Photolithographic dose determination by diffraction of latent image grating

A method of determining quantitatively the exposure levels for photoresists in semiconductor photolithography employs a specially designed grating pattern on a mask. The mask is first used to expose a series of LIM image gratings of different dosages. Then a normal plane wave at a longer wavelength is incident on these gratings one by one, and some nonzero order diffraction efficiency of the grating is measured to determine quantitatively the correct dosage to be used. This method can make a determination of exposure dosage, without knowledge of underlying film thickness and refractive index, and handle either resist thickness change or underlying film thickness/refractive index change or both.

DESCRIPTION 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to photolithiographic processes in 
the manufacture of semiconductor devices and, more particularly, to using 
a latent image grating to determine quantitatively the exposure dosage 
required in semiconductor photolithography for optimum processing. 
2. Description of the Prior Art 
In photolithographic processes, the photoresist line-width variation 
typically needs to be controlled within 10% of its nominal value. Very 
often this cannot be achieved due to the interference effect which causes 
the energy absorbed by the photoresist to fluctuate. The interference 
effect is caused by the thickness variation of the photoresist itself, the 
thickness variation of underlying film(s), and the optical property change 
of the underlying film(s), e.g., due to hot processes. This problem is 
aggravated when the exposure sources use short wavelengths, i.e., i-line 
and deep ultra-violet (DUV) wavelengths. Small thickness changes can 
correspondingly result in large variations in absorbed energy. 
Several methods have been used in the past to address the problem, among 
which are a "send ahead" wafer for each job. In this method, a series of 
dosages are used on each wafer, the wafer is developed and the line-widths 
corresponding to various dosages are measured to determine the optimum 
dosage. This method is accurate but time consuming and expensive. 
A similar method was proposed by 0. D. Crisalle, R. A. Soper, D. A. 
Mellichamp, and D. E. Seborg in "Adaptive control of photolithiography", 
SPIE, vol. 1464, pp. 508-526 (1991), with the same drawbacks. The problem 
can be eliminated with the addition of a top or bottom anti-reflective 
layer. This solution, however, adds an extra processing step and the 
material used makes the process more expensive. 
J. A. Bruce, R. K. Leidy, M. S. Hibbs, and B. J. Lin in "Characterization 
and prediction of line-width variation due to thin film interference 
effects", Proceedings KTI Microelectronics Seminar, pp. 1-13 (1989), 
describe a process in which the reflectivity from the wafer surface can be 
measured and used to determine qualitatively if a higher or lower dosage 
needs to be used for that wafer or job. However, this method is effective 
when only one film thickness is varying. Otherwise, an infinite number of 
thickness combinations corresponding to different absorbed energies can 
result in the same reflectivity, in which case it cannot be used to 
determine the dosage. 
Diffraction efficiencies of latent image (LIM) grating can be measured and 
used in conjunction with a rigorous Maxwell's equations' solver to 
determine the exposure dosage, as described by K. C. Hickman, S. M. 
Gaspar, K. P. Bishop, S. S. H. Naqvi, and J. R. McNeil in "Use of 
diffracted light from latent images to improve lithography control", SPIE, 
vol. 1464, pp. 245-257 (1991). However, one needs to know ahead of time 
the cause (e.g., which film thickness is varying) of the line-width 
variation and the optical indices of all relevant materials, so that the 
solver can perform the calculation. Neither of the two requirements can be 
easily achieved in production. 
Nomarski differential interference contrast (NDIC) microscopy can be used 
to measure LIM to determine the exposure dosage. Commercially available 
NDIC microscopes can be modified to perform this task. In-situ, real time 
dosage adjustment can be done by incorporating such hardware into a 
stepper. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a method of 
determining quantitatively the exposure dosage required in semiconductor 
photolithography which solves line-width variation problems due to thin 
film interference effect by dosage adjustment. 
According to the invention, a specially designed grating pattern on the 
mask is first used to expose, at the exposure wavelength (e.g., 365 nm), a 
series of LIM image gratings of different dosages. Then a normal plane 
wave at a longer wavelength (e.g., 633 nm) is incident on these gratings 
one by one, and some nonzero order diffraction efficiency of the grating 
is measured to determine quantitatively the correct dosage to be used. 
This invention can make a determination of exposure dosage, without 
knowledge of underlying film thickness and refractive index, and handle 
either resist thickness change or underlying film thickness/refractive 
index change or both.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION 
Referring now to the drawings, and more particularly to FIG. 1A, there is 
shown a conventional grating pattern on the mask 1 containing opaque lines 
and transparent spaces. The mask 1 is placed over a photoresist 2. A graph 
3 is illustrated below the photoresist 2 to show the measured diffraction 
efficiency as energy increases. In FIG. 1A the mask 1, photoresist 2 and 
graph 3 are shown at five time periods denoted a to e during exposure. 
Before exposure, the non-zero order diffraction efficiency measured by 
graph 3 is zero. Opaque regions 4 in mask 1 prevent part of the 
photoresist 2 from having any exposure to light. Areas 6 of the 
photoresist 2 below the transparent regions 5 of mask 1 become bleached as 
energy increases. As the exposure dosage increases, the measured 
efficiency increases and eventually saturates when the resist bleaching 
process is completed and its change of optical property stops. 
As can be seen from FIG. 1B, the mask 10 is not opaque, but allows some 
light transmission. Transparent regions 5 in mask 10 remain clear, but 
semitransparent regions 11, which replace opaque regions 4 in FIG. 1A, 
allow some light transmission. The invention uses a grating pattern so 
that what had been opaque lines become partially transparent (e.g., 
slightly greater than 0% to 70% of light transmission). As energy 
increases, the photoresist 12 eventually becomes fully bleached at time 
period e. The areas 14 of photoresist 12 below the semitransparent regions 
11 bleach at a slower rate than areas 15 below the transparent regions 5 
of the mask 10. Then at very large dosage, the diffraction efficiency is 
zero since the LIM grating is completely bleached. This effect allows 
clear reading of E.sub.max on the graph 13. In FIG. 1A, there is not a 
distinct quantity such as E.sub.max to characterize the exposure state of 
the resist. 
The diffraction efficiency reaches the maximum when the "optical 
distinction" of the two LIM regions is maximized, and the corresponding 
dosage is called E.sub.max. Notice the E.sub.max is not equal to imaging 
dosage, D.sub.image (i.e., dose to nominal CD). E.sub.max and E'.sub.max 
are measured quantities to determine what the new D.sub.image should be. 
If for a subsequent wafer/job the absorbed energy is reduced by a factor 
of .alpha. due to thin film interference effect, then E.sub.max measured 
will be increased by the same factor .alpha. since a higher dosage is 
required for the LIM grating to reach the same state of maximum optical 
distinction. Thus, D.sub.image can be adjusted by the same factor to 
compensate for the interference effect. 
This invention has an important feature in that it is transparent to the 
source of the interference effect. This is illustrated in FIGS. 2, 3 and 4 
in which the grating dimension and diffraction efficiency detection used 
are compatible, but not limited, to the ASML stepper's alignment system. 
Partial transmission (e.g., 25%) of energy 23 is assumed in FIG. 2 through 
semitransparent regions 21 of mask 20 above the photoresist 24, so the 
index (i.e., N1) change in the partial transmission region is four times 
slower than that (i.e., N2) in the transparent regions 22. FIG. 3 shows 
the hypothetical index change versus exposure energy absorbed by the 
resist, and FIG. 4 shows the simulated first-order diffraction efficiency 
from various combinations of films. A wide range of combinations of film 
and thickness were used and Maxwell's equations were solved numerically 
using the wave-guide model to calculate the data graphed in FIG. 4. The 
films are 
1.04 .mu.m resist on Aluminum (efficiency shown has been divided by 2 to 
fit in FIG. 4), 
resist of 0.96 .mu.m, 1.00 .mu.m, 1.04 .mu.m, 1.08 .mu.m on silicon, 
1.04 .mu.m resist on oxide (of 0.10 .mu.m, 0.14 .mu.m, 0.18 .mu.m) on 
silicon, and 
1.04 .mu.m resist on matched substrate. 
Peaks of all curves in the graph of FIG. 4 fall on the same line except for 
the aluminum substrate which is highly reflective. Therefore, wafers may 
need to be divided into two groups (i.e., of metallic substrates and 
dielectric substrates) and apply the invention to them separately. This 
can be done in production easily since wafers of metallic substrates are 
obvious to detect. Also, diffraction efficiency of a matched substrate may 
suffer from a weak signal level. However, substrates with low reflectivity 
usually do not suffer from the interference effect. Substrates of typical 
dielectric materials yield sufficient diffraction efficiency for 
detection. See, for example, K. C. Hickman et al., supra, and T. E. Adams, 
"Applications of latent image metrology in microlithiography", SPIE, vol. 
1464, pp. 294-312 (1991). 
Implementation of a preferred embodiment of the present invention is 
illustrated in the flow chart of FIG. 5, in which the frequency of 
updating the dosage depends on the nature of line-width variation. In the 
first step 51, D.sub.image is measured by conventional methods and 
E.sub.max is measured and stored. The next step 52 is a decision step to 
decide whether to proceed with preparing the wafer or job. If the result 
of this decision is "no", then the cycle ends. If the answer is "yes", 
then E'.sub.max is measured in step 53. An adjustment of dosage to 
##EQU1## 
is made in step 54, and the product chips are exposed with the new dosage 
in step 55. The process cycles back to the decision step 52. 
For gratings of small periods compared to wavelength, such as 2 .mu.m to 
.lambda.=633 nm, yield a small number of diffraction orders and large 
signal intensity, the opaque regions can be formed by a narrow Cr line 
(e.g. 0.4 .mu.m by a 0.48 NA i-line lens) below the resolution limit of 
the projection system so that the light intensity under the Cr line is not 
zero. For gratings of moderate to large periods compared to wavelength, 
such as 16 .mu.m to .lambda.=633 nm as used by the ASML alignment system, 
either the opaque line can be made by materials (e.g., the ones used for 
attenuated phase shifting mask) of partial transmissivity, or the Cr line 
can be made up of many fine Cr lines the below resolution limit. The 
latter is illustrated in FIG. 6, in which simulated aerial images of 
gratings with period=0.6 .mu.m, 0.4 .mu.m (wafer scale) and Cr lines of 
various widths are calculated. Thus, the partial transmissivity can be 
designed to suit resists of different bleaching characteristics. The 
smallest Cr width and its increment is 0.1 .mu.m (0.5 .mu.m on a 5.times. 
mask), which is within the capacity of modem E-beam writing equipment. 
Note that the ASML alignment system detects the first order diffraction 
efficiency from a grating on the wafer. Any optical system that detects 
non-zero diffraction efficiencies from a grating can be used in the 
practice of the invention. 
If the grating is of a large period, then instead of exposing a series of 
LIM marks the same mark should be reused to save real estate. This can be 
achieved since the accuracy of a modern stepper stage is below 10 nm, 
which is much smaller than the period. Thus, alignment accuracy of the 
stage moving back to the same mark is not a problem. 
While the invention has been described in terms of a single preferred 
embodiment, those skilled in the art will recognize that the invention can 
be practiced with modification within the spirit and scope of the appended 
claims.