Influence of anneals in oxygen ambient on stress of thick polysilicon layers

The stress and stress gradient in thick polysilicon layers used for high volume production of surface micromachined sensors depend strongly on the presence or absence of oxygen during high temperature annealing steps. This paper presents experimental results on the stress dependence applying various annealing processes. The stress and stress gradient can be changed reversibly by subsequent annealing steps in oxygen and nitrogen ambient, respectively, and this effect is independent of doping. The mechanisms responsible for this influence have been analyzed. It is concluded that the stress changes upon annealing can be attributed to the diffusion of oxygen atoms in and out of the thick polysilicon layers, as confirmed with secondary ion mass spectroscopy (SIMS) measurements. Applying this knowledge, with a careful selection of the annealing scheme, the stress state of the layers can be tailored according to the specific needs of the respective application. (C) 1999 Elsevier Science S.A. All rights reserved.