Effect of mechanical stress on the traps in silicon nitride thin films

Abstract

Silicon nitride (SiNx) films were prepared by plasma enhanced chemical vapor deposition (PECVD) and different traps were induced in the films by tuning the RF power ratio during film deposition. Different initial stresses were applied to study the relationship between the mechanical stress and trap density. In-situ stress measurements showed that the film densification occurred above 400 °C during the first thermal cycling. The trap density extracted by the capacitance-voltage measurements showed its lowest value (−0.10 × 1011 cm−2eV−1) at the initial compressive stress (−730 MPa) of the film, which was explained by the hydrogen elimination due to the ion bombardment during the deposition in the compressive film. The increase in trap density (1.60 × 1011 cm−2eV−1) for the tensile films (480 MPa) is attributed to the lowering of the ion bombardment, which increases the formation of Si-H bonds. Interestingly, the performed carrier lifetime measurements for the compressive stressed SiNx films are higher (1200 μs) than the tensile stressed SiNx films (750 μs) which validate the trap density calculations. Fourier transform infrared spectra and refractive index measurements showed that the Si/N ratio increased with the increasing initial stress. Overall, we predict that our current study facilitates the formation of highly endured nitride films for different electrical and optoelectronic applications.