Novel Technique for Critical Dimension Measurements of Phase-shift Masks Using Broadband Transmittance Spectra in Conjunction with RCWA

For the first time Rigorous Coupled Wave Analysis (RCWA) has been applied to the analysis of the transmittance spectra for the determination of critical dimension (CD) of phase-shift photomasks. The use of transmittance spectra proved to be instrumental in improving the sensitivity of the measurement to minor (sub-nanometer) changes in the width of the trench. We present a novel unique metrology solution based on the simultaneous measurement of broadband reflectance and transmittance, covering a wavelength range from 190 to 1000 nm, in one nanometer intervals. The analyses of both types of spectra are performed simultaneously, using Forouhi-Bloomer dispersion equations, in conjunctions with RCWA. The method provides accurate and repeatable results for critical dimensions, thickness, and optical properties (n and k spectra from 190 – 1000 nm) for all materials present in the structure. In the current study, the method described above was used to examine grating structures on ACI (After-Clean Inspection) phase-shift mask. The use of transmittance spectrum proved to be essential for the accurate measurement of the CD, since the transmittance spectrum is more sensitive to the change in line width, compared to the reflectance spectrum. The results were compared with the measurements taken on the same sample using conventional CD-SEM. The CD linearity study demonstrated excellent correlation with CD-SEM. The advantages of the optical reflectance and transmittance method are high throughput, non-destructive nature of the measurements and capability to measure a wider variety of structures pertinent to the photomask manufacturing process.