U.S. patent application number 10/674070 was filed with the patent office on 2005-03-31 for methods and systems to compensate for a stitching disturbance of a printed pattern in a maskless lithography system utilizing overlap of exposure zones with attenuation of the aerial image in the overlap region.
Invention is credited to Bleeker, Arno, Cebuhar, Wenceslao A., Kreuzer, Justin, Latypov, Azat, Vladimirsky, Yuli.
Application Number | 20050068514 10/674070 |
Document ID | / |
Family ID | 34376787 |
Filed Date | 2005-03-31 |
United States Patent
Application |
20050068514 |
Kind Code |
A1 |
Bleeker, Arno ; et
al. |
March 31, 2005 |
METHODS AND SYSTEMS TO COMPENSATE FOR A STITCHING DISTURBANCE OF A
PRINTED PATTERN IN A MASKLESS LITHOGRAPHY SYSTEM UTILIZING OVERLAP
OF EXPOSURE ZONES WITH ATTENUATION OF THE AERIAL IMAGE IN THE
OVERLAP REGION
Abstract
A method and system are provided for printing a pattern on a
photosensitive surface using a spatial light modulator (SLM). An
exemplary method includes defining two or more exposure areas on
the photosensitive surface, the exposure areas overlapping along
respective edge portions of the exposure areas to form an overlap
zone therebetween. Two or more exposure areas are exposed to print
an image therein, the exposing extending through the overlap zone.
The exposing within the overlap zone is then attenuated.
Inventors: |
Bleeker, Arno; (Westerhoven,
NL) ; Cebuhar, Wenceslao A.; (Norwalk, CT) ;
Kreuzer, Justin; (Trumbull, CT) ; Latypov, Azat;
(Danbury, CT) ; Vladimirsky, Yuli; (Weston,
CT) |
Correspondence
Address: |
STERNE, KESSLER, GOLDSTEIN & FOX PLLC
1100 NEW YORK AVENUE, N.W.
WASHINGTON
DC
20005
US
|
Family ID: |
34376787 |
Appl. No.: |
10/674070 |
Filed: |
September 30, 2003 |
Current U.S.
Class: |
355/77 ; 250/548;
355/52; 355/53; 355/55 |
Current CPC
Class: |
G03F 7/70091 20130101;
G03F 7/70466 20130101; G03B 21/18 20130101; G03F 7/70291 20130101;
G03B 27/42 20130101; G03F 7/70475 20130101 |
Class at
Publication: |
355/077 ;
355/052; 355/053; 355/055; 250/548 |
International
Class: |
G03B 027/32 |
Claims
1. A method to compensate for errors in a pattern to be printed on
a photosensitive surface using a spatial light modulator (SLM),
comprising: defining two or more exposure areas on the
photosensitive surface, the exposure areas overlapping along
respective edge portions of the exposure areas to form an overlap
zone therebetween; determining attenuation of at least one of a
predicted aerial image and a corresponding resist image; exposing
the two or more exposure areas to print an image therein, the
exposing extending through the overlap zone; and implementing the
determined attenuation during the exposing within the overlap
zone.
2. The method of claim 1, wherein the exposing includes applying a
laser dose.
3. The method of claim 1, wherein each exposure area corresponds to
one illumination source pulse.
4. The method of claim 1, wherein the attenuating step includes
active attenuation.
5. The method of claim 4, wherein the active attenuation step
includes dynamically adjusting pixels within the SLM to compensate
for deficiencies within the image.
6. The method of claim 1, wherein the attenuating step includes
passive attenuation.
7. The method of claim 6, wherein the passive attenuation step
includes use of at least one from the group including apodized
apertures, out of focus aperture/field stop, and pre-fabricated
modification of SLM pixels.
8. The method of claim 1, wherein the attenuating step is a
function of at least one from the group including illumination
mode, dimensions of the overlap zone, and dimensions of the
image.
9. An apparatus configured to compensate for errors in a pattern to
be printed on a photosensitive surface using a spatial light
modulator (SLM), the apparatus comprising: means for defining two
or more exposure areas on the photosensitive surface, the exposure
areas overlapping along respective edge portions of the exposure
areas to form an overlap zone therebetween; means for determining
attenuation of at least one of a predicted aerial image and a
corresponding resist image; means for exposing the two or more
exposure areas to print an image therein, the exposing extending
through the overlap zone; and means for implementing the determined
attenuation during the exposing within the overlap zone.
10. The apparatus of claim 9, wherein the means for exposing is
configured to apply a laser dose.
11. The apparatus of claim 9, wherein each area corresponds to one
illumination source pulse.
12. The apparatus of claim 9, wherein the means for attenuating is
configured to perform active attenuation.
13. The apparatus of claim 9, wherein the active attenuation
includes dynamically adjusting pixels within the SLM to compensate
for deficiencies within the image.
14. The apparatus of claim 9, wherein the means for attenuating is
configured to perform passive attenuation.
15. A computer readable medium carrying one or more sequences of
one or more instructions for execution by one or more processors to
perform a method to compensate for errors in a pattern to be
printed on a photosensitive surface using a spatial light modulator
(SLM), the instructions when executed by the one or more
processors, cause the one or more processors to perform the steps
of: defining two or more exposure areas on the photosensitive
surface, the exposure areas overlapping along respective edge
portions of the exposure areas to form an overlap zone
therebetween; determining attenuation of at least one of a
predicted aerial image and a corresponding resist image; exposing
the two or more exposure areas to print an image, the exposing
extending through the overlap zone; and implementing the determined
attenuation during the exposing within the overlap zone.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention generally relates to printing patterns
on photosensitive surfaces.
[0003] 2. Related Art
[0004] A printed pattern in a generic, maskless lithography tool is
formed from a sequence of exposures, or shots. Each shot results
from an image of a spatial light modulator (SLM) array being
projected onto the surface of a photosensitive surface, such as a
wafer substrate. This results in deposition of a dose, or a
quantity of irradiation from a light source, within a certain
exposure zone on this surface. Exposure zones are created when the
substrate surface is illuminated by flashes of light from the light
source. When the pattern extends beyond the boundaries of exposures
of a single SLM, the exposures are stitched together along adjacent
boundaries to form a completed pattern.
[0005] Stitching errors in the printed pattern occur near these
boundaries between adjacent exposure zones due to both geometrical
misalignments of the exposures and disturbances due to other
optical phenomena. Generally, stitching errors occur in printed
patterns due to spatial misalignment of the exposure zone on the
wafer from its expected position. Optical effects may also create
stitching errors, even in cases where the alignment may be perfect.
Even a small spatial misalignment of the shots, in the case of a
spatial misalignment, may result in a significant perturbation of
the printed pattern near the stitching line.
[0006] The optical effects are due to the fact that distribution of
the dose within each exposure zone is a result of an exposure by
partially coherent light. Since two adjacent exposure zones are
exposed at different times, the exposures are effectively
incoherent, thus creating the unwanted optical effects.
[0007] Therefore, what is needed is a solution to compensate for
stitching errors in printed patterns that occur near the stitching
line between adjacent exposure zones.
BRIEF SUMMARY OF THE INVENTION
[0008] Consistent with the principles of the present invention as
embodied and broadly described herein, the present invention
includes a method and system are provided for printing a pattern on
a photosensitive surface using a spatial light modulator (SLM). An
exemplary method includes defining two or more exposure areas on
the photosensitive surface, the exposure areas overlapping along
respective edge portions of the exposure areas to form an overlap
zone therebetween. Two or more exposure areas are exposed to print
an image therein, the exposing extending through the overlap zone.
The exposing within the overlap zone is then attenuated.
[0009] Further features and advantages of the present invention, as
well as the structure and operation of various embodiments of the
present invention, are described in detail below with reference to
the accompanying drawings.
DESCRIPTION OF THE FIGURES/DRAWINGS
[0010] The accompanying drawings, which are incorporated in and
constitute part of the specification, illustrate embodiments of the
invention and, together with the general description given above
and detailed description given below, serve to explain the
principles of the invention. In the drawings:
[0011] FIG. 1 is a block diagram illustration of a maskless
lithography system structured and arranged in accordance with an
embodiment of the present invention;
[0012] FIG. 2 is a diagrammatic perspective view of the exposure of
a photosensitive surface;
[0013] FIG. 3 is an illustration of deposition of a dose within a
number of exposure zones;
[0014] FIG. 4 is a graphical illustration of a stitching
disturbance near the stitching line in a desired uniform
pattern;
[0015] FIG. 5 is an illustration of the effects of a stitching
disturbance of an isolated dark line;
[0016] FIG. 6 is an illustration of an exemplary technique to
compensate for stitching errors not utilizing overlap of the
exposure zones;
[0017] FIG. 7 is a flow diagram of an exemplary method of
practicing the technique illustrated in FIG. 6;
[0018] FIG. 8 is an illustration of an exemplary technique to
compensate for stitching errors utilizing overlap of exposure zones
with attenuation;
[0019] FIG. 9 is a flow diagram of an exemplary method of
practicing the technique illustrated in FIG. 8;
[0020] FIG. 10 is an illustration of a technique to compensate for
stitching errors utilizing overlap without an explicit attenuation
producing an oscillating type pattern;
[0021] FIG. 10A is an illustration of the technique of FIG. 10
producing a checkerboard type pattern;
[0022] FIG. 11 is a flow diagram of an exemplary method of
practicing the technique illustrated in FIG. 10; and
[0023] FIG. 12 is a block diagram of an exemplary computer system
on which the present invention can be practiced.
DETAILED DESCRIPTION OF THE INVENTION
[0024] The following detailed description of the present invention
refers to the accompanying drawings that illustrate exemplary
embodiments consistent with this invention. Other embodiments are
possible, and modifications may be made to the embodiments within
the spirit and scope of the invention. Therefore, the following
detailed description is not meant to limit the invention. Rather,
the scope of the invention is defined by the appended claims.
[0025] It would be apparent to one skilled in the art that the
present invention, as described below, may be implemented in many
different embodiments of hardware, software, firmware, and/or the
entities illustrated in the drawings. Any actual software code with
the specialized, controlled hardware to implement the present
invention is not limiting of the present invention. Thus, the
operation and behavior of the present invention will be described
with the understanding that modifications and variations of the
embodiments are possible, given the level of detail presented
herein.
[0026] FIG. 1 is a block diagram view of a maskless lithography
system arranged in accordance with an embodiment of the present
invention. In FIG. 1, a maskless lithography system 100 includes a
control system 102. The control system 102 includes a computer
processor, a memory, and a user interface configured to enable a
user to input data for instructing the maskless lithography system
100 to produce a printed pattern.
[0027] The control system 102 is coupled to a pulsed light source
104 which provides pulses of light from a light source, such as an
excimer laser or some other suitable pulsed illumination mechanism.
The pulsed light source 104 is coupled to a beam relay system 106
which is typically an anomorphic system that includes a series of
lenses to create a desired numerical aperture in the light beam
produced by the pulsed light source 104. The pulsed light output
from the beam relay 106 is imaged onto a programmable array
108.
[0028] The programmable array 108 is configured to receive image
pattern data 110, representative of a desirable lithographic
pattern, and reflect light representative of the image to a
projection optics (PO) 109. The pattern data 110 is also known in
the art as mask layout data. The light reflected from programmable
array 108 passes through the PO 109 and then falls onto the
substrate 112. The function of the Projection Optics is (1) to form
an image of the object on the substrate and (2) to reduce the image
compared to the dimensions of the object. A pattern, representative
of the image data 110, is then imaged onto a photosensitive surface
112, such as a wafer substrate, which is being scanned at a
constant velocity. As understood by those of skill in the art, the
images that are to be projected onto the photosensitive surface 112
are contained in the programmable array 108 and may be changed by a
user via the control system 102.
[0029] The programmable array 108 can include an SLM, or some other
suitable micro-mirror array. By way of background, an SLM is an
array composed of a multitude of individually controlled pixels
(otherwise referred to as SLM elements). Each pixel can change its
optical properties in a controllable manner so that the field in
the object plane can be modulated. A typical SLM has square pixels
arranged in a rectangular array, with each pixel having an ability
to change only one of the parameters characterizing its optical
properties (one-parametric local modulation) within a certain
range.
[0030] For example, an existing SLM has 16.times.16 mm.sup.2
tilting mirrors arranged in a 2040.times.512 array and running at a
1 KHz refresh rate. The light modulation principles implemented in
different SLMs can be classified as transmittance modulation,
modulation by light deflection, phase shift modulation, de-focus
modulation, and/or a combination of several of the aforementioned
modulation types.
[0031] FIG. 2 is a diagrammatic perspective view of more detailed
aspects a maskless lithography system, such as the system 100 of
FIG. 1. In FIG. 2, a system 200 includes an SLM 202, a PO 203, and
a substrate 204 having a photosensitive surface. The SLM 202
includes mirror elements 206 configured to reflect light pulses 208
onto the substrate 204 within two-dimensional exposure regions 210.
The light pulses 208 are used to form a pattern 212 within the
exposure regions 210.
[0032] By way of background, the printed pattern in a maskless
lithography tool is formed from a sequence of two-dimensional
exposures or shots. Each of these two-dimensional shots results
from an image of a single SLM being projected to the surface of the
wafer, and it results in deposition of a dose within a certain
exposure zone. Additionally, each exposure is created by a single
pulse of light from the pulsed light source. Since the
two-dimensional exposure zones are stitched together edge-to-edge,
the stitching is very critical. A displacement of one exposure zone
on the order of a few nanometers can create pattern errors along
the edge that are clearly visible and detrimental to features
within the pattern. A single exposure can be performed in several
passes over a substrate surface, as illustrated in FIG. 3.
[0033] FIG. 3 is an illustration of the formation of a pattern
feature that extends across the boundary, or stitching line, of
adjacent exposure zones. In FIG. 3, stitched exposure zones 300
include exposure zones 301, 302, and 304. Each of these exposure
zones is created through deposition of a dose on a photosensitive
surface produced by a single light pulse from an illumination
source, such as the pulsed light source 104 of FIG. 1. That is,
within the duration of a single pulse, a photosensitive surface,
such as the substrate 204, is moved a predetermined distance
resulting in the deposition of the dose within each of the zones
301, 302, and 304.
[0034] An adjacent boundary of exposure zones 302 and 304 forms a
stitching line 306. A pattern feature 308 is formed within the
exposure zones 301, 302, and 304 and is positioned across the
stitching line 306. Optical effects or distortions of the feature
308 can occur due to the fact that distribution of a dose within
each exposure zone is the result of an exposure by a partially
coherent light. Subsequently, since the two adjacent exposure zones
302 and 304 are exposed at different times, these exposure zones
are effectively incoherent.
[0035] FIG. 4 is a graphical illustration of a stitching
disturbance near a stitching line in the desired uniform pattern in
a scenario where coherent illumination is assumed. For purposes of
illustration, examples of coherent illumination are used because
coherent illumination is considered to produce the most significant
stitching errors (disturbances) in printed patterns. The present
invention, however, is not limited to such an application.
[0036] In FIG. 4, a graph 400 illustrates that a uniformly bright
field is imaged in two exposures stitched at the origin. A first
exposure 402 results from the pixels to the right of the origin in
the object plane set to their absolutely bright state, while the
object plane field to the left from the origin is zero. The
resulting relative intensity distribution, or relative dose
variation, in the image plane is a well-known diffraction-limited
image of a semi-plane.
[0037] A second exposure 404 is a mirror image of the exposure 402
with respect to the origin. One can readily observe that the
relative dose value (or relative image intensity) at the origin for
both exposures is (1/4) for the case of coherent illumination. A
combination of the right-edge exposure 402 and the left-edge
exposure 404 produces an exposure 406 having a relative local-dose
value of (1/2) along a stitching boundary 407. The combination of
the exposures at the stitching line 407 with the relative local
dose value of (1/2) forms a stitching artifact or error 408.
[0038] In a practical sense, stitching artifacts, such as the
stitching artifact 408, will disturb the morphology of features
(e.g, the feature 308 of FIG. 3) that are formed across stitching
boundaries, such as the boundary 407. One such morphology
disturbance is the occurrence of variations in the line width of
lines used to form the printed patterns.
[0039] FIG. 5 is an illustration of the impact stitching errors can
have upon line-width variation in the absence of stitching-error
compensation techniques. More specifically, FIG. 5 is an
illustration of a test case showing the stitching disturbance of an
isolated dark line. In FIG. 5, an exemplary exposure 502 includes
an isolated horizontal dark line 503 on a bright background which
is approximately 2000 nanometers (nm) long (.about.9.lambda./NA),
where (.lambda.) is the light wavelength and (NA) is the numerical
aperture. The horizontal dark line 503 is formed using tilting
mirror pixels in the object plane.
[0040] In the example of FIG. 5, a size of each pixel, scaled to
the image plane, can be determined by the expression:
M*L=40 nm,
[0041] where (M) is the magnification factor and (L) is the pixel
length.
[0042] The line 503 is formed by two adjacent horizontal arrays
(rows) of mirrors tilted by 1 ( 0 = 2 * L )
[0043] and (-.alpha..sub.0), respectively, where (.alpha.) is the
tilt angle. The pixels of these rows act almost as absolutely dark
pixels. The background is bright and is formed by the pixels with
their mirrors being flat.
[0044] The pattern that forms the line 503 is exposed in three
exposures: 502, 504, and 506, in the manner illustrated in FIG. 3
above. The exposure 502 occurs during the first pass of the
substrate, and it exposes the entire line 503. The exposures 504
and 506 are two consecutive exposures during a second pass of the
substrate. Each of the two exposures 504 and 506 respectively
exposes one-half of the dark line 503. Dose distributions 504a and
506a, respectively associated with the exposures 504 and 506, are
shown near a stitching line resulting from each of the three
exposures. A sum 508 of the three exposures 502, 504, and 506 is
shown and includes an under-exposure area 509 along a stitching
line 509a.
[0045] A graph 510 provides an illustration of variations in the
line width near the stitching line 509a. In the example of FIG. 5,
the line width was computed using an image intensity threshold
resulting in a 70 nm line 512. As illustrated in the graph 510, the
variation, indicated by a line 514, reaches as high as +25 nm.
[0046] Although stitching errors can create anomalies that
significantly degrade printed patterns, as illustrated in the
examples of FIGS. 4 and 5, the present invention provides several
techniques to compensate for the effects created by these stitching
errors.
[0047] Stitching Compensation without Utilizing Overlap
[0048] FIG. 6 provides an illustration of an exemplary technique
600 to compensate for the effects of stitching errors. More
specifically, the technique of FIG. 6 provides stitching error
compensation without utilizing overlap of exposure areas. The
technique 600 permits the addition of an assist feature during the
second pass of the exposure of the substrate surface. Although in
the present application, the assist feature is added during the
second pass, any subsequent pass can be used as long as the pass in
which the assist feature is added does not have a boundary line at
the same location as a feature.
[0049] The assist features are added to the mask layout or pattern
data (e.g., the pattern data 110 of FIG. 1) input to the SLM to
produce the printed pattern. In the case of FIG. 6, assist features
are added to the pattern data in order to make the line at the
second pass thicker at a same position corresponding to earlier
passes to compensate for the effects of the stitching error.
[0050] In FIG. 6, a line is produced on the photosensitive surface
of a substrate during a first exposure (i.e., during a single pulse
of light from an illumination source). The line includes a left
segment 602 and a right segment 604, both formed during the first
pass. The left and right segments 602 and 604 form a line 606
having a stitching disturbance 608 (necking) across a stitching
line 609. To compensate for the stitching disturbance 608, an
assist feature 610 is added to the pattern data associated with
production of the line 606.
[0051] During a second pass of the substrate, the assist feature
610 produces a bulge at the stitching line 609 in a line 612 formed
in the object plane. Although the line 612 having the assist
feature 610 is formed in the object plane, it combines with the
line 606 to produce a line 614 in the image plane. The line 614 is
devoid of the stitching disturbance 608. In other words, the
thicker portion represented by the assist feature 610 compensates
for the resulting necking, or stitching disturbance 608, from the
first pass. More generally, if exposure zones of one pass are
shifted with respect to exposure zones during another pass, the
passes in which the feature is not affected by the stitching line
can be used to compensate for the stitching disturbance that
affected the feature.
[0052] During a scan, an exposure is produced in an adjacent
exposure zone. That is, the wafer is scanned once and then moves to
produce another exposure zone adjacent to the previous zone. A
subsequent pulse arrives and a subsequent exposure zone is formed.
Stitching errors occur, and can be detected, within these exposure
zones. The process of detecting, or predicting, stitching errors
within the exposure zones can be accomplished using several
techniques known to those of skill in the art.
[0053] Furthermore, pre-printing analyses of patterns are
accomplished using, for example, image modeling with the use of
modeling and simulation tools. Using these or similar modeling
tools, stitching error occurrences at the stitching boundaries can
also be simulated. The simulations can be performed apriorily or
real-time. Thus, techniques such as the technique 600 of FIG. 6 can
also be performed apriorily or real-time.
[0054] Another technique to compensate for stitching errors, that
does not utilize overlap of exposure areas, is active compensation.
With active compensation, exposures are performed by butting (i.e.,
without an overlap), and states of the SLM pixels in the vicinity
of the stitching line can be adjusted. That is, the SLM pixels in
the vicinity of the stitching line can be selected in such a way so
that stitching disturbances can be compensated for.
[0055] For example, stitching disturbances can create a widening of
the printed line in the vicinity of the stitching line, as a result
of under-exposure due to edge effects. Therefore, using brighter
states of the SLM pixels near the stitching line can reduce the
line widening effect. Thus, to compensate for widening of the
printed line, the states of the four SLM pixels, adjacent to the
stitching line, can be adjusted by a proportional amount.
Adjustment of a small number of pixels, however, will not always be
sufficient to compensate for the stitching effects. Adequate
compensation can still be achieved, however, using a larger number
of pixels near the stitching line with their adjusted states
computed from the solution of an inverse problem.
[0056] By adding the compensating data to the pattern data (e.g.,
the pattern data 110), the computation of the pixel states can be
adjusted in a manner such that the pixels print the desired
pattern. The calculation of pixel positions is determined in a
manner that takes into account positions of the stitching lines.
Pixel position determination also accounts for the fact that
exposure areas separated by these stitching lines are exposed in
different shots. Since the exposures are in different shots, there
is no coherence between the fields in these exposures. During
active compensation, however, there is a partial coherence inside
each exposure based upon the predetermined illumination mode.
[0057] FIG. 7 is a flow diagram of an exemplary method 700 of
practicing the technique 600 of FIG. 6. In one example, method 700
is carried out by a system 200 as described above, but is not
necessarily limited to this structure. In FIG. 7, a first exposure
of a photosensitive surface is performed in accordance with
predetermined image data (step 702). The first exposure occurs
during a first pass of the substrate surface and produces a first
image within a substrate area. In step 704, image deficiencies are
identified within a region of the first image. The image data is
adjusted to compensate for the identified image deficiencies (step
706). In step 708, a second exposure of the photosensitive surface
is performed in accordance with the adjusted image data during a
second pass.
[0058] Overlap if Exposure Zones with Attenuation
[0059] FIG. 8 provides an illustration of another exemplary
technique 800 to compensate for the effects of stitching
disturbances. The technique 800 utilizes overlap of exposure zones
and provides attenuation of the aerial image and within the overlap
region. In short, the technique 800 utilizes a small overlap of
exposure zones to compensate for the stitching disturbance of the
printed pattern occurring near the stitching line. The overlap area
near the stitching line receives an extra dose due to the extra
multiple exposures, and attenuation of the aerial image is
performed to compensate for these extra multiple exposures. The
attenuation can be performed either actively or passively.
[0060] Active attenuation is based on dynamically adjusting the
states of the pixels of the SLM array to perform the attenuation in
the overlap zone. This active attenuation can take into account the
illumination mode, width, and geometry of the overlap and the
specific pattern printed across the overlap zone, as well as other
factors.
[0061] Passive attenuation, on the other hand, uses modifications
of SLM related hardware to provide the attenuation. The methods to
achieve passive attenuation include, but are not limited to,
apodized aperture, out-of-focus/field stop, and prefabricated
modifications of the SLM elements.
[0062] APODIZED APERTURE. An apodized aperture can be introduced
either in front of the object or in an intermediate image plane.
This aperture has a variable transmission near the edge of the SLM
array, or its intermediate image, that ensures the attenuation of
the aerial image. Apodization is preferably performed near the
leading and trailing vertical edges to compensate for a spare
exposure in the overlap zone. Formulas describing the variation of
transmission can be dependent upon specific illumination conditions
(to provide better stitching illumination conditions). The formulas
can also be generic formulas, that work reasonably well for a wide
range of illumination conditions (for example, linear variation of
intensity transmission). These formulas are derived using
techniques known in the art.
[0063] Apodization can be passive (use of apodized aperture),
active (adjusting the pixels of the SLM array near the stitching
line), or a combination of these two. Passive apodization can be
performed either in the object plane or the (intermediate) image
plane.
[0064] OUT-OF-FOCUS/FIELD STOP. Out-of-focus aperture/field stop is
an alternative to apodized aperture, where an aperture that is
slightly out of focus can be used or a masking plate placed in
front of the SLM.
[0065] PREFABRICATED MODIFICATIONS. Prefabricated modification of
the SLM elements within the overlap zone is based on using an SLM
array in which the pixels falling within the overlap zone are
pre-modified in a way that ensures the desired attenuation of the
aerial image.
[0066] An example of a prefabricated modification of an SLM element
includes varying reflectivity of the micro-mirror surfaces within
the overlap zone for SLM arrays, using tilting or pistoning the
individual mirrors within the SLM array. The variation can be
discrete (constant reflectivity within each mirror, but varying
from mirror to mirror) or continuous. Varying reflectivity can be
generalized for an SLM array using any modulation principle (and
not utilizing micro-mirrors). For example, within the overlap zone,
pixels of an SLM array that use variations of transmission can be
pre-manufactured so that maximal transmission of their brightest
state varies in a desired way.
[0067] Another example of a prefabricated modification of an SLM
element includes a built-in modification of discrete states of the
SLM elements, or pixels. This modification assumes that the
discrete states (e.g., tilts of the tilting micro-mirrors) of the
SLM elements within the overlap zone are modified or shifted,
compared to the discrete states of other SLM elements. This
built-in modification technique is another example of passive
attenuation and can be applied to an SLM using any modulation
principle.
[0068] FIG. 8 is an example of active attenuation, as noted above.
Particularly FIG. 8 is an illustration of an exemplary technique
utilizing overlap of exposure zones with attenuation of the aerial
image, using an active attenuation approach. In FIG. 8, the
exposures 502, 504, and 506, shown in FIG. 5, are re-analyzed
applying an active attenuation approach. Here, a display 800 is
formed by combining the exposures 502, 504, and 506. For purposes
of illustration, the exposures 504 and 506 are overlapped by a
width of 10 pixels to produce a feature 802 within an overlap zone
804. Within the overlap zone 804, a linear attenuation of the
object plane field is implemented in accordance with one of the
passive attenuation techniques, noted above. A resulting image 807
is produced by combining the exposures 502, 504, and 506, after
utilizing the active attenuation. A graph 808 provides an
illustration of improvements realized by a reduction in the
line-width variations. Specifically, the graph 808 indicates that
line width variations 810 associated with the line 512, shown in
FIG. 5, can be reduced from the 25 nm (shown in FIG. 5) to less
than 5 nm.
[0069] FIG. 9 is a flow diagram of an exemplary method 900 of
performing active attenuation of an aerial image within the overlap
region. Using the method 900, two or more exposure areas on a
photosensitive surface are defined (step 902). The exposure areas
overlap along respective edge portions to form an overlap zone
therebetween. In step 904, the two or more exposure areas are
exposed to print an image therein, wherein the exposing extends
through the overlap zone. Next, the exposing that occurs within the
overlap zone is attenuated, as indicated in step 906.
[0070] Utilizing Overlap without Explicit Attenuation
[0071] FIG. 10 provides an illustration of an exemplary technique
1000 to compensate for stitching disturbances utilizing overlap
without an explicit attenuation. The technique 1000 is just one
exemplary approach utilizing overlap without an explicit
attenuation. More specifically, the technique 1000 produces an
oscillating stitching line along the stitching boundary. In other
words, the overlap zone is used to create a stitching border that
changes its direction in a zigzag pattern or some other
fashion.
[0072] In the technique 1000, exposure zones are overlapped. And
within the overlap zone, only one of the two overlapping pixels
carries the pattern. The other overlapping pixel is turned off. A
resulting printed pattern is thus distributed between two
overlapping exposure zones in such a way that the effective border
between the two adjacent exposure zones is, for example, a "shark
teeth" line or any other rapidly oscillating line. Such oscillating
lines represent a spatial averaging of the stitching disturbance
and, subsequently, a reduction of its effect on the printed
pattern.
[0073] The technique 1000 of FIG. 10 is just one example of an
oscillating stitching line approach. In FIG. 10, overlapping
exposure zones 1002, 1004, and 1006, are used to form a printed
feature 1005 along a stitching zone 1007. In the technique 1000,
however, selected pixels (within the pixels used to produce
overlapping exposures 1002, 1004, and 1006), are activated to
spatially average the energy associated with the stitching
disturbance. This spatial averaging creates a zigzag pattern 1009.
For example, within the exposure zone 1004, selected (overlapping)
pixels, within overlapping pixel sets, are representative of zone
portions 1004a, 1004b, and 1004c. These selected pixels are then
energized such that the pattern used to form the feature 1005 is
produced in one shot and only from pixels associated with the
exposure 1004.
[0074] Similarly, selected overlapping pixels that produce exposure
zone portions 1006a, 1006b, and 1006c, from the exposure 1006, are
used to form another segment of the pattern. When the exposures
1004 and 1006 are combined, an exposure 1008 is formed having
substantially reduced stitching artifacts within the overlap region
1010. Alternatively, the selected pixels within pixel sets used to
form the overlapping exposures can be alternately turned on and off
to produce some other pattern, such as a checkerboard pattern 1020
as indicated in FIG. 10A.
[0075] For example, the overlapping pixels can be distributed
between the exposures in a checkerboard pattern with white (on)
pixels belonging to one exposure and black (off) pixels belonging
to another exposure. Since relatively higher spatial frequencies
are used than in the case of the oscillating border (this spatial
frequency can also vary across the overlap zone if the pattern is
properly selected), better stitching can be produced.
[0076] In FIGS. 10 and 10A, the exposure zones are overlapped, and
within the overlap zone, only one of the two overlapping pixels
carries the pattern. The other overlapping pixel is turned off. The
printed pattern is thus distributed between two overlapping
exposures, and the printed pattern is preferably distributed
between these exposures with a relatively high spatial frequency.
Further, the overlapping exposures need not necessarily form an
oscillating connected border between two exposures.
[0077] FIG. 11 is a flow diagram of an exemplary method 1100 of
performing the present invention as illustrated in FIG. 10. In FIG.
11, two or more exposure areas are defined within a predetermined
region of a substrate surface. Each area corresponds to selected
pixels of the SLM (step 1101). In step 1102, an overlapping region
is formed between two or more exposure areas, the overlapping
region being defined by respective overlapping edges of the
exposure areas. The overlapping edges correspond to overlapping
pairs of the selected pixels from each area. In step 1104, the
pixels within each pair are alternately activated, such that only
one of the pixels within the pair is used to produce the
pattern.
[0078] The stitching disturbance can be compensated by properly
selecting the width of an overlap (without attenuation). For any
illumination mode, the butting (exposure with zero overlap) results
in under-exposure along the stitching line (e.g., for a coherent
illumination, the dose along the stitching line is 50% of its value
in the absence of the stitching effect). On the other hand, overlap
by a significant length (several times exceeding 2 ( ( M * NA ) )
)
[0079] without attenuation would result in a dose over-exposure
along the stitching line. If the width of the overlap zone is
properly selected between these two extreme values, an exact
location of the stitching line can receive the exact dose, which
would allow compensation of the disturbance near the stitching
line.
[0080] Overlap with a field stop close to the image plane will also
reduce the stitching/edge effects. The exposure zones are
overlapped, and a field stop is placed in close proximity to the
image plane so that this field stop blocks a part of the image in
the overlap zone that is most affected by the edge effects (the
outer part of the SLM image). The width of the exposure zone should
be large enough 3 ( >> ( M * NA ) )
[0081] to ensure that the edge effects can be blocked by the field
stop. Additionally, the pixels that are mostly blocked by the field
stop can be modulated to provide an extra compensation of the edge
effects remaining in the image that passes the field stop.
[0082] As a result, an image that reaches the image plane has
substantially zero or very minimal edge effects in it, and
stitching can be performed by butting the two images together.
[0083] One example of the overlap with a field stop is as follows:
an isolated line crossing the stitching boundary is printed, and
the pixels blocked by the field stop create a continuation of the
image of the line on the field stop. If the width of the layer of
blocked pixels is large enough (much greater than 4 ( M * NA ) )
,
[0084] the single exposure that creates an image of a semi-line
will result in a semi-line with a sharp edge, because the
edge-smearing effects are blocked by the field stop. If the second
half of the line is similarly imaged, the stitching can be
performed by butting the two images together.
[0085] Additional extraneous factors, noted below, will also affect
the impact of a stitching disturbance.
[0086] SMEARING OF THE IMAGE. Smearing of the image due to the
wafer motion during exposure is an additional phenomenon that will
affect the impact of a stitching disturbance. The exposure
mechanism in a maskless lithography tool may involve exposures by
short laser pulses that are performed on a wafer that moves with a
constant velocity. Such exposure mechanisms will result in smearing
of the image in the direction of motion.
[0087] The duration of the laser pulse must be small enough for the
smearing not to affect the printed pattern significantly. For
typical scanning speeds and laser pulse durations, the effect of
smearing should not exceed a small number of nanometers. This
effect can be further compensated by laser pulse synchronization.
At the same time, such smearing can be beneficial, as it will
naturally reduce the stitching disturbance occurring near the
stitching lines which are perpendicular to the direction of
scan.
[0088] JITTER OF THE LASER PULSES. Laser pulse jitter is another
factor that will affect the impact of a stitching disturbance. In
the exposure scenario described in the previous paragraph, the
laser pulse may be arriving too early or too late (laser jitter
phenomenon). The characteristic magnitude of the laser pulse jitter
is 3.sigma.=10 nanoseconds (nsec). As a result of these delays or
advances, the position of the exposure on the wafer resulting from
this laser pulse may be shifted in the direction of the wafer scan
or in the opposite direction. These small shifts can lead to
spatial misalignment of the stitched exposures, and this
misalignment (along with the misalignment of exposures occurring
due to other reasons) contributes to the stitching disturbance. The
techniques to compensate for stitching illustrated in the present
application, however, can be used to compensate the effects of
laser pulse jitter.
[0089] As stated above, the present invention can be implemented in
hardware, or as a combination of software and hardware.
Consequently, the invention may be implemented in the environment
of a computer system or other processing system. An example of such
a computer system 1200 is shown in FIG. 12.
[0090] The computer system 1200 includes one or more processors,
such as a processor 1204. The processor 1204 can be a special
purpose or a general purpose digital signal processor. The
processor 1204 is connected to a communication infrastructure 1206
(for example, a bus or network). Various software implementations
are described in terms of this exemplary computer system. After
reading this description, it will become apparent to a person
skilled in the relevant art how to implement the invention using
other computer systems and/or computer architectures.
[0091] The computer system 1200 also includes a main memory 1208,
preferably random access memory (RAM), and may also include a
secondary memory 1210. The secondary memory 1210 may include, for
example, a hard disk drive 1212 and/or a removable storage drive
1214, representing a floppy disk drive, a magnetic tape drive, an
optical disk drive, etc. The removable storage drive 1214 reads
from and/or writes to a removable storage unit 1218 in a well known
manner. The removable storage unit 1218, represents a floppy disk,
magnetic tape, optical disk, etc. which is read by and written to
by removable storage drive 1214. As will be appreciated, the
removable storage unit 1218 includes a computer usable storage
medium having stored therein computer software and/or data.
[0092] In alternative implementations, the secondary memory 1210
may include other similar means for allowing computer programs or
other instructions to be loaded into the computer system 1200. Such
means may include, for example, a removable storage unit 1222 and
an interface 1220. Examples of such means may include a program
cartridge and cartridge interface (such as that found in video game
devices), a removable memory chip (such as an EPROM, or PROM) and
associated socket, and the other removable storage units 1222 and
the interfaces 1220 which allow software and data to be transferred
from the removable storage unit 1222 to the computer system
1200.
[0093] The computer system 1200 may also include a communications
interface 1224. The communications interface 1224 allows software
and data to be transferred between the computer system 1200 and
external devices. Examples of the communications interface 1224 may
include a modem, a network interface (such as an Ethernet card), a
communications port, a PCMCIA slot and card, etc. Software and data
transferred via the communications interface 1224 are in the form
of signals 1228 which may be electronic, electromagnetic, optical
or other signals capable of being received by the communications
interface 1224. These signals 1228 are provided to the
communications interface 1224 via a communications path 1226. The
communications path 1226 carries the signals 1228 and may be
implemented using wire or cable, fiber optics, a phone line, a
cellular phone link, an RF link and other communications
channels.
[0094] In the present application, the terms "computer readable
medium" and "computer usable medium" are used to generally refer to
media such as the removable storage drive 1214, a hard disk
installed in the hard disk drive 1212, and the signals 1228. These
computer program products are means for providing software to the
computer system 1200.
[0095] Computer programs (also called computer control logic) are
stored in the main memory 1208 and/or the secondary memory 1210.
Computer programs may also be received via the communications
interface 1224. Such computer programs, when executed, enable the
computer system 1200 to implement the present invention as
discussed herein.
[0096] In particular, the computer programs, when executed, enable
the processor 1204 to implement the processes of the present
invention. Accordingly, such computer programs represent
controllers of the computer system 1200. By way of example, in the
embodiments of the invention, the processes/methods performed by
signal processing blocks of encoders and/or decoders can be
performed by computer control logic. Where the invention is
implemented using software, the software may be stored in a
computer program product and loaded into the computer system 1200
using the removable storage drive 1214, the hard drive 1212 or the
communications interface 1224.
CONCLUSION
[0097] Provided herein are several unique approaches to
compensating for the effects of stitching errors that can occur
near the stitching line in maskless lithography patterns. The
approaches include (a) techniques not utilizing overlap of exposure
areas, (b) techniques using overlap and attenuation, and (c)
techniques using overlap without explicit attenuation. These
approaches, either alone or in combination, significantly reduce
the stitching disturbances, optical anomalies, and other effects
that might otherwise be introduced into lithography patterns
printed on photosensitive surfaces.
[0098] The present invention has been described above with the aid
of functional building blocks illustrating the performance of
specified functions and relationships thereof. The boundaries of
these functional building blocks have been arbitrarily defined
herein for the convenience of the description. Alternate boundaries
can be defined so long as the specified functions and relationships
thereof are appropriately performed.
[0099] Any such alternate boundaries are thus within the scope and
spirit of the claimed invention. One skilled in the art will
recognize that these functional building blocks can be implemented
by analog and/or digital circuits, discrete components,
application-specific integrated circuits, firmware, processor
executing appropriate software, and the like, or any combination
thereof. Thus, the breadth and scope of the present invention
should not be limited by any of the above-described exemplary
embodiments, but should be defined only in accordance with the
following claims and their equivalents.
[0100] The foregoing description of the specific embodiments will
so fully reveal the general nature of the invention that others
can, by applying knowledge within the skill of the art (including
the contents of the references cited herein), readily modify and/or
adapt for various applications such specific embodiments, without
undue experimentation, without departing from the general concept
of the present invention. Therefore, such adaptations and
modifications are intended to be within the meaning and range of
equivalents of the disclosed embodiments, based on the teaching and
guidance presented herein. It is to be understood that the
phraseology or terminology herein is for the purpose of description
and not of limitation, such that the terminology or phraseology of
the present specification is to be interpreted by the skilled
artisan in light of the teachings and guidance presented herein, in
combination of one of ordinary skill in the art.
* * * * *