Spectral interferometry for thin film information by fitting Fourier transform spectrum

As the semiconductor industry grows, sub-micron films are becoming more important and more versatile. Characterizations of film thickness is essential in order to determine the optimum process conditions to minimize defects on the product. Spectral interferometry can directly obtain axial information about the sample. However, if the optical thickness of the film is less than 1 μm, the frequency domain information of the upper and lower surfaces overlap each other and cannot be separated, making it impossible to introduce the film thickness. We propose a spectral interferometry method for obtaining thin film information by fitting Fourier transform spectra to the thickness of thin films up to 1 μm. We built a Michelson-type interferometric structure and measured to obtain the interference spectrum between the reflected light of a thin film sample on a silicon substrate and a reference mirror. The Fourier transform spectrum reflecting the axial information was obtained by using the Fourier transform of the interference spectrum, and then the optical range difference between the sample and the reference mirror was obtained by peak finding. Further, a fitting model of the Fourier transform spectrum is obtained. Finally, the film thickness is obtained by minimizing the error function. We used INTERCONNECT to simulate the system and verified that the scheme is effective. In addition, we measured several film samples with different thicknesses, which are consistent with commercial ellipsometry measurements.