Control of residual stresses in as-grown polysilicon by multi-layer deposition: The "MultiPoly" process

LPCVD polysilicon thin films deposited between ∼550 and ∼600°C have an equiaxed microstructure (resulting from crystallization of an initially amorphous deposit) and contain ∼200 MPa residual tensile stresses after deposition, whereas polysilicon films deposited above ∼600°C have a columnar microstructure and contain ∼300 MPa residual compressive stresses after deposition. Both types of films also contain stress gradients. We have grown films containing multiple layers of polysilicon ("MultiPoly") by cycling the growth temperature between 570 and 615°C. The multilayer films thus formed are comprised of alternating tensile and compressive layers, and by control of the thickness of the individual layers, the overall stress of the polysilicon can display any value between that of the tensile layer and that of the compressive layer. We have focussed on producing films with zero overall residual stress, as measured by wafer curvature, and have characterized the microstructures by transmission electron microscopy (TEM) and X-ray diffraction (XRD). Because of the stress gradients present in both layers, devices made from films with zero overall residual stress may show distortions after release. We have measured the stress gradients in each type of layer and can design films with zero overall stress and zero overall stress gradients. It is also possible to design films with any level of overall (tensile or compressive) stress but with controlled displacements upon release.