Line width uniformity verification for 0.18 and below design rule reticles
T. Tan, S. C. Kuo and W. Shen
Taiwan Mask Corporation
N. Schumann, C. Wu
Applied Materials, Process Diagnostic and Control Business Group
ABSTRACT
A revolutionary CD error detection tool, the linewidth bias measurement (LBM), has been developed by the PDC
group of Applied Material to solve the localized CD error detection issue for 0. 18 m and below design rule reticles. In this
paper, we discussed, characterized and tested the LBM tool on both designed testers and real production reticles. Several
potential applications have been demonstrated and discussed.
Keywords: Localized CD error, LBM, inspection, CD uniformity, Applied Material
1.
QA ISSUES OF RETICLE CD CONTROL
Localized CD error has been a major threat for the photomask industry in the last several years' Because sampling
is used as the methodology to perform CD measurements, it is impossible to catch 100% of localized CD errors on the
production line due to the limited number of CDs measured within a reasonable measurement time. On the other hand, since
sensitivities of current inspection tools with traditional inspection method are larger than 0. 15 pm while CD requirements are
less than 0.04
it is also impractical to detect this type of CD defects. Without a proper control, this issue can certainly
affect product performance and overall yield dramatically. It has been considered as a potential showstopper for 0. 18 m and
below design rule reticle in the past.
2.
BASIC LBM CONCEPT
In a traditional die-to-database inspection, inspection grids are larger due to limitations on optics and inspection
speed requirement Each inspection pixel has certain gray levels from the nature of the CCD camera. For example, the
inspection grid can be 0.28 pm with 256 gray levels on Applied Material Orbot RT8000. The raw data of a pattern image is
represented by a two dimensional step function, as shown in the figure 1. To compare the inspection result with the database,
a pattern image must be reconstructed from the 2-D step function through a certain algorithm, which is different from tool to
tool. Inspection grid size, gray level and algorithm are directly related to the sensitivity of an inspection tool in the traditional
the-to-database inspection method.
Reconstructed image
0.28 urn grid
256 gray level
Algorithm
D-DB comparison
Step function of image
Database
Figure 1. Basic concept of a die-to-database inspection
Part of the 19th Annual BACUS Symposium on Photomask TechnoloQy
728
Monterey, California • September 1999
SPIE Vol. 3873 • 0277-786X!991$1O.OO
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In the real practice of a die-to-database inspection, variations, such as transmitted light intensity, affect the size of
the reconstructed image from plate to plate. A constant bias must be measured and applied to the reconstructed image for
each inspection. However, due to the CD errors on the inspected plate, there is still a location dependent residue bias exist
after applying the constant bias. This residue bias can represent the CD errors of the plate. On the other hand, the raw data of
this residue bias is just too noisy to meet the CD sensitivity requirement (<40 nm) because the image is reconstructed from a
2-D step function with a larger grid size (>150 rim).
To reduce the noise of the raw data, another algorithm is applied to average of the raw data over a region, typically a
200 tm x 200 m area. Because of the huge number of edges involved, noise level is greatly reduced and the resulted data
map can represent the CD error map with a good sensitivity over the whole inspection region. This is the basic LBM concept.
Considering the methodology ofLBM, we can simply characterize the nature ofLBM as follow:
Because the noise level is directly related with the number of edges in the average region, this method is good for
(1)
high-density patterns, such as memory patterns. We can expect a higher noise level and lower sensitivity on contact
levels or certain logic patterns.
This method can be applied to different generations of inspection tools. Smaller inspection grid size and higher gray
(2)
level can provide a better CD sensitivity due to the reduction in noise level on the reconstructed image.
LBM does not require any additional inspection. The data is collected at the same time of the normal inspection.
(3)
There is no penalty on the inspection speed. Therefore, it is a production worthy method.
3.
LOCALIZED CD ERROR TESTER
A tester with systematic localized CD errors was designed to test the sensitivity of LBM. Each 1 cm x 1 cm chip
contains 0.8 m line and space pattern over the whole chip. The center of each chip had an area (miss-size area) with design
CD errors, as shown in the figure 2a. On this 6"tester, we changed designed CD errors from 8.5 rim to 83.3 nm (multiples of
the Alta3000 writing grid) and the size of the miss-size area from 100 .tm to 5000 .tm, as shown in the figure 2b. Test plates
were written on an ETEC Alta3000 and processed through the standard 1P3700 resist process line. Standard die-to-database
inspection with LBM was then applied on an Applied Material RT8000 inspection tool.
Y=83.3 75 66.6 583 50 41.6 33.3 25 16.6 85 rim
1 cm ____________
X=5000
X=2000
TeArea:
x=1000
X=800
1
Med:
X=600
X=500
X=400
X=300
X=200
x=l00
Figure 2a. Tester cell design
Figure 2b. 10 x 10 6" tester
Figure 3a shows the result of LBM over the whole plate. Dark regions in the center of chips represent that LBM did
catch CD errors in those chips. Figure 3b shows the lower left corner of the tester, where small areas with larger CD errors
were designed. Figure 4c shows the upper right corner of the tester, where larger areas with smaller CD errors were designed.
Table 1 shows the minimum area size can be detected for each design CD error. In this test, LBM was surprisingly sensitive
to small CD errors. CD errors as small as 8.5 rim were detected over a region as small as 600 pm x600 pm. Although the line
density in our tester was very high -whichreduced the noise level, it can still be considered as a great result.
729
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Figure 3a. The LBM result ofthe localized CD error tester
Figure 3b. The lower left corner of the LBM result
Figure 3c. The upper right corner of the LBM result
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CD error introduced in the area (in nm) ___ ___
83.3 75.0 66.6 58.3 50.0 41.6 33.3 25.0 16.6 8.5
Mm. size of
area detected
100 100 100 200 200 300 300 300 500 600
(in u m)
Table 1. The minimum detectable area size for different detectable CD errors
4.
TYPICAL LBM RESULTS ON PRODUCTION PLATES
LBM has been tested on our production line with different types of products. Figure 4 shows the result from a
rejected plate due to CD errors. Large radial CD errors can be clearly seen on the result.
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Figure 4 The LBM result of a production plate (CD rejected) with radial CD errors
Figure 5a shows the result from one of our 0. 18 pin generation memory product with 0.8 pin horizontal line/space
pattern in each chip. Strip lines were observed. Figure 5b shows an enlarged region. The distance between strips of CD errors
was measured to be about 1 100 pin, identical with Alta3000 writing strip height This observation was confirmed on our
KMS400 CD measurement tool. It was believed Alta3000 writing caused these CD errors. This type of errors was observed
in many other plates with very high-density patterns. The non-linearity of the process at the proximity of the resist resolution
limit apparently amplified the effect of a small dose error (a similar effect as MEF). The current writer calibration method,
provided by the machine vendor, does not have a proper ability to detect this type of errors. It is especially important because
it only happen on the most advanced layers with high pattern density and tight CD specs.
731
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732
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5.
CONCLUSION
The technique of LBM can be considered as a major breakthrough of the industry in solving the localized CD error
problem. This paper shows some of its potential applications. Apparently, LBM can provide an even better CD information
than the traditional method on some layers. For example, instead of measuring hundreds of CDson a critical 0.18 generation
DRAM layers, we can just measure 3-5 CDs in addition to the LBM result — a more complete analysis than the former
method. The information provided by LBM can also open a new methodology to calibrate reticle writers as we mentioned
before.
ACKNOWLEDGEMENT
The authors want to acknowledge Kenny Yang and Andrew Wang of TMC for their help in process and CD
measurement. The authors also appreciate good suggestions from Wolfgang Staud of Applied materials.
REFERENCE
1. W. P. Shen, "Particle and contamination introduced localized CD errors", 18th annual symposium on photomask
technology and management, SPIE Vol. 3546, pp. 460-465, 1998.
733
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