Measurements of Corner Rounding in 2D Contact Holes on Phase-Shift Masks Using Broadband Reflectance and Transmittance Spectra in Conjunction with RCWA

For the first time, Rigorous Coupled-Wave Analysis (RCWA) is used for the analysis of both polarized broadband reflectance and transmittance spectra with the purpose of measuring the degree of corner rounding in 2D contact holes. The use of transmittance spectra proves to be advantageous for the characterization of the shape of the contact holes. In contrast with the conventional reflectance-only techniques, transmittance measurements prove to be more sensitive to the angstrom-level variations in the shape of the contact hole. Therefore, the new technique is capable of accurately determining the degree of rounding of the contact hole corners and characterizing a variety of shapes from perfectly round to perfectly square. Additionally, the high intensity of the transmitted spectra improves the signal-to-noise ratio and guarantees better repeatability of the results. For the current study, 2D arrays of square contact holes with 800 nm pitch are 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 (Rs and Rp) and two polarized transmittance spectra (Ts and Tp). The measured spectra are analyzed using the Forouhi-Bloomer dispersion equations, in conjunction with RCWA. The method provides accurate and repeatable results for the degree of corner rounding of the square contact holes. In addition, the method provides trench depth, critical dimensions, film thickness, and optical properties (n and k spectra from 190 – 1000 nm) of phase-shift photomasks. The results of the measurements are represented as high-resolution uniformity maps obtained for all the parameters mentioned above. The results show excellent correlation with conventional CD metrology techniques.