Optical Characterization and Process Control of Top Surface Imaging
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must be monitored as quickly as possible after the silylation process. A suitable measurement technique is non-destructive optical measurement. Optical differentiation of the unsilylated and silylated layers is possible due to the optical dispersion change with silicon incorporation. This
paper
describes
how the KLA-Tencor UV1280SE spectroscopic ellipsometer (SE) was used to monitor a resist silylation process developed by GENESIS.
11
Mask
EXPOSURE BY UV, E-BEAM
Resist
111
I--I
CROSSLINK BAKE
SILYLATION
DRY ETCH DEVELOP
Figure 1. Silylation Process Steps
THEORY Spectroscopic Ellipsometry Spectroscopic ellipsometry relies on the detailed analysis of the wavelength dependence of polarized light reflected from the sample. The reflected polarized light is incident on the detector after passing through an analyzer. In the rotating polarizer implementation, the polarization state of the incident light is continuously varied and the integrated intensity of the reflected polarized light at each wavelength is measured at the detector. The integrated intensities corresponding to a complete rotation of the polarizer are used to mathematically compute the standard ellipsometry parameters, Tan(y) and Cos(A) at each wavelength. These quantities are related to the two components Rp and Rs of the reflected polarized light, by the equation: Tan(,) exp(iA) = RP/Rs
(1)
where Rp is the component of polarization parallel to the plane of the incident and reflected beams, Rs is the component perpendicular to that plane. A mathematical regression analysis is performed between the measured Tan(y)-Cos(A) spectra and theoretical computation. The dielectric function is related to the optical constants by 6 = (n - ik) 2
(2)
where n is the refractive index, and k is the extinction coefficient.
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RESULTS Monitoring Thickness Using the UV-1280SE System The system used to perform the silylation process was the GENESIS STAR-200. The resist used in this study, MXP-7, was provided by the Microlithography Chemical Corporation. In a spectroscopic ellipsometry measurement on the UV-1280SE system, the theoretical ellipsometry spectra Tan(V) and Cos(A), are calculated based on the film stack and the dispersion of the individual layer. Through the regression of the theoretical spectra to the measured spectra, thickness and dispersion information can be 0.3 2 obtained. In this experiment, the harmonic oscillator model was C used to define the optical 1.75 C 02 "^F dispersion properties of the 0 0.2 .2 C resist films. Figure 2 shows 0 typical dispersion curves for 1.5 Z 'U (both the silylated and unn (un-silylated) .> 0.1 o0 0 silylated resist films. It is easy to W __ n (silylated) 1 .25 see that the most significant . k (un-silyMated) change of the dispersion occurs -- k (silyMated) w in the UV region. At a 1 wavelength of 633nm, the index 240 340 440 540 640 740 changes only slightly from 1.597 Wavelength (nm) to 1.539. •
An important role of the optical monitoring of the process is to predict thickness and material property change
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