Optical metrology of thick photoresist process for advanced 3D applications

In semiconductor and Micro-Electro-Mechanical Systems production, thick resist films are indispensable for circuit formation, chemical mechanical planarization, or wafer-level packaging applications. We investigate multiple metrology protocols that aim at meeting the needs for fast, accurate, full-wafer determination of the thickness of up to 100 μm thick resist layers deposited on silicon or glass wafers. We found very effective to use a two-step strategy consisting of: i/ an accurate determination of the optical characteristics of the photoresist based on prism-coupling technique, and ii/ the fast, full-wafer measurement of the wafers of interest, using high-resolution spectroscopic reflectometry (SR). The robustness of this strategy increased when analyzing the SR data with a Fourier transform (FT) based algorithm which combines FT-based filtering around the thickness of interest with spectral matching to improve measurement performance.