Recent results of studies on the mechanism of processes induced by the interaction of beams of (reactive) particles with surfaces in UHV, often combined with concurrent bombardment of the surface with low-energy ion or excimer laser beams, are reviewed. Angular-resolved mass spectrometry combined with time-of-flight studies on the desorbing products is used as a key diagnostic. A more complete picture is obtained using several other diagnostic tools to characterize the surface before, during or after the interaction. Interactions at Si and Cu surfaces will be emphasized. The review will deal successively with data for interaction with a single beam of Cl2, low-energy noble gas ions or excimer laser pulses, followed by data on the concurrent interaction of crossed beams of Cl2 and low-energy noble gas ions or excimer laser pulses with these surfaces. Some conclusions will be drawn.

We have deposited thin films of optical materials using ion beam sputtering and ion assisted deposition techniques. It is possible to obtain good quality film material deposited on substrates at temperatures lower than normally required. Ion assisted deposition influences film stoichiometry and packing density, which in turn determine optical and mechanical properties of the film material. We discuss two general indicators which appear helpful in predicting the degree to which these occur.

Various techniques which have been applied to modeling low-energy (≪ 1 keV) ion-solid interactions on an atomistic scale are described. In addition to their individual strengths, all such methods also have a number of drawbacks, both fundamental and practical. The range of validity, and the problems encountered external to this range, will be outlined for the different approaches. Finally, examples of molecular dynamics simulations of low-energy ion-solid interactions will be presented.

metal-alkyl molecules provied an early source molecule for photodeposition of metal. They continue to be of interest for patterned and unpatterned laser deposition and for conctact formation on compound semiconductors. This article provides a brief review of what has been learned about the surface photo-physic of this class of molecules based on several years of study with uV and iR absorption spectroscopy, UHV surface analysis, and thermal and photodesorption amass spectroscopy.

Studies of the thermal and photon-induced surface chemistry of dimethyl cadmium (DMCd) and dimethyl tellurium (DMTe) under ultrahigh vacuum conditions have been performed for substrate temperatures in the range of 133 K to 295 K. Results on GaAs(100) and Si(100) surfaces indicate that for DMTe, the predominant adspecies, dimethyl tellurium, can be photodissociated to a metal adspecies at both 193 and 248 nm. For DHCd, the major adspecies, monomethyl cadmium, is unreactive to photon stimulation.

The deposition of W films by ArF laser-induced chemical-vapor deposition (LCVD) was investigated as a function of incident laser power, WF6 and H2 partial pressures, and substrate temperature. The deposition of W films by LCVD is discussed dividing that into two parts, thermal CVD (TCVD) and photon assisted CVD (PhCVD). The rate of PhCVD has been defined as the difference between the rates with and without laser irradiation. The reaction orders for PhCVD are 1, 0 with respect to WF6 and H2 partial pressures, respectively, and the rate linearly increases with increase in laser repetition rate. The activation energy in PhCVD is 0.17 eV. These facts indicate that, in LCVD, PhCVD takes place independently of TCVD and that the deposition rate in PhCVD is determined by the formation of F radicals in the dissociation of WF6 molecules by laser irradiation.

MOS capacitors with LCVD-W gates were fabricated and their characteristics were compared with those with sputtered-W gates. It was shown that the level of contamination due to mobile ions in the capacitor with the LCVD-W gate was extremely low.

Laser photodissociation of trimethylindium on a quartz substrate has been studied by the laser-induced fluorescence and photofragment mass-spectrometric techniques. Translational energy distributions of photofragments are measured.

Using dimethylaluminum hydride as the source gas and an ArF laser as the light source, aluminum thin films with low electrical conductivity were deposited via photochemical reactions. To emphasize surface reactions, vapor pressure was kept low with a t pical value of 6.7×10−3 Pa. The laser energy density was typically 23 mJ/cm2 per pulse. The deposition rate of aluminum films became reasonably high above a substrate temperature of 90 °C and increased thereafter with substrate temperature. The electrical resistivity was about four times greater than the bulk aluminum value. A strongly nonlinear dependence of the deposition rates on laser energy density was observed. The incremental change in thickness per pulse increased with the inverse of the pulse repetition rate, which indicated, together with area selectivity, the importance of surface photolysis of absorbed molecules.

We use tunable UV laser light in the region 200-320 nm, produced by frequency doubling the output of a dye laser for the decomposition of organometallic compounds. This method has been applied to TMA, trimethylaluminum AI(CH3)3. Only theTMA monomer absorbs UV light for λ>220nm. TMA decomposes by one-photon absorption mainly into two channels: aluminum plus organic fragments and aluminummonomethyl plus organic fragments. The ratio [A1]/[AICH3] is wavelength dependent. Finally, we present a mechanism to explain the photolysis of trimethyl compounds of group III elements (Al,Ga,In).

The influence of laser fluence and carbon monoxide partial pressure on the deposition rate of thin Cr(C,O) films was investigated. The films were deposited from Cr(CO)6 mixtures with CO and Ar using a KrF excimer laser. The observed results are discussed with respect to the relative importance of gasphase and surface processes. The results reveal that under the present experimental conditions chromium atoms produced in the gas phase play only a minor role as direct film precursors in KrF laser-induced film deposition.

The spatial and temporal distribution of Cr* in a laser-CVD arrangement has been studied by laser-induced fluorescence (LIF). Low excited Cr*(a5S2) atoms are produced by dissociation of Cr(CO)6 by a KrF laser ( λ = 248 nm) via two photon absorption. An observed disappearance of Cr* after photolysis is due to reaction with photofragments, quenching via collision relaxation and deposition. The diffusion constants for Cr* in He/Cr(CO)x mixtures could be determined. The Cr* density in the vicinity of a sample drops sharply by heating the sample. The decrease is due to thermal decomposition of Cr(CO)6 with an activation energy of 0.11 eV.

Fine metal particles are deposited on a surface by explosive decomposition of organometallic compounds with single pulse laser irradiation. This new technique has the following characteristics:

(1) High quantum efficiency of ca. 105, that depends on the size of the reaction chamber used and the concentration of organometallic compounds.

(2) Formation of uniform spherical particles of ca. 0.3 µm size under some reaction conditions.

(3) Rapid formation of metal particles. Most of the starting materials are decomposed within a few miliseconds.

The key points of this decomposition reaction are:

(1) Formation of high density active species in the gas phase by the irradiation with high power lasers.

(2) Propagation of a thermal chain reaction.

A one-dimensional model for pyrolytic laser direct-writing of metal features from thin metalorganic films is presented. The model extends previous results for scanning laser crystallization by including separate but coupled mass and energy balances and allowing for variations in the optical properties and thermal conductivity in the film as it decomposes. A finite element approach is used to obtain solutions to the steady state and transient model forms. The model predicts the existence of multiple steady states for a range of laser powers when the optical absorbance is a nonlinear function of the fractional conversion in the film. These predictions agree qualitatively with multiple steady states that have been observed in direct-writing of palladium features from palladium acetate films. Experimental results that demonstrate multiplicity in the palladium acetate system are presented. The model also predicts the existence of periodic solutions that correspond to the periodic features that have been reported for direct-writing of gold features from organogold films.

The laser-induced chemical vapor deposition (LCVD) of aluminum metal has been achieved via the pyrolytic decomposition of trimethylamine aluminum hydride (TMAAH). This material is a volatile, crystalline solid which is non-pyrophoric, in contrast to many other aluminum precursors. Laser-driven pyrolysis of TMAAH enables the selective deposition of high purity, highly conducting aluminum deposits. The volatility of the TMAAH precursor is directly responsible for the rapid rates of aluminum deposition and permits rapid scan velocities to be utilized. The relationship between the chemical structure of TMAAH and the high purity of the aluminum deposits is described.

Two relaxation techniques are described, both of which use surface temperature modulation as their basis. In these techniques, modulation of laser light intensity results in a modulation of surface reaction rates. The first technique, a diffusive transport relaxation technique, has been developed to study the chemical reactions occurring under the high pressure conditions used in laser-induced chemical vapor deposition. Perturbation of the reaction rate in a high pressure reactor produces transients in the gas phase reactant and product concentrations for the case of gas phase transport-limited deposition. The product species are differentiated from species formed from the reactant during ionization and fragmentation due to the phase-sensitive nature of the detection scheme. The system response is examined theoretically by examining the response of the gas phase reactant and product concentrations to the modulated surface reaction. The second technique is a new technique which has been developed for obtaining detailed information concerning reaction energetics and pathways of elementary surface reactions. This technique also employs a laser to generate a surface temperature modulation. The laser light is incident on a pure metal film which is mounted within an ultra high vacuum chamber. By modulating the laser intensity, the surface chemical reaction rates are modulated. The combined use of surface temperature modulation and appropriate linearization methods allows a quantitative analysis of the overall reaction energetics to be made. and is used here to investigate the surface processes in a thermal laser-induced reaction.

Metallic gold was deposited on several different substrates by laser pyrolysis of a goldcontaining ink. The gold was deposited in the form of lines by translating the substrate during laser pyrolysis; linewidths ranged from 10 Am to 1.5 mm. This process was performed in open air. Prior to annealing, the films contain 40 to 50 % carbon impurity and the resistivity is extremely high. After a 1 hour anneal in air, the carbon content is reduced to < 5 %, and the resistivity is reduced to roughly 13 times bulk gold. Further improvement may be possible with a different annealing schedule.

An investigation is reported of the laser surface.cladding of an alloy of nominal composition Ni-4Fe-30Cr-6A1 (wt%) on a mild steel substrate, using a continuous powder feed of the elemental powder mixture into the melt pool. The effect of laser processing conditions has been investigated in relation to clad dimensions, bonding, dilution level, hardness and microstructural features. A 2 kW CW C02laser was used working at 1.6 kW power and 2.5 mm beam diameter at different traverse speeds ranging from 1.8 to 25 mm/s: clad layers were produced in the range of 0.15-3 mm thick. Fine microstructures were obtained (corresponding to the rapid solidification rates). Good metallurgical bonding was observed between the clad layer and substrate within the range of specific energies, 50-130 J/mm2. For all the conditions studied, no cracking was observed in single tracks; low specific energy conditions gave little porosity.

We compare three approaches to excimer laser assisted growth of GaAs: 1) Film growth is dominated by gas phase photolysis of deposition precursor gases at 193 nm under low pressure metal organic chemical vapor deposition (MOCVD) conditions. This approach leads to rapid deposition at low substrate temperature, but with very low spatial resolution (∼1 cm). 2) Growth is controlled by laser induced pyrolysis of adsorbed triethylgallium (TEGa) at 193 nm under metal organic molecular beam epitaxy (MOMBE) conditions. Films grown under these conditions have the potential for high spatial resolution (i.e. submicron), but geometric constraints in MBE conflict with the requirement of a short working distance between the lens and substrate, making it difficult to attain this limit. 3) Areaselective growth is controlled by laser induced pyrolysis of adsorbed TEGa at 351 nm (XeF excimer laser) under low pressure MOCVD conditions. This approach combines the advantage of laser controlled surface chemistry with the potential for much shorter working distances.

Selected area growth of GaAs occurs in this latter mode because TEGa dissociatively chemisorbs at 400°C to form a stable layer which decomposes further under laser irradiation to liberate hydrocarbon products. The Ga left behind on the surface reacts with As2 and As4 (formed by pyrolysis of trimethlyarsine or triethylarsine in a side tube) to grow GaAs in irradiated areas. Since the gas does not absorb at 351 nm the process is highly areaselective. Patterned films with feature sizes of ∼70µm (limited by the mask dimensions) were grown by this method. Interference between the incident beam and light scattered along the surface also causes a substructure of parallel lines to form on the features with a line spacing about equal to the laser wavelength 0.35µm. This indicates that the ultimate spatial resolution is comparable to that predicted by thermal diffusion calculations (∼0.3µm).

We report X-ray photoelectron spectroscopy (XPS) studies of excimer laser stimulated decomposition of triethylgallium (TEGa) and trimethylgallium (TMGa) adsorbed on Gastabilized GaAs(100) surfaces in ultrahigh vacuum. TEGa and TMGa dissociatively chemisorb on GaAs at room temperature, whereupon irradiation by an excimer laser (at 193 or 351 nm) leads to further dissociation and desorption of carbon-containing species. The carbon removal rate (per laser pulse) decreases as carbon is removed suggesting multiple reaction sites, coverage dependent Arrhenius parameters, or second-order reactions. Based on the dependence of the rate on laser wavelength and fluence, we conclude that at low fluence, a two-photon electronic excitation of the adsorbate occurs, while at high fluence, laser induced pyrolysis dominates.

Laser induced chemical vapor deposition (LCVD) of GaAs has been achieved at low temperature using trimethylgallium and arsine precursors. The deposited film traces the path of the Ar+ laser, thus making it attractive for several applications. The localized thermal expansion resulting from the laser induced temperature rise has to be elastically accomodated in order to prevent lattice distortion during the LCVD process. We report on the growth conditions that can be used without the occurence of plastic deformation in the epitaxial films. A model is presented to explain the thermal expansion induced distortion during the deposition process and will be compared with experimental results.