Laser Chemical Vapor Deposition-Applications In Materials Processing

AbstractLaser chemical vapor deposition (LCVD) uses a focused laser beam to locally heat the substrate and drive the CVD deposition reaction. Several different deposition reactions and substrates have been examined as a function of intensity and irradiation time using a CC>2 laser source: Ni on SiO2, TiO2 on SiO2 , TiC on SiO2 , and TiC/on stainless steel. LCVD film thicknesses range from 20ym. Deposition rates of mm/min have been observed for LCVD Ni and TiC and 20 um/min for LCVD Ti02 The diameter of the deposited films is dependent on irradiation conditions and can be as small as 0.10 the laser beam diameter. The LCVD films exhibit excellent physical properties such as adherence, con­ ductivity, hardness and smoothness.The advantages of LCVD are the same as other laser processing techniques, i.e., control of area and depth heated, rapid heating and cooling, and cleanliness. Possible applica­ tions of this technique include: formation of ohmic contacts and localized protective coatings in semiconductor devices; localized coatings and dopants for waveguide optics; surface hardening and alloying of machine surfaces; welding of ceramic materials; and generation of new materials.Chemical vapor deposition (CVD) is a technique for depositing coatings or growing layers on a substrate that is widely used in the semiconductor, optics and refractory materials industries. In conventional CVD gaseous compounds which do not react at room temperature are passed over a substrate heated to a temperature at which either the reac- tants decompose or combine with other constituents to form a layer on the substrate.The use of a laser as the heat source for chemical vapor deposition offers several distinct advantages: a) spatial resolution and control; b) limited distortion of the substrate; c) the possibility of cleaner films due to the small area heated; d) avail­ ability of rapid, i.e., non-equilibrium, heating and cooling rates; and e) the ability to interface easily with laser annealing and diffusion of semiconductor devices and laser processing of metals and alloys. It should be possible to generate deposits of almost any material that can be deposited by conventional CVD and probably some which cannot.The deposition of material by thermal decomposition using a laser heat source was first reported for graphite from a hydrocarbon vapor in 1972-^- at the Third International Con­ ference on Chemical Vapor Deposition. At the same conference the following year, D. M. Mattox^ reported the LCVD of W by H2 reduction of WFg using a multimode CW CO2 lasertranslated across a SiC^ substrate. More recently, the LCVD of polycrystalline Si from SiH4 by Christensen and Lakin3 , and CoO from cobalt acetyl acetonate and air by Steen4 has been reported. In both cases, a CO2 laser source and glass or quartz substrates were used.Experimental ProcedureLaser chemical vapor deposition (LCVD) is best described by referring to Figure 1. The laser is focused through a transparent window and the transparent reactants onto an absorbing substrate, creating a localized hot spot at which the reaction takes place. The absorptivities of the reactants and the substrate determine the laser wavelength which is used.Three different cases have been examined as a function of intensity and irradiation time using a CO2 laser source: deposition of a reflecting film on an absorbing substrate, Ni/SiO2; an absorber on an absorbing substrate, TiO2 and TiC/SiO2; and an absorber on a reflecting substrate, TiC/stainless steel.Initial experiments in LCVD on SiO2 substrates at the Center for Laser Studies were carried out using the experimental apparatus illustrated in Figure 2. The laser was a CW CO2 rated nominally at 20 W, but usually run at about 10 W. Attenuation was provided by a ZnSe Brewster angle polarizer using two Brewster angle plates at opposing angles to minimize beam walk-off. Although the beam profile without the attenuator is approximately