The kinetics of processes for laser-photochemically depositing localized thin films are discussed, both in the low-intensity regime, where photoreactions are rate limiting, and in the high-intensity regime, where mass transport is rate limiting. Comparisons are made between processes relying on gas-phase and surface-phase photochemistry. Examples are given of recent studies of photoreactions confined entirely to surface phases, and preliminary results of experiments performed in the mass-transport-limited regime are reported.
An ArF excimer laser was used to photochemically deposit thin films of silicon dioxide, silicon nitride, aluminum oxide and zinc oxide at low temperatures (100-500°C) for microelectronic applications. High depo-sition (>1000 A/Min) rates and conformal step coverage were obtained. The hydrogen bonding, pinhole density, index of refraction, etch rate, and breakdown voltage have been measured for the Si02 and silicon nitride films. The effect of substrate temperature and ArF (193 nm) surface photons on the physical, chemical and electrical properties of Si02 films have been investigated.
In neutralization and negative-ion formation from positive ions scattering from a solid surface, a laser can be used to control the nature of resonant, near-resonant, and even nonresonant transfer of electrons from the conduction band. These spectral characteristics can be achieved by variation of only the laser frequency and intensity.
Experiments are described in which semiconductor or metal films are grown from the vapor phase by photodissociating or photoionizing diatomic or polyatomic molecules. The photolysis of GeH4 at 248 nm Mw = 5 eV) is initiated by a two photon process that liberates the germylene radical GeH2. Spatially and temporally-resolved concentration profiles for several excited states of GeH and atomic Ge have been measured near the substrate. Thin indium films have been deposited on nickel substrates by dissociatively ionizing indium monoiodide (InI) to produce In+ - I- ion pairs. The dynamics of ion pair production for thallium iodide or InI vapor photoexcited at 193 nm have been studied by combining an excimer laser with microwave absorption techniques.
We present results on the 77K photodecomposition of Fe(C0)5 to produce Fe films using synchrotron and excimer laser radiation. Analysis of these films included in situ photon-stimulated ion desorption (PSID) and total electron yield (TEY) spectroscopies, and ex situ Auger electron spectroscopy (AES), scanning electron microscopy (SEM), X-ray fluorescence, and resistivity measurements. For the films grown using synchrotron radiation, the various ion products were identified and their intensities monitored during photolysis. For the films grown using excimer laser radiation, appropriate laser power densities and Fe(C0)5 pressures produced adherent metallic films which contained less than 13 atomic % oxygen and carbon contamination.
The photochemistry of Fe(CO)5 adsorbed on Si02 at submonolayer coverage has been studied. IR and UV-visible spectra show that the only significant product is Fe3(C0)12 rather than Fe2(C0)9, which is the product observed upon irradiation of Fe(C0)5 in the gas or liquid phase or in solution. Evidence is presented that the intermediate reacting to give Fe3(CO)12 on the Si02 surface is a species designated Fe(C0)4(Si02), which denotes a complex between Fe(C0)4 and the terminal surface hydroxyl groups or siloxane bridging oxo groups.
In laser chemical vapor deposition (LCVD), a laser is used to drive a deposition reaction by locally heating the substrate. Although the reactant systems used may be similar to conventional CVD, the film growth characteristics may differ in several ways. The changes in the optical properties of the film/substrate during deposition must be considered as the amount of laser energy absorbed determines the surface temperature and therefore the deposition rate. Also affecting the deposition rate is the diffusion of reactants to the reaction zone. Because of the small area heated in LCVD, higher surface temperatures can be accessed before diffusion and convection limit the deposition rate. For favorable reactant systems, very rapid deposition rates (greater than 100 um/sec) and scan speeds for line deposition (greater than 10 cm/sec) can be achieved.
General principles of laser direct-write deposition processes are reviewed. Device interconnection of CMOS gate arrays by means of computer-controlled, laser-induced thermochemical surface reactions is described. Interconnection quality parameters are related, and processing rate considerations are discussed.
We report on the laser-assisted deposition of metals by pyrolytic and photolytic dissociation of metal alkyls. A Kr+ laser is used to pyrolytically deposit Al, Cd, and Zn on optically absorbing substrates; deposition rates are typically a few um/s. A frequency-doubled Ar+ laser is used to deposit Ga by photolytic decomposition of the parent molecules on quartz substrates. Cadmium deposition is achieved by photolysis of the parent molecules adsorbed on the quartz substrate using the visible output of a Kr+ laser. The time evolution of these processes is studied by measuring in situ the optical transmission during deposition. We have observed different morphologies which depend on the deposition mechanism.
In order to enhance the deposited area and to improve the uniformity of hydrogenated amor phous silicon (a-Si:H) films, obtained from photodissociation of silane molecules by CO2 laser radiation, two new different experimental approaches are investigated. One of these utilizes a high power (≈ 1 KW) CW CO2 laser with uniform intensity distribution in a rectangular beam cross section; the other consists in a continuous scanning, along a horizontal plane parallel to the substrate, of a low power (≈ 100 W) gaussian laser beam. Preliminary results about p and n doping of the photodeposited material by boron and pho-sphorous ion implantation proved its high doping efficiency and its structural similarity to the chemical vapor deposition produced material.
A process for depositing a-Si:H films from CO2 laser-heated gases has been demonstrated and modelled, the properties of resulting films have been investigated extensively. Film growth rate is determined by the peak gas temperature, defined by an energy balance between the absorption of the laser beam and thermal conduction to the substrate and the cell walls. The hydrogen content and neutral spin density follow an equilibrium function of the substrate temperature. The optical and electronic properties also depend on the substrate temperature.
The localized deposition of metal and metal oxide films by laser thermal decomposition of a solid organometallic film has been investigated. Several different laser sources have been used to irradiate commercially available organometallic resinate films applied using standard photoresist techniques on glass and quartz substrates. The focused laser causes localized decomposition of the organometallic film and the unreacted film is removed with solvent. Decomposition occurs stepwise with the initial state being insoluble but opaque and nonconducting. Continued irradiation results in metallic appearing films. Film dimensions considerably less than the laser spot diameter could be be produced under appropriate irradiation conditions.
Evaporation of materials by means of lasers is a technique that has reached maturity as an optical coating and semiconductor epitaxial layer growth technology and as such has been applied to a large class of materials. Using cw and pulsed CO2 lasers, several oxides, fluorides and semiconductors were deposited with good optical properties. Mixtures of the above were obtained by using multiple laser evaporation targets. The dielectric films were characterized for their optical properties and semiconductor films for their structural and electrical properties. Laser evaporation conditions and characterization results of the films will be explained.
A glow discharge electron beam has been used to deposit silicon dioxide (Si02) and silicon nitride (Si3N4) films for microelectronic applications. Electron beam assisted CVD is a new technique in which the reaction volume is defined mainly by the geometry of the electron beam and offers the possibility of uniform deposition over large areas. The Si02 films were deposited in silane-nitrous oxide-nitrogen mixtures, and the Si3N4 films were deposited in silane-ammonia-nitrogen mixtures. The films were deposited with a 2-4 kV electron beam parallel to the sample, at 0.1-1 Torr pressures, and at substrate temperatures from 50-400°C. The index of refraction, sthoichiometry, pinhole density, etch rate, conformal step coverage, and hydrogen bonding were measured.
A photochemical process is described that has potential for the manufacture of very large scale integrated circuits. A prototype reactor is described and the results of experimentation are presented and discussed. The deposition of a Si02 thin film by the photochemical reaction of silane and nitrous oxide was found to be a surface reaction but the data has not differentiated between a desorption or adsorption process. The reaction is photosensitized with mercury and it is shown that at high mercury concentrations, a mercurous oxide forms. Modifications to the prototype reactor are presented showing the excellent thickness uniformity possible. Criteria for VLSI manufacture by photochemistry is also presented.
A CO2 laser pyrolysis technique has been used to prepare ultrafine (< 0.1p diameter) boron-silicon powders with different boron concentrations. These powders have been used as a spin-on boron diffusion source for silicon wafers. The spin-on colloidal suspension is prepared by mixing the powder with a thermally degradable polymer binder, polymethyl-methacrylate (PMMA), and an organic vehicle, cyclohexanone. Thin, uniform films are spun-on using a standard photoresist spinner. Two different procedures are followed in diffusing the boron from the boron-silicon powder. In the first process, the boron is diffused by heating the wafer in an argon ambient (1000-1260°C). The excess dopant layer is removed by oxidation (02) and subsequent etching (HF). In the second process, the powder is first converted to a borosilicate glass layer by oxidation, followed by diffusion in an argon ambient. Some experiments using commercially available boron nitride powder as a diffusion source are also discussed.
Pulsed laser irradiation of thin metal films sputtered onto a magnesium alloy substrate produces surface compositions which exhibit improved pitting corrosion resistance to aqueous chloride ion relative to untreated magnesium. Preliminary results suggest a relationship between the composition of the laser produced surface plasma, as determined by atomic emission spectroscopy, and the corrosion properties of the laser melted magnesium surface. A photoacoustic technique for monitoring the energy deposited in a sample during laser surface treatment will also be described.
Laser-induced or -enhanced chemical etching of solid surfaces has provided an important new tool in recent years for materials processing. Studies have been reported on a wide range of substrates, including semiconductors, metals, ceramics, glasses and thin organic films. In this paper the current status of the field is reviewed. Characteristics of laser etching of various materials are described. Activities related to applications and to fundamental understanding of the process are also discussed.
The dynamics of laser stimulated etching of Ge by Br2 have been studied by characterization of the desorbed products. The product time-of-flight distribution was measured with a quadrupole mass spectrometer, providing both product identification and velocity analysis. Transient reflectivity measurements were used to determine when the melt threshold for Ge was exceeded. At low Br2 pressure, etching at a rate of - 0.1 - 0.2Å per pulse was observed above the melt threshold. The principal product observed was GeBr2, whose translational energy distribution deviated slightly from Maxwell-Boltzmann: the observed distribution was relatively rich in high energy species (i.e. hyperthermal).
This paper reports work towards the development of new UV laser-induced radical-etching processes for the efficient and selective removal of: (1) polycrystalline (poly)-silicon layers deposited on silicon dioxide substrates, (2) tungsten layers deposited on either silicon or silicon dioxide substrates, and (3) silicon dioxide layers deposited on either silicon or aluminum substrates.
We report a new method of laser-controlled etching in which the radiation from a pulsed-ultraviolet excimer laser is used to etch inorganic insulators exposed to plasma species that have been produced in a glow discharge sustained in silane. The maximum etch rate achieved was 50 nm/min for thin-film silicon dioxide.