Innovative application of the RCWA method for the ultra-sensitive transmittance-based CD measurements on phase-shift masks

Abstract

ABSTRACT A novel scatterometry method, based on broadband m easurements of reflectance and transmittance spectra is presented. For the first time Rigorous Coupled Wave Analysis (RCWA) algorithm is applied to the analysis of the transmittance spectra for the determination of trench depths, critical dimensions, profiles, film thicknesses, and optical properties (n and k spec tra from 190 – 1000 nm) of phase-shift photomasks. It is shown that very small structural and/or material variations, which are difficult to detect with reflectance (R) only measurements, can be readily distingu ished with transmittance (T) measurements. For the current study, a spectrophotometer-based instru ment (n&k R-T Scatterometer) was used, capable of collecting four continuous spectra during one measurement – two polarized reflectance spectra (R s and R p ) and two polarized transmittance spectra (T s and T p ). The light source of the spectrophotometer was equipped with a rotating polarizer, facilitating TE and TM polarizations of the measurement beam. The analysis was performed using Forouhi-Bloomer dispersion equations, in conjunctions with RCWA algorithm, applied simultaneously to reflectance and transmittance spectra. The method provided accurate and repeatable results of above stated parameters, for all materials present in the structure. A linearity study based on this novel reflectance-transmittance scatterometry method, demonstrated excellent correlation with the target values and the conventional CD-SEM measurements and improved repeatability compared to the traditional reflectance- only measurements. The ad vantages of the method are high throughput, non-destructive natur e of the measurements, and capabilit y to measure a wider variety of structures pertinent to the photomask manufacturing process. Keywords: Optical metrology, transmittance measurements, critical dimensions, broadband reflectometry, Forouhi-Bloomer dispersion equations, RCWA, CD linearity, repeatability.