Excimer-laser etching on silicon

Studies have been made of poly- and single Si etching induced by excimer-laser irradiation of the silicon surfaces in halogenated gases. Etching was investigated for different conduction types, impurity concentrations and crystallographic planes. Chlorine atoms accept electrons generated in photoexcited, undoped p-type Si, thus becoming negative ions which are pulled into the Si. However, the n⁺-type Si is etched spontaneously by Cl^– as a result of the availability of conduction electrons. Fluorine atoms, with the highest electronegativity, take in electrons independent of whether the material is n- or p-type. And thus, the easy F^– ion penetration into Si causes spontaneous etching in both types. New anisotropic etching for n⁺ poly-Si is investigated because of its importance to microfabrication technology. Methyl methacrylate (MMA) gas, which reacts with Cl atoms, produces a deposition film on the n⁺ poly-Si surface. The surface, from which the film is removed by KrF (5 eV) laser irradiation, is etched by Cl atoms, while the film remains on the side wall to protect undercutting. However, with the higher photon energy for the ArF (6.4 eV) laser, the Si-OH bonds are broken and electron traps are formed. These electrontrapping centers are easily annealed out in comparison to the plasma-induced centers. Pattern transfer etching for n⁺ poly-Si has been realized using reflective optics. The problems involved in obtaining finer resolution etching are discussed.     

Photochemical etching of GaAs with Cl2 induced by synchrotron radiation

The quality and efficiency of etching at room temperature is determined in the wavelength range from 105 nm to 300 nm by replicating a mask on a GaAs(100) wafer and by wavelength selection of synchrotron radiation with filters and a monochromator. A good anisotropy and selectivity is found for Cl₂ pressures from 10^–2 mbar up to 1.5 mbar, but above 3 mbar the selectivity is lost. Efficiencies for stimulation of the chlorination reaction and for desorption are separated and an optimal efficiency for stimulation of about 100 removed Ga and As atoms per photon is obtained around a wavelength of 122 nm at 1.5 mbar. Growth of reaction products on the surface occurs for short wavelengths and transport processes through layers up to a thickness of 350 nm are relevant. The efficiency and quality of etching can be improved by additional desorption with long wavelengths especially with lasers.     

Laser-induced chemical etching of silicon in NF3 atmosphere

Chemical etching of single-crystal Si in an NF₃ atmosphere is performed by continuous irradiation with an Ar⁺ laser at 514.5 nm. The etching process proves to be a thermally stimulated chemical reaction between solid Si and NF₃ gas. The experimental results show how the depth and width of the etched grooves depend on laser power, scan speed, and gas pressure. The etch rates observed may exceed 25 m/s.     

Laser-induced chemical etching of silicon in chlorine atmosphere

Laser-induced chemical etching of single-crystalline (100) Si in Cl₂ atmosphere has been investigated for continuous Ar⁺ and Kr⁺ laser irradiation at around 351 nm, and at 457.9, 488.0, 514.5, and 647.1 nm. For laser irradiances below 10⁵ W/cm² the etching mechanism is non-thermal, and is based on photo-generated electron-hole pairs within the Si surface and Cl atoms produced within the gas phase. The experimental results are compared with model calculations.     

Surface-science aspects of plasma-assisted etching

Plasma-assisted etching methods have been used in the manufacture of integrated circuits for more than 10 years and yet the surface-science aspects of this technology are poorly understood. The chemistry must be such that the reactive species generated in the plasma react with the surface being etched to form a volatile product. The chemistry is usually dominated by atoms, molecular radicals and low-energy (20–500 eV) positive ions. In microstructure fabrication, the positive ions are accelerated from the plasma towards the etched surface arriving essentially at normal incidence. Thus, the bottom surface of a very small feature being etched is subjected to both energetic ions and reactive neutral species, whereas the sidewalls of the feature are exposed to reactive neutral species only. The role of the energetic ions is primarily to accelerate the reaction between the neutral species and the etched surface (i.e., accelerate the etch rate), thereby reducing the steady-state top-monolayer coverage of the etching species on the etched surface. On the sidewalls, however, the reacting-species coverage is a saturation coverage. The present understanding of some of the surface-science aspects of this complex environment will be summarized, often using the Si-F system as an example, and some phenomena which are not well understood will be described.     

Dopant-induced excimer laser ablation of poly(tetrafluoroethylene)

Clean ablation of poly(tetrafluoroethylene) (PTFE) at etch rates in excess of 7µm/pulse has been achieved with an excimer laser using 308nm radiation and a 25 ns pulse width. This was accomplished by doping the ultraviolet-transparent PTFE polymer with polyimide. Ablation rates were investigated as a function of fluence in the range from 1 to 12J/cm² and dopant levels up to 15% (wt/wt). Results show that at a given fluence there exists an optimum absorption coefficient _max, for which maximum ablation rates are achieved. The value of _max was found to decrease with increasing fluence. The relationship between _max and fluence was determined from existing ablation rate models and found to compare favorably with empirical results.     

Laser-assisted etching of gallium arsenide in chlorine at 308 nm

Xenon chloride (308 nm) excimer laser-assisted etching of GaAs (100) in Cl₂ was demonstrated and characterized with respect to laser and gas parameters. The etch rate increased linearly with laser fluence from thresholds in the range of 50 to 75 mJ/cm² to the highest fluence studied, 650 mJ/cm². For a laser fluence of 370 mJ/cm², the etch rate varied with Cl₂ pressure reaching a maximum at a Cl₂ pressure of about 2 Torr. The etch rate decreased monotonically with Ar buffer gas pressure because of redeposition of GaCl₃ products into the etched channel. The redeposited GaCl₃ affected the etch rate and the etch morphology. The etch rate and morphology also varied with laser repetition rate. The mobility of chlorine on the surface also plays an important role in the etching mechanism.     

Laser-induced etching of polycrystalline Al2O3TiC in KOH aqueous solution

Laser-induced etching of polycrystalline Al₂–O₃TiC material by a tightly-focused cw Ar ion laser has been investigated in a KOH solution with different concentrations. It is found that the KOH concentration can strongly affect the etching quality where low KOH concentration can result in rough and irregular patterns. Laser-induced etching of polycrystalline Al₂O₃TiC in a KOH solution is found to be a photothermal reaction in which a threshold laser power exists. With an appropriate set of etching parameters, well-defined grooves can be obtained with clean side walls and with an etching rate up to several hundred micrometers per second. The etching behavior is also found to depend on laser scanning direction. It is also found that the grains in the polycrystalline Al₂O₃TiC material play an important role in the etching dynamics and etching quality. This etching process is believed to be applicable to the formation of a slider surface of magnetic heads in the future.     

Photoluminescence of Anisotropically etched silicon

Visible room-temperature luminescence of Anisotropically Chemically Etched (ACE) silicon under spontaneous chemical surface modification in HNO₃:HF solution is reported. The material is investigated by SEM, AES, IR transmission and Raman scattering methods.     

Laser projection-patterned etching of (100) GaAs by gaseous HCl and CH3Cl

Laser projection-patterned etching of GaAs in a HCl and CH₃Cl atmosphere performed using a pulsed KrF-excimer laser (=248 nm, =15 ns) and deep-UV projection optics (resolution 2 m) is reported. The etching process carried out in a vacuum system having a base pressure of 10^–6 mbar is shown to result from a purely thermochemical reaction. Etching takes place in two steps: (i) between the laser pulses, the etchant gas reacts with the GaAs surface-atomic layer to form chlorination products (mainly As and Ga monochlorides), (ii) local laser surface heating results in the desorption of these products allowing further reaction of the gas with the surface. The influence of the etching parameters (laser energy density, gas pressure and pulse repetition rate) on the etch rate and the morphology of the etched features was studied. Etch rates up to 0.15 nm per pulse, corresponding to the removal of 0.5 GaAs molecular layer, are achieved. The spatial resolution of the etching process is shown to be controlled by the heat spread in the semiconductor and by the nonlinear dependence of the etch rate on the surface temperature. As a result, etched features smaller or larger than the projected features of the photomask are achieved depending on the laser energy density. Etched lines having a width of 1.3 m were obtained at low fluences by the projection of 2 m wide lines onto the GaAs surface.     

Laser-assisted etching of diamonds in air and in liquid media

2 O, (CH₃)₂SO). Diamond samples are virtually transparent at this wavelength, and the coupling of laser radiation to diamond is via the formation of a thin graphitized layer at the diamond surface. The etching rate in liquid media is slightly higher than in air at otherwise equal conditions and is as high as 50 μm/s for etching with a scanning laser beam. Raman spectra measurements carried out on diamond samples etched in air show the presence of glassy carbon on the surface, whereas for samples etched in a liquid the diamond peak at 1332 cm^-1 dominates with significantly lower intensity of the glassy carbon peak. Electroless copper deposition on the laser-etched features is studied to compare the catalytic activity of the diamond surface etched in air with that etched in liquids. Possible mechanisms responsible for the observed difference both in the structure of the etched area and in the electroless Cu deposition onto the surface etched in various media (air or liquids) are discussed. Received: 2 August 1996/Accepted: 7 January 1997     

Laser photochemical etching of silicon

Argon laser induced chemical etching of single crystal silicon with chlorine is studied. Etch rates are determined as a function of gas pressure, crystal orientation, laser power and wavelength. Analysis of gas phase and surface products by Fourier transform IR and X-ray photoelectron spectroscopy are used to probe the reaction mechanism. Contrary to previous reports, no thermally enhanced etch rate is observed for Si (111) and the presence of oxide on the surface is found to inhibit etching even at high laser power.     

Laser-induced dry etching of integrated InP microlenses

Received: 4 June 1997/Accepted: 13 June 1997     

Laser-induced desorption and etching processes on chlorinated Cu and solid CuCl surfaces

Time-resolved mass spectrometry is used to study the desorbed species due to laser-induced etching of a solid CuCl and a chlorinated Cu surface. The observed desorption threshold, mass distribution and kinetic energies of the desorbed atoms and molecules at 355 and 532 nm radiation show that the laser-induced etching process is not simply thermal evaporation. It is suggested that competing nonthermal mechanisms due to electronic excitations may be very important in laser-induced desorption and etching. These processes are different for a solid CuCl and a chlorinated Cu surface. For laser-induced etching of Cu surfaces, chlorination of Cu is essential; however, formation of stoichiometric CuCl is not necessary. Excess Cu in the surface layer is responsible for the observed different etching behavior of a chlorinated Cu and a solid CuCl surface. The effect of laser radiation on these surfaces and possible etching mechanisms are discussed based on the experimental observations.     

Suppression of silicon (001) surface reactivity using a valence-mending technique

It is experimentally shown that, by terminating dangling bonds on Si(001) with a monatomic layer of selenium, the chemical reactivity of the surface is suppressed. In the case of nickel silicidation, transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy reveal that Se passivation suppresses Ni silicidation by over 100 °C as compared to the bare Si(001) surface. The formation of Ni subsilicide (Ni₂Si) is not observed on Se-passivated Si(001). This interfacial silicidation appears to be linked with changes in electrical behavior of the interface between titanium and Se-passivated Si(001), which we reported previously.     

Laser-induced dry chemical etching of Mn-Zn ferrite in CCl2F2 atmosphere

Maskless etching of Mn-Zn ferrite in dichlorodifluoromethane (CCl₂F₂) by Ar⁺-ion laser (514.5 nm line) irradiation has been investigated to obtain high etching rates and aspect-ratio of etched grooves. The etching reaction was found to be thermochemical. High etching rates of up to 360 m/s, which is about one order of magnitude higher than that in a CCl₄ gas atmosphere and even higher than that in a H₃PO₄ solution, have been achieved. A maximum aspect-ratio of 6.9 was obtained.     

Laser-induced etching and deposition of W using a-SiO<Subscript>2</Subscript> microspheres

A regular lattice of a-SiO₂ microspheres on a quartz support is used as a microlens array for laser-induced surface patterning by etching and deposition of W in atmospheres of WF₆ and WF₆+H₂, respectively. Received: 22 July 2002 / Accepted: 30 July 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +43-732/2468-9242, Email: dieter.baeuerle@jku.at     

Chemical etching of silicon by CO2-laser-induced dissociation of NF3

The pulsed infrared laser dissociation of NF₃ is reported for the first time, and is used to investigate silicon etching. The role played by collision-enhanced multiple-photon absorption and dissociation is considered, with data on the nonlinear decrease of the absorption cross-section with increasing pulse energy and increasing pressure presented. Using an experimental arrangement in which the laser beam is focussed parallel to the surface, the dissociation process induces spontaneous etching of silicon. Fluorinecontaining radicals diffuse from the focal volume to the surface where a heterogeneous chemical reaction occurs. Etching was monitored by use of a quartz-crystal microbalance upon which a thin film of amorphous silicon was deposited. For a surface with no previous exposure to the photolysis products, dissociation causes the formation of a surface layer prior to the onset of etching. X-ray photoelectron spectroscopy demonstrates this to be a fluorosilyl layer possessing a significant concentration of SiF₃ and SiF₄. In contrast, a surface already thickly fluorinated does not form a thicker layer once laser pulsing commences again. In this case, etching starts immediately with the first pulse. The etch yield dependencies on several parameters were obtained using silicon samples possessing a thick fluorosilyl surface layer. These parameters are NF₃ pressure, laser wavenumber, pulse energy, buffer gas pressure, and perpendicular distance from focal volume to surface. Modeling of the etch yield variation with perpendicular distance shows the time-integrated flux of radicals impinging on the surface to be inversely proportional to the distance. Attempts at etching SiO₂ under identical conditions were unsuccessful despite the evidence that thin native oxide films are removed during silicon etching.     

Gold and platinum diffusion: The key to the understanding of intrinsic point defect behavior in silicon

The study of gold and platinum diffusion is found to allow the separate observation of the intrinsic point defects, i.e., of silicon self-interstitials and of vacancies. The diffusion of gold in float zone (FZ) silicon is found to be dominated by the kick-out mechanism for temperatures of 800° C and higher. The diffusion of platinum in FZ silicon is described by the kick-out mechanism for temperatures above approximately 900° C, whereas for temperatures below approximately 850° C the dissociative mechanism governs platinum diffusion. As a result of numerical simulations, we suggest a complete and consistent set of parameters which describes the diffusion of platinum in silicon in the temperature range from 700° C to 950° C and the diffusion of gold in the temperature range from 800° C to 1100° C. The generation or recombination of self-interstitials and vacancies is found to be ineffective at least below 850° C. The concentration of substitutional platinum is determined by the initial concentration of vacancies at diffusion temperatures below 850° C. Platinum diffusion below 850°C can be used to measure vacancy distributions in silicon quantitatively.     

Realization of silicon quantum wires by selective chemical etching and thermal oxidation

Ultra-fine silicon quantum wires with SiO₂ boundaries were successfully fabricated by combining SiGe/Si heteroepitaxy, selective chemical etching and subsequent thermal oxidation. The results are observed by scanning electron microscopy. The present method provides a very controllable way to fabricate ultra-fine silicon quantum wires, which is fully compatible with silicon microelectronic technology. As one of the key processes of controlling the lateral dimensions of silicon quantum wires, the wet oxidation of silicon wires has been investigated, self-limiting wet oxidation phenomenon in silicon wires is observed. The characteristic of the oxidation retardation of silicon wires is discussed.