Polysilicon thin film process

A method of forming a thin polysilicon diaphragm comprising the steps of:

• (a) depositing silicon nitride in a layer on a front surface of a base of single crystal silicon having front and back parallel surfaces;
• (b) depositing silicon in a film onto the surface of the silicon nitride layer by decomposition of silane at substantially 580.degree. C. or less under conditions such that the film deposited has substantially uniform fine structure;
• (c) depositing another layer of silicon nitride over the silicon film deposited on the front surface of the substrate;
• (d) annealing the substrate and the films deposited thereon at a temperature between about 650.degree. C. and 950.degree. C. and for a time such that the strain in the deposited polysilicon film is at a selected tensile strain;
• (e) exposing the single crystal silicon of the substrate on the back surface of the substrate in an area underlying the polysilicon film which will be the desired area of the diaphragm;
• (f) anisotropically etching the exposed area of the back surface of the substrate and terminating the etch on the silicon nitride layer deposited on the front surface of the substrate to provide a free diaphragm which includes the polysilicon film.

A method of making strain controlled polysilicon film diaphragms or beams comprising the steps of:

• (a) providing a substrate having an unannealed film of amorphous or polycrystalline silicon formed thereon, the film having a structure such that when the film is annealed the average grain size of the crystals will be no longer than about 300 Angstroms;
• (b) annealing the film by heating the film and substrate at a temperature between about 650.degree. C. and 950.degree. C. for a time selected to yield a desired tensile strain level in the film;
• and
• (c) etching away the substrate under a portion of the annealed film so that that portion forms a free film which is in tensile strain and is supported at its edges by the substrate.