Chemistry of Nitrogen-Silane Plasmas
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CHEMISTRY OF NITROGEN-SILANE PLASMAS DONALD L.SMITH, ANDREW S. ALIMONDA and FREDERICK J. VON PREISSIG Xerox Palo Alto Research Center, 3333 Coyote Hill Rd., Palo Alto, CA 94304 ABSTRACT The N2 -SiH 4 rf glow-discharge plasma has been analyzed by line-of-sight mass spectrometry of species impinging on the deposition electrode, including N atoms. Properties of SixNyHz films deposited from this plasma have been examined. At high rf power and low SiH 4/N2 , almost all of the SiH 4 is consumed by reaction with N atoms at the film surface and becomes incorporated into the film. No Si-N precursor species are seen in the gas phase. This is in contrast to the NH3 -SiH 4 plasma, where the Si(NH 2)3 radical is the key precursor. If power is insufficient or SiH 4 flow is excessive, Si2Hm species are generated in the plasma. Under optimized conditions, films slightly N-rich with no Si-H bonding and only 7% H (as N-H) can be deposited at high rate. The film tensile stress of the NH3 process is absent in the N2 process due to the lesser amount of condensation that takes place during deposition. However, trench coverage is much better in the NH3 process. INTRODUCTION "Silicon nitride" thin film (SixNyHz) deposited by plasma-enhanced chemical vapor deposition (PECVD) has been studied for over two decades and is widely used in the semiconductor industry. Yet until recently, little was known about the plasma chemistry of the deposition process. Earlier this year, we reported the first analysis of the plasma chemistry of silicon nitride PECVD from ammonia and silane, the most commonly used reactant mixture [1]. In the present work, the nitrogen-silane process is analyzed using the same reactor and techniques, and is compared to the results for ammonia. Others have found that the N2 process results in films with much lower H content, with z = 5-10% vs. 20-30% for the NH3 process. Low mobile H content is important when the film is used as a passivant for MOS devices. However, the NH3 process is more widely used, because the N2 process has been found more difficult to control and to give low deposition rates [2, 3]. It will be seen below that these problems can be avoided when the plasma chemistry is understood and properly controlled. For comparison with the N2 process, our results for NH3 -SiH 4 will be reviewed first [1, 4]. When the plasma rf power is sufficiently high, both NH3 and SiH 4 become activated, and when there is sufficient excess NH3 , almost all of the SiH 4 reacts with it to form tetra-aminosilane, Si(NH 2 )4 , and the triaminosilane radical, Si(NH 2 )3 . The latter is the dominant precursor species for film growth. It decomposes on the surface and in a "condensation zone" beneath the surface in a process whereby an NH2 and a H from neighboring precursor radicals combine to form an NH3 molecule, which is evolved into the plasma. The resulting Si and N dangling bonds pull together to propagate the Si-N network, and this creates tensile stress. Films deposited under these conditions are N-rich (y/x>4/3), and the ratio of
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