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Linewidth measurement of photoresist structures continues to be an integral part of the wafer fabrication process for exposure and development control of all critical process steps. The introduction of 1 pm geometries into a routine production environment, such as the Hewlett-Packard NMOS III VLSI processl, poses formidable measurement problems. Traditional brightfield linewidth measurement techniques, which were adequate for circuit geometries of 3-4 μm and above, have been generally unacceptable for 1 pm complex structures. The principal limitation is that the reflected image profile depends not only on the resist line parameters but also on the optical properties of the substrate and all transmissive sub-layers. As a consequence, even a carefully optimized brightfield measurement system will measure identical resist lines differently due to normal process variations in the substrate and subsurface layers. In-process SEM measurements can be functionally adequate but our experience in routine wafer fabrication has demonstrated severe drawbacks with respect to cycle time, reliability, operator skill and space requirements. To overcome these problems a new technique using fluorescence was investigated. The fluorescence principle was successfully reduced to practice on PMMA photoresist lines, incorporating a bleachable dye (Coumarin 6). The PMMA is the bottom layer resist in a two-layer production photoresist process described by K. Bartlett, et al. 2. The fluorescence technique has all the advantages commonly associated with optical measurements and in addition it virtually eliminates the measurement variations due to the sub-strate and sub-layers. The paper will cover: 1. The parameters of the sample and measurement systems that have an impact on linewidth determination. 2. The experiments and statistical methods to systematically identify problems and optimize critical dimension measurements. 3. The results obtained with brightfield optical metrology. 4. The theory and advantages of fluorescence metrology. 5. The results obtained on production VLSI wafers using fluorescence measurement of critical dimensions
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