Measuring system of A, B, and C photoresist parameters

The concept of parameters A, B, and C of photo resist and outline of measuring theory and an instrument have been reported by Dill et al. in 1975. Since then, parameters A, B, and C have been used in simulation of profile or quality control of photoresist. In practice, however, a dedicated measuring instrument is hardly used and parameters are calculated from transmittance measured with a spectrophotometer instead. Dill et al. used a substrate of matched refractive index, but quartz substrate without matching its refractive index to photoresist usually is employed for convenience reasons. In this paper, experiments have been made to clarify the difference between Dill's authentic method and conventional method using a newly developed instrument (ABC-Analyzer) and a conventional spectrophotometer. Light of broad band from an Xe-Hg lamp was filtered through a narrow bandpass filter and exposed to a photoresist-coated sample. Then temporal change of transmittance was measured and stored in a computer. Parameters A, B, and C were determined automatically according to Dill's model using the transmittance data. Calculated parameters through two different procedures were compared. Parameters A, B, and C obtained by using the developed instrument were lower than those obtained by using a spectrophotometer. The difference comes from the difference in calculation method for transmittance. In other words, the transmittance measured with spectrophotometer is different from that based on Dill's definition. Furthermore, maximum differences in parameters A and B were 5 and 45 percent, respectively, between those obtained from the samples using an unmatched quartz substrate and a refractive index matched substrate. This is caused by multiple interference between reflected light from the photoresist/substrate interface and that from the back side of the substrate. It was confirmed that it is important to minimize this effect in order to improve accuracy in measuring A, B, and C.