U.S. patent application number 11/090643 was filed with the patent office on 2005-08-04 for method of evaluating reticle pattern overlay registration.
This patent application is currently assigned to NANYA TECHNOLOGY CORPORATION. Invention is credited to Hsiao, Chih-Yuan, Mao, Hui-Min, Wu, Wen-Bin.
Application Number | 20050168740 11/090643 |
Document ID | / |
Family ID | 34076550 |
Filed Date | 2005-08-04 |
United States Patent
Application |
20050168740 |
Kind Code |
A1 |
Wu, Wen-Bin ; et
al. |
August 4, 2005 |
Method of evaluating reticle pattern overlay registration
Abstract
A method for evaluating reticle registration between two reticle
patterns. A wafer is defined and etched to form a first exposure
pattern, by photolithography with a first reticle having a first
reticle pattern thereon. A photoresist layer is formed over the
wafer and defined as a second exposure pattern, by photolithography
with a second reticle having a second reticle pattern thereon. A
deviation value between the first and second exposure patterns is
measured by a CD-SEM. The deviation value is calibrated according
to the scaling degree and the overlay offset to obtain a
registration data. The reticle registration between the two reticle
patterns is evaluated based on the registration data.
Inventors: |
Wu, Wen-Bin; (Taoyuan Hsien,
TW) ; Hsiao, Chih-Yuan; (Taipei, TW) ; Mao,
Hui-Min; (Taipei, TW) |
Correspondence
Address: |
QUINTERO LAW OFFICE
1617 BROADWAY, 3RD FLOOR
SANTA MONICA
CA
90404
US
|
Assignee: |
NANYA TECHNOLOGY
CORPORATION
TAOYUAN
TW
|
Family ID: |
34076550 |
Appl. No.: |
11/090643 |
Filed: |
March 25, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11090643 |
Mar 25, 2005 |
|
|
|
10792345 |
Mar 3, 2004 |
|
|
|
Current U.S.
Class: |
356/401 |
Current CPC
Class: |
G03F 9/7019 20130101;
G03F 9/7011 20130101; G03F 7/70633 20130101 |
Class at
Publication: |
356/401 ;
716/019 |
International
Class: |
G01B 011/00; G06F
017/50 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 2, 2003 |
TW |
TW92124185 |
Claims
1-22. (canceled)
23. A method for fabricating a wafer sample for inspection of a
critical dimension scanning electron microscope (CD-SEM),
comprising the steps of: forming a first pattern on a wafer by a
first reticle; forming a photoresist layer on the wafer; and
patterning the photoresist layer to form a second pattern with a
second reticle, thereby forming a wafer sample for CD-SEM
inspection.
24. (canceled)
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to semiconductor fabrication,
and in particular to a method of checking overlay registration
between every two reticle patterns for photolithography.
[0003] 2. Description of the Related Art
[0004] Each lithography step uses a pattern referred to as a layer,
such as a(n) patterned conductive layer, semiconductor layer or
insulating layer. In order to make semiconductor devices, each
photolithography reticle or mask corresponding to a certain
structural pattern must be aligned with the semiconductor substrate
for overlay registration before exposure.
[0005] Conventionally, corresponding alignment marks or features
are set on a semiconductor substrate, i.e. a wafer, and the reticle
respectively for alignment. Often alignment marks are included in
other layers, as the original alignment marks may be obliterated as
processing progresses. It is important for each alignment mark on
the wafer to be labeled so it may be identified, and for each
pattern to specify the alignment mark (and the location thereof) to
which it should be aligned. By providing the location of the
alignment mark, it is easy to locate the correct feature in a short
time. Each layer should have an alignment feature so that it may be
registered to the rest of the layers.
[0006] Generally, reticle providers usually provide registration
specifications for patterns on the reticles. The exposure is
performed by aligning the alignment marks directly. FIG. 1 shows a
conventional alignment for exposure between a reticle pattern and a
wafer. Four alignment marks 12 are disposed on a wafer 10. Four
alignment marks 22 on four corners of an exposure pattern 20 on a
reticle are positioned to align with the four alignment marks 12,
thereby ensuring the pattern 20 is transferred precisely to the
predetermined area on wafer 10.
[0007] During the exposure, alignment between the reticle pattern
20 and the wafer 10 is accomplished by alignment marks thereon.
However, inherent errors within the reticle pattern 20 cannot be
adjusted by the exposure alignment. For semiconductor devices
requiring multi-level alignment, inherent error addition between
two continuous or discontinuous layer patterns may exceed the
original specification. Due to shrinking feature sizes, the
tolerances for overlay registration of the reticle pattern to the
wafer are also reduced.
[0008] Conventionally, alignment registration is inspected by
preparing thin sections of the testing layers formed on a wafer and
viewing by an X-SEM (X-ray scanning electron microscope). FIG. 2 is
a profile of an X-SEM section, showing the overlay registration
between three layers. The disadvantage of X-SEM sections is they
can only show the overlay registration of certain cross-sections of
the wafer, not deviations of the alignment in a whole picture. In
addition, when CD-SEM is utilized for inspection, designed to
inspect critical dimension (CD) for semiconductor devices, the
conventional bottom anti-reflection coating (BARC) widely used for
photolithography improvement interferes with the detection signals
from CD-SEM, thereby causing difficulty in viewing the overlay
registration by CD-SEM
SUMMARY OF THE INVENTION
[0009] One object of the invention is to provide a method for
evaluation overlay registration of reticle patterns.
[0010] Another object of the invention is to provide a method for
evaluating overlay registration between two discontinuous reticle
patterns.
[0011] Still another object of the invention is to provide a method
for fabricating a wafer sample for CD-SEM inspection of overlay
registration.
[0012] To achieve the objects, the present invention provides a
method for fabricating a wafer sample for evaluating the overlay
registration between reticle patterns. A first pattern is formed on
a wafer by photolithography with a first reticle having a first
reticle pattern thereon. A photoresist layer is then formed on the
wafer. The photoresist layer is patterned to form a second pattern
by photolithography with a second reticle having a second reticle
pattern thereon. Deviations are measured between the first and
second patterns on the wafer along X-, Y- or X and Y axes. A
scaling value and an overlay offset of the deviations are
calibrated to obtain an overlay registration value. Whether the
registration value is out of a specification is determined
according to the calibrated values.
[0013] In an embodiment, a bottom anti-reflection layer (BARC) can
be formed between the wafer and the photoresist layer. After the
photoresist layer and the bottom anti-reflection layer are
patterned as the second pattern, the bottom anti-reflection layer
is removed by over-etching to provide a clearer profile of the
first and second patterns for measurement.
[0014] A detailed description is given in the following embodiments
with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The present invention can be more fully understood by
reading the subsequent detailed description and examples with
references made to the accompanying drawings, wherein:
[0016] FIG. 1 shows a conventional alignment for exposure between a
reticle pattern and a wafer;
[0017] FIG. 2 is a conventional profile of an X-SEM section,
showing the overlay registration between three layers;
[0018] FIG. 3 is a flowchart of a method for evaluating overlay
registration according to an embodiment of the invention;
[0019] FIGS. 4A to 4E are cross-sections showing the process for
fabricating a wafer sample with two mask patterns according to an
embodiment of the invention;
[0020] FIGS. 5A and 5B are top views showing the measurement of
pattern overlay on a wafer sample by a CD-SEM according to an
embodiment of the invention;
[0021] FIGS. 6A to 6C illustrate a sampling method on a wafer
sample for measuring deviations of the patterns thereon according
to an embodiment of the invention;
[0022] FIG. 7A shows a scaled pattern formed by a reticle pattern
according to an embodiment of the invention;
[0023] FIG. 7B shows the overlay offset between two patterns
according to an embodiment of the invention;
[0024] FIGS. 8A and 8B are figures showing deviation curves of FIG.
6C along X- and Y-axis respectively; and
[0025] FIGS. 8C and 8D are calibrated figures of FIGS. 8A and 8B
respectively after calibrating the scaling values and overlay
offsets.
DETAILED DESCRIPTION OF THE INVENTION
[0026] The present invention is applicable to series of reticles
for photolithography to check the overlay between two continuous or
discontinuous reticle patterns. For example, the reticle can
comprise a pattern thereon defining active regions (AA), gate
layers (GC), deep trenches for capacitors (DT), contact openings
(CS), bit line openings (CB) or a layer of interconnection on a
semiconductor substrate. It is of note that the present invention
is not limited thereto, being also applicable to reticles with
other patterns, depending on the construction of the corresponding
semiconductor device.
[0027] According to the present invention, two continuous reticle
patterns, such as reticles for deep trenches for capacitors and the
reticle for active regions, or two discontinuous reticle patterns,
such as those of deep trenches for capacitors and for gate layers.
The deviating orientation between the patterns depends on their
corresponding layout. Generally, the deviation may be along X- or
Y-axes, or X- and Y-axes, depending on which two reticle patterns
are overlaid.
[0028] FIG. 3 is a flowchart of a method for evaluating overlay
registration according to an embodiment of the invention. The
embodiment is described in accordance with FIG. 3, illustrating an
evaluation of overlay registration between a reticle pattern for
deep trench capacitors and a reticle pattern for active regions.
FIGS. 4A to 4E are cross-sections showing the process for
fabricating a wafer sample with the two reticle patterns.
[0029] In step S302, a first pattern is formed on a wafer 400 by
photolithography with a first reticle having a first reticle
pattern thereon, as shown in FIG. 4A. A photoresist layer 404 is
deposited on the surface of the wafer 400. Preferably, a bottom
anti-reflection layer (BARC) 402 is formed on the wafer 400 before
forming the photoresist layer 404 for improving the quality of
photolithography. A reticle with a reticle pattern of deep trench
capacitors (not shown) is utilized to form a patterned photoresist
layer by photolithography. The wafer 400 is then etched to about
1000 .ANG. with the patterned photoresist layer 404 as a mask to
form deep trenches 406 for capacitor on the wafer 400 as shown in
FIG. 4B. The patterned photoresist layer 404 and bottom
anti-reflection layer (BARC) 402 are then removed, forming a wafer
400 with deep trenches 406.
[0030] In step 304, a photoresist layer is then formed on the
wafer. Preferably, a bottom anti-reflection layer (BARC) 408 is
formed on the wafer 400 to fill the deep trenches 406 and then the
second photoresist layer 410 is formed on the BARC layer 408.
[0031] In step S306, the photoresist layer is patterned to form a
second pattern by photolithography with a second reticle having a
second reticle pattern thereon, as shown in FIG. 4D. A reticle with
a reticle pattern for defining active regions (not shown) is
utilized for patterning the photoresist layer 410 by
photolithography to form an active region pattern 412. The exposed
anti-reflection layer (BARC) 408 is then etched with the patterned
photoresist layer 410 as a mask to form active region openings. In
a preferred embodiment, the exposed anti-reflection layer (BARC)
408 is etched for about 5 seconds to form a clearer pattern of
active regions 412, as shown in FIG. 4E.
[0032] According to the above steps, the pattern on the first
reticle is transferred to wafer 400 and the pattern on the second
reticle is transferred to the photoresist layer 410 on wafer 400,
thereby simplifying the fabrication of a wafer sample for overlay
registration.
[0033] According to the above steps, the wafer sample as shown in
FIG. 4E can be measured from the top view thereof to evaluate the
overlay accuracy between the deep-trench pattern 406 on the wafer
400 and the active-region pattern 412 on the photoresist layer 410.
However, a wafer sample of any two reticle patterns can be formed
accordingly and the invention is not limited thereto. In addition,
over-etching of exposed anti-reflection layer (BARC) 408 also helps
reduce interference during observation with a CD-SEM.
[0034] After the wafer sample is obtained, step S308 measures
deviations between the first and second patterns on the wafer along
X-, Y- or X and Y axes. The definition of X- and Y-axes can be
pre-determined by marking a first axis of the wafer sample and then
a second axis perpendicular to the fist axis.
[0035] FIGS. 5A and 5B are top views showing two types of patterns
overlaid on a wafer sample of the invention. FIG. 5A shows a
pattern overlay between a gate-layer (GC) pattern and a deep-trench
(DT) pattern according to an embodiment of the invention. The wafer
sample of GC and DT can be observed by a CD-SEM from the top. As
shown in FIG. 5A, in the region I of the wafer sample, the DTs
exceeding GCs along y-axis are measured. The pattern deviation of
region I of DTs is: 1 Y l = ( b 1 - b 2 ) / 2 + ( b 3 - b 4 ) / 2
2
[0036] Due to the DT pattern in a unit of two deep trenches,
deviation thereof is also the average of two. However, the
deviation in the DT pattern can still be calculated solely in
accordance with the layout of the deep trenches, as follows: 2 Y l
- 1 = ( b 1 - b 2 ) 2 and Y l - 2 = ( b 3 - b 4 ) 2
[0037] FIG. 5B shows a pattern overlay between an active-region
(AA) pattern and a deep-trench (DT) pattern according to another
embodiment of the invention. The wafer sample of AA and DT can be
observed by a CD-SEM from the top. As shown in FIG. 5B, in the
region III of the wafer sample, the deviations between DTs and AAs
are measured along both x and y axes. As shown in FIG. 5B, the
pattern deviation of region III of DT to AA along x axis is: 3 X m
= ( a 1 - a 2 ) 2
[0038] The pattern deviation of region III of DT to AA along y axis
is: 4 Y m = ( b 1 - b 2 ) 2
[0039] Table 1 lists various overlay patterns, and the deviated
orientations, i.e. deviation axis, and corresponding wafer sample
structures thereof.
1 TABLE 1 The overlay deviation patterns axis Wafer sample
structure AA-DT X and Y A wafer sample with a DT pattern GC-DT Y A
wafer sample with a DT pattern CB-AA X A wafer sample with a AA
pattern CB-GC Y A wafer sample with a GC pattern CS-GC Y A wafer
sample with a GC pattern M0-CB X A wafer sample with a TEOS
pattern
[0040] The "wafer sample structure" in Table 1 represents the first
reticle pattern transferred to the wafer of the two reticle
patterns. For example, to measure the overlay between reticle
patterns of AA and DT, the DT pattern is formed first on the wafer
sample and the reticle pattern of AAs transferred to the
photoresist layer on the wafer. As shown in Table 1, sequences to
form the two overlaid patterns may be different from conventional
sequences for fabricating a semiconductor device. However, the
sequence in which pattern is formed on a wafer sample depends on
the observation under a CD-SEM. Moreover, because millions or even
billions of units may be formed on a wafer sample, sampling of the
measurement under a CD-SEM is further exemplified by FIGS. 6A to
6C.
[0041] FIG. 6A shows a pattern formed by a stepper, which transfers
a reticle pattern to the wafer sample 400 step-and-repeatedly 24
times to form a rectangular pattern 600. Due to the increased size
of wafers, the pattern on a reticle is transferred to the wafer for
A exposures to form a pattern, such as 24 exposures as shown in
FIG. 6A. The transferred sub-patterns 601-624 construct a pattern
600. In an embodiment, only 6 sub-patterns, including sub-patterns
601, 606, 619, and 624 on the four corners of the pattern 600 and
sub-patterns 609 and 616 on the central area of the pattern 600,
are selected for overlay measurement under a CD-SEM. Generally, the
sampled sub-patterns B for measurement will not exceed the total
sub-patterns A, i.e. the transferred patterns from the reticle
pattern. The preferred sampling locations include the sub-patterns
at the corners and in the central area of the pattern 600.
Generally, the two reticle patterns are patterned by the same
stepper and the sub-patterns transferred aligned with each
other.
[0042] FIG. 6B illustrates measurement of a selected sub-pattern
619 in FIG. 6A. Plural points are selected along the axis to which
the two patterns will deviate. For example, because the AA pattern
deviates from the DT pattern along both X- and Y-axes as shown in
FIG. 5B, sub-pattern 619 is divided into M rows along X-axis and P
columns along Y-axis. Each row of the sub-pattern 619 is divided
into N points along X-axis and each column of sub-pattern 619 into
Q points of sub-pattern 619. If GC-DT patterns are measured, the
sub-pattern 619 is only divided into P columns along Y-axis with Q
points on each column due to the deviation of GC-DT patterns only
along Y-axis.
[0043] As shown in FIG. 6B, two rows on the opposite sides of
sub-pattern 619, X1 and X2, are selected with 16 points of equal
distance on each row. Two columns on the other opposite sides of
sub-pattern 619, Y1 and Y2, are selected with 12 points of equal
distance on each column. The rows and columns of the sub-pattern
619 can be selected from the lateral area of the sub-pattern 619 or
by an equal distance on the surface of the sub-pattern 619. If
there is no AA-DT pattern overlaid on the selected point, the AA-DT
pattern nearest the selected point is taken for overlay
measurement.
[0044] FIG. 6C shows the results of measurement of the sampled
points of rows X1 and X2 and columns Y1 and Y2 with a CD_SEM in
accordance with FIG. 5C.
[0045] As shown in FIG. 3, in step S310, a scaling value and an
overlay offset of the deviations are calibrated to obtain an
overlay registration value between the first and second
patterns.
[0046] To measure the overlay registration between the two reticle
patterns, e.g. AA and DT patterns, precisely, the scaling error and
overlay offset during exposure is calibrated. These errors
generally result from operation or tool error during
photolithography. As shown in FIG. 7, scaling error results from
the level shift of reticle 700, thereby forming scaled pattern 710
on the wafer sample. The overlay offset results from misalignment
between the reticle pattern and wafer. As shown in FIG. 7B, the
overlay offset between the DT pattern to AA pattern is d1 along X
axis and d2 along Y axis.
[0047] FIGS. 8A to 8D show original and calibrated deviation curves
of FIG. GC along X and Y axes respectively. Deviations in the
points on the selected rows and columns shown in FIG. 6C are
linearly regressed, and the original regression curves of rows X1
and X2 are shown in FIG. 8A and the original regression curves of
columns Y1 and Y2 are shown in FIG. 8B.
[0048] FIGS. 8C and 8D are calibrated figures of FIGS. 8A and 8B
respectively after calibrating the scaling values and overlay
offsets.
[0049] The scaling error of each point of the original regression
curves can be calibrated. The scaling error of row X1 can be
calibrated as follows:
M'.sub.(X1)n=m.sub.(X1)n-(n-1).times.S.sub.X1;
[0050] wherein m.sub.(X1)n is the deviation of the n.sup.th point
on row X1, S.sub.X1 is the slope (S) of the regression curve of row
X1, and M'.sub.(X1)n is the deviation with scaling calibration of
the n.sup.th point on row X1.
[0051] Similarly, the scaling error of row X2 can be calibrated as
follows:
M'.sub.(X2)n=m.sub.(X2)n-(n-1).times.S.sub.X2
[0052] Similarly, the scaling error of column Y1 can be calibrated
as follows:
P'.sub.(Y1)q=p.sub.(Y1)q-(q-1).times.S.sub.Y1;
[0053] wherein p.sub.(Y1)q is the deviation of the q.sup.th point
on column Y1, S.sub.Y1 is the slope (S) of the regression curve of
column Y1, and M'.sub.(Y1)q is the deviation with scaling
calibration of the q.sup.th point on column Y1.
[0054] Similarly, the scaling error of column Y2 can be calibrated
as follows:
P'.sub.(Y2)q=p.sub.(Y2)q-(q-1).times.S.sub.Y2
[0055] According to the above formulas, the slope (S) of the linear
regression curve of each selected row or column is the scaling
error of the patterns.
[0056] The calibration of overlay offset of each selected row and
column on the sub-pattern 619 is performed. In an embodiment, the
overlay offsets of X1, X2, Y1 and Y2 are calibrated as follows: 5 O
X1 = n = 1 N M ( X1 ) n ' N O X2 = n = 1 N M ( X2 ) n ' N O Y1 = q
= 1 Q P ( Y1 ) q ' Q O Y2 = q = 1 Q P ( Y2 ) q ' Q
[0057] The average value of the deviations calibrated by scaling is
the overlay offset of each selected row or column. The deviations
calibrated by scaling of each point on X1, X2, Y1 and Y2 are
further calibrated with the overlay offsets
O.sub.X1.multidot.O.sub.X2.multidot.O.sub.Y1 and O.sub.Y2
respectively to obtain registration data thereof as follows:
M.sub.(X1)n=M'.sub.(X1)n-O.sub.X1
M.sub.(X2)n=M'.sub.(X2)n-O.sub.X2
P.sub.(Y1)q=P'.sub.(Y1)q-O.sub.Y1
P.sub.(Y2)q=P'.sub.(Y2)q-O.sub.Y2
[0058] The curve of registration data is shown in FIGS. 8C and 8D.
FIGS. 8C and 8D show the registration data of the points in X1 and
X2, and Y1 and Y2 respectively, with the calibration of scaling
error and overlay offset. Generally, the patterns on two reticles
correspond with each other, after the calibration, the curve of the
registration data is 0, i.e. overlapping with the X- or Y-axis.
However, curves in FIGS. 8C and 8D are not zero, indicating
registration of reticles of AA and DT do not 100% correspond to
each other. This may result from inherent pattern deviation on the
reticle AA and/or reticle DT.
[0059] As shown in FIG. 3, in step S312, it is determined whether
the registration value is out of a specification. The registration
value indicates the overlay accuracy of the reticle pattern. If the
registration value shows the overlay error is an inherent error of
a reticle, it will be difficult to compensate or calibrate the
error during photolithography. The registration data of the
selected sub-patterns 601, 606, 619, 624, 609 and 616 of pattern
600 are measured and calibrated with scaling error and overlay
offset through steps S308 to S310. Final registration data is
further evaluated by statistic methods to determine whether the
registration data of two reticles is within a specification. If
not, the reticles may need to be reproduced.
[0060] While the invention has been described by way of example and
in terms of the preferred embodiments, it is to be understood that
the invention is not limited to the disclosed embodiments. To the
contrary, it is intended to cover various modifications and similar
arrangements (as would be apparent to those skilled in the art).
Therefore, the scope of the appended claims should be accorded the
broadest interpretation so as to encompass all such modifications
and similar arrangements.
* * * * *