Investigation ofthe effect of processing steps on stress in a polysilicon structuralmembrane

Abstract

Stress control in low-pressure chemical vapour deposition(LPCVD) polysilicon is important for micromechanical processesinvolving surface micromachining. This paper outlines workdetermining the effect of processing steps on the stress in blanketfilms of sensor polysilicon after deposition, implant and annealprocessing. The measured stress in the blanket films was thencorrelated with interferometry images of the microfabricated pressuresensor devices after each processing step.There were small reductions in the measured tensile stress, in theblanket films, for a number of deposition steps. However, a reflowof the borophosphosilicate glass (BPSG) interlayer dielectric, at950 °C, caused the most significant change in stress,resulting in a transition from tensile to compressive forpolysilicon deposited at higher temperatures. The layer in contactwith the polysilicon during processing influenced the final measuredstress in the polysilicon blanket film. For devices withtetraethylorthosilicate oxide (TEOS) in contact with the polysiliconmembrane during processing, the blanket film was tensileafter sealing and compressive after processing. When the layer incontact with the polysilicon blanket film during processing wasLPCVD polysilicon or LPCVD nitride, the final measured stress was tensile.The measured blanket film values corresponded well to theinterferometry images of the fabricated pressure sensor devices.For a TEOS layer in contact with the polysilicon membrane duringprocessing, the devices exhibited a deflected, tensile membranedevice after sealing and a buckled, compressive polysilicon membranedevice after processing. When the layer in contact with thepolysilicon membrane during processing was LPCVD polysilicon orLPCVD nitride, the patterned devices were deflected, exhibitingtensile post-sealing characteristics. At the end of processing,these membranes remained deflected.