The Relationship between Intrinsic Stress of Silicon Nitride Films and Ion Generation in A 50 KHz RF Discharge
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THE RELATIONSHIP BETWEEN INTRINSIC STRESS OF SILICON NITRIDE FILMS AND ION GENERATION IN A 50 kHz RF DISCHARGE
K. AITE*, J. HOLLEMAN*, J. MIDDELHOEK* AND R. KOEKOEK** *University of Twente, ICE Lab., P.O.B. 217, 7500-AE Enschede, Netherlands. Netherlands.
The
**Tempress BV., Marconistraat 14, 7903-AG Hoogeveen, The
ABSTRACT Silicon nitride films prepared by the Plasma Enhanced Chemical Vapor Deposition 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 N2/NH3 /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 N2 /NH3 /SiH 4 discharge was the partial pressure variation with ithth 2ntesit pak vaiaton used for measuring the N+ peak intensity 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.
Mat.Res. Soc. Symp. Proc. Vol. 130. c1989 Materials Research Society
348
INTRODUCTION PECVD silicon nitride is a hard material, impermeable to mobile ions, mainly used for passivation and scratch protection in the field of integrated circuits fabrication [1]. Therefore, mechanical stability of PECVD silicon nitride films is an essential property. Large internal stresses are the cause of irreversible degradation of the film which can occur during deposition or after subsequent annealing steps. High stress can cause cracking of the nitride film. A strong compressive stress results in voids and cracks in the aluminum lines. It is well known that the internal stresses of PECVD silicon nitride films are related to the discharge frequency [2-3], and are generally compressive or tensile when deposited at low or high frequencies, respectively. The mechanical properties of thin films can be altered by the implantation or incorporation of impurities during or after growth [4,5]. This work shows the influence of nitrogen incorporation to the SiH 4 -NH3 gas mixture in a low frequency (50 kHz) rf discharge on the intrinsic stress of the resulting silicon nitride films. The increase of the intrinsic stress due
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