Local Stress Measurement in Thin Thermal SiO2 Films on Si Substrates

A novel method for local stress determination in SiO2 has been developed. The method consists in etching of SiO2 in a manner to create an oxide bridge and separating SiO2 from silicon in the bridge area. In this way, released oxide allows strain calculation and determination of compressive stress in the direction of the longitudinal axis of the bridge. Stress calculations limited to an area as small as 4×10−6 in. 2 have been obtained. The limiting factors to the accuracy of stress determination are the use of a bulk quartz modulus of elasticity, and the accuracy of the measurement of bridge deflection. The thinner the SiO2 film, the higher the accuracy of strain determination. Local stress as high as 1.78×1010 dyn/cm2 is detected in 272-Å-thick film. The compressive stress determined by this method indicates strong thickness dependence in the films below 1500 Å; the thinner the thermal oxide, the higher the average composition stress to its Si/SiO2 interface value. The method is particularly useful for the oxide films below 1000 Å. Evidence has been obtained that, in very thin thermal SiO2 films, crystal orientation of compressive stress does exist.