Matthias Fuertsch

Comprehensive study of processing parameters influencing the stress and stress gradient of thick polysilicon layers

Thick polysilicon layers (greater than 10 micrometer), grown in an epitaxial reactor, are highly desirable for surface micromachining applications. The mechanical properties of these layers were studied extensively by characterizing the stress and stress gradient. The stress profile and texture were insensitive to variations of deposition parameters both of the polysilicon seed layer and the epitaxial process, and were influenced to a small degree by doping with phosphorous. Annealing in oxygen ambient resulted in compressive stresses up to 80 MPa and stress gradients of -10 MPa/micrometer. Subsequent annealing in nitrogen reduced the stress and stress gradient, but it can be reversed by re-annealing in oxygen. A model based on diffusion of oxygen is presented explaining the influence of the annealing on stress. Oxygen atoms diffuse into polysilicon during an anneal in the oxygen ambient, introducing compressive stress. Upon annealing in the nitrogen ambient, oxygen is released from the polysilicon layers due to the partial pressure of oxygen at the annealing temperature. The diffusion of oxygen atoms out of the layers results in a partial reversal of the mechanical effects. This insight gives the possibility to tailor the stress of thick polysilicon layers within certain limits to the specific needs of the application.