Thin film ellipsometry metrology

Abstract

A wide variety of commercial ellipsometers are available in the market today. They all measure the change in the state of polarization of light on reflection, but the measurement techniques adopted vary from instrument to instrument. Further, the models used to evaluate the thickness and refractive index of the oxide film during analysis of measurement data vary in complexity. The two main techniques of measurement are single wavelength ellipsometry and spectroscopic ellipsometry. The NIST Standard Reference Materials available today are based on conventional single wavelength ellipsometry. We discuss the challenges encountered in providing reference materials by using spectroscopic ellipsometry. First, the limits of conventional single wavelength ellipsometric determination of film thickness are investigated and then possible new technologies are explored. We present a discussion comparing the different types of instruments available and how their unique designs affect the accuracy of thickness determinations. Manufacturing, and accurate determination, of thickness of films this thin (<10 nm) is a challenging task. Results from independent ellipsometric measurements on two different types of instruments are compared for the case of ultra thin thermally grown silicon dioxide films on silicon crystal substrates. Stability curves for the thickness of thin dielectric films over a period of two years are also presented.