Engineering in- and out-of-plane stress in PECVD silicon nitride for CMOS-compatible surface micromachining

This paper presents notable improvements in the ability to control and distinguish the composite stress components within plasma enhanced chemical vapour deposition (PECVD) silicon nitride. Wafer curvature measurements complemented by stress structure fabrication and characterisation has enabled detailed analysis of in- and out-of-plane stress. Analytical modelling has allowed clarification of the relative contribution to the wafer curvature attributed solely to the stress gradient, which is of the order of 10-5 microns. Therefore the measured wafer curvature (due to composite stress), can be thought as a true representation of the actual wafer curvature due solely to the in-plane stress of the deposited thin film. This work represents a considerable advance compared with our previously published stress characterisation work on PECVD silicon nitride, which relied solely on wafer curvature measurements. However, the fabricated ring-beam and fixed-fixed structures were unable to resolve the in-plane stress component in high out-of-plane stress regimes. As predicted, at the zero stress gradient point, the fixed-fixed structures did measure an in-plane longitudinal compressive stress of -50MPa, which agrees well with wafer curvature measurements. Both stress components may now be repeatably controlled to realise tensile or compressive stresses (in-plane longitudinal) and positive or negative stress gradients (out-of-plane), by varying the RF deposition power. This new methodology allows for optimisation of the material for specific applications and in addition enhances the accuracy of micromechanical device models.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.