Ultra-thin films of silicon dioxide are formed on silicon surfaces at room temperature by direct bombardment with an oxygen-containing ion beam at energies of 150 eV or less. The process of film growth is studied.through ellipsometric measurements of their properties as a function of ion energy and dose, oxygen partial pressure and substrate temperature. Typical oxide thicknesses of the order of 50 Å are obtained by three minute or longer exposures to beams of current density 135 μA/cm2. Ion-beam grown oxides are compared to conventional thin oxide films grown at elevated temperatures and show the same stoichiometry. The growth rate decreases rapidly after a continuous oxide film has been formed. Performing the ion bombardment at elevated substrate temperatures leads to only small enhancement of the oxide growth indicating non-thermally driven reaction kinetics.

A silicon surface exposed to NF3 gas was irradiated with an ArF excimer laser beam. The reaction products on the surface and their chemical bonding features were studied by in-situ x-ray photoelectron spectroscopy at each step of the photochemical etching. It was found that SiFX (1≤X≤4) units and molecular fluorine exist in the reacting surface region. The surface Si-Si bonds attacked with fluorine are progressively fluorinated and the final surface products are mainly SiF4 and SiF3. A possible mechanism of fluorine etching is discussed on the basis of a valence electron transfer (VET) model.

The general utility and selectivity of photochemical dry etching processes which require direct participation of photogenerated carriers is largely determined by the electronic properties of the semiconductor. The laser which is used to produce the carriers responsible for etching can also provide in situ measurement of some of the important electronic properties which influence the etching process.

Deep trenches have been etched in crystalline silicon with polarization-controlled, variable-curvature walls. Scan speeds of up to 10mm/s have been demonstrated. A model of the etching process has been developed which is based on a local, melt-enhanced etch rate. Comparisons of model predictions and experimental data are presented.

Ultraviolet laser-induced, radical-etching processes developed by us can provide practical etch rates and selectivities for most of the important film layer combinations used in silicon microelectronic devices. These processes have been demonstrated in simple proximity and projection exposure experiments to produce etch features on surfaces with dimensions of a few tenths of a micrometer.

Mechanistic studies suggest that, in our etching processes for polysilicon and molybdenum, fluorine atoms responsible for etching are primarily produced from the precursors COF2 and NF3 on the surface rather than in the gas phase. The predominant production process appears to be photodecomposition of surface adsorbed precursor. Contributions due to precursor pyrolysis or precursor reaction with photogenerated charge carriers are found to be unimportant.

Dry etching of fused SiO2 in the presence of several etchants using CO2 and CO2 + Ar+ lasers yields controlled, rapid removal rates of 10 Ao – 500 μm/sec and smooth, high quality surfaces. Etching occurs mainly by thermal ablation of SiO2 due to strong CO2 laser absorption. The addition of the Ar+ laser, which is absorbed by Br2 etchant to yield Br atoms, increases the etch rate as a result of a combination of photochemical and gas-phase heating effects.

Very rapid room-temperature photochemical etching of n-type GaAs was achieved in aqueous hydrofluoric acid in conjunction with ultraviolet laser illumination. The etch rates of ˜500 μm/min represent an order of magnitude increase in etch rates over previously reported results for solutions that contained no hydrofluoric acid. Furthermore, incorporation of nitric acid into the hydrofluoric acid solution resulted in smooth etched surfaces thus allowing deep, waveguiding etching. This rapid process was used to etch deep, large-area structures in GaAs samples.

Laser-assisted etching techniques have been developed for alloyselective patterning of GaAs/AlGaAs layered structures. Such techniques are potentially very important for the fabrication of a large variety of heterostructure devices. Studies have been carried out with several liquid etchants and have utilized a CW dye laser tuned to a wavelength above the band edge of GaAs, but below that of the AlGaAs layer (e.g., λ = 0.85 μm) to etch GaAs. Using the laser focal spot to spatially define the etched area, features exhibiting very smooth interface surfaces were produced in MBE-grown epitaxial layers. A high degree of selectivity for GaAs over A1(0.4)Ga(0.6)As, low background etch rates, and high feature etch rates (> 2 μm/min) were demonstrated. Patterned etching with AZ 1350J photoresist exhibited good pattern resolution and nearly vertical sidewall profiles. Etchant mixtures of dilute H2S04:H202:H20 and HNO3:H20 produced qualitatively different results, with the HNO3:H20 mixture exhibiting a tendency to leave a residual GaAs layer at the interface, even for long etch times. Surface analysis techniques, including Auger spectroscopy, profilometry, and interference contrast microscopy have been used to characterize the etched structures, and a simplified analytical model has been used to predict the qualitative dependence of etch rate on layer thickness.

Lithium niobate exhibits low reactivity with most chemical etchants. Consequently, etching a LiNbO3 surface to produce optical structures such as ridge waveguides or grooves for fiber coupling normally requires relatively slow processes such as ion milling. We have developed a laser-driven chemical etching process for etching highly unreactive ionic solids based on the fusion of salts In the molten phase and show that the etch rate can be more than 100 times faster than ion milling rates. This process involves spatially localized melting of LiNbO3 by high-power density laser pulses with photon energies in excess of the band gap of LiNbO3. While molten, LiNbO3 undergoes reaction with a surface coating of KF to form niobium oxyfluoride anions by fusion of the salts. The resulting solid is highly water soluble. The insolubility of LiNbO3 permits subsequent removal of only the irradiated area by rinsing in water. Surface morphology is determined by laser power density. The process exhibits a wavelength dependence.

Ultraviolet laser radiation at 248 nm and 266 nm was used to ablate low density (0.04 to 0.14 g/cm3) microcellular polystyrene and TPX foams at laser fluences between 0.1 and 1000 J/cm2, The dependence of the etch rate on laser fluence and foam density is consistent with a linear absorption mechanism for the initial stages of polystyrene ablation. Studies of the morphology and chemical composition of the ablated foams as a function of laser fluence and foam density show that the degree of melting increases at the higher foam densities. Blackbody and radical specie contributions to optical emission from the ablated foam were readily resolved for correlation with laser fluence, foam density, absorption spectra, and etch depth. Temperatures in excess of 3000 °K are calculated from emission spectra observed during the ablation of polystyrene foams. From this data we conclude that chemical and thermal changes in the ablated foam are influenced by the absorption spectrum as well as the density of the foam.

Excimer laser induced ablative decomposition of polyimide and poly(methylmethacrylate) at high fluences (> 1 J/cm2) is discussed. It is shown that 0.4 μm sized features can be imaged in polyimide using ArF laser pulses. Preliminary results from experiments in which copper particles (<1 μm in size) have been deposited by exposing thin films of a copper formate and glycerol paste to KrF laser radiation are presented.

It has been recognized for some time that the doping level in silicon influences etch rate in plasma environments[1–8]. We have now been able to reproduce and investigate these doping effects in a modulated-beam, mass spectrometer system described previously [9] using XeF2 as the etchant gas. The phenomena which have been observed in plasma reactors containing fluorine atoms are also observed in our experiments. The data has led to a model which explains the major trends.

Soft x-ray core-level photoemission was used to study the reaction layer formed on a Si(111) surface during steady-state etching by XeF2. These surfaces show shifted Si 2p core levels which correspond to particular silicon fluoride species residing in the reaction layer. Spectra were collected in constant final state (CFS) mode for electron kinetic energies ranging from 2 eV to 90 eV. The use of CFS mode allowed the electron escape length to remain constant during individual spectra. The electron escape length for the reaction layer as a function of kinetic energy was then determined by monitoring the intensity of the bulk silicon signal. Thus, by analyzing the proportions of each silicon fluoride species observed in the individual CFS spectra as a function of the electron escape depth in the reaction layer, the distribution of the silicon fluoride species in the layer was obtained. The distribution determined placed SiF at the interface between the reaction layer and the bulk silicon, and the remaining species, SiF2, SiF3 and SiF4, successively layered on top. This observation leads to a model for the structure of the reaction layer.

Etching of silicon by molecular fluorine is accelerated when trace quantities of copper are present on the surface. Copper is a residue formed when sputter-deposited aluminum (containing 0.5 % copper) is selectively removed by HF from the silicon surface. The temperature dependence of the etching rate was studied in the range 60–290°C. At temperatures higher than 80°C copper causes a ˜100-fold increase in the rate of etching of the underlying silicon (100), compared to unmetallized samples. Above 180°C, F2 exhibits a higher absolute etch rate than equivalent concentrations of fluorine atoms. Preliminary results for other metal contaminants and etchant gases indicate that silver also accelerates F2 etching, and copper enhances etching by NF3. The results are interpreted in terms of a catalytic mechanism.

Prototypical silicon deposition and etching reactions have been observed in gas phase reactions of size selected bare silicon cluster ions at room temperature. Laser evaporation of bulk silicon just outside the ion cell of a Fourier transform mass spectrometer produces abundant positive and negative silicon cluster ions. These cluster ions are trapped inside the cell for subsequent study of their bimolecular reactivity with various neutral reagents. Deposition type reactions occur with silanes, e.g. CH3SiH3, and increase the number of silicon atoms in the cluster. Etching reactions occur in reactions with certain halogen or oxygen containing reagents. For example, NO2 and XeF2 react to destroy the silicon clusters by sequential loss of a silicon atom. Overall the reactivity of small silicon cluster ions correlates with chemistry which occurs at two distinct types of dangling bonds in the clusters: either a lone pair of electrons or a single unpaired electron.

The reactivity of XeF2 and CF3 on thermally grown SiO2 surfaces has been investigated. CF3 radicals were generated by infrared multiple-photon dissociation of C2F6 using a pulsed CO2laser. X-ray photoelectron spectroscopy (XPS) was used to determine the chemical state of the surface after exposure to the gases. Both CF3 and XeF2 are relatively inert to annealed SiO2 surfaces.- Ion damaged surfaces are much more reactive. Implications for ion enhancement of etching are discussed. XPS spectra reveal fluorine, from XeF2 bonded to both silicon and oxygen. The CF3 radical appears to undergo little dissociation upon chemisorption on SiO2.

Kinetic energy and angular distribution measurements give information about the emission mechanism of desorbed particles. For a sputtering mechanism, the energy and angular distribution is usually assumed to be of the form: Φ(E,θ)∝ E cosθ/(E+U)3 . This expression is valid for sputtered atoms and cannot be immediately applied to the sputtering of molecules, where the energy is shared between internal and external degrees of freedom. A model is presented for the kinetic energy and angular distribution of sputtered polyatomic molecules. The formalism is related to an earlier model for sputtered clusters by Können, Tip and De Vries [1]. It is assumed that the individual atoms of a molecule are given an initial momentum according to linear collision cascade properties. If the molecule does not dissociate, it is treated as an entity and carried as such through a planar surface potential barrier. The model shows a continuous transition from a fragmentation dominated behaviour at high kinetic energies to a collision-cascade-like behaviour at low kinetic energies of the sputtered molecules. The model is compared with experimental data on sputtering of Kr2, SiCl and SiF.

The etch yield vs n-type doping level up to 1.5 × 1021 As/cm3 was measured. Also, the etch yield as a function of ion energy (300 to 1500 eV) for the heaviest doped samples (1.5 × 1021 As/cm3) was measured. Only a small 10–15 percent enhancement in etch rate over low doped silicon was observed. Channeling experiments with Rutherford backscattering spectrometry showed 84 percent of the arsenic atoms were in substitutional positions in the silicon crystal. In addition, silicon was implanted with arsenic to a dose of 1.25 × 1017 #/cm2 and then laser annealed. This produced samples with a 5000 angstrom layer of uniformly doped silicon at five atomic percent. Channeling experiments showed these samples to have 82 percent of the dopant in substitutional sites. Samples were then masked with chrome and etched at varied pressures, temperatures and ion energies. Within our experimental resolution no isotropic etching was observed. This small doping dependence of the etching behavior of n-type silicon in reactive ion beam etching suggests that the rate limiting step for Cl2 RIBE is different than for Cl2 RIE, rf-plasma and laser assisted etching. A discussion of this difference is given assuming a much lower concentration of atomic chlorine in RIBE.

Etch inhibitor layers are the key to anisotropy for a number of dry etch processes, yet little is known about these layers because of the difficulty in analyzing them on the side walls where they form. In this paper we will show that an Al grid suspended above an etching surface can be used to suppress ion bombardment and allow the inhibitor to form on large horizontal surfaces which can be easily analyzed. The effect of ion bombardment on the nature of the inhibitor layer can also be elucidated using this technique. In conjunction with X-ray Photoelectron Spectroscopy, this method was used to look at the polymeric inhibitor formed during Si etching in SF6/C2CIF5 with and without the presence of photoresist.

A chemically assisted ion beam etching (CAIBE) technique is described which employs an ion beam from an electron bombardment ion source and a directed flux of ClF3 neutrals. This technique enables the etching of tungsten foils and films in excess of 40 μm thick with good anisotropy and pattern definition over areas 5 mm2, and with a high degree of selectivity. (100) tungsten foils etched with this process exhibit preferred orientation etching, while polycrystalline tungsten films exhibit high etch rates approximately 80% that of (100) orientation tungsten.