Photoélasticimétrie infrarouge assistée par ordinateur : Mesure des contraintes résiduelles dans le silicium polycristallin

Abstract

The polysilicon ribbon (E.P.R.) produced by electron bombardment of silicon powder in vacuum show high residual stresses which are damaging for the electrical properties; moreover, the brittleness of the ribbons are incompatible with the thermal treatments used in the solar cells elaboration process. Then, it becomes necessary to know the values and the repartition of the ribbon stresses which depends upon the pulling conditions. Our studies are divided into two processes; that is, thermal gradient measurement upon the ribbons during their elaboration with opticalpyrometry or I.R. thermography and stress spreading analysis after crystallization.This paper presents the new method used for stress analysis. The measurements are relative to the first order stresses and the infrared photoelasticity is used with the technique of digital image processing so that the collection, calculation, and analysis of the photoelastic fringe patterns can be automatically realized. A microcomputer calculates the values at any point of the sample for obtaining finally a pseudocolor imaging of the spreading of the residual stresses in the ribbons. The optical absorption coefficient a of silicon is of the order of 20 cm–1 at 1.06 µm ; this value leads to a “transparency” compatible with the photoelastic measurements by transmission and to sufficient sensitivity of the silicon vidicon or C.C.D. camera tube. Moreover, the relative stress-optic coefficient C of silicon is of the same order as that of plexiglas (20 Brewsters); so, it is possible to observe the isoclinic and isochromatic fringe patterns with a good degreee of resolution.