Chemical Reaction Pathways for the Deposition of Amorphous Silicon-Hydrogen Alloys by Remote Plasma Enhanced CVD
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CHEMICAL REACTION PATHWAYS FOR THE DEPOSITION OF AMORPHOUS SILICON-HYDROGEN ALLOYS BY REMOTE PLASMA ENHANCED CVD GN PARSONS, DV TSU and G LUCOVSKY Department of Physics, NC State Univ, Raleigh, NC 27695-8202 ABSTRACT We have grown thin films of a-Si:H alloys by Remote Plasma Enhanced CVD (Remote PECVD) and have studied the deposition process by mass spectrometry. We find that the concentration of silane fragments (SiHx x=0 to 3) and higher silanes (e.g. disilane Si 2 H6 ) in the gas phase is below our detection limit of =0.5%. Bias experiments and a comparison of the a-Si:H deposition rate with the known concentration of silane in the gas phase suggest that in Remote PECVD, silane molecules, SiH4 *, vibrationally excited in the gas phase or on the deposition surface may lead dirtectly to a-Si:H film deposition. INTRODUCTION Amorphous silicon-hydrogen (a-Si:H) alloy films have been deposited by a variety of thermal and plasma enhanced techniques. These techniques include: 1) reactive magnetron sputtering (1-2] (MS), in which silicon atoms are sputtered in the presence of hydrogen from a silicon target using an argon plasma ; 2) conventional glow discharge [3] (GD) or Direct PECVD, in which the silane reactant gas is directly plasma excited and the substrate is immersed in the plasma glow; 3) triode glow discharge [4,5], where a grid separates the substrate from the silane plasma; 4) ECR Plasma deposition, where a magnetic field confines the silane plasma in a region removed from the substrate surface [6]; and 5) Remote PECVD [7], where helium flows through a plasma excitation region and mixes with silane introduced downstream in a region removed from the plasma glow, resulting in film deposition on a substrate outside of the helium plasma. The specific plasma processes, reaction pathways and gas phase precursors leading to film deposition have not been unambiguously determined for the Direct PECVD process. Although the SiH3 radical has been suggested, and is often "accepted" as the dominant precursor leading to high quality a-Si:H films in Direct PECVD [8,9], its role has recently been questioned [10]. The objective of the research described here is to determine the nature of the precursor species and the reaction pathways leading to the deposition of a-Si:H in the Remote PECVD process. This process is "easier" to study and analyze than Direct PECVD because the silane reactant gas is not directly plasma excited. We observe that this reduces the number of active precursor species and therefore reduces the number of reaction pathways leading to film deposition. REMOTE
PLASMA
a-Si:H
DEPOSITION
To produce Remote PECVD a-Si:H films, helium is excited in an inductively coupled rf plasma (13.56 MHz) separated by more than 10cm from a heated substrate. Excited helium species (e.g., He metastables and ions) and electrons flow from the plasma region into the deposition chamber. Silane (diluted with 90% Ar or He) is introduced outside the plasma region through a 'showerhead' gas
Mat. Res. Soc. Symp. Proc. Vol. 131. ,1989
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