Strain studies in LPCVD polysilicon for surface micromachined devices

Polycrystalline silicon (polysilicon) has emerged as a preferred material f or surface micromachined MEMS applications because of its compatibility wit h standard CMOS process. The important parameters of polysilicon films for sensor-actuator devices are the residual stress and stress gradient. For fr ee standing microstructures, it is important to reduce the stress in the fi lm. In order to exploit the advantage of polysilicon for MEMS applications, it is essential to develop a process to obtain low-stress polysilicon film s. In the present work, we have investigated the effect of deposition param eters on 2-4 mu m thick LPCVD polysilicon films using a specially designed spiral structure for strain measurements. The films were deposited in the t emperature range of 580-630 degrees C at pressures 180 to 320 mTorr. The ru le of post-deposition annealing at 1000 degrees C in N-2 ambient on strain reduction has been investigated using conventional furnace annealing and ra pid thermal annealing (RTA). The as-deposited films show significant strain under all the deposition conditions investigated. The strain is reduced to insignificant values after 100 min of furnace annealing. In case of RTA, s imilar strain values are achieved in only 30 s of annealing. The overwhelmi ng superiority of RTA over furnace annealing in terms of the thermal budget of the process has been clearly demonstrated. The effect of polysilicon do ping with boron or phosphorus has also been studied for applications in ele ctrically conducting microstructures. The final strain values after anneali ng are about 1 x 10(-4), which is the limit of resolution of the strain mea surement scheme used in the present investigations. (C) 1999 Elsevier Scien ce S.A. All rights reserved.