J. Phys. Chem. 1993,97, 12937-12948
12937
Kinetics of S i c C V D Surface Decomposition of Silacyclobutane and Metbylsilane A. D.Johnson,t J. Perrin,* J. A. Much,' and D. E. Ibbotson' AT& T Bell Laboratories, Murray Hill, New Jersey 07974 Received: September 22, 19930
Silicon carbide thin films have been deposited on Si( 100) from the single-source reagents silacyclobutane (SCB, c-C3H&iH2) and methylsilane (MeSiH3). Thermal decomposition kinetics were studied in a low-pressure (50 mTorr), cold-wall CVD reactor over the temperature range 600-1 150 OC. Partial pressure analyses based on mass spectrometry indicate the dominant decomposition channels for SCB and MeSiH3 are consistent with the formation of the isomeric intermediates H2Si=CH2 and HSiCH3, respectively. Deposition rates using the two reagents were comparable at all temperatures, reaching 80 A/s at 1050 OC. It is apparent from the lattice vibrational spectrum and X-ray diffraction data that S i c deposited from MeSiH3 is more crystalline. These structural differences suggest a distinct difference in the surface reaction mechanism which is inconsistent with isomeric equilibration. SCB deposits carbon-rich material (C/Si = 1. l), becoming stoichiometric above 1000 OC, while that deposited from MeSiH3 is Si-rich (C/Si = 0.8) at all temperatures. The hydrogen content of SCB films decreased from 6 to 1 atom 8 with increasing temperature. Results were similar with MeSiH3, but with half as much hydrogen. Modeling of heat and mass transfer at the substrate shows that decomposition occurs exclusively at the surface, allowingsurface reaction probabilities to be measured as a function of temperature. Activation energies for SCB and MeSiH3 decomposition are 41 and 53 kcal/mol, respectively. A mechanism for SiC/CVD is proposed that is consistent with the observed kinetics and products of SCB and MeSiHs decomposition.
1. Introduction The mechanical and electronic properties of silicon carbide are presently being exploited in thin film applications ranging from X-ray lithography to diodes and transistors that operate in high-temperatureand hostile environments. Because S i c is likely to play an increasingly strategic role, it is important that the growth of this material be studied and tailored for specific applications. Recently, efforts have been directed at designing precursors specifically for S i c CVD.ls2 An ideal sourcematerial is one that decomposes at low temperatures to form a film having the desired film properties. Although many of the applications for which S i c has been considered require stoichiometric material? films are usually deposited from a two-component mixture4(e.g. SiH4and C3Hg) with a composition that is sensitive to temperature, pressure, and source gas flow.%' Here, we describe the kinetics of low-pressure CVD of Si(l,,C, from two source gases: silacyclobutane(c-C~HaiH2.SCB) and methylsilane (CH3SiH3, MeSiH3). Both sources contain the constituent atoms Si and C; hence the S i c composition will to a large extent be controlled by their decomposition chemistry rather than process variables such as temperature, flow, and pressure. Material deposited from SCB and MeSiH3 has been characterized with respect to their composition and structure using Rutherford backscattering spectrometry (RBS), elastic recoil detection (ERD), Fourier transform infrared (FTIR), and X-ray diffraction (XRD) (section 3). We demonstrate that in a cold-wall, low-pressure reactor, film growth is dominated by source decomposition at the surface of the substrate (AppendixA). The surface decomposition of SCB and MeSiH3 is studied by monitoring product partial pressures during S i c CVD with a quadrupole mass spectrometer (section 4). Partial pressure analysis allows the composition of the gaseous and hence the solid products of SCB and MeSiH3 decomposition Present address: Air Productsand Chemicals, Inc.,Allentown, PA 18 195. * Labratoire de Physique des Interfaces et des Couches Minces,CNRS, Ecole Polytechnique, F-91128 Palaiseau Cedex, France.
*Abstract published in Advance ACS Abstracts, November 15, 1993.
0022-365419312097-12937304.00/0
to be determined. The Si(1,$, composition, estimated from this partial pressure analysis, is then compared to thin film stoichiometriesdetermined from RBS measurements. Deposition rates on a Sic-coated susceptor, obtained from the depletion of SCB and MeSiH,, and those on a Si( 100) substrate, measured by laser interferometry, are in good agreement and are used to determine surface reaction probabilities as a function of substrate temperature and source pressure (section 5 ) . From these kinetic measurements, the ratedetermining step for S i c CVD is identified and a mechanism is proposed that is consistent with both the products of SCB and MeSiH3 decomposition and the Si(,-,)C, composition and structure (section 6). 2. Experimental Section 2.1. CVD Reactor. SCB and MeSiH3 were pyrolyzed in a cold-wall, low-pressureCVD reactor (Figure 1) having an internal volume of 3750 cm3 and evacuated by a 250-cfm roots blower backed by a dual-stage mechanical pump. Reactor pressure was measured with a 10-Torr Baratron and controlled with an exhaust valve located downstream of the reactor. The source gases for S i c CVD were SCB (Dow Corning Corp., 99%) and MeSiHl (Hiils America Inc., 97%) whose flows were controlled by mass flow controllers calibrated by the rise of pressure in a known volume. Because gas composition depends upon the degree to which the source is depleted, the initial pressure was set at a substrate temperature where there is no decomposition (
