Design and Operation of ETA, an Automated Ellipsometer

The design and operational features are described for a computer-assisted ellipsometer (called ETA for Ellipsometric Thickness Analyzer), developed to provide reliable, real-time measurement of field-effect transistor gate insulator thickness in a manufacturing environment. ETA illuminates the sample with light of fixed polarization and uses a rotating analyzer to measure the polarization of the reflected light. Sample alignment is done automatically by ETA, so that usually no operator adjustments are required. Fourier analysis of the light transmitted by the analyzer is used to reduce noise and enhance measurement precision. In its normal mode of operation (incident light linearly polarized at 45°), ETA can measure single and double-layer films of SiO
2
and Si
3
N
4
in the thickness range of 300 to 800 Å with precision comparable to that of conventional ellipsometers. Other modes of operation, which make use of a fixed-position compensator in the incident light path, allow precise measurement of thin films (0 to 300 Å) and permit use of ETA as a general-purpose ellipsometer. The typical time interval required for wafer alignment, data acquisition, analysis and recorded output of film thickness is about five seconds, and the measurement reproducibility is typically about 1 Å.