Note: Descriptions are shown in the official language in which they were submitted.
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IN-LINE HOLOGRAPHIC MASK FOR MICROMACHINING
FIELD OF THE INVENTION
The present invention relates to methods and apparatus for microlithography,
photopatterning, machining and materials processing and, more particularly, to
high-
efficiency in-line holograms that combine the functions of a lens and a
standard amplitude
mask in one device.
to BACKGROUND OF THE INVENTION
There are many industrial applications and processes that require precise
patterning
of a workpiece, two such applications being, for example, fabricating
microcircuits, and
forming circuit board interconnections. For instance, the demand for compact
electronics
packaging has seen the means for forming interconnections among microcircuits
evolve
from the use of peripheral interconnections (i.e., connections around the edge
of the
package) to the use of flexible ball grid arrays (BOA) on the surface of the
package. This
newer BOA packaging and thin, flexible interconnection method requires the
creation of
an array of hundreds of vias (i.e., holes) on the order of 25 m diameter in a
thin multi-
layer laminate insulating layer, such as polyimide (for example, KAPTON
polyimide, sold
under this trademark by DuPont).
Traditional means for accomplishing precise patterning of a workpiece by
micromachining include mechanical drilling, chemical etching, contact
printing, and
projection photolithography. In recent years, however, lasers have been shown
to be a
valuable and often preferred means for performing high-precision
micromachining because
of their directionality, coherence, high intensity and high photon energy.
The specific interaction between the laser beam and the workpiece depends on
the
laser wavelength and the material comprising the workpicce. For instance,
infra-red
wavelength and visible wavelength laser beams focused to a small spot on the
workpiece
provide intense localized heating which vaporizes most workpiece materials.
However,
such localized heating can have the undesirable side-effect of thermally
damaging the
workpiece. On the other hand, ultraviolet (UV) wavelength lasers (such as
excimer lasers)
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provide photons with sufficient energy to excite the electrons that form the
molecular
bonds of certain workpiece materials such as polyimide. Sufficient excitation
of the
bonding electrons with a tightly focused beam results in the localized
disassociation of the
material with little or no heating of the workpiece. This process is referred
to as
"ablation."
In a typical laser-based micromachining application, a laser is used to
irradiate the
surface of a workpiece in order to form a desired pattern thereon or therein.
One method
of laser-based micromachining involves a mask-based step-and-repeat operation,
wherein
the mask is illuminated with a laser beam, and a projection lens images the
mask onto the
workpiece. While this method is capable of forming small well-defined spots
and is well-
suited for forming arbitrary shapes or figures, the method is inefficient with
its use of
available light because the mask blocks a portion of the beam in order to form
the pattern.
Also, the step-and-repeat method is time-consuming, particularly when hundreds
or
thousands of spots need to be patterned on each of a multitude of workpieces.
is Another method of laser-based micromachining involves scanning a laser beam
over the workpiece with a flying-spot scanning apparatus. However, this
apparatus is
fairly complex and expensive, and is generally not well-suited for forming
arbitrary shapes
and figures, and it has limited processing capacity or "thruput" (up to about
1000
holes/second) because of its serial mechanical nature.
To increase "thruput" (the number of workpieces that can be processed in a
given
time interval) and to simplify the apparatus for step-and-repeat laser
micromachining,
there have been recent efforts to develop laser micromachining methods and
apparatus that
employ various types of multiple-focusing means for simultaneous drilling
multiple holes
(i.e., forming holes in "parallel" rather than serially). Such means include
conventional
lenses, fresnel zone plates (FZP's), computer-generated holograms (CGHs),
diffractive
optical elements, and binary phase gratings.
Because there is some confusion in the patent literature regarding the
definition of
the above multiple-focusing means, the following definitions are used herein.
A FZP is a plate with concentric transparent and opaque annular rings or ring
sections that transmit and block alternating Fresnel zones on a wavefront
thereby allowing
the transmitted light to positively interfere and come to a focus. An FZP can
also be
made with refractive zones instead of opaque zones, so that the phase of the
light is
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changed to be in phase with the other zones, rather than simply being blocked.
For FZP's
used to create an image other than a single focus spot, the zone pattern is
calculated and
then produced by digital means and lithography, as is referred to as a
"kinoform."
A holographic optical element*(HOE) is an optical component used,to modify
light
s rays by diffraction, and is produced by recording an interference pattern of
two laser
beams and can be used in place of lenses or prisms where diffraction rather
than refraction
is desired.
A hologram is a continuous diffracting region created by two or more
interfering
beams in which the phase information of the wavefronts in the object is
converted to
intensity or phase variations. The continuous diffracting region can also be
computer-
generated. Each point on the hologram contains information about the entire
object, and
thus any portion of the hologram can, in principle, reproduce the entire three-
dimensional
image of the object via wavefront reconstruction.
Diffractive optical elements (DOEs) have zones of refraction, phase shift, or
amplitude modulation with a scale that allows for the directional control of
diffraction
effects. A DOE can have a focusing effect as in an FZP, or it can have more
complicated
effects such as chromatic correction or aspherical distortion correction.
Diffracting optical
elements are made using computation to describe the zones of diffraction, and
then
producing these zones in a suitable substrate surface by means of diamond
turning or by
lithographic processes common to semiconductor manufacturing or injection
molding.
A binary optical element is a diffracting optical element having a binary or
"flat-
top" zone profile.
In addition, the phrase "in-line" as used herein denotes a geometry in which
is
coaxial, i.e., disposed along a common axis.
Laser micromachining methods and apparatus employing the above multiple-
focusing means are generally faster and more efficient than step-and-repeat
micromachining, contact printing, and projection photolithography. However,
these
multiple-focusing apparatus and methods also have their own shortcomings and
limitations.
U.S. Patent No. 5,223,693 to Zumoto et al. discloses an in-line optical
projection
micromachining apparatus. The apparatus comprises a mask having apertures and
reflective parts in between, and a hemispherical reflective member for
returning the light
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reflected off the reflective parts of the mask back toward the open areas of
the mask. A
projection lens is used to image the mask onto a workpiece. While this system
operates
in-line, it is fairly complex because the projection lens for most
applications would not be
a single lens element, but a multi-element well-corrected lens system capable
of imaging
very small features. In addition, when the mask features to be patterned are
small relative
to the total area of the mask, the amount of light transmitted by the mask
will be
relatively low, even with the hemispherical reflective member present.
U.S. Patent No. 5,481,407 (the '407 patent) to Smith et at. discloses a laser-
based method and apparatus for creating small holes having a desired shape
(e.g., circular,
to square, oval, etc.) by laser ablation. The focusing means is a segmented
array of FZPs,
wherein the form of the individual FZPs comprising the segmented array
determines the
shape of the holes. While this technique allows for a multitude of holes to be
patterned
simultaneously with a single exposure, it is not well-suited for patterning
generalized
"non-hole" type objects, i.e., objects having significant physical extent.
This is because
= each FZP in the FZP segmented array is designed to bring light to a small
focus at a
designated location on the workpiece, rather than to form an image of an
extended object
on the workpiece. This is disadvantageous because each of the multitude of
discrete FZP
elements needs to be aligned to a specific location on the workpiece.
Moreover, there are practical shortcomings with the focusing means disclosed
in
the `407 patent. For instance, the image-forming properties of a segmented
lens array are
disadvantageous in an industrial environment. Generally, when a workpiece is
patterned
with a laser micromachining apparatus, material on the workpiece is ejected
from the
surface during patterning and can become deposited on the image-forming means
of the
apparatus. When the image-forming means is a lens-type array (e.g., an FZP
array), the
deposited material can obscure a portion of the array, resulting in a
diminution of image
quality in the patterns formed by the obscured array lens elements. This
problem can be
particularly troublesome when the ablated material is transparent, because the
deposited
material will create a phase error over portions of the lens-type array which
is difficult to
detect by visual inspection. To prevent ejected material from depositing on
the image-
;o forming means, a pellicle or other protective surface can be introduced
into the apparatus.
However, such modifications make the apparatus more complex and costly.
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Also, because the FZPs are discrete, it is difficult to make a mask that will
print
structures in close proximity.
The publication "Laser Machining with a Holographic Lens," Applied Optics,
Vol. 10, No. 2, February 1971 by J.M. Moran discloses the concept of using a
hologram
illuminated by a laser for machining single and multiple spots on a workpiece.
Using a
hologram as a mask is advantageous in that it can have multiple- focal
distances because
of the three-dimensional nature of the holographic image. A hologram also has
the
advantage of being able to create not only sharply focused points, but
extended images
which can be patterned into or onto a workpiece. Moreover, there is no need to
compute
an "array" of segmented areas to achieve repetitive patterning, as a hologram
can comprise
a substantially continuous diffracting region recording of the wavefronts from
disparate
features on an object. In other words, a hologram is not a segmented array.
Rather, each
portion (or, alternatively, large portions) of the hologram contributes to the
creation of the
image formed. Indeed, a hologram can be cut into pieces, with each piece being
capable
of reproducing, in toto, the entire image (albeit from a limited set of
angles). This
property makes holograms very advantageous over discrete arrays of focusing
elements
because if part of the hologram is obscured by for example material ejected
from the
workpiece, the first-order net effect of the obscuration is a diminution in
the overall
intensity of the entire image, rather than the loss of resolution of the
individual sub-
images.
While the hologram in the above-cited publication has the above-mentioned
advantages, it is used off-axis, meaning that the illuminating beam, hologram,
and
workpiece are not in-line. An in-line geometry is preferred for most
manufacturing
applications, as the apparatus is simpler to fabricate and less costly than an
off-axis
apparatus. Also, the method of patterning with an in-line apparatus is less
complex, as
precise alignment between the workpiece and the hologram is more easily
achieved.
Moreover, an in-line geometry allows for the hologram to be "replayed" with a
beam
having a wavelength different from the wavelength used in its construction
with minimal
impact on aberrations. In addition, some manufacturing processes require an in-
line
geometry because of the geometry of the existing installed base of expensive
manufacturing apparatus. Also, for many applications, e.g., drilling vias for
microcircuit
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interconnections, the vias must have an axis perpendicular to the surface of
the workpiece
in order for the various layers of the microcircuit to be properly
interconnected.
U.S. Patent No. 5,612,986 to Howells et at. (the `986 patent) discloses a
method of
performing X-ray lithography using holographic images from a computer-
generated on-
axis hologram. However, the method disclosed in the `986 patent requires a
computer-
generated hologram (which restricts the types of images the hologram can form
and is
computationally intensive), is only for forming images smaller than 0.25 m,
and
apparently only works at X-ray wavelengths.
U.S. Patent No. 4,668,080 to Gale et al. (the '080 patent) discloses an
apparatus
to for forming a periodic pattern in a layer of photosensitive material, the
apparatus
comprising a lenticular array of lenslets and a means for scanning a beam of
light
sequentially through each lenslet in the array. The '080 patent also discloses
an apparatus
where the lenslets in the lenticular array are holograms, and where the array
of holograms
is sequentially scanned by a light beam scanning means.
The publication "High-resolution image projection at visible and ultraviolet
wavelengths," by I.N. Ross et al., Applied Optics, Vol. 27, No. 5, pg. 967
(March 1, 1988) discusses the construction of a holographic test mask having
resolution
test-patterns recorded therein, and then patterning the test-patterns in
photoresist by
illuminating the holographic test mask with a laser. While this technique
exploits the
aforementioned advantages of a hologram, the recording of the hologram and
subsequent
patterning steps are accomplished off-axis.
The publication "Photosensitized polystyrene as a high-efficiency relief
hologram
medium," by F. M. Schellenberg et al., SPIE Vol. 1051 Practical Holography Ill
(1989),
discloses using holograms off-axis for photoablation using high-powered
lasers. The
holograms were reflection holograms formed in t-BOC, a plastic material with
limited
damage threshold to deep ultra-violet wavelengths.
The publication "A technique for projection x-ray lithography using computer-
generated holograms" by C. Jacobsen and M.R. Howells, J. App. Phys. 71 (6) 15
March 1992, discusses a holographic approach to x-ray projection lithography
using an in-
line hologram generated by computer. However, this publication only provides
computer
simulations of the imaging and contemplates an in-line CGH, which is time-
consuming.
Indeed, while the theoretical aspects of in-line holograms have been explored,
the actual
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fabrication of in-line holographic masks for practical industrial use is truly
daunting. The
fact that persons skilled in the art of holography have not, to date, actually
optically
fabricated and used an in-line holographic mask suitable for micromachining in
an
industrial environment is testimony to the difficulty involved in applying in-
line
holographic methods to an industrial environment.
Therefore, there exists a need for high-efficiency optically fabricated in-
line
holograms suitable for industrial use from the infra-red to the deep ultra-
violet region of
the electromagnetic radiation spectrum for patterning a workpiece.
to SUMMARY OF THE INVENTION
The present invention is an optically made, high-efficiency in-line
holographic
mask (ILHM) for in-line holographic patterning of a workpiece, and apparatus
and
methods for performing same. The ILHM of the present invention combines the
imaging
is function of a lens with the transmission properties of a standard amplitude
mask, obviating
the need for expensive projection optics. In forming the ILHM, two types of
object masks
are used: type I object masks, which are non-opaque except for one or more
substantially
transparent elements, and type 11 object masks, which are substantially opaque
except for
one or more substantially transparent elements. The one or more substantially
transparent
20 elements can be phase-altering, scattering, refracting or diffracting. The
present invention
has application for wavelengths ranging from the infra-red (1R) to the x-ray
region of the
electromagnetic radiation spectrum.
In one aspect of the invention, an ILHM for patterning a workpiece is formed
by a
process comprising the steps of providing an illumination source for
generating a coherent
25 illumination beam directed along an axis, and then providing a non-opaque
object mask
(i.e., a type I object mask) having a semi-transparent layer with an optical
density between
0.1 and 5 and one or more substantially transparent elements for creating
object
wavefronts when the illumination beam is incident thereon. Next, the object
mask is
disposed in the illumination beam, and a holographic recording medium is
provided in the
30 illumination beam adjacent the object mask. The next step involves
illuminating the
object mask with said illumination beam, thereby causing the object mask to
allow
undiffracted reference wavefronts to pass therethrough, and also causing the
one or more
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transparent elements to create object overlapping wavefronts which interact
with the
undiffracted reference wavefronts to create an interference pattern. The
preferred beam
ratio (intensity) between the reference wavefronts and the object wavefronts
is from 0.1:1
to 100:1. Finally, the interference pattern is recorded in the holographic
recording
medium so as to create a substantially continuous diffracting region.
In another aspect of the present invention, an ILHM capable of patterning a
workpiece is formed by a process comprising the steps of providing an
illumination source
for generating a coherent illumination beam directed along an axis, then
providing a
substantially opaque object mask (i.e., a type If object mask) having one or
more
to substantially transparent elements for creating object wavefronts when the
illumination
beam is incident thereon. Next, the object mask is disposed in the
illumination beam, and
a holographic recording medium is provided in the illumination beam adjacent
the object
mask. The next step involves illuminating the object mask with the
illumination beam,
thereby causing the one or more elements to create overlapping object
wavefronts. Next,
is a reference beam is provided that is coherent with the illumination beam
and that has
reference wavefronts that are in-line with the object wavefronts and that
interact with said
object wavefronts so as to create an interference pattern. The preferred beam
ratio between
the reference wavefronts and the object wavefronts is from 0.1:1 to 100:1.
Finally, the
interference pattern is recorded in the holographic recording medium so as to
create a
20 substantially continuous diffracting region.
Another aspect of the present invention is a method of creating a pattern on a
workpiece comprising the steps of providing a source of illumination for
generating a
reconstruction beam having a reconstruction beam wavelength and extending
along an
axis. Next, an in-line holographic mask that creates a holographic image
corresponding
25 to a pattern when illuminated with said reconstruction beam is disposed on
the axis. Next,
a workpiece is disposed on the axis adjacent the in-line holographic mask.
Finally, the in-
line holographic mask is illuminated with the reconstruction beam so as to
form the
holographic image on the workpiece and impart the pattern to the workpiece.
A further aspect of the present invention is a method of patterning a
workpiece
30 using an ILHM whereby the wavelength of light used to construct the ILHM is
different
from the wavelength of light used to pattern the workpiece.
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Another aspect of the present invention is an apparatus for patterning a
workpiece
comprising a source of illumination for generating a reconstruction beam
extending along
an axis, an in-line holographic mask disposed on the axis adjacent the source
of
illumination, and a workpiece holder disposed on the axis adjacent the in-line
holographic
mask.
An additional aspect of the present invention is an apparatus for patterning a
workpiece, comprising a source of illumination, a workpiece holder, a lens
having an
object plane and an image plane and an in-line holographic mask capable of
forming a
real image. The lens is disposed between the source of illumination and the
workpiece
holder, the mask is disposed between the source of illumination and the lens.
The lens
then relays the real image to be at or near the workpiece holder.
BRIEF DESCRIPTION OF THE DRAWINGS
is
FIG. I is a schematic side view of an apparatus for patterning a workpiece
using an ILHM
of the present invention;
FIG. 2 is a schematic side view of an apparatus for forming an ILHM of the
present
invention using a type I object mask;
FIG. 3 is a schematic side view of an apparatus for forming an ILHM of the
present
invention using a type 11 object mask;
FIG. 4 is a schematic side view of an apparatus similar to the apparatus of
FIG. 3 except
that it employs a beamsplitting cube as a beam combiner;
FIG. 5a is a schematic side view of a first apparatus for forming a hologram 1-
11 of a
type II object mask as part of a first two-step process for forming an ILHM of
the present
invention;
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FIG. 5b is a schematic side view of a second apparatus for forming a hologram
1-11 of a
type II object mask as part of a second two-step process for forming an ILHM
of the
present invention;
FIG. 6 is a schematic side view of an apparatus for forming an ILHM of the
present
invention from a hologram H I formed using the apparatus of FIG. 5a or FIG.
5b;
FIG. 7 is a schematic side view of the apparatus of FIG. I being used for
patterning a
workpiece with an ILHM formed using the apparatus of FIG. 6;
FIG. 8 is a schematic side view of an apparatus for forming a hologram H I
using a
type II object mask as part of a third two-step process for forming an ILHM of
the present
invention;
FIG. 9 is a schematic side view of an apparatus for forming an ILHM of the
present
invention from a hologram HI formed using the apparatus of FIG. 8;
FIG. 10 is a schematic side view of an ILHM of the present invention as used
in the
apparatus in FIG. I in combination with a projection lens;
FIG. I1 is a perspective schematic view of a phase mask with an array of phase
indentations as phase-altering elements;
FIG. 12 is a plot of the spatial intensity distribution of a real image
constructed from an
ILHM formed using a type I phase object mask of FIG. 11;
FIG. 13 is a perspective schematic view of a scattering mask with an array of
scattering
centers as scattering elements;
FIG. 14 is a schematic side view of a diffracting object mask having an array
of lenslet
elements, and illustrates how the lenslets produce converging and diverging
wavefronts
from incident plane wavefronts;
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FIG. 15 is a partial perspective schematic view of a diffracting object mask
with an array
of grating elements;
FIG. 16 is a schematic side view of the apparatus of FIG. I with a type I
diffracting
object mask of FIG. 15 being used to form an ILHM of the present invention;
FIG. 17a is a cross-sectional view of a transparent object mask substrate
having an opaque
layer with apertures formed therein;
FIG. I7b is a cross-sectional view of the object mask substrate of FIG. 17a,
with a layer
of negative photoresist deposited atop the opaque layer and in the apertures;
FIG. 17c is a cross-sectional view of the object mask substrate of FIG. 17b in
an
apparatus for providing two interfering plane wave beams that interfere within
the
negative photoresist layer to form a grating (not shown) within the apertures;
FIG. 17d is a cross-sectional view of the object mask substrate of FIG. I7c
after the
negative photoresist layer is developed and shows a negative photoresist
grating formed
within the apertures, thereby resulting in a type 11 grating mask;
FIG. 17e is a cross-sectional view of the object mask of FIG. 17d, but with
the opaque
layer removed to form a type I diffracting object mask; and
FIG. 18 is a side-view of a diffracting object mask wherein a grating is
formed on the
back side of the substrate.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is an optically made, high-efficiency in-line
holographic
mask (ILHM) for in-line holographic patterning of a workpiece and apparatus
and
methods for performing same. The ILHM of the present invention combines the
imaging
function of a lens with the transmission properties of a standard amplitude
mask, obviating
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the need for expensive projection optics. Because the ILHM of the present
invention is
optically made (as opposed to computer generated), a wide variety of features
of varying
complexity can readily be patterned on a workpiece. Further, the ILHM is not
limited to
discrete phase or transmission values as an approximation to the holographic
interference
pattern, but contains a wide range of phase information present in the
diffracted
wavefronts from a real object. Further, it will be understood that the present
invention has
application for wavelengths ranging from the IR to the x-ray region of the
electromagnetic
radiation spectrum.
For applications in the DUV and UV wavelengths, materials such as fused
silica,
calcium fluoride and lithium flouride may be employed. For applications in the
IR,
materials such as germanium, silicon, zinc selenite or zinc sulfide may be
employed. For
applications in the visible wavelength, any number of well-known optical
quality glasses
may be employed, such as quartz or Bk-7. For applications in the x-ray region,
materials
such as diamond, mylar or beryllium may be employed. These materials are
substantially
Is transparent and resistent to damage at the given wavelengths, even for for
high fluence.
In the. present invention, the term "high efficiency" refers to an ILHM's
ability to
redirect a high percentage of a beam of light normally incident the ILHM,
thereby
forming a real image of sufficient intensity to pattern a workpiece disposed
coaxial with
and adjacent the ILHM and opposite the coherent beam. Also, as used herein,
the phrase
"patterning a workpiece" is a general way of describing a multitude of
industrial
applications of the present invention, such as drilling holes or other
features by
photoablation to form interconnects in microcircuits, or for forming apertures
in thin
membranes (e.g., ink-jet cartridge membranes), and the like. To facilitate
patterning of
the workpiece, a layer of light-sensitive material may be employed.
The theory and operation of holograms is described in the book "Optical
Holography" by R. Collier, C. Burckhardt, and L. Lin, published by Academic
Press, Inc.,
San Diego, CA 92101 (ISBN 0-12-181052-6). Briefly, a hologram is a
substantially
continuous diffracting region created by recording, in a light-sensitive
recording medium,
the interference pattern created by two coherent light beams: a first light
beam (referred to
herein as the "illumination beam") comprised of coherent wavefronts that
scatter or
diffract from an object, resulting in an "object beam" having "object
wavefronts," and a
second "reference beam" comprised of coherent "reference wavefronts," also
coherent with
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the object wavefronts. The diffraction region is described as being
"substantially
continuous" because the recording medium will generally not be able to record
the full
range of intensities incident thereon. Thus, there results a small amount of
discontinuity
in the interference pattern due to the limited sensitivity of the recording
medium.
s Typically, the object reference beams originate from the illumination beam,
so that
coherence between the beams is readily achieved. The reference beam (usually
comprised
of plane-waves, but not necessarily so) allows phase and amplitude information
contained
in the object wavefronts to be preserved in the recording medium. This is
achieved by
converting the phase information contained in the object wavefronts into
amplitude
variations in the form of a complex interference pattern formed in the
recording medium.
Generally, when two wavefronts with amplitudes A,(x,y) and A,(x,y) interfere,
the overall
irradiance distribution I(x,y) is given by I(x,y)= ; A, + A, ;' = A, 2 + A, 2+
2RE(A,A,*).
The term 2RE(A,A,*) (where "RE" denotes "the real part of") represents the
overlap of
the two amplitudes and is the "interference term." The hologram may be
constructed by
simultaneously exposing the entire object with the illumination beam and the
entire
recording medium with the reference beam. Alternatively, the illumination beam
may
scan the object while the reference beam simultaneously scans the recording
medium.
Once a hologram is formed or "constructed," the object recorded therein is
"reconstructed" or "replayed" by illuminating the hologram with a second
coherent beam,
called the "reconstruction" beam, usually of the same wavelength, and usually
with
wavefronts that are "conjugate" (the reverse wavefront and direction) to the
wavefronts in
the reference beam. For a plane-wave reference beam described by the equation
R(x) = Aoexp(ikx), where A. is a constant amplitude factor, k = 2n/A (lambda
being the
wavelength of the reference light beam), and x is the distance along an x-
axis, the
conjugate beam is the complex conjugate of R(x), written as R*(x)= Aoexp[-
ikx].
When the hologram is illuminated with a reconstruction beam that is identical
to
the reference beam, the substantially continuous diffracting region imparts a
wavefront
onto the reconstruction beam that is identical to the object wavefronts. The
result is that
the object wavefronts appear to be "released" from the hologram as they
propagate away
from a point where the object was originally located, thereby forming a three-
dimensional
"virtual" image.
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When the hologram is illuminated with a reconstruction beam that is the
conjugate
of the reference beam, the substantially continuous diffracting region imparts
a wavefront
onto the reconstruction beam that is identical to the conjugate of the object
wavefronts.
The result is that the object wavefronts appear to be released from the
hologram in
"reverse," resulting in the original object being "reconstructed" as a three-
dimensional real
image in space. It is this real iinage that is usable for patterning a
workpicce.
For producing a real image from an in-line hologram, there is no difference,
in
theory, whether the reconstruction beam is identical to or conjugate to the
illumination
beam. However, for the sake of convention, the reconstruction beam in the
present
invention is shown as being conjugate to the illumination beam. One way this
is achieved
with an in-fine hologram is by inserting the hologram into the reconstruction
beam with its
backside facing the beam.
It is important to note that multiple holograms can be recorded in a single
holographic recording medium. The superimposed holograms, upon reconstruction,
will
form independent real images. These images can be displaced spatially from one
another
to create a particularized three dimensional irradiance distribution.
Apparatus for Patterning a Workpiece Using an ILHM
FIG. I is a schematic side view of an apparatus 10 for patterning a workpiece
using an ILHM. The apparatus 10 includes the following elements, all disposed
coaxially
along axis 20: a source of illumination 24, beam expanding and collimating
optics 28, an
ILHM 32 with front surface F and back surface B, a workpiece holder 36 capable
of
holding a workpiece 40, the workpiece as shown having a front surface - S
disposed in a
plane PI a distance d' from front surface F of ILHM 32. ILHM 32 has a
substantially
continuous diffraction region, as discussed in more detail below. Also shown
in the
Figure is a light beam 44 (i.e., a beam of wavefronts) emanating from source
of
illumination 24, reconstruction beam 48 with reconstruction wavefronts 52, and
diffracted
wavefronts 56.
Source of illumination 24 may be, for example, a coherent light source, such
as a
laser, for example a Krypton-ion laser, which operates at a wavelength of 413
nm, or an
Excimer laser operating at 248 rim or 193 rim. While it is preferred that
source of
illumination 24 be coherent, in practice perfect coherence is unattainable and
also
14
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sometimes not desirable. Also, it will often be preferred that source of
illumination 24 be
only "substantially" coherent. It will be understood that the word "coherent"
as used
herein encompasses "substantially coherent," which is how the term is used in
practice by
those skilled in the art of holography. Moreover, it will be understood that
for the present
invention source of illumination 24 can have a wavelength ranging from the IR
through
the DUV region of the radiation spectrum.
Beam expanding and collimating optics 28 are used to increase the size of
light
beam 44 so as to be of a suitable dimension relative to ILHM 32 and workpiece
40, to
filter undesirable high spatial frequencies thereby increasing beam
uniformity, and to
to collimate light beam 44 after it is expanded, thereby forming
reconstruction beam 48.
Beam expanding and collimating optics 28 may include, for example, a first
converging
lens, a spatial filter (i.e., a small axial aperture) and a second converging
lens (none
shown). It will be appreciated by one skilled in the art that any one of a
number of
optical systems can serve as beam expanding and collimating optics 28.
A collimated beam, by definition, has planar wavefronts that are perpendicular
to
their direction of propagation. In the embodiment of apparatus 10 of FIG. 1,
reconstruction beam 48 is shown to be collimated and comprised of
reconstruction
wavefronts 52 that are planar and perpendicular to optical axis 20.
It will be appreciated by those persons skilled in the art that in making ILHM
32
and then reconstructing or "replaying" it, the illumination and reconstruction
beams need
not consist of plane waves. In fact, it will often be preferable in practice
to determine
beforehand the exact form of the wavefronts of the reconstruction beam to be
used and, in
anticipation, tailor the reference beam wavefronts to be the conjugate of the
reconstruction
beam wavefronts. This eliminates (or significantly reduces) aberrations
induced by a
mismatch between the wavefronts in the reference and reconstruction beams. For
the sake
of simplicity, the discussion hereinafter presumes and the accompanying
Figures show the
illumination reference and reconstruction beams to consist of plane waves.
With continuing reference to FIG. 1, ILHM 32 is disposed in apparatus 10 with
back surface B facing reconstruction beam 48. This is so that reconstruction
beam 48 is
"conjugate" relative to ILHM 32, assuming the collimated reference beam used
in making
ILHM 32 was incident front surface F. Patterning of workpiece 40 is achieved
by
illuminating ILHM 32 at normal incidence with reconstruction beam 48,
whereupon the
CA 02761126 2011-12-06
substantially continuous diffracting region that constitutes ILHM 32
transforms
reconstruction wavefronts 52 into diffracted wavefronts 56, which converge at
surface S of
workpiece 40 to form a real image of sufficient intensity and definition
(e.g., substantially
free of diffraction artifacts) to precisely pattern- workpiece 40.
One of the main advantages of in-line patterning of a workpiece using ILHM 32
of
the present invention is that the wavelength of the light used to construct
the ILHM 32
and to pattern a workpiece using the ILHM 32 need not be the same. This is
because an
ILHM 32 has the property that using a reconstruction beam with a different
wavelength
than that of the illumination beam results only in an axial displacement of
the real image
and does not introduce significant aberrations. If, when constructing an ILHM
32, the
distance between the object and the recording medium is d and the wavelength
of the
illumination (i.e., construction) and reference beams is A,, then replaying
the ILHM 32
with a reconstruction beam wavelength of A, results in a real image being
formed at a
distance d' = [A,IA]d from the ILHM 32. This property is advantageous because
it allows
for ILHM 32 to be formed at a wavelength best suited for making a hologram
(e.g., perhaps visible light from a Helium-Neon laser at 633 nm, an Argon-ion
laser at
513 rim, or a Krypton-ion laser 413 nm), and then patterning the workpiece
using a
wavelength best suited for ablating a particular workpiece material (e.g., UV
light from an
excimer laser at 248 rim to ablate pholoresist or thin plastic or a frequency
doubled diode
pumped laser at 355 nm or 266 nm to ablate polyimide). For example, in the
present
invention, in one experiment, the spacing d is set at 60 millimeters (mm) in
fabricating an
ILHM 32 with an illumination and reference beam wavelength of A, = 413 rim.
ILHM 32
is then used in apparatus 10 of FIG. I to pattern a workpiece disposed at a
distance
d' = 100 mm from the ILHM 32, using a reconstruction beam wavelength A, = 248
rim.
Apparatus and Processes For Forming an ILHM
The process for forming ILHM 32 depends on the type of object used. In the
present invention, the object is a specialized "object mask," i.e., a planar
substrate
(substantially transparent, semi-transparent or opaque) having one or more
specially
designed elements corresponding to (but not necessarily identical to) the
feature or features
to be patterned on a workpiece. In fact, as will be seen below, the one or
more
specialized elements on the object mask are designed to create object
wavefronts arising
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from light passing through the one or more elements and being dispersed by the
process of
either phase-alteration, scattering, refraction, or diffraction, or a
combination thereof,
which ultimately results in a large interference term in the interference
pattern recorded in
the ILHM 32.
Generally speaking, there are two types of specialized object masks used in
the
present invention to form ILHM 32: (i) object masks that are otherwise
substantially
transparent or semi-transparent and that have one or more substantially
transparent phase-
altering, scattering, refracting, or diffracting elements. These object masks
are capable of
transmitting a sufficient portion of the illumination beam to serve as a
reference beam are
referred to hereinafter as "type I" object masks; and (ii) object masks that
are substantially
opaque and that have one or more substantially transparent phase-altering,
scattering,
refracting, or diffracting elements. These object masks do not transmit a
reference beam
and are referred to hereinafter as "type 11" object masks. In other words, a
type 11 object
mask is the same as a type I object mask, with the exception that a type II
object mask
does not transmit a sufficient portion of the illumination beam to serve as a
reference
beam.
With type I object masks, a precise balance of intensities of the object
wavefronts
and the reference wavefronts is required to obtain a sufficiently large
interference term.
One method of achieving this balance is to provide the type I object mask with
a semi-
transparent layer of material, such as a thin layer of metal or dyed polymer,
that covers
the object mask in the area not covered by the one or more object mask
elements. This
semi-transparent layer serves to attenuate the transmitted illumination beam,
thereby
providing a desired intensity balance between the object wavefronts and
reference
wavefronts. For the type I object masks of the present invention, a dark
chrome layer
having an optical density in the range (logarithmic) of 0.1 to 5.0 provides
the proper
substrate transmission to properly balance the diffracted object and reference
wavefront
intensities. Preferred values for the beam ratio (reference beam to object
beam) are from
0.1:1 to 100:1. The precise value of the optical density for the semi-
transparent material
to achieve a desired beam ratio will depend on the number, and shape, of the
mask
elements and is best determined by trial and error.
With type 11 object masks, an in-line reference beam is not readily available
via
partial transmission of the illumination beam through the object mask.
However, an in-
17
CA 02761126 2011-12-06
line reference beam is provided by directing a portion of the illumination
beam around the
object mask, and then bringing it back in-line with the original illumination
bean. In this
arrangement, filters can be used to obtain the proper beam ratio in the range
set forth
above. A detailed description of several type I and type 11 specialized object
masks, along
with the process steps for forming them and using them in constructing an ILHM
32
according to the present invention is provided below.
Using it Type I Object Mask
FIG. 2 is a schematic side view of an apparatus 100 for forming an ILHM 32
using a type I object mask according to the present invention. Apparatus 100
is similar to
apparatus 10 of FIG. 1, and includes the following elements, all disposed
coaxially along
axis 120: a source of coherent illumination 124, beam expanding and
collimating
optics 128, a type I object mask MI, and a holographic recording medium
(hereinafter,
simply "recording medium") 140 with front surface F and back surface B,
wherein front
surface F is disposed in a plane P2 distance d from the surface of object mask
MI closest
to recording medium 140. The distance d in apparatus 100 will be the same as
the
distance d' in apparatus 10 (see FIG. 1) when the wavelength of coherent
source of
illumination 24 and source of coherent illumination 124 are the same.
Also shown in FIG. 2 is a coherent beam 144 emanating from source of coherent
illumination 124, illumination beam 148 with illumination wavefronts 152,
overlapping
object wavefronts 156, and reference beam 158 having reference wavefronts 160.
Reference wavefronts 160 are the illumination wavefronts 152 that pass through
object
mask MI attenuated but otherwise unaltered. Overlapping object wavefronts 156
arise
from the phase-alteration, scattering, refraction, or diffraction of
illumination wavefronts
152 by the one or more elements (not shown) on the "object," i.e., object mask
MI.
Recording medium 140 may be photoresist, photopolymer, silver-halide emulsion,
or any
other light-sensitive medium known to be suitable for recording a hologram at
the
particular wavelength of the illumination beam.
With continuing reference to FIG. 2, in a preferred embodiment of the present
invention, an ILHM 32 is formed using a type I object mask in apparatus 100 by
a
process comprising the steps of (a) providing coherent illumination beam 148
directed
along axis 120; (b) providing a type I object mask; (c) inserting the type I
object mask
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within coherent illumination beam 148; (d) providing recording medium 140
adjacent
object mask MI opposite source of coherent illumination 124; (ej illuminating
object
mask MI with illumination beam 148, thereby creating overlapping object
wavefronts 156
and reference wavefronts 160; and (f) recording, in the recording medium a
substantially
continuous diffracting region comprising the interference pattern created by
overlapping
object wavefronts 156 and reference wavefronts 160. The process for recording
the ILHM
32 is described in more detail below.
Using a Type II Object Mask
FIG. 3 is a schematic side view of an apparatus 200 for forming an ILHM 32
using a type II object mask Mil according to the present invention. Apparatus
200
comprises all the elements of apparatus 100 of FIG. 2, except that type I
object mask Ml
is replaced with type If object mask MII. Apparatus 200 further includes a
first
beamsplitter 204, a first mirror 208, a second mirror 212, and a second
beamsplitter 216.
First beamsplitter 204 is disposed along axis 120 immediately adjacent beam
expanding
and collimating optics 128 and opposite source of coherent illumination 124,
and diverts a
portion of illumination beam 148 from axis 120 to create a reference beam 158
having
reference wavefronts 160. Reference beam 158 circumvents object mask MII by
being
reflected by mirror 208, which reflects reference beam 158 to mirror 212,
which in turn
reflects reference beam 158 toward second beamsplitter 216. Second
beamsplitter 216 is
disposed between object mask M11 and a recording medium 140 and is oriented
such that
it acts as a beam combiner by directing reference beam 158 along axis 120
towards
recording medium 140 so that reference wavefronts 160 are in-line with object
wavefronts 156.
Beamsplitters 204 and 216 may each be for example, a pellicle or plate
beamsplitter with a reflective coating tuned to the wavelength of coherent
source of
illumination 124. Alternatively, beamsplitters 204 and 216 may be large
beamsplitting
cubes, or such other beamsplitting and beam combining apparatus or techniques
that are
well-known in the art.
FIG. 4 is a schematic side view of an alternate apparatus 300, similar to
apparatus 200, for forming an ILHM 32 according to the present invention, but
that
employs a beamsplitting cube 216' as a beam combiner. Beamsplitting cube 216'
is
19
CA 02761126 2011-12-06
useful for minimizing spurious interference fringes, or "noise" in the ILHM 32
caused by
reflections from interfaces within apparatus 300, or by mechanical vibration.
Apparatus 300 comprises the elements of apparatus 100 in FIG. 2, and further
includes a
first beamsplitter 304, disposed in coherent beam 144, which creates a second
coherent
beam 144'. Second coherent beam 144' reflects off first and second mirrors 308
and 312
and passes through a second beam expanding and collimating optics 128',
thereby forming
a reference beam 158. This arrangement could also be used in apparatus 200
shown in
FIG. 3. Reference beam 158 is then directed to beamsplitter cube 216', which
acts as a
beam combiner by directing reference beam 158 to be in-line with optical axis
120.
to Illumination of object mask Mll with illumination beam 148 results in the
creation
of overlapping object wavefronts 156. Overlapping object wavefronts 156 pass
directly
through beamsplitting cube 216' and are combined therein with in-line
reference
wavefronts 160 in reference beam 158. The resulting interference pattern is
recorded in
recording medium 140 as an ILHM 32, as described above.
Beamsplitting cube 216' includes faces Fl, F2 and F3 that are substantially
the size
of object mask M11 and recording medium 140. In a preferred embodiment, object
mask
Mll and recording medium 140 are in contact with and indexed-matched to faces
FI and
F2, respectively, of beamsplitting cube 216'. Index matching between object
mask MII
and beamsplitting cube 216' and/or beamsplitting cube 216' and recording
medium '140
may be achieved using a suitable adhesive or fluid (e.g., epoxy or index-
matching oil) as a
temporary mount. In addition, faces FI through F3 of beamsplitting cube 216'
may have
an anti-reflection coating to enhance transmission of light therethrough.
One of the main advantages of using apparatus 200 or 300 to form an ILHM 32 is
that reference beam 158, though ultimately in-line, can be adjusted in
intensity while
directed off axis from axis 120 to provide the precise intensity balance
between
illumination beam 148 and reference beam 158 necessary to maximize the
efficiency of
the ILHM 32.
With reference to apparatus 200 of FIG. 3 or apparatus 300 of FIG. 4, in a
preferred embodiment of the present invention, an ILHM 32 is constructed by a
process
comprising the steps of. (a) providing illumination beam 148 directed along
axis 120;
(b) providing a type 11 object mask MII; (c) disposing object mask MII within
illumination
beam 148; (d) providing a recording medium 140 adjacent object mask MIi and
opposite
CA 02761126 2011-12-06
source of coherent illumination 124; (e) illuminating object mask MII at
normal incidence
with illumination beam 148, thereby creating overlapping object wavefronts
156;
(f) simultaneous with step (e), directing portion 144' of coherent beam 144
around object
mask Mll using, for example, mirrors and beamsplitters, thereby forming in-
line reference
beam 158 having reference wavefronts 160 that are in-line with overlapping
object
wavefronts 156 (and thus normally incident recording medium 140); and (g)
recording, in
recording medium 140, an ILHM 32 having a substantially continuous diffracting
region
comprising the interference pattern created by overlapping object wavefronts
156 and
reference wavefronts 160. The manner in which this substantially continuous
diffracting
region is formed will be more apparent following the more detailed description
of masks
MI an MII, provided below. The process for recording ILHM 32 in recording
medium
140 is also described in more detail below.
FIGS. 5a, 5b and FIG. 6 are schematic side views of apparatus 400, 450 and 500
respectively, used for forming an ILHM 32 using a type 11 object mask and a
two-step
process, wherein the first step involves forming a first hologram Hl, and the
second step
involves forming an ILHM 32 using first hologram HI.
With reference now to FIG. 5a, apparatus 400 includes all the elements of
apparatus 100 of FIG. 2 and further includes a beamsplitter 304 disposed in
coherent
beam 144 to form a second coherent beam 144' to be used to form illumination
beam 148.
A mirror 308 redirects coherent beam 144' along an axis 420 oriented at an
angle 0 with
respect to optical axis 120 and also directs the beam through expanding and
collimating
optics 128', thereby forming illumination beam 148. Meanwhile, coherent beam
144 is
directed along axis 120 and passes through beam expanding and collimating
optics 128
thereby forming a reference beam 158. A type II object mask MII is disposed in
illumination beam 148 along axis 420 such that the angle between the object
mask MII
surface normal 422 and axis 420 is 0. A first recording medium 440 is disposed
coaxial
with and perpendicular to axis 120 in a plane parallel to object mask MII, at
the location
where axes 120 and 420 intersect. Recording medium 440 has a front surface F'
and back
surface B', and is disposed with front surface F' facing reference beam 158.
Axis 420
passes through the centers of object mask MII and recording medium 440, and
recording
medium surface normal 424 and axis 420 form an angle 0. Also, an axis 426
parallel to
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CA 02761126 2011-12-06
axis 120 and separated therefrom by a distance L passes perpendicularly
through the
center of object mask MII.
When illumination beam 148 illuminates object mask MII, overlapping object
wavefronts 156 are created, which propagate toward first recording medium 440
along
axis 420. Hologram H 1 is formed by recording, in recording medium 440, the
interference pattern created by overlapping object wavefronts 156 and
reference
wavefronts 160.
With reference now to FIG. 5b, apparatus 450 is an alternate embodiment of
apparatus 400 of FIG. 5a for forming hologram HI as a first step in forming an
ILHM 32
of the present invention using a two-step process. Apparatus 450 comprises the
same
elements as apparatus 400 in FIG. 5a, except that the elements are combined to
form a
different beam geometry than that of apparatus 400. In apparatus 450,
illumination
beam 148 is directed along an axis 426 and is normally incident object mask
MII.
Overlapping object wavefronts 156, created by illuminating object mask Mil
with
illumination beam 148, are preferentially directed at an angle 8 towards first
recording
medium 440 along axis 420 by the one or more mask elements (scattering,
refracting or
diffracting, not shown in the Figure and discussed in more detail below) on
object mask
MII or by a directional diffuser or grating (neither shown) on or near object
mask MII.
Hologram HI is formed by recording, in recording medium 440, a substantially
continuous
diffracting region comprising the interference pattern created by the
overlapping object
wavefronts 156 and reference wavefronts 160 in reference beam 158.
With reference now to FIG. 6, the second step for forming ILHM 32 using first
hologram H1 formed in the first step using either apparatus 400 or 450 is now
described.
Apparatus 500 has essentially the same geometry as that of apparatus 450 of
FIG. 5b, and
for the sake of simplicity, is shown to have the same elements as apparatus
450, except'
that object mask MII and first recording medium 440 are removed. In addition,
the
distance L' separating parallel axes 426 and 120 is greater than L. Hologram H
I now
serves as the mask for forming an ILHM 32 using apparatus 500, and is disposed
in
illumination beam 148" with back surface B' facing illumination beam 148"
(note that
illumination beam 148" in apparatus 500 is the same as reference beam 158 in
apparatus 450) and can also be considered to be a reconstruction beam. A
second
recording medium 540 with front surface F and back surface B is disposed in
reference
22
CA 02761126 2011-12-06
beam 158" along axis 426 in a plane substantially parallel to hologram H I
(note that
reference beam 158" in apparatus 500 is the same as illumination beam 148 in
apparatus 450). An axis 544 passing through the respective centers of hologram
H1 and
recording medium 540 forms an angle 0 with respect to hologram HI surface
normal 546
and recording medium surface normal 548, as shown.
Hologram HI is disposed such that when it is illuminated by illumination
beam 148", diffracted wavefronts 156" form an in-focus real image 550 in an in-
focus real
image plane 554, which is parallel to face F of recording medium 540. The
center 556 of
in-focus real image 550 is located a distance L from axis 120. Second
recording
medium 540 is disposed in a defocused real image plane 558 a defocus distance
x
(e.g., 60mm) away from in-focus real image plane 554 and in line with
reference
beam 158" along axis 426, so that a defocused real image (not shown) of in-
focus real
image 550 and reference wavefronts 160" are recorded in recording medium 540
as an
ILHM 32. Because diffracted wavefronts 156" propagate along axis 544,
recording a
is centered defocused real image in recording medium 540 at a distance x from
best-focus
real image plane 554 requires that axis 426 be separated from axis 120 by an
distance, L' = L + xtanO.
In a preferred embodiment of the present invention, an ILHM 32 is constructed
by
a two-step process using apparatus 400 of FIG. 5a and apparatus 500 of FIG. 6,
comprising the steps of (I) with reference to apparatus 400 of FIG. 5a, (a)
providing a
first coherent reference beam 158 directed along axis 120; (b) providing a
first coherent
illumination beam 148 directed along axis 420 which forms an angle 0 with axis
120;
(c) providing a type II object mask MII disposed in first illumination beam
148 and along
axis 420 such that the object mask MII surface normal 422 and axis 120 form an
angle 0;
(d) providing first recording medium 440 disposed along axis 120 adjacent
object
mask MII, and opposite source of coherent illumination 124, such that axis 420
passing
through the respective centers of object mask MII and recording medium 440
form as
angle 0 with their respective surface normals 422 and 424; (e) illuminating
object mask
MII with first illumination beam 148 along axis 420, thereby creating object
wavefronts 156 that propagate towards first recording medium 440 along axis
420; (f)
simultaneous with step (1)(e), illuminating first recording medium 440 with
first reference
beam 158 along axis 120; (g) recording, in recording medium 440, a first
hologram 1-11
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CA 02761126 2011-12-06
having a substantially continuous diffracting region comprising the
interference pattern
created by overlapping object wavefronts 156 and reference wavefronts 160.
Then, with
reference to apparatus 500 of FIG. 6, (2)(a) providing a second coherent
illumination
beam 148", the same as or conjugate to first reference beam 158, directed
along axis 120;
(b) providing a second coherent reference beam 158" directed along axis 426,
which is
parallel to axis 120; (c) disposing hologram HI in illumination beam 148"
perpendicular
to axis 120; -(d) providing a second recording medium 540 disposed
perpendicular to axis
426 in defocused real image plane 558, such that axis 544 passing through the
respective
centers of hologram HI and recording medium 440 forms an angle 6 with their
respective
surface normals 546 and 548; (e) illuminating hologram H 1 with second
illumination
beam 148" along axis 420, thereby creating overlapping diffracted wavefronts
156" that
propagate towards recording medium 540 along axis 544; (f) simultaneous with
step
(2)(e), illuminating recording medium 540 with reference beam 158"; and (g)
recording, in
recording medium 540, an ILHM 32 having a substantially continuous diffracting
region
comprising the interference pattern created by overlapping diffracted
wavefronts 156" and
reference wavefronts 160". The process for recording ILHM 32 recording medium
540 is
described in more detail below.
Alternatively, with reference to apparatus 450 of FIG. 5b, in another
preferred
embodiment of the present invention, step (1) in the above two-step process
for
contracting ILHM 32 comprises the steps of. (a) providing a first coherent
reference
beam 158 directed along axis 120; (b) providing a first coherent illumination
beam 148
directed along axis 426, which is parallel with axis 120; (c) providing a type
II object
mask MIi disposed in first illumination beam 148 along axis 426; (d) providing
first
recording medium 440 perpendicular to axis 120 adjacent object mask MII and
opposite
source of coherent illumination 124, and disposed such that axis 420 passing
through their
respective centers form an angle 0 with their respective surface normals 422
and 424;
(e) illuminating object mask Mil with first illumination beam 148 along axis
426, thereby
creating overlapping object wavefronts 156 that propagate towards first
recording medium
440 along axis 420 due to one or more directionally scattering, refracting, or
diffracting
elements on object mask M1I or a directional diffuser or grating on or near
object mask
Mil; (f) simultaneously with step (1)(e), illuminating first recording medium
440 with
reference beam 158 along axis 120; and (g) recording, in recording medium 440,
a first
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hologram HI having a substantially continuous diffracting region comprising
the
interference pattern created by overlapping object wavefronts 156 and
reference
wavefronts 160. The process for recording ILHM 32 in recording medium 440 is
described in more detail below.
FIG. 7 is a schematic side view of the apparatus 10 of FIG. I being used for
patterning a workpiece with ILHM 32 formed using either of the above-described
two-step
processes. Workpiece 40 is placed in workpiece holder 36 with its front
surface S a
distance d' away from front surface F of ILHM 32. In this arrangement, best-
focus real
image 550 is reproduced at surface S of workpiece 40, but the center 560 of
best-focus
real image 550 is displaced perpendicular to axis 20 by a distance y = d'tan6.
Thus, the
workpiece can be patterned with best-focus real image 550 centered on front
surface S of
workpiece 40 by either displacing workpiece 40 by a distance y = d'tan6 with
respect to
ILHM 32, or by shifting ILHM 32 a distance -y = -d'tanO with respect to
workpiece 40.
With reference now to FIGS. 8 and 9, there is shown schematic side views of a
apparatus 600 and 650, respectively, for forming an ILHM 32 using a type II
object mask
and a third two-step process, wherein the first main step involves recording
two
interference patterns in a single hologram Hl using apparatus 600 of FIG. 8,
and the
second main step involves forming an ILHM 32 using hologram HI as an object
mask in
apparatus 650 of FIG. 9.
Apparatus 600 of FIG. 8 is identical to apparatus 400 of FIG. 5a, except that
object mask Mill is disposed along axis 120 rather than axis 420, so that
object
wavefronts 156 are normally incident recording medium 440. In forming hologram
HI
using apparatus 600 as part of the first main step, two sub-steps are
required. The first
sub-step is illuminating object mask MII with illumination beam 148, and
simultaneously
illuminating recording medium 440 with reference beam 158 directed along axis
420,
thereby recording in recording medium 440 a first interference pattern from
the
interference of overlapping object wavefronts 156 and reference wavefronts
160. The
second sub-step is removing object mask MII, and illuminating recording medium
440
with illumination beam 148 and reference beam 158, thereby recording in
recording
medium 440 a second interference pattern that overlaps the first interference
pattern.
When illumination beam 148 and reference beam 158 consist of plane waves, the
second
interference pattern is a grating. Thus, the hologram III formed in recording
medium 440
CA 02761126 2011-12-06
actually contains two super-imposed holographic recordings: one of object mask
Mll and
one of the interference pattern formed by illumination beam 148-and reference
beam 158.
The second main step is creating the ILHM 32 using apparatus 650 of FIG. 9 and
hologram H I as formed in the first main step, as an object mask. Apparatus
600 includes
source of coherent illumination 124 which provides illumination beam 148"
directed along
axis 654, which intersects axis 120 at an angle 0. Hologram H I is disposed
perpendicular
to axis 120 where axis 654 intersects axis 120, and with its back surface B'
facing
illumination beam 148". A recording medium 660 having a front side F and a
backside B
is disposed along axis 120 adjacent hologram HI and opposite illumination beam
148" in a
to defocused real image plane 662. Front surface F' of hologram HI and front
surface F of
recording medium 660 are separated by a distance z. When hologram HI is
illuminated
with illumination beam 148", two wavefronts are formed: diffracted wavefronts
156"
arising from diffraction by the first superimposed recorded interference
pattern, and
reference wavefronts 160" arising from diffraction by the second superimposed
recorded
interference pattern. An in-focus real image 664 is formed at an in-focus real
image
plane 668 a distance q from front surface F' of hologram HI. Recording medium
660 is
disposed with its front surface F parallel to in-focus image plane 668 at
defocused real
image plane 662, and a distance d = z-q away from in-focus real image plane
668, so that
a defocused real image (not shown) and reference wavefronts 160" are recorded
in
recording medium 660 as an ILHM 32. It should be noted that recording medium
660 can
also be placed in a defocused real image plane between hologram 1-II and in-
focus real
image plane 668. The ILHM 32 is then used in apparatus 10 of FIG. I to pattern
workpiece 40, as described above.
An important advantage of the ILHM 32 formed using the third two-step process
as described above is that it can be readily used in combination with a
projection lens.
With reference to FIG. 10, there is shown such an ILHM 676 as used in
apparatus 10 of
FIG. I wherein apparatus 10 further includes a projection lens 680 placed
between ILI-IM
676 and workpiece 40. Illuminating ILHM 676 with reconstruction beam 48 forms
a real
image 682 at object plane 684 of projection lens 680, which is then projected
by
projection lens 680 to form real image 682' on surfaces of workpiece 40 placed
at the
image plane of projection lens 680. Thus, it will be apparent that ILHM 676
(or, in fact,
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CA 02761126 2011-12-06
any of the ILHMs of the present invention) can be used in place of a standard
mask used
in existing photo patterning tools utilizing a projection lens.
Thus, with reference to apparatus 600 of FIG. 8 and apparatus 650 of FIG. 9,
in a
preferred embodiment of the present invention, an ILHM 32 is constructed as a
two-step
process. With reference to apparatus 600 of FIG. 8, the first step comprises
(a) providing
a first coherent illumination beam 148 directed along axis 120; (b) providing
a type II
object mask MII along axis 120; (c) providing a first recording medium 440
adjacent
object mask MII and opposite source of coherent illumination 124; (d)
illuminating object
mask Ml! with illumination beam 148, thereby creating overlapping object
wavefronts
to 156; (e) simultaneous with step (1)(d), illuminating first recording medium
440 with first
reference beam 158 along an axis 420, which forms an angle 6 with respect to
axis 120;
(f) recording in first recording medium 440, as hologram HI, a first
interference pattern
created by the interference between first overlapping object wavefronts 156
and reference
wavefronts 160; (g) removing object mask MII and further recording in first
recording
medium 440, as hologram HI, a second interference pattern, superimposed on the
first
interference pattern, created by the interference between illumination beam
148 and
reference beam 158.
Then, with respect to apparatus 650 of FIG. 9, the second step of the process
involves (a) providing a second coherent illumination beam 148" conjugate to
reference
wavefronts 158 of step (1)(e), above, and directed along axis 654; (b)
disposing
hologram HI in second illumination beam 148" along axis 120; (c) providing a
second
recording medium 660 coaxial with and adjacent first hologram 141 and opposite
illumination beam 148" along axis 120, in defocused real image plane 662 a
distance d on
either side of in-focus real image plane 668 in which in-focus real image 664
from
hologram Hl is formed; (d) illuminating first hologram H1 with second
illumination
beam 148" thereby forming (i) diffracted wavefronts 156" which form in-focus
real
image 664 at in-focus real image plane 668 as a result of the diffraction of
second
illumination beam 148" by the first recorded interference pattern in hologram
HI, and
(ii) diffracted reference wavefronts 160" coherent with diffracted wavefronts
156", formed
by the diffraction of second illumination beam 148" by the second recorded
interference
pattern in first hologram HI; (e) recording, in second recording medium 660,
an ILHM 32
having a substantially continuous diffracting region comprising the
interference pattern
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CA 02761126 2011-12-06
from diffracted wavefronts 156" (which form a defocused real image at surface
F of
second recording medium 660), and diffracted reference wavefronts 160".
In forming hologram Hl in accordance with any of the three preferred two-step
processes set forth above, a preferred recording step includes recording
hologram FI1 on a
plate coated with silver halide, using an exposure between 200 to 600
ergs/cm2,
developing in Ilford developer for 3 minutes, bleaching in EDTA bleach until
clear, and
then drying the plate in a graded alcohol bath (50%, 75% and 100%). Hologram
HI can
also be formed in UV materials like photopolymer or photoresist.
Recording the ILHM
In the preceding discussion, the process for recording an ILHM in the
holographic
recording medium is only mentioned in passing. Additional detail concerning
how this
recording is achieved is provided in this section.
To create a durable ILHM 32 according to the present invention, a recording
medium such as photoresist is deposited on a quartz or fused silica substrate.
After
exposure, the recording medium is developed, and the substrate is etched
using, for
example, a reactive ion-etch (RIE) process or ion beam milling, in order to
transfer the
interference fringes recorded in the recording medium into the substrate.
Adjustment of
the etch parameters allows for the tailoring of the profile that is
transferred into the
substrate. The process of transferring the pattern recorded in the recording
medium into a
substrate that is durable, easily cleaned, and that has a low thermal
expansion, high
UV transparency, and high refractive index uniformity (e.g., optical quality
quartz or fused
silica) makes for an ILHM 32 that is eminently suitable for use in industrial
applications
because of its resistance to environmental affects, and high damage threshold
to UV and
DUV wavelengths.
A recording medium suitable for use in the present invention comprises, for
example, Shipley 505 or 1805 photoresist spun onto a quartz or fused silica
substrate to a
thickness of about 10,000 Angstroms, The precise thickness of the layer can be
varied to
obtain a desired developed thickness prior to etching. The layer is then soft-
baked at
95 degrees Centigrade for 30 minutes. In one example of this process, a
recording
medium is used to record an ILHM 32 of a mask having an array of substantially
transparent phase-altering elements on a substantially transparent substrate
(this type of
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CA 02761126 2011-12-06
mask is discussed in greater detail below). The ILHM 32 is formed by
illuminating the
mask with a coherent illumination beam having a wavelength of 413 nanometers
(nm) and
providing an exposure of about 20 to 50 millijoules/cm2. The photoresist is
then
developed with Shipley MF319 maximum resolution developer, wherein the develop
time
was adjusted to yield the best resist profiles without "clipping" or
"bottoming out" of the
profiles. By adjusting the exposure time t, the intensity I of the beam, and
the developing
procedure, the exposure e = I x t can be adjusted such that the interference
pattern is
optimally recorded in the recording medium, which results in a high-efficiency
ILFIM 32.
Type I and Type II object masks of the Present Invention
The present invention uses four different specialized type I and type II
object
masks in forming an ILHM 32, called phase-altering, scattering, refracting and
diffracting
masks, in reference to the four different kinds of elements that make up each
specialized
mask. Each kind of element is substantially transparent and is designed to
spread light
transmitted through the element. It will be apparent to one skilled in the art
that the
degree to which the individual elements spread light passing therethrough can
be tailored,
and even made directional, by adjusting the relevant parameters pertaining to
each type of
element, as described below.
a. Phase object mask
FIG. 11 is a perspective schematic view of a phase mask 700 comprising a
planar
transparent substrate 704 having an array 708 of cylindrical-shaped
indentations 712 of
depth D, a diameter 716, and a center-to-center spacing 720. Indentations 712
are phase-
altering elements that alter the phase of coherent light passing through mask
700.
Indentations 712 are shown in FIG. 11 as being cylindrical for the sake of
example. In
practice, however, indentations 712 can be any desired shape. Indentations 712
may be
formed in substrate 704 by any one of a number of techniques known in the art
of mask-
making, such as first coating the substrate with a layer of chrome, then
providing a layer
of photoresist on top of the chrome, then exposing the photoresist with a
pattern consisting
of an array of circular shapes (or any desired shape), then developing the
photoresist, then
etching the chrome, then ion milling or reactive ion etching or liquid etching
the exposed
substrate, and then removing the remaining chrome and photoresist. Removing
the
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CA 02761126 2011-12-06
chrome from the substrate results in a type I phase mask. Alternatively, a
sufficiently thin
layer of chrome may be left on substrate 704 to reduce transmission of the
mask to
achieve a better beam ratio between the intensity of the transmitted reference
wavefronts
and the object wavefronts. A type II phase mask is formed by leaving a
sufficiently thick
layer of chrome on substrate 704 so that light does not pass through the
regions of the
substrate surrounding indentations 712.
With continuing reference to FIG. 11, the etch depth D into substrate 704 is
adjusted according to the wavelength of illumination and amount of phase-shift
required
for the particular application. The relationship between the depth D of phase
to indentations 712 and the amount of phase shift C in radians at a wavelength
A induced by
phase indentations 712 is D = A (D/((n-1)27r ], where n is the index of
refraction of
substrate 704.
As an alternative to forming phase indentations, phase-shifting islands
deposited on
top of substrate 704 may be used. These island may be formed, for example, by
sputtering quartz or another type of glass or UV transparent polymer onto
substrate 704
using known lithographic techniques. In addition, other shapes besides
circular phase
indentations 712 may be used. The particular shape of the one or more phase
objects is
determined by the desired pattern to be formed in the workpiece, and whether
that pattern
can be created by one or more phase elements on the mask.
FIG. 12 is a plot of the spatial intensity distribution 726 of a real
holographic
image reconstructed from an ILHM 32 fabricated using apparatus 100 of FIG. 2
and a
type I phase object mask 700 of FIG. 11. Phase object mask 700 was an optical-
quality
quartz substrate 704 with cylindrical-shaped indentations 712 having a
diameter 716 of
100 m and a center-to-center spacing 720 of 1200 n. As can be seen from the
plot, the
resultant spatial intensity distribution 726 has high-intensity peaks 730 well
above the
noise regions 734. This mask proved to be very well-suited for drilling holes
in a plastic
membranous workpiece.
b. Scattering object mask
FIG. 13 is a perspective schematic view of a type I scattering object mask 750
comprising a planar transparent substrate 754 having an array 758 of small,
circular
scattering elements 762, each having a diameter 766 and separated by center-to-
center
CA 02761126 2011-12-06
spacing 770. Scattering elements 762 can be thought of as consisting of many
very small
randomly oriented facets or phase-shitting elements (not shown), which can be
sufficiently
small so that each scattering element 762 constitutes a diffuser. With
reference to FIG. 2,
when scattering object mask 750 is placed in apparatus 100 and illuminated by
illumination beam 148, the individual scattering elements 762 each distributes
a small
portion of illumination beam 148 over recording medium 140 as a set of small,
randomly
directed plane waves, The collection of these sets of plane waves from each
scattering
element 762 constitutes overlapping object wavefronts 156. When overlapping
object
wavefronts 156 are combined with reference wavefronts 160 and recorded in
recording
to medium 140, the result is an ILHM 32 having a substantially continuous
diffracting region
comprising an interference pattern with a large interference term. When an
ILHM 32 thus
formed is replayed in apparatus 10 of FIG. 1, the light from reconstruction
beam 48 is
recombined at the surface S of workpiece 40 (i.e., at plane P1) as small
circular images
capable of patterning workpiece 40.
With continuing reference to FIG. 13, scattering elements 762 are formed on
scattering object mask 750 by any one of a number of techniques known in the
art of
mask-making, such as first covering the substrate 754 with a chrome layer,
then depositing
a layer of photoresist on top of the chrome layer, then exposing the desired
pattern into
the photoresist, then developing the photoresist, then etching the chrome,
then removing
the photoresist, and then liquid etching the exposed substrate using a dilute
mixture of
ammonium bifluoride and barium sulfate, to produce frosted scattering elements
having a
desired scattering distribution (e.g., lambertian). This results in a type II
scattering object
mask wherein the area 776 of substrate 754 surrounding scattering elements 762
is
covered with an opaque chrome layer. A type I scattering object mask is formed
by the
additional step of removing the chrome layer so that area 776 is transparent
or semi-
transparent. The etchant dilution, temperature, time of the etching, and the
material of
substrate 754 determines the amount of surface roughness and hence the
scattering profile
of scattering elements 762. A preferred amount of surface roughness is between
0.311m
and 3 m. Scattering elements 762 can also be made to scatter or diffuse light
in a
preferred direction by introducing a wedge into each element, or imparting a
preferred
etch or facet direction to each element.
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CA 02761126 2011-12-06
c. Refracting object mask
FIG. 14 is a schematic side-view of a refracting object mask 800 comprising a
planar transparent substrate 804 having an array 808 of lenslets 812. Lenslets
812 are
shown as positive piano-convex lens elements having a focal length f, a
diameter 814, and
a center-to-center spacing 816. Lenslets 812 convert plane illumination
wavefronts 152
into converging spherical wavefronts 152', which after a distance f from
refracting object
mask 800 become diverging and spherical wavefronts 152". Diverging and
spherical
wavefronts 152" are akin to those formed from diffraction by a very small
aperture.
Diverging and spherical wavefronts 152" from adjacent lenslets 812 in lenslet
array 808
can be made to overlap more or less strongly by choosing the appropriate
lenslet
diameter 814, center-to-center spacing 816, and focal length f. Recording an
ILHM 32 of
wavefronts 152" generated by diffraction with small apertures is difficult
because the
amount of light transmitted is often too small to achieve the proper intensity
balance
between the object wavefronts and the reference wavefronts. However,
refracting
mask 800 transmits sufficient light through lenslets 812 to allow for a
holographic
recording. The result is an ILHM 32 with a substantially continuous
diffracting region
comprising an interference pattern with a large interference term. Thus, in
forming the
ILHM 32 of the present invention using refracting object mask 800, the
distance d in
FIG. 2 should satisfy the condition d > f (see in FIG. 14) so that diverging
spherical
wavefronts 152" overlap at recording medium 140. Then, in patterning a
workpiece using
apparatus 10 of FIG. I with an ILHM 32 thus formed, the distance d' will be
given by the
relation d' = d - f.
With continuing reference to FIG. 14, in a type I refracting object mask, the
regions 818 between lenslets 812 are transparent or semi-transparent. In a
type II
refracting object mask, regions 818 are sufficiently opaque to prevent any
transmitted light
from being recorded in the ILHM 32. When an ILHM 32 is formed using a type I
or
type II refracting object mask according to the present invention and is
replayed in
apparatus 10 of FIG. I, an array of sharp points of light are formed at plane
P1.
A first method of forming a type I refracting object mask is first providing a
substrate such as quartz, then depositing a layer of positive photoresist on
its upper
surface, then exposing a pattern consisting of an array of circles in the
photoresist, then
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CA 02761126 2011-12-06
developing the photoresist to create holes in the photoresist, then
isotopically etching the
quartz until the holes take on a concave shape.
A second method of forming a type I refracting object mask is first depositing
a
layer of negative photoresist on a substrate, then exposing the photoresist
with UV
illumination through a mask having an array of circular apertures, then
developing the
photoresist layer thereby creating an array of cylindrical photoresist
islands, then heating
the photoresist islands so that they melt and spread to form the lenslet
elements 812, as
shown in FIG. 14. For example, Shipley S 1827 photoresist, which has a melting
temperature of 135 C, can be used to form piano-convex lenslets with a
diameter of
25 m, a thickness of 2.2 m, and an index of refraction of about 1.6.
A method of forming a type 11 refracting object mask or a semi-transparent
type I
refracting object mask is first providing a transparent substrate, such as
quartz then coating
the upper surface of the substrate with a layer of semi-transparent or opaque
chrome, then
depositing a layer of positive photoresist on top of the chrome layer, then
exposing the
desired patterns in the photoresist, then developing the photoresist, then
etching the
chrome layer to create apertures in the chrome layer, then depositing another
layer of
negative photoresist exposing from the back and then developing so that the
resist now
fills the chrome spaces, then melting the photoresist layer until the
photoresist island takes
on a convex shape.
Negative piano-concave lenslets 812 may also be formed by creating
hemispherical
indentations in substrate 804. A first method of forming a type I or type 11
refracting
object mask having negative piano-concave lenslets includes the steps of
coating
substrate 804 with a thick layer of photoresist, then exposing the photoresist
with a mask
having an array of shaped apertures or features with a specified optical
density gradient,
developing the photoresist thereby creating an array of shaped indentations in
the
photoresist.
A second method of forming the same mask includes the steps of coating
substrate 804 with a suitably soft transparent material, such as acrylic
polymer, and then
impressing hemispherical indentations into the photoresist by contacting it
with a template.
For piano-concave lenslets, the focal length f of each lenslets is negative,
which
results in the creation of diverging wavefronts, so the distance d can be any
value (see
FIG. 14).
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CA 02761126 2011-12-06
d. Diffracting object mask
FIG. 15 shows a section of a diffracting object mask 850 comprising a planar
transparent substrate 854 with a front surface 856 and a back surface 858. On
front
surface 856 there is an array 860 of grating elements 864 each surrounded by a
region
868. In a type [ diffracting object mask, region 868 is transparent or semi-
transparent. In
a type II phase mask, region 868 is opaque. Grating elements 864 are shown in
the
Figure as circular in shape with a diameter 872 and a center-to-center spacing
874, with a
periodic linear grating enclosed therein. Generally, grating elements 864 can
have any
shape (e.g., square, rectangular, triangular, polygonal, crossed, etc.) and,
more generally,
can be any diffracting structure for redirecting, diverging, or converging
light passing
therethrough, such as a .hologram.
Referring now to FIG. 16, there is shown apparatus 100 of FIG. I with type I
diffracting object mask 850 of FIG. 15 being used to form an ILHM 32 according
to the
present invention. When diffracting object mask 850 is illuminated with
illumination
beam 148, each grating element 864 creates a set of wavefronts 156 that
propagate in a
plurality of well-defined directions determined by the precise nature of each
grating
element 864. In the Figure, only the zeroeth-order wavefronts 876 and first-
order
wavefronts 878 with angle 66 are shown. Zeroeth-order wavefronts 876 are
undiffracted
but attenuated illumination wavefronts 152 that serve as reference wavefronts
in forming
the ILHM 32. The proper choice of grating frequency v of grating elements 864
and
distance d will result in overlapping object wavefronts 156 from each grating
structure 864
at recording medium 140, resulting in an ILHM 32 having a substantially
continuous
diffracting region with a large interference term. For a diffraction grating,
the diffraction
angle 0b is determined from the relation sinO,= Xv.
An ILHM 32 formed using a type I or type II diffracting object mask 850 of
FIG. 15 with, for example, a crossed grating and a circular outer shape
produces an array
of circular images, each containing an array of bright spots within. This type
of image is
useful for ablating areas on a workpiece, since the bright spots represent a
high-
concentration of energy. As an example, a type li diffracting object mask is
fabricated
wherein grating elements 864 are each a uniform grating having a period of 50
cycles/mm,
a diameter 872 of 100 m, and a center-to-center spacing 874 of 1200 m. The
[LHM 32
formed from this mask using apparatus 100 of FIG. 2 is then used to pattern a
work-piece.
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CA 02761126 2011-12-06
With reference now to FIGS. 17a-e there is shown the steps of a preferred
method
for creating a type I or type II diffracting object mask. With're-ference to
FIG. 17a, the
method includes the steps of providing substrate 854 and coating front surface
856 with an
opaque layer 890 (such as dark chrome), then forming apertures 892 of an
arbitrary shape
in opaque layer 890 so as to expose underlying substrate 854 using standard
techniques
known in the art (e.g., coating with photoresist, exposing the photoresist,
developing the
photoresist and then etching the opaque layer).
Referring now to FIG. I7b, the next step is coating opaque layer 890 with a
negative photoresist layer 894 such as AZ OMR 83.
to Referring to FIG. 17c, the next step is exposing photoresist layer 894
through back
surface 858 with two interfering plane wave beams 896 and 898 propagating at
angles (i
and y with respect to an axis 900, which passes perpendicular through the
center of
substrate 854. Plane wave beams 896 and 898 are generated by the standard
techniques,
such as the one shown in the Figure, which includes providing a light beam 44
emanating
from coherent source of illumination 24 and dividing the beam with a
beamsplitter 304,
thereby forming two coherent beams 44 and 44', which are directed by
beamsplitter 304
and mirror 308, respectively, to beam expanding and collimating optics 28 and
28' located
in beams 44 and 44', respectively. Plane wave beams 896 and 898 are then made
to
interfere within photoresist layer 894 at an angle = p-+ y.
With reference now to FIG. 17d, grating element 864 with grating lines 904 is
formed in photoresist layer 894 within each aperture 892. The spacing a
between adjacent
grating lines 904 is given by E = A/sin4), where X is the illumination
wavelength from
coherent source of illumination 24. At this point in the fabrication process,
opaque
layer 890 can be left in place to create a type II diffracting object mask as
shown in
FIG. 17d. Alternatively, opaque layer 890 can be stripped to form a type I
diffracting
object mask, as shown in FIG. 17e, comprising groupings of grating lines 904
on
surface 856 of substrate 854.
An alternate embodiment of diffracting object mask 850 is shown in FIG. 18.
Type 11 grating mask 950, shown in side-view, has the same elements as those
shown in
FIG. 17a, and further includes a grating 954 on backside 858. Grating 954 is
formed by
following essentially the aforementioned steps 17a-e in forming grating mask
850, except
that negative photoresist layer 894 is deposited on backside 858 rather than
on opaque
CA 02761126 2011-12-06
layer 890 (see FIG. 176), plane wave beams 896 and 898 (see FIG. 17c) are made
to
interfere in the negative photoresist layer deposited on backside 858, and
that opaque
layer 890 is left in place on substrate 854. Thus, diffracting object mask 950
functions
essentially in the same way as diffracting object mask 850, except that the
grating
structure 954 in diffracting object mask 950 is continuous and displaced from
apertures 892 by the thickness of substrate 854.
Exemplary Process of Forming an ILHM
Now described is an example of a process for forming an ILHM 32 with a semi-
transparent phase object mask, where the ILHM 32 could be used, for instance,
to perform
excimer laser ablation of 2 mil (0.0002" thick) polyimide film (such as
DuPont's KaptonTM'M).
This ablation procedure is presently routinely performed using more complex
and expensive
phototools in the production of flexible circuits of the type used in
microelectronics
packaging for semiconductor chips used in consumer devices such as cellular
telephones and
portable computers. The ILHM 32 formed using the present specific process
example can
also be utilized in the production of inkjet printer nozzles, liquid crystal
displays,
alphanumeric markings on metals, ball grid array packages in ceramic or
plastic. Also, with
lower power (i.e., non-ablative) illumination, the ILHM 32 of the present
specific process
example could be used to expose photoresist or other photoactive materials
placed on silicon
or other substrates. Many other uses will be apparent to one skilled in the
art, including the
pattering of three dimensional structures with high aspect ratios.
The first main step in forming the ILF M 32 of the present specific process
example is
preparing the phase object mask. In the preferred process, the density of the
chrome on the
mask should be such that the beam ratio between the undiffracted illumination
beam (i.e., the
reference beam) and the object beam (i.e., diffracted object wavefronts)
caused by the phase-
shifting and diffraction of the incident illumination beam by the etched phase
structures is
approximately 3: 1. Continuing with a description of the preferred process, a
quartz substrate
is then coated with chrome to an optical density of 0.6, and a layer of
Shipley 1800 series
photoresist is deposited atop the chrome layer. The photoresist is then
exposed with UV
light in a contact copy jib (such as that manufactured by Oriel Corporation)
using an electron
beam patterned master mask with a dark field and clear features. The
photoresist is then
developed in Shipley MF312 developer diluted 1:1 with DI water. The chronic is
then
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CA 02761126 2011-12-06
etched in Transene type 1020 chromium mask etchant. The quartz in the
resulting open
areas is then etched using a 10% concentration of buffered hydrofluoric acid
to a depth
equivalent to a 180 degree phase shift in the wavelength used for the
exposure. For
413 nm light this depth is approximately 210 rim. Reactive ion etching can
also been used
for this step where less undercut of the chrome layer is desired.
The second main step is to place the phase mask in a collimated beam of light
(e.g., a Lambda-Physik EMG 104 excimer laser emitting at 248 nm) and
holographically
exposing an excimer grade fused silica plate (Corning 7940 or Heraeus
Supersil) coated
with Shipley SPR511 A photoresist, disposed a distance of 5 cm away from the
phase
object mask. The photoresist is then developed in Shipley MF319 developer and
the
quartz substrate reactive ion etched to an average depth of approximately 250
nm with
Freon 14 and oxygen (approx 8%) using the photoresist as a mask.
Alternatives and variations to any of the above-mentioned specific method can
be
employed to make a ILHM 32 that will result in the same functionality and thus
still be
within the spirit and scope of the present invention. In fact, while the
present invention
has been described in connection with preferred embodiments, it will be
understood that it
is not limited to those embodiments. On the contrary, it is intended to cover
all
alternatives, modifications, and equivalents as may be included within the
spirit and scope
of the invention as described herein.
37
