Laser chemical processing for microfabrication makes use of chemical reactions and phase changes confined to micrometer-scale dimensions at vapor/solid and liquid/solid interfaces. When conditions are established for rapid interfacial reactions, processing speeds can become limited by diffusive transport. These diffusion-limited rates, however, increase by many orders of magnitude as the microreaction geometry is reduced to micrometer lengths using well-focused visible and UV beams. A calculation of the geometric scaling of such rates is summarized here. For realistic conditions, rate increases of up to four decades are predicted relative to corresponding reactions on semi-infinite plane surfaces.

Mechanisms for laser-driven pyrolytic deposition of micron-scale metal structures on crystalline silicon have been studied. Models have been developed to predict temporal and spatial properties of laser-induced pyrolytic deposition processes. An argon ion laser-based apparatus has been used to deposit metal by pyrolytic decomposition of metal alkyl and carbonyl compounds, in order to evaluate the models. These results of these studies are discussed, along with their implications for the high-speed creation of micron-scale metal structures in ULSI systems.

Laser pyrolysis at short wavelengths is a powerful tool for micron-sized one-step local deposition of insulating, semiconducting and metallic materials from the gas phase. Various structures, e.g. stripes on different substrates, and rods of various lengths and diameters, have been produced. The morphology of the deposited material, the deposition rate, and the dimensions (typically 1-300 μm) of the structures were investigated quantitatively as functions of laser irradiance local temperature, laser focus diameter, scanning velocity, and gas pressure.

The deposition of Ga on quartz substrates by photodissociation of trimethylgallium has been investigated. Light from a frequency doubled Ar-ion laser beam (257.2 nm) was focused onto quartz substrates. The Ar-ion laser was operated under cw or mode-locking conditions. The deposition rate has been investigated as a function of averaged UV power densities in the range of 1.8 – 1750 w/cm2 and is shown to depend critically on the laser operating mode. For cw operation, the maximum growth rate within the focal zone is limited to 17 Å/s whereas under mode-locking conditions, no saturation is observed and the growth rate increases linearly with the laser power up to 70 Å/s at 1.75 kW/cm2.

Photodecomposition of organometallic gases has been shown to be potentially useful for high resolution direct metal deposition. However, a number of problems, particularly the low writing rates, must first be solved. An understanding of the deposition mechanism is essential to this task. In particular, the decomposition must be shown to occur either in the gas phase or in the adsorbed layer. This question can be resolved by the dependence of the writing rate on spot size. Measurements of the process show that once the deposition begins the bulk of the deposit occurs from decomposition in the gas phase.

The purpose of the paper is to examine the basic processes involved in the laser-enhanced chemical etching of solids. Specifically, the process of chemisorption, the reaction between the adsorbate and substrate atoms and the vaporization of product species affected by the laser radiation are discussed. It is shown that the laser method can provide important insight into the gas-surface reaction mechanisms. In addition, a number of examples are given to demonstrate the potential of the technique for applications to material processing. Some current studies on the laser-induced chemical etching of materials relevant to microelectronics are reviewed. Certain practical experimental approaches are also considered.

Submicrometer gratings have been etched in GaAs and CdS crystals which have been immersed in an oxidizing etch and illuminated with interferring laser beams. A resolution of 170 nm was obtained. At high laser intensity and with prolonged etching time the surface properties of the material are degraded. The use of in-situ optical measurements of grating parameters allows ready optimization of the grating fabrication process.

N-type GaAs substrates have been exposed to zinc atmospheres at 600 – 800 deg.C and focused light from a Krion laser at 647.1 nm. Etch pits are generated at optical densities of 0.2 – 10 KW/cm2 and exposure times of 5 – 60 minutes. They are characterized by a smooth profile with typical diameters of 50 – 80μm and depths of 0.1 – 50 μ The etch depth depends strongly on the laser power and on the zinc and arsenic vapor concentrations. The onset of etching is characterized by an increase in surface reflectivity of up to 10 per cent which is probably due to the formation of a thin liquid metal film. The growth of this film is initiated by optically generated holes at the semiconductor surface. The film is not saturated and dissolves GaAs from the substrate. In addition to etching, thermal diffusion of zinc can be observed in samples subjected to long exposure times or high temperatures.

The photochemical etching of chromium-doped and n-doped <100> GaAs in HNO3 and KOH is examined in the wavelength region of 334 to 514 nm from an argon-ion laser. The etching process is found to be not thermally controlled. The etch rates of chromium-doped GaAs agree with a diffusion-controlled model of the photochemically produced holes. For both types of GaAs, HNO3 is found to produce morphologically superior results.

Raman spectroscopy is a powerful tool for evaluating semiconductor crystal characteristics with a diffraction–limited spatial resolution of approximately 0.5 μm, comparable to device structure dimensions. Examples are presented of the use of Raman scattering to measure stress variations across Sion-insulator and Si-on-sapphire stripes with dimensions down to 2 μm and, to probe the spatial variation in the effectiveness of annealing of a Si layer deposited over metal device structures. Detailed measurements of the variation in the Raman spectra of thin crystalline Si films, with thicknesses down to 3.0 nm, are discussed. Measurements made on roughened Si surfaces are presented which show that surface morphology on a submicron spatial scale results in enhanced Raman intensities.

The frequency of the phonon line in the Raman scattering spectrum recorded during CW laser-beam heating of Si was used as a characteristic of the lattice temperature inside the laser spot. It is shown that Raman scattering is a good temperature probe up to the laser power approaching optical damage of Si.

An experimental technique has been developed to study the electrical properties of semiconductor grain boundaries (GBs) by a focused laser beam. The laser beam is trained on a GB while the photoconductivity of the sample is measured. Both the steady-state and transient signals are recorded as functions of temperature. From these data, we obtain well-defined GB parameters, including the barrier height, interface charge density, trap energy and thermal capture cross-section. This technique allows us to examine localized regions of individual GBs in a semiconductor with multiple grains.

The laser-induced luminescence of direct gap semiconductors can be measured with high spatial (1 μm) and temporal (< 1 ns) resolution. A profiling technique is described for in-situ measurements during laser processing. It allows the evaluation of temperature, carrier density and band gap energy within the focal zone. The technique has been applied to (Al,Ga)As heterostructure material subjected to highly focused cw-Kr-ion laser radiation. The measurements reveal low temperature rises (<200 deg.C) at high excitation densities (>1kw/cm2) and an inhomogeneous distribution of optically excited carriers for concentrations above 2.1017 cm−3.

An excimer laser is used to photochemically deposit thin films of silicon dioxide, silicon nitride, aluminum oxide, and zinc oxide at low temperatures (100–350deg;C). Deposition rates in excess of 3000 Å/min and conformal coverage over vertical walled steps were demonstrated. The films exhibit low defect density and high breakdown voltage and have been characterized using IR spectrophotometry, AES, and C-V analysis. Device compatibility has been studied by using photodeposited films as interlayer dielectrics, diffusion masks, and passivation layers in production CMOS devices.

Additionally, we have deposited metallic films of Al, Mo, W, and Cr over large (>5 cm2) areas using UV photodissociation of trimethylaluminum and the refractory metal hexacarbonyls. Both shiny metallic films as well as black particulate films were obtained depending on the deposition geometry. The black films are shown to grow in columnar grains. The depositions were made at room temperature over pyrex and quartz plates as well as silicon wafers. We have examined the resistivity, adhesion, stress and step coverage of these films. The films exhibited resistivities at most ∼20 times that of the bulk materials and tensile stress no higher than 7 × 109 dynes/cm2

Si3N4 films have been deposited on Si by using 193 nm ArF excimer laser radiation to initiate the reaction of SiH4 and NH3 at substrate temperatures between 200–600°C. Stoichiometric films having physical and optical properties comparable to those produced using low-pressure chemical vapor deposition (LPCVD) have been produced. The dielectric properties of the films are at present inferior to those of LPCVD material.

Evidence for the selective formation of surface silanol induced by ultraviolet irradiation in the intensity range between 3 × 107 W/cm2 and 108 W/cm2 is presented. Enhanced metallic deposition (Ti) on appropriately prepared samples is also reported.

U.V. laser radiation from a krypton ion laser was used to dissociate manganese carbonyl [Mn2 (CO)10*] at the surface of a ZnS thin film. The resulting Mn deposit was thermally diffused into the film to form a ZnS:Mn thin film electroluminescent (TFEL) device with spatially selective Mn doping. Process parameters and techniques are described as well as electroluminescence measurements on display elements. Finally we suggest applications of this process to future TFEL displays.

Chemical vapor deposition (CVD) and plasma etching are important gas-phase techniques used in fabricating semiconductor devices. These processes frequently involve poorly understood multicomponent gas-phase reactions which control reproducibility and product quality. Laser spectroscopic techniques have recently been developed to investigate CVD and plasma etching. These methods offer several advantages for probing complex systems. A comparison of various probing techniques will be presented, and recent results of laser spectroscopic investigations of plasma etching and CVD of silicon and III-V compounds will be reviewed.

Laser-induced fluorescence experiments have been carried out during CF4/O2/H2 plasma etching of Si and SiO2. Measurements of relative CF2 radical concentrations as a function of rf power, frequency, pressure, and gas composition are reported. The results are correlated with etch rates of Si and SiO2. The balance between CF2 and F concentrations is shown to influence the etching process strongly.

A study of the UV photochemistry of organometallic molecules in the vapor and adsorbed phases is discussed. The gas-phase spectra is interpreted on the basis of molecular orbital theory and experimental data. The absorption spectra of the surface adlayers are determined by computer subtraction of the gas-phase spectra and an intepretation is provided. The influence of metalsubstrate microstructure on UV photochemical reactions of metal alkyls is briefly discussed.