Lithography
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Spectroscopic Critical Dimension (SCD) Metrology for CD Control and Stepper Characterization Authors: John Allgair, Motorola, APRDL & Pedro Her rera, KLA-Tencor Corporation Co-authors: R. Hershey, L. Litt, D. Benoit; Motorola, APRDL, A. Levy, U. Whitney; KLA-Tencor Corporation
Smaller device dimensions and tighter process-control windows have created a need for CD metrology tools to detect and measure changes in feature profiles that are becoming critical to inline process control and stepper evaluation for sub-0.18 µm technology. Spectroscopic CD is an optical metrology technique that can address these needs. This work describes the use of a spectroscopic CD metrology tool to measure a sub-0.18 µm gate level focus and exposure matrix in order to characterize the lithography process window. The results include comparison to the established inline CD SEM, as well as profiles from a cross-section SEM. Repeatability, long-term stability, and matching data from a gate-level nominal process are also presented.
Introduction
Smaller device dimensions and tighter process control windows have created a need for metrology tools that measure more than just one-dimensional critical dimension (CD) features. The need to easily detect, identify, and measure changes in feature profiles is becoming critical to controlling current and future semiconductor lithography and etch processes. Measuring changes in sidewall angle and resist height, as well as detecting subtle phenomena such as line-rounding, t-topping, and resist footing, is now as important as the traditional CD line-width measurement. This additional profile information can be used to enhance process-control mechanisms and can also be used to evaluate and characterize the performance of a stepper/track module. Traditional CD metrology techniques give no indication of a measured feature’s sidewall angle or height. Spectroscopic CD is an optical metrology technique that can address these needs. SCD is based on spectroscopic ellipsometry (SE), an accepted and 50
Fall 2001
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widely used optical technique for measuring film thickness and film properties. This work describes the SCD measurement technique and its use in measuring a sub-0.18 µm gate-level focus and exposure matrix to characterize the lithography process window. SCD results are compared to results from a CD SEM and a cross-section SEM to determine if SCD is able to measure accurately the feature behavior through changes in focus and exposure. Furthermore, SCD is used to monitor features outside the process window to determine if it can detect and identify out-of-control process conditions. Repeatability, long-term stability, and matching data from a gate-level nominal process are also presented. These repeatability and stability tests were performed to verify SCD meets the roadmap requirements for current and future semiconductor processes. SCD Measurement
The SCD measurement technique is summarized in Figures 1 and 2. Gratings on the production wafers