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Title: Real-time control of photoresist thickness uniformity during the bake process
Authors: Lay, Lee Lay
Schaper, Charles
Khuen, Ho Weng 
Issue Date: 2000
Citation: Lay, Lee Lay,Schaper, Charles,Khuen, Ho Weng (2000). Real-time control of photoresist thickness uniformity during the bake process. Proceedings of SPIE - The International Society for Optical Engineering 4182 : 54-64. ScholarBank@NUS Repository.
Abstract: As the line-width goes sub-100 nm, process windows for all lithographic processes become smaller. Process control and monitoring of the process parameters become increasingly important and necessary as small variation in process variables such as exposure dose, temperature, resist thickness, developer concentration, etc. may cause the final critical dimension to differ from the specification. In this paper, process monitoring and control during the softbake process is implemented with an array of thickness sensors and a multi-zones thermal module. Analyzing the oscillatory reflectance spectrum over a spectrum of 450-850 nm using a multi-wavelength spectrometer, resist thickness loss during the bake process is monitored in real-time. A fast and efficient algorithm is implemented using Least Square Estimation for analyzing the reflectance signal. With these in-situ thickness measurements estimated at the rate of 0.1 seconds, thickness control during the bake process is made possible, provided that a thermal module can rapidly change temperature. Feedback control using model-based control strategy is used to improve thickness non-uniformity at the end of the bake process. This is achieved by manipulating the temperature distribution across the bakeplate to cause the different rates of solvent removal from the resist. With this strategy, the softbake process compensates thickness non-uniformity during the coating process. Convergence of resist thickness at two points is demonstrated. The resist thickness converges with a small thickness difference of a few angstroms by the end of the softbake process. On the average, there's a significant reduction in thickness non-uniformity when real-time thickness control is implemented. Resist thickness at three zones is controlled to converge to a target thickness with very small non-uniformity.
Source Title: Proceedings of SPIE - The International Society for Optical Engineering
ISSN: 0277786X
DOI: 10.1117/12.410103
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