A Review of Thin-film Thickness Measurements using Optical Methods

This paper reviews earlier studies focusing on thickness measurements of thin films less than one micrometer thick. Thin films are a widely used structure in high-tech industries such as the semiconductor, display, and secondary battery industries. Typical non-destructive and non-contact techniques for measuring the thickness of thin films are spectral reflectometry (SR) and spectroscopic ellipsometry (SE). SR can measure the thin-film thickness with a simple layout. With the combination of SR and optical interferometry, the simultaneous measurements of thin film and 3D surface profiles or thick layer have been proposed and demonstrated. For an analysis and verification of SR, several works including artificial intelligence algorithms and uncertainty evaluations have been published. SE can measure thinner thicknesses with more information pertaining to the polarization state, incident angle, wavelength, and etc. According to the type, location, and number of elements that make up the basic optical layout, ellipsometers can be classified into five types. Based on a mathematical model of the ellipsometric transfer quantity, the operational principle and measurement procedure are discussed. To ensure measurement reliability, the uncertainty components of the SE were evaluated. With the development of high-tech industries in the future, thin-film thickness measurement techniques can be expected to find wider use with faster measurement speeds, a higher dynamic range, and better measurement reliability.