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We have studied the influence of different process parameters on the composition and mechanical properties of silicon-nitride layers deposited in a SiH4-NH3-N~ plasma. The layers were deposited at temperatures between 300 ~ and 600~ at total pressures between 65 and 195 Pa and RF frequencies between 0.1 and 20 MHz. The Si/N ratio and the density of the as-deposited layers were determined by means of RBS. IR spectroscopy was used to measure the hydrogen content and Si-H/N-H ratio of the plasma nitride layers. It is shown that hydrogen content, density, and mechanical stress of plasma silicon-nitride layers strongly depend on deposition conditions. To explain the experimental data, a simplified reaction model is given. With this model, differences in densities, hydrogen content, and stress can be related to cross-linking reactions and to the extent of ion bombardment which takes place at different excitation frequencies.Plasma silicon-nitride layers are widely used in the IC technology for passivation of devices. The layers act as barriers against the diffusion of impurity ions such as sodium, and they are also used for scratch protection during mounting operations. The electrical and mechanical properties of plasma silicon-nitride layers depend strongly on composition, density, and mechanical stress and, therefore, on the deposition conditions. Composition and stress measurements of plasma silicon-nitride layers have been reported (1-4), but attempts to make a more systematic study and quantitative evaluation of the results are relatively, scarce. Therefore, a systematic experimental approach was applied to study composition, density, and stress of plasma silicon-nitride layers prepared under various deposition conditions.In this paper, we report on the influence of deposition temperature, gas pressure, gas phase composition, and RF frequency on the composition, density, and stress of plasma silicon-nitride layers. The experimental data will be described by a simplified reaction model.

The authors have studied deposition of plasma silicon nitride layers as a function of gas-phase composition for the SiH4-NH3-x system, where x is N~, Ar, or H2. Most of the depositions were performed at 300~ and at an operating frequency of 50 kHz. The deposited layers were characterized with Anger electron spectroscopy in combination with ion-bombardment, Rutherford backscattering, and infrared spectroscopy. It is shown that the ratio of the amount of atomic hydrogen atoms bonded to silicon and nitrogen atoms is only a function of the silicon-nitrogen ratio of the deposited layers. It is further shown that the etch rate of plasma silicon nitride layers in buffered HF depends primarily on the silicon-nitrogen ratio and the density of the deposited material.

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