Microstructure and properties of silicon nitride thin films deposited by reactive bias magnetron sputtering

Abstract

Silicon nitride (SiNx) thin films have been deposited by radio frequency (rf) magnetron sputtering of a silicon target in reactive nitrogen-argon atmospheres without intentional substrate heating. The influence of negative substrate bias Vs on the microstructural, compositional, chemical, mechanical, and optical properties of the SiNx films was systematically investigated. An extensive analysis of the films was carried out using ellipsometry, transmission electron microscopy (TEM), atomic force microscopy (AFM), Rutherford backscattering spectrometry, secondary ion mass spectrometry (SIMS), Fourier transform infrared (FTIR) spectroscopy, ultraviolet-visible spectroscopy, stress and chemical etch rate measurements. TEM and AFM studies revealed that films produced at low bias voltages had a porous columnar structure containing large void, typical of zone 1, but that films produced at higher bias voltages had relatively smooth surfaces with a highly condensed structure, typical of zone T. Both FTIR and SIMS analyses showed that an extremely small amount of hydrogen was contained in the SiNx films deposited at Vs over −75 V, resulting from the film densification by energetic bombardment. It was also found that the amount of argon incorporated in the film increased with increasing bias voltage, whereas the oxygen content decreased. As the substrate bias voltage was increased, the mechanical internal stress in the SiNx films became increasingly compressive and saturated at a value of about 1.8×1010 dyne/cm2 at higher bias voltages. This was found to be well correlated with the increased argon content and the film densification. The lowest etch rate in buffered hydrofluoric acid, approximately 72 Å/min, was observed with the application of a substrate bias of −50 V. A further reduction in etch rate could be achieved by annealing at 900 °C for 1 h in a N2 ambient. The optical band gap of the SiNx films varied from 4.85 to 4.39 eV depending on the bias voltage.