermistor platform 32. Again, the reflector 30
preferably
extends over most of or the entire area of the substrate underneath the
thermistor
platform 32. However, in Figs 8 to 10, the absorber assembly 26 further
comprises an
additional optical absorber 68 comprising a set of additional elongated
resonators 70.
The additional optical absorber 68 extends on the thermistor platform 32 under
the
thermistors 36 and the electrode structure 38 such that the thermistors 36 and
the
electrode structure 38 each lies over and in vertical alignment with one or
more of the
additional elongated resonators 70.
In this embodiment, it will be understood that the additional optical absorber
68 can
absorb at least some of the electromagnetic radiation that has not been
absorbed on
its first passage through the absorber assembly 26 and thermistor assembly 24
and
that has been reflected by the reflector 30. The additional optical absorber
68 thus
acts as a physical barrier that prevents or at least impedes the
electromagnetic
radiation reflected by the reflector 30 from reaching the thermistors 36 and
the
electrode structure 38. It is noted that the number, length and arrangement of
the
additional elongated resonators 70 of the additional absorber 68 may or may
not be
identical to those of the elongated resonators 50 of the optical absorber 28.
It is also
to be noted that due to the provision of the additional absorber 68, the
support
structure 34 includes four dielectric layers 40a to 40d.
CA 02875303 2014-12-17
29
Referring now to Figs 11A, 11B and 12A, 12B, different exemplary
implementations of
the optical absorber 28 will now be described. In particular, it will be seen
that by
selecting the length and orientation of each elongated resonator 50, and by
allowing
these two parameters to differ from one resonator to another, the optical
absorber 28
can be designed to provide a wavelength- and polarization-selective absorption
spectrum, which exhibits one or more absorption peaks in specific regions of
the
electromagnetic spectrum, for example in the terahertz region.
Figs 11A and 11B are respectively a top plan view of an exemplary
implementation of
the optical absorber 28 and a schematic representation of its absorption
spectrum
plotted as a function of wavelength. The set of elongated resonators 50 of the
optical
absorber 28 is divided in three resonator subsets 56a to 56c. Of course, the
number
of subsets may differ in other embodiments. Also, while all the resonators 50
are
parallel to one another, this need not be the case in other embodiments, as
will be
discussed further below. In Fig 11A, the elongated resonators 50 from
different
subsets 56a to 56c have different lengths, thus determining different
absorption
spectra. Also, in the illustrated embodiment, the elongated resonators 50
consist of
dipole-like antennas each of which embodied by a pair of wire-shaped
conductors 64a, 64b arranged parallelly in an end-to-end relationship, with a
load
resistor element 66 connecting the two adjacent ends. Under the assumption
that
there is little or no mutual interaction between the elongated resonators 50,
the
absorption spectra of the three resonator subsets 56a to 56c combine, with
minimal
coupling therebetween, to produce a three-peak absorption spectrum of the
optical
absorber 28, as depicted in Fig 11B, where the resonance wavelengths are
linearly
related to the corresponding resonator lengths. It will be understood that
while the
three absorption spectra do not overlap appreciably in Fig 11B, various
degrees of
overlap can be provided in other embodiments.
CA 02875303 2014-12-17
Referring to Figs 12A and 12B, in some embodiments, the optical absorber 28
may
not only have a resonance wavelength that depends linearly on its length, but
also be
used to selectively absorb components of a linearly-polarized incident
electromagnetic radiation. In such a case, the optical absorber 28 can be
referred to
5 as "polarization-sensitive". In order to illustrate the effect of
polarization sensitivity,
Fig 13 depicts theoretical absorption spectra plotted as a function of
wavelength of an
optical absorber that includes a set of aligned and identical resonators and
that is
exposed to electromagnetic radiation polarized along (solid line) and
perpendicularly
(dashed line) to the resonators. A non-limitative example of such an optical
10 absorber 28 is provided in Figs 1 to 3. It is seen that for the range of
wavelengths
illustrated in Fig 13, resonant absorption takes place for parallel
polarization, while
negligible absorption occurs for perpendicular polarization, thereby
indicating a
preferential absorption of the electromagnetic radiation polarized along the
resonators. It will be understood that a microbolometer pixel such as the one
depicted
15 in Figs 1 to 3 may be used to determine the direction of polarization of
linearly-
polarized incident electromagnetic radiation by rotating the microbolometer
pixel
along an axis perpendicular to the plane containing the elongated resonators
and by
analyzing the variation of the detected signal as a function of the rotation
angle.
20 In Figs 12A and 12B, a top plan view of another exemplary implementation
of the
optical absorber 28 and a schematic representation of its absorption spectrum
plotted
as a function of wavelength are illustrated, respectively. In this embodiment,
each
elongated resonator 50 of the optical absorber 28 extends along one of a first
and a
second orthogonal axis, respectively designated as the x and y axes in Fig
12A. The
25 resonators 50 are divided in a first subset 56a extending along the x
axis and a
second subset 56b extending along the y axis, the resonators 50 in different
subsets
having different lengths. It will be understood that the resonators 50 of the
first
subset 56a are configured to preferentially absorb a first component of the
electromagnetic radiation polarized along the x axis, and that the resonators
50 of the
CA 02875303 2014-12-17
,
31
second subset 56b are configured to preferentially absorb a second component
of the
electromagnetic radiation polarized along the y axis.
The schematic absorption spectra of the optical absorber of Fig 12A exposed to
electromagnetic radiation polarized along the x axis (solid line) and along
the y axis
(dashed line) are depicted in Fig 12B. It is seen that the absorption spectrum
for
electromagnetic radiation polarized along the x axis has an absorption peak at
a first
resonance wavelength determined by the resonator length associated with the
first
subset 56a. Likewise, the absorption spectrum for electromagnetic radiation
polarized
along the y axis has an absorption peak at a second resonance wavelength
different
from the first resonance wavelength and determined by the resonator length
associated with the second subset 56b. Therefore, in the illustrated
embodiment the
components of the electromagnetic radiation polarized along the x and y axes
are
resonantly absorbed by the optical absorber 28 at different wavelengths.
It is to be noted that, while the resonator lengths associated with the first
and second
subsets are different in the embodiment of the Figs 12A and 12B, this need not
be the
case in other embodiments. In particular, it will be understood that a
polarization-
insensitive optical absorber could be achieved in a configuration where the
set of
elongated resonators is divided in two subsets of perpendicularly oriented
resonators
having the same length. In such a case, the components of incident
electromagnetic
radiation polarized along the two subsets of resonators are resonantly
absorbed by
the optical absorber at the same wavelength.
In summary, the different configurations of the optical absorber discussed
above have
been presented for illustrative purposes only and should not be construed so
as to
limit the scope of the present invention. Indeed, various other configurations
could be
devised for the optical absorber, depending on the specifics of each
application, for
CA 02875303 2014-12-17
,
32
example in terms of operating wavelength range, polarization sensitivity or
insensitivity, and single- or multi-frequency requirements.
Microbolometer array
Referring now to Fig 14, in accordance with another aspect of the invention,
there is
provided a microbolometer array 58. The microbolometer array 58 includes a
plurality
of arrayed uncooled microbolometer pixels 20 such as described above. While
Fig 14
depicts a four by four pixel array 58 for clarity, it will be recognized that
in other
embodiments, the total number of microbolometer pixels 20 in the array 58
could be
higher or lower depending on the intended application.
For example, in some embodiments, the microbolometer array 58 may include
microbolometer pixels 20 arranged in an array of between 40x30 and 1280x960
pixels, wherein the spacing between two nearest-neighbor microbolometer pixels
20
(e.g., the pixel pitch) may be between about 12 and 312 pm. It should also be
noted
that, while the microbolometer pixels 20 are arranged to form a two-
dimensional array
in the embodiments of Fig 14, they may alternatively be configured as a linear
array
or be provided at arbitrary locations that do not conform to a specific
pattern.
It will be understood that the overall absorption spectrum of the
microbolometer array
results from the combination, and ideally the direct superposition, of the
absorption
spectra of its individual pixels. Consequently, providing the array with a
certain
absorption spectrum can be achieved, in principle, by tailoring the absorption
spectra
of the individual pixels during fabrication or design. As discussed above, in
some
embodiments, tailoring the absorption spectra of an individual pixel can, in
turn, be
accomplished simply by selecting the length and/or orientation of the
elongated
resonators of that pixel.
CA 02875303 2014-12-17
33
In some embodiments, the absorption spectra of the pixels can be designed by
selecting the resonator lengths so as to provide an array with a continuous
broadband
absorption spectrum or an absorption spectrum including a plurality of
continuous
absorption bands. For example, in a non-limitative embodiment, an array can be
designed in which each row or column is configured for absorption at one
particular
resonant wavelength corresponding to one particular resonator length, so that
by
combining the relatively narrow responses of each row or column, an array with
a
broader absorption spectrum can be obtained, for example in the terahertz
region.
Referring to Figs 15A, 15B and 16A, 16B, two exemplary embodiments of the
microbolometer array 58 will now be described. In both cases, the array 58
includes a
plurality of microbolometer pixels 20 arranged in a matrix of rows and
columns. It
should be noted that, as used herein, the terms "row" and "column" may be used
interchangeably depending on the orientation of the array 58. Figs 15A and 16A
provide top plan views of the two embodiments of the array 58, depicting the
length
and orientation of the elongated resonators 50 of the optical absorber 28 of
each
pixel 20, while Figs 15B and 16B depict schematic absorption spectra plotted
as a
function of wavelength of the array 58 of Figs 15A and 16A, respectively. It
will be
seen that each embodiment can provide a broadband absorption spectrum by
careful
tailoring of the absorption spectra of the individual pixels. As mentioned
above,
tailoring the absorption spectrum of a given pixel 20 can involve selecting
the length
and orientation of the elongated resonators 50 for that pixel 20.
First, in the exemplary embodiment of Figs 15A and 15B, the pixels 20 of the
array 58
are divided in four subsets 60a to 60d of pixels, each of which corresponding
to one
particular column of the array 58. In particular, the pixels 20 of the array
58 are
designed such that the elongated resonators 50 have identical lengths within
each
subset 60a to 60d, but different lengths in different subsets 60a to 60d, with
resonator
length increasing going from left to right in Fig 15A. It will be understood
that while, for
CA 02875303 2014-12-17
34
simplicity, the number of resonators 50 depicted in Fig 15A is the same for
all the
pixels 20, this need not be the case in other embodiments. For example, in the
case
of pixels 20 with shorter resonators 50 (e.g., the leftmost column in Fig
15A), more
resonators 50 could be provided to increase the effective surface of the
optical
absorber 28 and, thus, contribute to enhancing the absorption of
electromagnetic
radiation.
As illustrated in Fig 15B, the pixels 20 within a same column exhibit
identical
absorption spectra. Meanwhile, the pixels 20 from different columns exhibit
different
absorption spectra having different resonant wavelengths, where the different
resonant wavelengths are linearly related to the different resonator lengths.
More
particularly, the leftmost absorption spectra in Fig 15B is associated with
the leftmost
column in Fig 15A, the second leftmost absorption spectra in Fig 15B is
associated
with the second leftmost column in Fig 15A, and so on.
The resonator lengths are further selected so that pixels 20 in adjacent
columns have
different but partially overlapping absorption spectra, as depicted in Fig
15B. Such a
partial overlap can be achieved by selecting resonator lengths that vary
gradually
enough between adjacent columns to maintain a sufficiently small separation
between the corresponding spectra. The partially overlapping spectra from each
pair
of adjacent columns combine to form a continuous broadband absorption spectrum
(thick solid line) of the array 58, which can advantageously exhibit a nearly
constant
maximum value over a relatively broad wavelength range.
It is to be noted that the broadening of the spectrum of the array 58 compared
to that
of a pixel 20 remains relatively modest in Fig 15B, due to the fact that, for
simplicity,
the array 58 depicted in Fig 15A includes only four columns of pixels.
However, in the
case of a more typical array provided with as many as a thousand or more
columns,
the broadening can be much more important. For example, using the techniques
CA 02875303 2014-12-17
described herein, the absorption spectrum of an array of 80x60 pixels could be
broadened to cover a wavelength range between 50 and 300 pm (i.e., from 6 to
1 THz).
5 It should also be noted that, while the pixels 20 having partially
overlapping
absorption spectra in Fig 15B correspond to pixels 20 that are spatially
adjacent in the
array 58 of Fig 15A, this need not be the case in other embodiments. In fact,
using
the techniques described herein, a broadband microbolometer array could be
obtained regardless of any relationship between spectrum overlap and pixel
10 proximity, as long as the elongated resonators of a number of pixels
have different
lengths that determine different absorption spectra whose combinations forms a
continuous broadband absorption spectrum. However, it will be understood that
providing adjacent pixels with partially overlapping absorption spectra can be
advantageous in some situations, for example when the array is employed in two-
15 dimensional spatial scanning applications, such as security screening and
spectroscopy applications.
Referring now to the exemplary embodiment of Figs 16A and 16B, the pixels 20
of the
array 58 are divided in two groups 62a, 62b of pixels, the first group 62a
20 corresponding to the pixels 20 in the left half of the array 58 and the
second
group 62b corresponding to the pixels 20 in the right half of the array 58.
The
resonators 50 of the pixels 20 in the second group 62b are shorter than those
of the
pixels 20 in the first group 62a. Also, due to these shorter lengths, the
pixels 20 in the
second group 62b each include a larger number of resonators 50 than the pixels
20 in
25 the first group 62a so as to provide a higher fill factor for optical
absorption. As
discussed below, for each group 62a, 62b, the absorption spectra of the pixels
20 of
the group combine to form a respective broader, continuous absorption band in
the
absorption spectrum of the array 58 as a whole. In other words, the absorption
CA 02875303 2014-12-17
36
spectrum of the array 58 includes two relatively broad absorption bands
separated by
a spectral transmission window in which absorption is relatively low.
The elongated resonators 50 of the pixels 20 in the first and second groups
62a, 62b
extend along a first and a second orthogonal axis, which are respectively
designated
as the y and x axes in Fig 16A. Accordingly, the pixels 20 in the first and
second
groups 62a, 62b are configured to predominantly absorb a first and a second
component of the electromagnetic radiation polarized along the first and
second
orthogonal axes, respectively. For this reason, the spectra depicted in Fig
16B
correspond to schematic absorption spectra of the array 58 plotted as a
function of
wavelength in the case of electromagnetic radiation linearly polarized along
the y axis
(thick solid line) and along the x axis (thick dashed line), respectively.
In Fig 16A, the first group 62a of pixels 20 is further divided in three
subsets 60a to
60c. The first subset 60a includes the four pixels 20 of the first and second
rows, the
second subset 60b includes the two pixels 20 of the third row, and the third
subset 60c includes the two pixels 20 of the fourth row. The pixels 20 of the
first
group 62a are designed such that the elongated resonators 50 have identical
lengths
within the same subset, but different lengths in different subsets.
Accordingly, the
pixels 20 within each subset of the first group 62a exhibit identical
absorption spectra
having identical resonance wavelengths. Meanwhile, the pixels 20 from
different
subsets exhibit different absorption spectra having different resonant
wavelengths,
where the different resonant wavelengths are linearly related to the different
resonator
lengths. In the illustrated embodiment, the resonator lengths and, thus, the
resonance
wavelengths decrease from the first subset 60a to the third subset 60c.
The resonator lengths are further selected so that the pixels 20 in the first
and second
subsets 60a, 60b have partially overlapping absorption spectra, and likewise
for the
second and third subsets 60b, 60c. As in the embodiment of Fig 15B discussed
CA 02875303 2014-12-17
37
above, this partial overlap can be achieved by selecting the resonator lengths
so they
vary sufficiently slowly between the first and second subsets 60a, 60b and
between
the second and third subsets 60b, 60c to maintain a sufficiently small
separation
between the corresponding spectra. The partially overlapping spectra from the
pixels 20 of the three subsets 60a to 60c of the first group 62a combine to
form a
continuous and broader absorption band (thick solid line) in the absorption
spectrum
of the array 58, which can advantageously exhibit a nearly constant maximum
value
over a relatively broad wavelength range.
In Fig 16A, the second group 62b of pixels 20 is divided in two subsets 60d,
60e,
which include the pixels 20 of the third and fourth columns of the array 58,
respectively. The pixels 20 of the second group 62b are designed such that the
elongated resonators 50 have identical lengths within the same subset (and
thus
identical absorption spectra), but different lengths in different subsets (and
thus
different resonant wavelengths). The resonator lengths are further selected so
that
the pixels 20 in the first and second subsets 60d, 60e have partially
overlapping
absorption spectra, which can be achieved by selecting the resonator lengths
in the
first and second subsets 60d, 60e to be sufficiently close to each other. The
partially
overlapping spectra of the two subsets 60d, 60e of pixels 20 combine to form
another
continuous and broader absorption band (thick dashed line) in the absorption
spectrum of the array 58.
Those skilled in the art will appreciate that the two embodiments of the
microbolometer array described above have been presented for illustrative
purposes
only, and should not be construed so as to limit the scope of the present
invention.
Indeed, various other design rules could be used by which the absorption
spectrum of
each individual pixel is tailored for absorption at specific wavelength and/or
polarization in order to provide a microbolometer pixel array with a broadband
or
CA 02875303 2014-12-17
38
multi-band, wavelength- and/or polarization-selective absorption spectrum, in
particular in the THz region.
For example, while the elongated resonators 50 within each individual pixel 20
are
assumed, for simplicity, to be parallel and identical to one another in Figs
15A and
16A, they could be allowed to differ, both in length and orientation, in other
embodiments (see, e.g., the pixels depicted in Figs 11A and 12A). Also, while
the
arrays 58 depicted in Figs 15A and 16A include different groups of pixels, in
other
embodiments, an array where all the pixels are identical and have identical
absorption
io spectra could also be used without departing from the scope of the
invention.
Depending on the intended application, the microbolometer array may be
configured
so that the absorption spectrum of each individual pixel, row or column of
pixels, or
groups or clusters of pixels is independently tailored during fabrication or
design. In
particular, many configurations in terms of the length and orientation of the
elongated
resonators are possible and useful in practicing the techniques described
herein. For
example, an array with a broadband or multi-band absorption spectrum can be
obtained by tailoring the resonator length within each pixel or row, column or
group of
pixels for absorption at one particular wavelength with a relatively narrow
bandwidth.
Alternatively or additionally, an array with either a polarization-sensitive
or insensitive
absorption spectrum can be designed by carefully selecting the orientation of
the
elongated resonators within each pixel.
Of course, numerous modifications could be made to the embodiments described
above without departing from the scope of the present invention.
