A Special Case of Using Ellipsometry to Measure the Thickness of Oxide on Polysilicon: II . Application

Abstract

Experimental results presented in this work verify the application of a visible light single wavelength ellipsometric method for measuring the thickness of oxide on polysilicon (poly-ox). The method requires a special test structure with a thin oxide under the poly. These results confirm that the uncertainty of the poly-ox thickness determination is approximated by the thickness of the oxide under the polysilicon. Test structures were generated with 100A oxides under 5000A polysilicon. With the proper choice of the values of A ~ and ~~ for the film-free surface, the poly-ox thickness determined by ellipsometry using the single-film approximation agreed within 100A of the thickness measured by Auger electron spectroscopy. In the semiconductor manufacturing environment, poly oxidation processes are usually monitored with singlecrystal test wafers. Since the differential oxide thickness on poly relative to single-crystal depends on the several process conditions (poly deposition, doping, annealing, oxidation), the single-crystal monitors do not accurately represent the results on the polysilicon. The new method permits measurement of poly-ox thickness directly after oxidation and therefore accounts for the interaction of the previous process conditions with the conditions of oxidation that is so important to the resultant poly-ox thickness.

A Special Case of Using Ellipsometry to Measure the Thickness of Oxide on Polysilicon: I . Theoretical Considerations

“…Measurements made by us in the manner described in Ref. (2) indicate that a value ofN = 3.80 -0.355j is appropriate. Palik, "Handbook of Optical Constants of Solids," Academic Press, Orlando, FL (1985), reports an imaginary part of the index to be 0.36 for amorphous silicon.…”

“…We shall also discuss what affects the degree of uncertainty. The accompanying work (2) presents experimental measurements demonstrating that this method actually works with a degree of uncertainty acceptable in many applications.…”