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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to an apparatus for exposing a photosensitive layer
through a photomask carrying microscopic patterns, such as of an
integrated circuit, and more particularly to a light exposure apparatus
suitable for use in photolithographically printing a micro-circuit on a
semiconductor substrate.
The exposure of a photosensitive layer such as a photoresist layer applied
on a substrate is associated with two objectionable problems of damaging
the printed image, one of which is caused by the diffraction of light, and
another of which is due to the standing wave occurring in the
photosensitive layer.
Conventionally, it has been known that negative films carrying microscopic
patterns, when printed by the contact-printing method, provide no
perfectly reproduced patterns because of the influence of diffraction
which appears in a phenomenon as arising from the insufficient contact
between the negative film and the photosensitive layer. This phenomenon is
serious in the non-contact printing method which has been developed to
prevent damage of the photomask as well as of the wafer which will be
caused each time when the photomask is brought into intimate contact with
the photosensitive layer applied on the wafer. An apparatus which has
overcome this drawback by utilizing the physical phenomenon that one
diffraction pattern can be cancelled by superposition of another
diffraction pattern known in German laid open for inspection application
2,140,549.
On the other hand, when the light flux impinging upon the wafer is of a
monochromatic light, the standing (stationary) waves are caused to appear
in the photosensitive layer applied on the wafer surface, resulting in
formation of wrinkles on the hardened portions of the developed and
processed photosensitive layer of the wafer. In order to overcome this
drawback, it is desirable to provide a light-permeable layer of a
specified thickness and having an index of refraction almost equal to that
of the photosensitive layer located between the photosensitive layer and
the wafer and to expose this photosensitive layer to a light flux
containing many different wavelength rays.
SUMMARY OF THE INVENTION
An object of the invention is to provide a light exposure apparatus for
printing in which the number of diffraction patterns (fringes) which are
capable, upon superposition, of cancelling one another is increased to
produce an integrative effect on elimination of the diffraction patterns,
whereby the shape of the improved pattern is made more smooth.
Another object of the present invention is to provide a light exposure
apparatus having provision for varying the inclination angles of
illumination light fluxes impinging upon the photomask in accordance with
the line width of the microscopic pattern carried on the photomask.
Still another object of the invention is to provide a light exposure
apparatus of the character described which is constructed so as to
facilitate the incorporation of the optical means for compensating the
standing wave.
The term "integrative effect" herein used refers to an infinite
superposition of diffraction patterns which is effected by continuous
distribution of incident angles of the rays impinging upon the photomask.
However, in the practical case, it is difficult to make a light flux of
rays with continuously varying angles of incidence. Accordingly, in one
embodiment of the invention as shown in FIG. 2, such integrative effect is
approximated by a finite superposition of diffraction patterns which is
effected by use of a light flux of rays with discontinuously distributed
angles of incidence.
Further, it is desirable to vary the inclination angles at which the rays
impinge upon the photomask in accordance with the average width of the
lines constituting the microscopic pattern and with the distance between
the photomask and the photosensitive layer. To achieve this, the present
invention contemplates the use of an iris diaphragm or a diaphragm plate
having a number of openings of different diameter for varying the size of
aperture of the apparatus with variation in the maximum inclination angle.
The optical means for compensating the standing wave will be described
later in connection with a specific example of the embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation showing the principle of the
invention.
FIG. 2 is a schematic sectional view of a preferred embodiment of a light
exposure apparatus for printing according to the present invention.
FIG. 3 is a top view of a fly's eye lens assembly usuable in the apparatus
of FIG. 2.
FIGS. 4 and 5 are enlarged sectional views of each lens element of the
fly's eye lenses assembly of FIG. 3.
FIG. 6 is a schematic sectional view showing a specific example of the
embodiment employing a filter for compensating the standing wave.
FIGS. 7A and 7B are schematic representations showing the effect of the
filter for compensating the standing wave.
FIG. 8 shows the spectral characteristics of usually avialable
photoresists.
FIG. 9 is a spectrum of a usual mercury vapor lamp.
FIG. 10 shows a spectral sensitivity characteristic of an example of the
photoresist.
FIG. 11 is a transmittance characteristic of a standing wave compensating
filter.
FIG. 12 shows another example of the effect of the standing wave
compensating filter.
FIG. 13 is a fragmentary sectional view of the apparatus of the invention.
FIG. 14 is a schematic sectional view of a illumination compensating filter
usuable in the apparatus of FIG. 13.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 shows the action of the apparatus of the invention, wherein 1 is the
surface of a photosensitive layer, 2 is a photomask having a transparent
region 3 formed therein serving as a slit for a light flux of rays 4. A
continuity of slit 3 forms a micro-circuit. The interval between surface 1
and mask 2 is in the range of 6 microns to 30 microns, and the width of
the slit 2 is about 1.5 microns to 6 microns. The illumination light flux
4 is composed of rays with respective different inclination angles with
respect to the normal line of the photomask 3. 5 is a diffraction pattern
produced with collimated rays passing through the slit 3 at right angles
with respect to the plane of the photomask 2. For rays passing through the
slit 3 at angles deviated from the right angle by discrete degrees, a
number of patterns of similar profile to that of pattern 5 are produced in
respective positions displaced from one another by slight discrete
distances, so that the superposition of these patterns provides a pattern
6. It is to be understood that the quality of image of the photomask
projected on the photosensitive layer is mainly deteriorated by the
secondary maxima of each of the diffraction patterns.
As shown in FIG. 2, a schematic example of an optical system for producing
an illumination light flux 4 of FIG. 1 constructed according to the
present invention comprises a spherical concave mirror 10, a light source
11, positioned at the center of curvature of mirror 10, a first condenser
12 positioned on the side of light source 11 opposite to mirror 10 to
collimate the rays coming from the light source 11, and a fly's eye lens
assembly 13 positioned in the path of the collimated rays from the
condenser lens 12.
The detailed structure of the fly's eye lens assembly 13 is illustrated in
FIG. 3, and each of the lens elements 13a, 13b, 13c, . . . is a negative
lens having a short focal length as shown in FIG. 4. A number of lens
elements of identical dimension are sandwiched between a pair of metal
plates 20 having openings of a diameter slightly smaller than that of each
lens element in alignment with each other together with not shown spacers.
It is preferred to increase the number of lens elements in a single
assembly 13 and to arrange them in as close relation to each other as
possible. In a practical case, however, this number may be a compromise
with the various manufacturing factors. The fly's eye lens assembly 13 is
disposed in the apparatus of FIG. 2 with the lens element 13a being
coaxially aligned with the optical axis of the apparatus and with the
other lens elements 13b and 13c being radially spaced therefrom. The
dashed line F indicate a focal plane containing the virtual focal points
or origins of the lens elements. Instead of using the negative lenses as
the lens elements of the assembly 13, positive lens elements as shown in
FIG. 5 may be used. In this case, the focal plane containing the real
focal points of the positive lens elements is formed below the assembly
13.
The apparatus further includes a second condenser lens 15 arranged in
coaxial alignment with the optical axis of the apparatus with the focal
point of the condenser lens 15 being at the focal plane F to collimate the
rays from the assembly 13 onto a photomask 16 positioned in the position
corresponding to the entrance pupil of the second condenser lens 15.
Positioned behind the photomask 16 is a photoresist layer 17 applied on a
semiconductor wafer 18. A predetermined spaced apart relationship between
mask 16 and photoresist layer 17 is firmly established by mask support
means 19.
The action of the apparatus of FIG. 2 will next be explained. The rays
emanating from the light source 11 are deflected in passing through the
first condenser lens 12, becoming rays parallel to one another and to the
optical axis thereof. Therefore, it is important to increase the
efficiency of the light energy of light source 11 by use of the condenser
lens 12. Next, a light flux of parallel rays enters the fly's eye lens
assembly 13 which divides it into many component fluxes while causing each
component flux to diverge. The component flux from the central lens
element 13a includes a central ray L.sub.1 and marginal ray L.sub.2 which,
after having passed through the second condenser lens 15, become parallel
to each other and enter the mask 16 at right angles thereto. The lens
element 13c arranged in the near field of the circumference of assembly 13
provides a component flux including a central ray L.sub.3 and a marginal
ray L.sub.4 which after having passed through the second condenser lens 15
become parallel to each other, entering the mask 16 at inclination angles
of .theta. with respect to the normal line of the mask 16. A marginal ray
L.sub.5 exiting from the lens element 14b enters the mask 16 at an
inclination angle smaller than that of the marginal ray L.sub.4 exiting
from the lens element 13c. It is to be understood from the foregoing
consideration that the maximum inclination angle .theta. available in the
apparatus depends upon the distance l between the axis of the lens element
13c and the axis X of the assembly 13 as shown in FIG. 3. Therefore, when
a diaphragm device 14 positioned adjacent the assembly 13 is operated to
decrease the size of aperture of the apparatus, the maximum inclination
angle .theta. is decreased.
With the apparatus of FIG. 2, the mask is exposed to a number of oncoming
waves equal to the number of lens elements in the fly's eye lens assembly
13. In order that these waves are effectively superimposed on the mask, it
is necessary to prevent occurrence of vignetting in the light flux between
the fly's eye lenses assembly and the mask. The mask has to be disposed in
coincidence with the entrance pupil of the second condenser lens. The
fly's eye lens assembly is designed to have a diameter determined in
accordance with the maximum inclination angle for the rays necessary for
compensating the secondary maxima and to be constructed with the lens
elements each of which has a F-number determiend by taking into account
the diameters of the second condenser lens and the fly's eye lenses
assembly. In practice, it is impossible to interchange the assembly each
time when one mask is replaced by another one having a slit width
different from that of the former mask. But it is preferred to change the
inclination angle of the rays. On this account, in the embodiment of the
invention, there is provided the diaphragm device 14 which is controllable
to optimize the maximum inclination angle. Because the operation of the
diaphragm gives no adverse influence on the other optical parts, the
entire system can be operated under very stable conditions. It will be
understood from the foregoing description that the mask 16 is illuminated
by rays with various inclination angles as shown in FIG. 1 to thereby
remove the influence of diffraction on the image of the mask projected on
the surface of the photoresist layer 17.
As the fly's eye lens assembly functions both as a beam divider and as a
divergent lens for each split component flux, it becomes a diffusing plate
such as ground glass or opal glass when the number of split component
fluxes approaches infinite. However, the ground glass or opal glass
generally has a far better divergence than the lens element, that is, it
lacks directivity, so that the light amount of the light source must be
increased to effect equivalent results. When a diffusing plate is employed
instead of the fly's eye lens assembly in the apparatus of FIG. 2, it has
to be disposed with its diffusing surface coinciding with the front focal
point of the second condenser lens.
The construction and arrangement of the apparatus of FIG. 2 is further
adapted for incorporation of optical means capable of compensating the
standing wave. For facilitating the incorporation of the standing wave
compensating means, the condenser lens system in the apparatus is divided
into two parts, the first one of which provides a light flux of parallel
rays directed to the fly's eye lens assembly, and the second one of which
converges the light fluxes exiting from the assembly. The standing wave
compensating means is removably inserted in the path of the light flux of
parallel rays between the first and second parts of the condenser lens
system.
Referring now to FIG. 6, there is shown a specific example of the
embodiment of the invention as including a light source 21, a concave
mirror 22 facing the light source 21, a focal condenser lens 23 of
positive power arranged on the side of the light source 21 opposite to
mirror 22, this condenser lens being composed of a positive lens element
23a and a negative lens element 23b, a filter 24 for compensating the
standing wave, and a multiple flux generating lens assembly 25 constructed
in the form of a negative lenslet network such as a fly's eye lens
assembly mentioned above. This multiple flux generating lenses assembly 25
may be otherwise constructed, for example, in the form of an optical fiber
plate of which the fiber has the refractive index varying with radius, a
diffusing plate, or a direction-selective diffusing element. The apparatus
of FIG. 6 further includes a diaphragm device 26 positioned adjacent and
in from of the assembly 25, a convergent optical system or a lens 27, an
illumination compensating filter 28, a mask 29 carrying microscopic
patterns and a wafer 30 having successively coated thereon a transparent
layer and a photosensitive resist layer, all of these parts being arranged
in coaxial alignment with the optical axis of the apparatus. A lamp of
high brightness and capable of producing acute rays suited for a given
photosensitive resist material is suitable as the light source 21. The
mecury vapor lamp has found wide acceptance to utilize short wavelength
rays such as the g line, h line, or 365m.mu. line. The effective use of
these rays is an important factor in designing the apparatus. In this
respect, the mirror 22 also contributes an additional increase in the
efficiency of the lamp to the apparatus by directing to the condenser lens
23 the rays which would not otherwise enter the condenser lens 23. Another
factor in efficiently collecting the light emerging from the lamp is to
decrease the F-number of the first condenser lens 23. The F-number itself
is defined by the ratio of the focal length to the lens diameter. The
diameter of the lens cannot be increase so much because of the limited
bulk of the apparatus. Therefore, the embodiment of the invention employs
as the first condenser lens group a lens having a decreased focal length
as composed of a positive lens element and a negative lens element
arranged in this order from the front with respect to the illumination
light. Another advantage of such construction of the first condenser lens
23 is that the light source 21 radiates rays in all directions which are
deflected in passing through the front positive lens element 23a to
converge, and the converging rays from lens element 23a are made parallel
to one another by the rear negative lens element 23b to thereby facilitate
the arrangement and construction of the parts following the condenser lens
23. The position of the standing wave compensating filter 24 is not
confined to that shown in FIG. 6, but arrangement of the filter 24 behind
the first condenser lens 23 is advantageous in decreasing the diameter of
the filter 24. This filter is removably inserted into the light flux and
can be readily replaced by another filter 24' having different
characteristics.
The action of the standing wave compensating filter will now be explained.
In order to increase the resolving power of the photosensitive material
such as photoresist which is widely used, it is necessary to decrease the
thickness of the photosensitive layer applied on the wafer to as small a
level as possible. The presently available coating technique using a
spinner can apply a uniform coating of a thickness in the order of
microns. On the other hand, the line widths of the pattern to be printed
is also in the order of microns. Therefore, the accurate control of the
thickness of the photoresist layer is very important in fabricating the
integrated circuits. Further, it is to be noted that both the line widths
of the pattern and the thickness of the photosensitive layer are in the
order of microns, that is, they are near to the wavelengths of light. The
I.C. printing operation requires in a sense a very high accuracy near to
the margin of the performance of the optical system so that the minute
defects in reproduction of the line widths must be taken into account.
Such problems are encounted in each of the projecting, contact-printing
and non-contact printing methods. As the common problem, mention may be
made of the standing wave produced in the photoresist layer.
The substrate on which an I.C. pattern is transferred by printing is
usually made of silicon or metal such as aluminum, so that the wave of
light entering the photosensitive layer interferes with the wave of light
reflected therefrom to form a standing wave. When the exposure of the
photoresist is optimized for improvement of the resolving power by taking
into account the presence of the standing wave, wrinkles are produced in
the hardened portions of the developed and processed photoresist layer.
When the exposure is increased to prevent the wrinkles from occurring, the
resolving power has to be sacrificed to some extent because of the
over-exposure. The formation of wrinkles is largely dependent upon the
contrast of the image, so that this problems is avoidable in the
contactprinting method which ensures high contrast, but becomes serious in
the non-contact printing or lens projecting methods.
As stated above, the formation of the standing wave is caused by the
reflection wave from the substrate on which the photoresist layer is
applied. The standing wave has an energy distribution along the thickness
of the photoresist layer. Such as energy distribution within the
photoresist layer deteriorates the uniformity of the exposure in the
direction along the thickness of the layer, whereby the hardening degree
of the photosensitive layer varies with distance from the surface of the
substrate to result in unsatisfactory image reproduction due to increased
roughness of edges of the pattern and formation of wrinkles and, in
extreme case, the peeling-off of the photoresist layer from the substrate.
For minimizing the influence of the standing wave, it is effective to
employ a multicolor lighth source capable of producing a plurality of
different wavelength acute rays. In this case, a number of standing waves
are produced for respective color lights which are incoherently
superpositioned on one another to produce a beat with the result that the
peaks and valleys of the standing waves tend to cancel each other out at a
portion of the entire length along the thickness so that the resultant
compound wave in this portion is considerably uniform in the direction
along the thickness. Therefore, if a transparent layer is provided between
the photosensitive layer and the substrate, the position of the
photosensitive layer can be adjusted to occupy this portion by the
thickness control of the transparent layer, whereby a uniform exposure is
effected. In order to effectively remove the influence of the standing
wave by use of a multicolor light source, it is further necessary to carry
out a systematic evaluation of the spectral characteristics of the light
source and the spectral sensitivity of the photosensitive material. For
example, when an exposure is made by use of two lights of different
wavelength, it is desirable that the influences of these two lights on the
photosensitive layer are identical to each other as determined after the
development. If so, the place in which the peaks and valleys of the
standing waves cancel each other out is more preferable for the uniform
exposure. However, the position of the place in which the cancelling
occurs most effectively is dependent upon the reflection characteristics
of the substrate, the thickness and refractive index of the intermediate
transparent layer and the thickness and refractive index of the
photosensitive layer. In order to secure the effective cancelling in this
place, it is further necessary to control the intensity ratios or
intensity distribution of the lights of different wavelength which are
used in the exposure. This situation will be better understood by
reference to FIGS. 7A and 7B, wherein FIG. 7A shows a resultant compound
wave occurring when g-line and h-line have identical effects on the
photoresist, and FIG. 7B shows a resultant compound wave occurring when
h-line has an effect two times as large as that of the g-line. It will be
appreciated that the conditions of FIG. 7A are more preferable for
elimination of influence of the standing wave.
On the other hand, the process for fabricating IC devices is to superimpose
a series of patterns by a succession of alignment and fabricating steps
until the finished product is constructed. In this process, photoresists
are used as the photosensitive material, whereby the number of
photoresists used in a single run is not always limited to only one, but
two or more photoresists may be used when a multiplicity of patterns of IC
is produced on a wafer. When different photoresists are used as the
photosensitive material, it is not required to change the optical system
itself, but from the systematic standpoint of view, it is necessary to
alter the spectral characteristics of the system. FIG. 8 shows spectral
sensitivity curves of three photoresists. It is to be understood that the
spectral characteristics of the system has to be largely varied in
accordance with the shape of the spectral sensitivity curve of the used
photoresist.
The actual printing optical system usually employs a color
temperature-invariable light source which does not enable the system to
embody different spectral characteristics for different photoresists.
According to this invention, the control of the intensity ratio or
intensity distribution of light rays available from the light source for
the purpose of effectively compensating the standing wave is effected by
interchanging the standing wave compensating filter in response to a
change in the spectral characteristics of the photoresist. This will be
described in more detail in connection with FIGS. 9, 10 and 11. FIG. 9
shows the spectrum of a mercury vapor lamp. This spectrum is characterized
as having bright lines peculiar to mercury, namely, 365.01m.mu.-line, the
h-line (404.66m.mu.), g-line (435.84m.mu.) and e-line (546.07m.mu.), for
example. The intensities of the 365m.mu.-line, h-line and g-line in the
spectrum are so large that the intensities of the other lines may be
neglected. For this reason, it may be said that the effective exposure is
made with only these bright spectral lines. In printing microscopic
patterns, the g-line and shorter wavelength lines than the g-line are
frequently used, because of the blue sensitivity and good resolving power
of the photoresist material. FIG. 10 shows an example of the spectral
characteristics of a photoresist. Most of the presently-available
photoresists are adapted to be sensitive to near-ultraviolet rays. This
example of the photoresist in FIG. 10 is sensitive to rays having
wavelengths longer than 430m.mu.. Accordingly, the bright spectral lines
substantially usable for the exposure of the photoresist are limited to
three lines, namely, the h-line, 365m.mu.-line and 334m.mu.-line. Now
assuming that two of these lines, namely, h-line and 365m.mu.-line are
used in an exposure, the influence of each actual spectral line on the
photoresist may be estimated by a relation between the intensity of the
respective spectral line. In the combination of a mercury vapor lamp of
FIG. 9 and the photoresist of FIG. 10, both the spectral characteristics
of the lamp and spectral sensitivity of the photoresist are larger at the
365m.mu.-line than at the h-line, so that the standing wave is more
susceptible to 365m.mu.-line rays. In order to eliminate the influence of
the standing wave, therefore, the apparatus of the invention employs a
standing wave compensating filter having a transmittance characteristic as
shown in FIG. 11 which compensates the spectral characteristics of the
lamp so as to equalize the influences of the 365m.mu.-line and the h-line
to each other to effect optimum result.
In addition, the interchangeability of the standing wave compensating
filter in the apparatus makes it possible to adjust the position of the
relatively flat portion of the resultant compound wave in accordance with
the position of the photosensitive layer relative to the substrate, when
the design parameters for the intermediate transparent layer made of
SiO.sub.2, for example, located between the substrate and the
photosensitive layer are previously given. For example, the first place in
which the peaks and valleys of the standing waves with two lights of
different wavelengths counting from the substrate approaches the substrate
as the difference between the two wavelengths is increased. In the case of
a smaller wavelength difference, the position of the first place is
farther from the substrate, but the image of the first place is
advantageously widened. Therefore, as the number of usable wavelengths
increases, the possibility of minimizing the debasing influence of the
standing wave is increased provided that a suitable combination of lights
of different wavelength determined by taking into account the spectral
sensitivity of the photoresist is selected by use of a corresponding
filter and at the same time the intensity ratio of the lights is suitably
controlled.
FIG. 12 shows a resultant compound wave occurring when the actions of the
g-line, h-line and 365m.mu.-line rays are balanced with one another,
wherein the place in which the feature of the standing wave is minimized
is nearer to the substrate as compared with those of FIGS. 7A and 7B. It
is to be understood that the provision for compensating the standing wave
made in the interchangeable filter of the apparatus of the invention gives
a great advantage of increasing the percentage of finished products which
will be found acceptable.
Referring again to FIG. 6, the multiple flux generating assembly 25 is
positioned in front of the second condenser lens system 27 so that the
exit rays from the assembly 25 impinge upon the condenser lens system 27
at incident angles uniformly distributed in a certain range. The standing
wave compensating filter 24 and the multiple flux generating assembly 25
are designed to have respective diameters slightly larger than the
diameter of the light flux exiting from the first condenser lens 23. The
diaphragm device 26 positioned adjacent and behind the assembly 25
functions as a means for controlling the maximum angle of incidence of the
rays impinging upon the mask 29. The second condenser lens system 27 is
shown as comprising a single convex lens by which the many component
fluxes generated by the assembly 25 are again collected to effectively
illuminate the mask 29. It is required that the diameter of the lens 27 be
large enough to secure a certain angle of incidence on the mask and to
collect a large proportionn of the rays emanating from the assembly 25.
For example, as shown in FIG. 13, the second condenser lens 27 collects
the rays exiting from the fly's eye lens assembly 25 without vignetting,
and permits the mask 29 to be positioned in the entrance pupil thereof. It
is to be understood that a decrease in the diameter of the flux of
parallel rays which is effected by the first condenser lens system 23 is
advantageous in decreasing the effective aperture of the second condenser
lens system 27.
The photosensitive materials for use in printing microscopic patterns are
usually photoresists. However, the photoresist layers are very susceptible
to uneven illumination and, therefore, it is preferred to limit the
unevenness to less than 5 percent. The avoidance of vignetting by the
aforesaid arrangement is an important factor in providing uniform
illumination. But, in the practical apparatus, a satisfactory uniform
distribution of illumination light energy on the mask cannot be effected
by only the avoidance of vignetting. On this account, the apparatus of the
invention further employs an ND filter for compensating the uneven
illumination on the mask. An example of the ND filter is shown in FIG. 14,
in which a compound plate which functions as a ND filter is constructed
from a plano-convex lens element 31 made of a material having a larger
absorbance for rays to which the photoresist material is exposed and a
plano-concave lens element 32 made of a material having a smaller
absorbance arranged in mating relation to each other. In this case,
different compensation functions can be imparted to the compound plate by
using lens elements 31 and 32 having different axial thicknesses and radii
of curvature. Another example of the ND filter is a transparent plate on
which a metal such as aluminum or chrominum is evaporated so as to impart
to the plate a continuous or discontinuous distribution of density.
It will be seen from the foregoing description that the present invention
provides a light exposure apparatus of which the form and construction is
suitable for incorporation of optical means for preventing obscurity at
the edges of the pattern image and for compensating the standing wave in
the photosensitive layer, and in which the actinic rays from the light
source are permitted to pass through the photomask to the photosensitive
layer in a manner such that a microscopic pattern can be effectively
printed on the photosensitive layer.
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