The Relationship between Intrinsic Stress of Silicon Nitride Films and Ion Generation in A 50 KHz RF Discharge

Abstract

Silicon nitride films prepared by the Plasma Enhanced Chemical VaporDeposition technique (PECVD) are widely used in microelectronics. The intrinsic stress value of the silicon nitride films is a key factor which determines their reliability. A low frequency (50 kHz) RF discharge and a N 2 /NH 3 /SiH 4 gas mixture were used to deposit silicon nitride films on 4-inch silicon wafers in a horizontal hot-wall reactor at a temperature of 350 °C. The compressive stresses in the deposited films were found to increase linearly with the nitrogen ionization created in the glow discharge. The nitrogen ion bombardment at the film surface induces ion implantation into the deposited film which results in film expansion. The stresses were deduced by the interference fringes technique and from surface profiler measurements. Optical Emission Spectroscopy of the N 2 /NH 3 /SiH 4 discharge was used for measuring the N+ peak intensity variation with the 2 pak ntesit vaiaton ithth partial pressure of the nitrogen gas. When the nitrogen ionization was increased, a shift of the main infrared transmission peak corresponding to the Si-N bond in the silicon nitride films was observed using Fourier Transform Infrared Spectroscopy (FTIR). The Si-H and N-H infrared peaks in the films were not affected by the ion bombardment. A linear relationship was found between the wavenumber of the main infrared Si-N peak vM ( 835-875 cm" 1 ) and the intrinsic stress ai of the deposited nitride films with the slope dvM/doi -4.85x10" 8 cm' 1 /Pa.

LPCVD against PECVD for micromechanical applications

“…When stress-free films are needed the residual stress can be further compensated by ion bombardment or increase of the Si-N ratio. Ion bombardment by low-frequency plasma excitation [11,20,4] or ion implantation [10,11] and the increase in the Si-N ratio [13,12,16,17,14,15] causes compressive stress. Ion bombardment can be part of the PECVD process.…”

“…Some CVD film properties are also changed when the Si-N ratio is changed [14,12]. The refractive index can be varied from 1.8 to 2.6; the density [15] and the etch rate is also affected.…”