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Optically Measuring Stress in Thin Films

IP.com Disclosure Number: IPCOM000048372D
Original Publication Date: 1982-Jan-01
Included in the Prior Art Database: 2005-Feb-08
Document File: 2 page(s) / 24K

Publishing Venue

IBM

Related People

Kaus, G: AUTHOR [+4]

Abstract

Magnitude and orientation of stress in thin transparent films are determined from stress-induced birefringence.

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Optically Measuring Stress in Thin Films

Magnitude and orientation of stress in thin transparent films are determined from stress-induced birefringence.

Illumination beam 1 is linearly polarized in polarizer 2, passes a lambda/2- plate 3 and beam splitter 4 to impinge on substrate 5 covered by a thin transparent film 6 (e.g., Si with an SiO(2) top layer). If birefringent, film 6 splits beam 1 into an ordinary and an extraordinary beam that are phase-shifted relative to each other according to the value of the birefringence and the thickness of layer 6. The reflected beam is directed by beam splitter 4 to an electro-optic crystal 7, compensating the phase difference after an appropriate voltage has been applied. If properly compensated, the output of light detector 9, arranged behind a polarizer 8 is at a minimum. Details of the compensation method are described in German Patent 2851750.

To determine the orientation to the principal axes of birefringence and hence for stress, lambda/2-plate 3 is oriented such that no modulation is obtained at detector 9. The ratio of the two principal stresses is obtained by measuring the detector output after rotating lambda/2-plate 3 by 45 degrees.

The optical method allows an easy determination of stress in thin and ultra- thin films that is very difficult to measure otherwise (based on X-ray diffraction or on the mechanical bending of the substrate due to film stresses).

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