Alternative Reactants for the Laser-Assisted Deposition of Silicon Nitride on Metals
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ALTERNATIVE REACTANTS FOR THE LASER-ASSISTED DEPOSITION OF SILICON NITRIDE ON METALS. Julian P. Partridge and Peter R. Strutt. The University of Connecticut, Institute of Materials Science, Box U-136, Storrs, CT 06268. ABSTRACT An alternative approach is described for the laser synthesis of silicon nitride layers using relatively innoccuous, non-pyrophoric reactants such as hexamethyltrisilazane. The pyrolysis process produces 2000 A particles of silicon nitride and the reaction mechanisms are invesigated using infrared spectroscopic analyses. The morphology of the deposited layer is critically dependent on process conditions due to considerable differences in the coupling efficiencies of the nitride deposit and the underlying metallic substrate. The existence of a laser-generated plasma above the metal is discussed in the light of microstructural features observed. INTRODUCTION The majority of advances in the preparation of silicon nitride surface layers have been made in conventional thermal CVD (l-4);this involves passing reactants such as SiF 4-NH 3, SiH 4-NH3 , or SiCI4-NH 3 over heated substrates to produce amorphous or a-phase Si3N 4. The laser synthesis of amorphous Si0 2 , Si3N 4 and SiNHY has been demonstrated using excimer lasers (5-7) and, more recently, crystalline silicon nitride powders with 2000 A particle diameter have been formed using tuned CW and pulsed CO 2 lasers
(8-11).
Most laser syntheses of silicon nitride to date have involved the reaction of SiH 4 with N 2 or NH 3 , the only by-product being hydrogen which is removed from the reaction zone by Xigh gas flow rates (9). Aside from the handling problems associated with the pyrophoric silane gas, satisfactory and controllable deposition rates require critical control of process parameters such as reactor pressure, gas composition, and laser mode and wavelength (10). Lasers offer several advantages in the synthesis of ceramic powders, however, including (a) high rates of reaction, (b) short times at high temperature (typically 0.1 sec at 1100 'C , (c) small interaction zones resulting in localized deposition, and (d) effectively cold-wall reactors with minimal contamination or corrosion. It has been shown that the SiC14-NH 3 CVD reaction to form Si 3N 4 involves the polymerization of Si(NH) 2 followed by pyrolysis of the polymers thus formed (12). This has led recently to an extension of laser Si3N4 synthesis to include the use of non-toxic, non-pyrophoric organosilicon prepolymers in conjunction with CO2 lasers (13, 14). The selection of a suitable organometallic precursor for the synthesis of Si3N 4 involves a balance between two competing factors. The first is that the reactant have a suitable vapor pressure whilst exhibiting sufficient chemical stability. The incorporation of metallic ligands such CH 3 within the structure inhibits hydrolysis whilst affording a degree of volatility. The second factor is the elimination of unwanted reactant fragments as gaseous by-products. Nitrides prepared by RF plasma polymerization and subsequent pyro
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