THE INFLUENCES OF ROUGHNESS ON FILM THICKNESS MEASUREMENTS BY MUELLERMATRIX ELLIPSOMETRY

The accuracy of measurement of the thickness of uniform thin films on solid substrates by null ellipsometry is severely limited when the sub strate is rough. It is impossible to separate these two effects experi mentally with the null ellipsometer, and there is no theoretical basis or generally used model available to separate these effects. Thus, a dual rotating-compensator Mueller matrix ellipsometer has been constru cted to carry out film thickness measurements on rough substrates. Mea surements were made on a set of specially prepared specimens of 8630 s teel, roughened by grit blasting with aluminum oxide. Grit sizes and b lasting pressures were varied to produce 11 different roughness values ranging from 0.01 to 1.295 mu m R(a), as measured with a stylus trace r device. Upon each of the 11 roughness groups, films of magnesium flu oride were overlaid to thicknesses of 89, 180, 254, and 315 nm. One se t of specimens was left uncoated. Experimental results for film thickn ess measurements on rough surfaces matched the ideal (for smooth surfa ces) form well for roughnesses up to 0.13 mu m R(a), at most angles of incidence. For rougher specimens, significant deviations in results w ere observed for all but the largest angles of incidence. The nonideal data were attributed to the cross-polarization effects of surface geo metry, and apparent depolarization. The resolution of thickness measur ements was 1 nm for polished specimens, and decreased continuously to 10 nm for the roughest specimens examined.