| 摘要 |
A method and apparatus for optically determining a physical parameter of an underlayer such as the underlayer refractive index Nu, extinction coefficient ku and/or thickness tu through a top layer having a first top layer thickness t1 and an assigned refractive index index nt and coefficient of extinction kt. The values of index nt and extinction coefficient kt can be estimated, optically determined or assigned based on prior knowledge. In a subsequent step a first reflectance R1 is measured over a wavelength range DELTA lambda by using a test beam spanning that wavelength range. Then, a second reflectance R2 of the top layer and underlayer is measured using the test beam spanning wavelength range DELTA lambda at a second top layer thickness t2. In a calculation step the physical parameter of the underlayer is determined from the first reflectance measurement R1, the second reflectance measurement R2, and the assigned or predetermined thickness values t1, t2, and the refractive index nt. and coefficient of extinction kt of the top layer. A dispersion model can be used in this calculation step. Alternatively, with transmissive samples, a first and second transmittance T1, T2 can be used.
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