Polarized transmittance-reflectance scatterometry measurements of 2D trench dimensions on phase-shift masks

For the first time, polarized broadband transmittance (T) plus reflectance (R) measurements, combined with the Rigorous Coupled-Wave Analysis (RCWA) and the Forouhi-Bloomer dispersion equations for n and k, were used to measure 2D trench dimensions. This is in contrast to traditional scatterometry, which is based on reflectance-only measurements. T and R were measured from 190 to 1000 nm in one-nanometer intervals. Inclusion of the transmittance measurements proved to be advantageous, because there is a greater sensitivity of the T spectra to the sub-nanometer structural and/or material variations, which are difficult to detect with R-only measurements. Furthermore, the intensity of T is much higher than the intensity of R, resulting in a much improved signal-to-noise ratio, since intensity is proportional to number of photons reaching the detector, which in turn is proportional to the signal. Thus, the higher the intensity, the higher the signal-to-noise, and the better the repeatability and reproducibility of the results. For the current study, 2D arrays of square and circular contact holes of various pitches were measured on an After-Clean-Inspection (ACI) phase-shift mask, using a spectrophotometer-based instrument, capable of collecting four continuous spectra during one measurement - two polarized reflectance spectra (R-s and R-p) and two polanized transmittance spectra (T-c and T-p). The measured spectra were analyzed using the Forouhi-Bloomer dispersion equations, in conjunctions with RCWA algorithm, applied simultaneously to R and T polarized spectra. The method provided accurate and repeatable results for contact hole depths, critical dimensions film thicknesses and n and k spectra. High-resolution uniformity maps were obtained for all the parameters mentioned above.