Stress in undoped LPCVD polycrystalline silicon

The effect of processing conditions on stress in undoped LPCVD polycrystalline silicon films was investigated. Films were deposited at temperatures between 605 and 700 degrees C and pressures from 300 to 550 mtorr, with varying silane flow rate and deposition time. Low temperatures produced tensile films, while temperatures greater than about 620 degrees C resulted in compressive stress. Film thickness and deposition pressure also affect the stress rate. By removing film layers with a plasma etch, the stress profile through the film thickness was determined. Tension results when silicon atoms deposit in the amorphous state and subsequently crystallize, but thermal stress, due to differences in expansion coefficient between the film and substrate, is ruled out as significant contributor to the film stress. By superposing the crystallization stress and a compressive intrinsic stress component that decreases with temperature, the observed stress trend is obtained.<>