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The significant optical absorption of most currently available commercial single layer 193 nm resists, even at a coating thickness of 0.4 micrometer, implies increased sensitivity to process control fluctuations of the kind that negatively impact critical dimension (CD) uniformity, process latitude, resist sidewall profile, and line edge roughness. These problems, although less severe on reflective substrates, are particularly acute on wafers with bottom anti-reflection coatings (BARCs), which are useful in CD control. With different intensity of light reaching different levels in the resist film on a BARC, a gradient is thus established in the extent of the chemical amplification reactions on which semiconductor lithography is based. The result is the familiar sloped sidewall profile and poor CD uniformity after the resist is developed. Further, with most of the photoacid generators and the polymer resins in the 193 nm resists having very low quantum efficiencies and significant absorption at 193 nm, respectively, most of the absorbed light in the resist is used up in energy dissipative processes, instead of in generating photoacids which catalyze the chemical amplification chemistry of these resists. One approach to overcome this absorption problem is to use significantly thinner resist films, but etch considerations preclude such option as these materials do not have very good etch stability. The purpose of this paper is to quantitatively assess the impact of absorption on the process control of sub- 0.15 micrometer features patterned on a full field ASML 193 nm scanner, interfaced to a TEL MARK-8 track. Optical properties of different resist films/BARC stack combinations are characterized by UV spectroscopic ellipsometry and broad band spectrometry, and sidewall profiling is done by atomic force microscopy.
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