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K. VEDAM(a)*

, P.J. McMARR(a)

and J.M. BENNETT(b)

(a) Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802; and (b) Michelson Laboratory, U.S. Naval Weapons Center, China Lake, CA 93555 ABSTRACT

Well characterized rough surfaces of aluminum films have been studied by the nondestructive technique of Spectroscopic Ellipsometry (SE). The roughness of the aluminum specimens had been characterized earlier by Total Integrated Scattering and Stylus Profilometry techniques to obtain numerical estimates of rms roughness and autocovariance lengths. The present SE measurements on these specimens were carried out at a number of angles of incidence in the range 30-800 and at a number of discrete wavelengths in the spectral range 300-650nm. The SE results were then analyzed by the scalar theory of diffraction from random rough surfaces by treating the surface as a simple random rough surface. The results of such analyses of the SE measurements are compared with the results of the earlier characterization techniques. INTRODUCTION

Optical technologies today require a detailed knowledge of the surface properties of the materials employed in critical components. These surfaces can, and often do, have surface roughness that can adversely affect the performance of an optical system. There is need to be able to measure surface roughness in the order of angstroms rms with high accuracy. Bennett [1,21 has recently reviewed available instruments which can measure surface roughness. These fall under two general types: noncontact interferometric and contact stylus instruments. Smooth optical surfaces usually have isotropic random roughness [3-5] and the height distribution they exhibit is often taken to be a Gaussian function [6,7]. Under these conditions, reflectometry techniques can be used to characterize these surfaces. In this study we have used Spectroscopic Ellipsometry (SE) as well as a stylus-type surface profiling instrument and total integrated scattering (TIS). The SE measured parameters A and P were analyzed using the scalar scattering theory of Ohlidal and Lukes (OL) [8,9]. This analysis yields the rms surface slope, tano, with respect to a smooth reference standard. The rms surface roughness, a, can be estimated from the measured TIS using the 2 relation TIS=l-exp(-4rra/%) . Here X is the wavelength of light. These two parameters, tano and a, are related by the expression (10) tano= V"2(a/T), where T is the autocovariance length. T can be derived [6] from the autocovariance function, G(x), which itself is calculated from the measured surface profile as discussed below. The results of these different techniques will be presented here and discussed for the case of 15 Al thin films with different degrees of surface roughness. EXPERIMENTAL PROCEDURE

Samples 38.6mm in diameter and -9.5mm thick made of Zerodur, fused quartz, float glass, or dense flint were optically polished using a variety of techniques (fresh feed or bowl feed pitch polishing, felt lap polishing) and different types a