Design and Operation of ETA, an Automated Ellipsometer

Abstract

The design and operational features arc 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 Si0 2 and Si 3N, in the thickness range of 300 to 800 A 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 A)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 I A.472

Ellipsometry and Its Applications in Stoichiometry

“…The modern type photometric ellipsometry technique, with the application of the stepper motor, photodetector and computer control system, appeared in early 1970s [19,20]. This type of ellipsometry technique no longer needs to adjust the polarizers in the light path to a specific azimuth and make the light reach the detector for a particular polarization.…”

Conventions and formulas for using the Mueller-Stokes calculus in ellipsometry

“…Although both the x-to-y [10,11] and y-to-x [3,9] ratios are found in the literature, the latter appears more frequently and is adopted here. Following the notation of Azzam and Bashara [9] -6-3.…”

The influence of cell window imperfections on the calibration and measured data of two types of rotating-analyzer ellipsometers

“…The rotating analyzer is speed-controlled [4] with an accuracy within 5 ppm at an angular frequency of 66.6 Hz. When using the intensity method, the periodic analog detector signal is converted into digital form on sampling instants given by a general timing circuit [2,3]. In a special computer program these converted intensities are averaged and Fourier-analyzed and the ellipsometric A and \1/ values are calculated from the two Fourier coefficients, a and b.…”

“…[2]. More complicated expressions, taking into account small polarizer imperfections, the calibration azimuth of Arot, and errors introduced by using a low-pass filter have been developed by Aspnes [9].…”

“…The polarizer-sample-rotating analyzer (P&Irot) and the polarizer-sample-rotating analyzer-fixed analyzer (PSA,,~) ellipsometers belong to the subgroup with a rotating analyzer. The first measures phase and irradiance (intensity method) [2,3], the second two phases (phase method) [4]. The effect of azimuth-angle errors, component and cell window imperfections on the ellipsometric parameters has been reviewed by Azzam and Bashara [S].…”