U.S. patent application number 10/810406 was filed with the patent office on 2004-09-16 for process for detecting defects in photomasks.
Invention is credited to Burdorf, James, Pierrat, Christophe.
Application Number | 20040179726 10/810406 |
Document ID | / |
Family ID | 24797993 |
Filed Date | 2004-09-16 |
United States Patent
Application |
20040179726 |
Kind Code |
A1 |
Burdorf, James ; et
al. |
September 16, 2004 |
Process for detecting defects in photomasks
Abstract
The present invention provides a process for performing
automatic inspection of advanced design photomasks. In a preferred
embodiment, an aerial image of a portion of a photomask is
generated. A simulated image corresponding to original pattern data
is also generated. The aerial image and simulated image are then
compared and discrepancies are detected as possible defects.
Inventors: |
Burdorf, James; (Meridian,
ID) ; Pierrat, Christophe; (Boise, ID) |
Correspondence
Address: |
WONG, CABELLO, LUTSCH, RUTHERFORD & BRUCCULERI,
P.C.
20333 SH 249
SUITE 600
HOUSTON
TX
77070
US
|
Family ID: |
24797993 |
Appl. No.: |
10/810406 |
Filed: |
March 26, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10810406 |
Mar 26, 2004 |
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09135110 |
Aug 17, 1998 |
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09135110 |
Aug 17, 1998 |
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08696652 |
Aug 14, 1996 |
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5795688 |
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Current U.S.
Class: |
382/144 |
Current CPC
Class: |
G01N 21/95607 20130101;
G03F 7/70666 20130101; G03F 1/36 20130101; G03F 1/84 20130101; G03F
7/7065 20130101 |
Class at
Publication: |
382/144 |
International
Class: |
G03C 005/00 |
Claims
We claim:
1. A process for detecting defects in masks comprising: generating
an aerial image of a portion of a mask; generating a simulated
image corresponding to original pattern data used to create said
mask; and comparing said aerial image to said simulated image.
2. A process for detecting defects in masks as defined in claim 1
wherein said simulated image is generated from original pattern
data taking into account expected distortions and corner rounding
due to image processing.
3. A process for detecting defects in masks as defined in claim 1
wherein said simulated image is obtained by generating an aerial
image of a mask design used to generate a portion of the mask with
which it is compared.
4. A process for detecting defects in masks as defined in claim 1
wherein said mask is generated using proximity effect correction
techniques.
5. A process for detecting defects in masks as defined in claim 4
wherein said mask is generated using optical proximity effect
correction techniques.
6. A process for detecting defects in masks as defined in claim 4
wherein said mask is generated using x-ray proximity effect
correction techniques.
7. A process for detecting defects in masks as defined in claim 4
wherein said mask is generated using ion beam proximity effect
correction techniques.
8. A process for detecting defects in masks as defined in claim 4
wherein said mask is generated using e-beam proximity effect
correction techniques.
9. A process for detecting defects in masks as defined in claim 1
wherein said photomask includes phase shifting techniques.
10. A process for detecting defects in masks as defined in claim 1
wherein said mask includes proximity effect correction techniques
and phase shifting techniques.
11. A process for detecting defects in masks as defined in claim 1
wherein said mask comprises a photomask.
12. A process for detecting defects in masks as defined in claim 1
wherein said masks are used in the manufacture of integrated
circuits.
13. A process for detecting defects in masks as defined in claim 1
wherein said mask comprises an x-ray mask.
14. A process for detecting defects in masks as defined in claim 1
wherein said mask comprises a stencil mask for ion projection
lithography.
15. A process for detecting defects in masks as defined in claim 1
wherein said mask comprises a mask for electron beam projection
lithography.
16. A process for detecting defects in masks as defined in claim 1
wherein said aerial image and said simulated image are generated
out of focus.
17. A process for detecting defects in photomasks comprising:
generating an aerial image of a portion of a photomask; generating
a simulated image corresponding to original pattern data used to
create said photomask; and comparing said aerial image to said
simulated image.
18. A process for detecting defects in photomasks as defined in
claim 16 wherein said simulated image is generated from original
pattern data taking into account expected distortions and corner
rounding due to image processing.
19. A process for detecting defects in photomasks as defined in
claim 16 wherein said simulated image is obtained by generating an
aerial image of a mask design used to generate the portion of the
photomask with which it is compared.
20. A process for detecting defects in photomasks as defined in
claim 16 wherein said photomask is generated using optical
proximity effect correction techniques.
21. A process for detecting defects in photomasks as defined in
claim 16 wherein said photomask includes phase shifting
techniques.
22. A process for detecting defects in photomasks as defined in
claim 16 wherein said photomask includes proximity effect
correction techniques and phase shifting techniques.
23. A process for detecting defects in photomasks as defined in
claim 16 wherein said aerial image and said simulated image are
generated out of focus.
24. An apparatus for detecting defects in photomasks comprising: an
aerial image measurement system for generating an aerial image of a
portion of a photomask; a simulated image generating system for
generating a simulated image corresponding to original pattern data
of said photomask; and a comparator for comparing said aerial image
and said simulated image.
25. An apparatus for detecting defects in photomasks as defined in
claim 22 wherein said image simulator comprises an aerial image
measurement system.
26. An apparatus for detecting defects in masks comprising: means
for generating an aerial image of a portion of a mask; means for
generating a simulated image corresponding to original pattern data
used to create said mask; and means for comparing said aerial image
with said simulated image.
27. An apparatus for detecting defects in photomasks comprising:
means for generating an aerial image of a portion of a photomask;
means for generating a simulated image corresponding to original
pattern data used to create said photomask; and means for comparing
said aerial image with said simulated image.
Description
BACKGROUND OF THE INVENTION
[0001] This invention was made with government support under
Contract No. MDA 972-92-C-0054 awarded by Advanced Research
Projects Agency (ARPA). The government has certain rights in this
invention.
[0002] The present invention relates to processes for inspecting
photomasks to detect defects. More particularly, the present
invention relates to an automatic inspection system for detecting
defects in photomasks.
[0003] Advances in capacity in semiconductor chips have generally
been the result of decreases in the size of features on a chip. The
lateral dimensions of features are generally defined by
photolithographic techniques in which a detailed pattern is
transferred to a photoresist by shining light through a photomask
or reticle.
[0004] In recent years, phase shifting masks have been developed to
improve photolithographic processes. Phase shifting masks increased
image contrast and resolution without reducing wave length or
increasing numerical aperture. These masks also improve depth of
focus and process latitude for a given feature size.
[0005] With phase shift photolithography, the interference of light
rays is used to overcome the problems of defraction and improve the
resolution and depth of optical images projected onto a target.
With this technology, the phases of the exposure light at the
target is controlled such that adjacent bright areas are preferably
formed 180.degree. out of phase with each other. Dark regions are
thus produced between the bright areas by destructive interference
even when defraction would otherwise cause these areas to be lit.
This technique improves total resolution at the target.
[0006] Another method that has been developed to produce masks for
use in the fabrication of semiconductors containing small features
is optical proximity effect correction ("OPC"). In this method,
changes are made to the binary mask's layout so that it will print
more clearly. Because of the limited resolution of the current
photolithographic tools (i.e., steppers), the patterns defined on
the photomask are transferred into the resist on the wafer with
some distortions referred to as optical proximity effects. The main
consequences in term of line width control are: corner rounding,
difference between isolated and semi-isolated or dense patterns,
lack of CD linearity or where small features print even smaller
than their expected size compared to large features, and line end
shortening where the length of a line having a small line width
becomes smaller than its expected size.
[0007] Moreover, optical proximity effects are convoluted with
subsequent processing step distortions like resist processing, dry
etch proximity effects and wet etch proximity effects. In order to
achieve a sufficient line width control at the wafer level, the
mask designs are corrected for proximity effects, namely re-entrant
and outside serifs are used to correct rounding and the edges of
the patterns are moved to correct line width errors. Another
technique consists in adding small, non-printing features, referred
to as subresolution features, in order to correct line width
errors. In some cases, these features can also improve the process
latitude of the printed resist patterns.
[0008] Printable defects in photomasks and reticles have
historically been a source of defects that have reduced die yields.
With current photolithographic techniques, printable defects in the
photomasks, are repeated many times over the surface of a
semiconductor wafer and can result in substantial yield losses.
Accordingly, it is important to detect and correct as many defects
as possible in the photomasks.
[0009] Defects in photomasks can arise from many different sources.
For example, certain defects such as bubbles, scratches, pits and
fractures can be contained in the raw glass substrates. Defects can
also be formed in the chrome layer by particulate inclusions, pin
holes or voids, and excess material.
[0010] As advances have been made in photomask design such as phase
shifting and OPC, it has become harder to detect defects in the
photomasks. However, defect detection and correction has become
increasingly important. Previously, masks were checked by exposing
and developing an image on a resist layer on a plain quartz wafer.
The resulting pattern was then inspected. However, there was no
die-to-database inspection with this system.
[0011] Automatic photomask defect detection systems have been
developed and are commercially available. These include systems
such as the KLARIS system by KLA Instruments Corp. and the
Chipcheck system by Cambridge Instruments. Inspection tools such as
KLA and Orbot systems are also available for die-to-die inspection
of the printed image on wafers. These systems are limited by the
fact that the inspection is performed at 1.times. (versus 4.times.
or 5.times. for most advanced reticles). The maximum allowable
defect size is smaller and a complete inspection is not possible in
the case of a single die reticle as die-to-database capability is
not available on these systems.
[0012] In the KLA system, light is transmitted through the
photomask and detected by a CCPD image sensor. This image is then
compared to the image from a database or compared to the image from
another die on the mask. If one comparison of a die to the database
is performed, the remaining comparisons on the mask can all be
die-to-die inspections that relate back to the initial
comparison.
[0013] These prior art systems are generally limited to basic mask
designs and have limited capability of checking advanced designs
such as those containing optical proximity effect corrections and
phase shifting layers.
[0014] Because of the importance in detecting and correcting
photomask defects, it would be a significant advancement in the art
to provide an automatic process for detecting defects in advanced
photomask designs. Such a process is disclosed and claimed
herein.
SUMMARY OF THE INVENTION
[0015] The present invention provides an automatic process for
detecting printable defects in masks. The invention is particularly
useful in analyzing advanced photomask designs such as those which
include optical proximity effect corrections and phase shifting
layers.
[0016] In a preferred embodiment, a mask design is generated from a
binary mask layout. The mask design is then used to generate a
photomask such as by suitably etching a chrome layer on a quartz
plate. The present invention provides a process for detecting any
defects that are formed in the photomask. In a preferred
embodiment, an aerial image of the photomask is generated. This is
then compared with a simulated image of the binary mask layout
which has been adjusted to account for expected distortions and
corner rounding caused by image processing of the mask and wafer.
Any discrepancies between the aerial image and the simulated image
are likely due to defects in the photomask.
[0017] In a second preferred embodiment, the aerial image of the
photomask is compared with a simulated aerial image of the mask
design. Again, any discrepancies between these two images are
likely due to defects in the photomask.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a schematic illustration of a feature of a
photomask design illustrated at different stages according to a
first embodiment of the present invention.
[0019] FIG. 2 is a schematic illustration of a feature of a
photomask design illustrated at different stages according to a
second preferred embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0020] The present invention provides a process for performing an
automatic inspection of advanced design photomasks to detect
printable defects which might cause fatal flaws in semiconductor
dies. The invention is best understood by reference to the attached
drawings in which like parts are designated with like numerals.
[0021] Referring first to FIG. 1, a feature of a semiconductor mask
design is generally designated at 10. Feature 10 forms part of a
binary mask layout. From this layout, features on an advanced mask
design are generated. Feature 12 corresponds to feature 10 but is
obtained by applying optical proximity effect correction techniques
to feature 10. Feature 12 is then used to generate a corresponding
feature on a photomask.
[0022] Feature 14 corresponds to feature 12 as it appears in the
chrome on the photomask. During fabrication, a defect 16 was formed
in the design. Defect 16 comprises excess chrome which remains on
the quartz plate. However, it will be appreciated by those skilled
in the art that the process of the present invention can also be
used to detect other types of defects such as missing chrome,
contamination, glass damage, phase defects, transmission errors and
even poor repairs made to a defective mask.
[0023] In order to detect any defects, an aerial image 18 is
generated from feature 14 on the photomask. Aerial images can be
generated using a system comparable to the commercially available
MSM-100 aerial image measurement system manufactured by Carl Zeiss,
Inc. This system is set up to analyze actual masks under optical
conditions that are essentially equivalent to those of a stepper of
interest, but greatly magnified. As the exposure light is shown
through the mask and magnified, a UV sensitive CCD camera is used
for data capture.
[0024] A simulated image 20 of feature 10 is also generated and
takes into account expected distortions and corner rounding due to
image processing of the mask and wafer.
[0025] Image 20 can also be the result of the convolution of
feature 10 with some convolution function(s) representing, but not
limited to, the aerial image, the mask fabrication process and OPC
corrections. For example, the aerial image can be generated by
various software programs such as FAIM produced by Vector
Technologies, DEPICT produced by TMA, and SPLAT produced by The
University of California, Berkeley.
[0026] Aerial image 18, which is generated using a threshold such
that dimensions of image 18 match the dimensions of image 20, is
then compared to simulated image 20. Incongruity 24, which
corresponds to defect 16, will be identified during the comparison
as a discrepancy between the two images.
[0027] Reference is next made to FIG. 2 which illustrates a second
preferred embodiment of the present invention. In this embodiment,
a feature 10 of binary mask design is again used to generate a mask
design feature 12. This mask design is then used to generate
feature 14 on a photomask and an aerial image 18 is generated from
the image on the photomask.
[0028] However, in this embodiment, simulated image 30 is generated
as a simulated aerial image of mask design image 12. Aerial image
18 is then compared to this simulated image 30 to obtain a
comparison 32 where any incongruities 34 will appear as
discrepancies between the two images. Image 30 can also be the
result of the convolution of feature 10 with some convolution
function(s) representing, but not limited to, the aerial image, the
mask fabrication process, OPC corrections, etc.
[0029] While the invention has been described with respect to mask
designs using optical proximity effect correction techniques, it
will be appreciated by those skilled in the art that it can also be
applied to other advanced mask designs such as those using phase
shifting layers. The invention can be used either to analyze known
defects or to do an automated inspection over an entire mask
surface. Additionally, while the above description has been limited
to the analysis of a single feature, it will be appreciated that
blocks of multiple features can be analyzed.
[0030] In addition to photomasks, the present invention can be used
for x-ray masks, stencil masks for ion projection lithography,
masks for electron beam projection lithography, etc. The techniques
of the present invention can also be applied to imaging systems
other than those used in the manufacture of integrated
circuits.
[0031] While the invention has been described with respect to the
presently preferred embodiments, it will be appreciated by those
skilled in the art that changes and modifications could be made to
the disclosed embodiments without departing from the spirit or
scope of the invention. For example, the inspection technique and
aerial images could be performed out of focus in order to detect
defects that mainly print out of focus such as phase defects.
Additionally, other techniques of mathematical processing of the
data can be used to generate images 20 and 30. Further, the
simulated image of the mask can be generated during inspection or a
portion of the simulation can be performed before inspection and
the remainder during inspection. Accordingly, all changes or
modifications which come within the meaning and range of
equivalency of the claims are to be embraced within their
scope.
* * * * *