Optimization of a low-stress silicon nitride process for surface-micromachining applications

A detailed examination of the effects of deposition parameters, using LPCVD, and subsequent processing on the characteristics of silicon nitride is presented. The properties investigated are deposition rate, refractive index, etch rate and intrinsic strain. The chemical composition of the material is determined using XPS and EPMA. A close relationship between the chemical composition and mechanical properties is observed. The ratio of process gas flow (using NH3 and SiH2Cl2) is shown to have a strong effect on all properties with deposition pressure having a secondary effect. As the gas-flow ratio NH3/SiH2Cl2 is ranged from 0.176 to 1 the silicon content changes from Si/N = 0.95 to 0.86, yielding a change in strain levels from 350 mu epsilon to 3000 mu epsilon. Further increase in NH3 yields only minor changes in silicon to nitrogen ratio and thus only minor changes in the film characteristics. Additional thermal processing is shown to have a considerable effect on the mechanical properties of the material, X-ray studies suggest that this to be due to volume shrinkage of the layer and not phase transformations involving crystallographic changes. Tuning of the firm properties through the processing parameters is shown.