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.     

Laser-photoetching characteristics of polymers with dopants

The photoetching behavior of poly(methylmethacrylate), poly(dimethylglutarimide) and chlorinated poly(methylstyrene) doped with pyrene and 4-aminobenzoylhydrazide excited by 308 nm excimer-laser pulses has been studied. Some common laser-etching characteristics including the reduction of the threshold fluence for ablation, the enhancement of etching efficiency and the existence of optimal conditions regarding the laser fluence and dopant concentration for generating clean and smooth etching patterns are identified. The photoetching mechanism and the potential application of the doping technique to material processing are discussed.     

Contribution to time-resolved enhanced chemical etching and simultaneous annealing of ion implantation amorphized silicon under intense laser irradiation

Surfaces of single-crystal silicon wafers are amorphized by high-dose phosphorous ion implantation. These surfaces of the wafers, immersed in concentrated KOH, are laser-chemically etched by pulse irradiation of a ruby laser. Simultaneously, the remaining parts of the amorphous layer are annealed. The time dependence of the etching process enhanced during pulse irradiation is recorded and analysed. Reasons for the etching rates which differ between amorphous and single-crystal silicon are given on the basis of experimental and numerical results.     

Laser-induced desorption of Na-dimers

We find that Na-dimers are desorbed in a thermal process if rough Na surfaces are irradiated with pulsed laser light of λ=532 nm. In contrast, for light of λ=355 nm, Na₂ can be detached in a non-thermal reaction at low laser fluences. This is concluded from the kinetic energy distributions of the dimers determined by time-of-flight measurements using a second laser at λ=248 nm for photoionization. The transition from non-thermal to thermal desorption at large fluences of the laser light can also be identified. Received: 23 July 1996 / Accepted: 26 August 1996     

Laser-induced desorption of overlayer films off a heated metal substrate

The temperature-induced desorption of adsorbed overlayer films with thicknesses between 4 and 200 ML off a suddenly heated metal substrate is studied using molecular-dynamics simulation. We observe that the rapid heating vaporizes the surface-near part of the overlayer film. The initial heating-induced thermoelastic pressure and the vapor pressure in the vapor film drive the remaining film as a large relatively cold cluster away from the surface. In our simulations, the material present in the developing vapor film amounts to roughly 2 ML and is quite independent of the overlayer film thickness. For cluster thicknesses beyond 40 ML, the desorption time increases only little with film thickness, while the resulting cluster velocity decreases only slightly.     

Surface doping of semiconductors by pulsed-laser irradiation in reactive atmosphere

Intense pulsed-laser irradiation in a suitable chemical atmosphere can produce a significant incorporation of chemical species from the environment to the surface molten layer. This process has been used to produce p-n junctions in silicon and GaAs irradiated, respectively, in PCl₃ and SiH₄ atmospheres. A modelling of the incorporation process, taking into account the solid-liquid-solid transition of the surface layer, has been developed following both a numerical and a semi-analytical approach. The modelling of the doping process gives results in a reasonably good agreement with the experimental doping profiles, obtained by irradiating Si samples in PCl₃ atmosphere.     

Laser-induced etching of silicon

Conventional fabrication method of porous silicon is anodisation of single crystal silicon in hydrofluoric acid. In this report, we show that it is possible to fabricate porous silicon by laser-induced etching. An earlier report by us has demonstrated the dependence of porous silicon photoluminescence characteristic on the etching laser wavelength [1]. Here we used 780 nm line from a diode laser as the etching source, and the optimum etching conditions were obtained. A simple model was proposed to explain the etching process. Scanning Electron Microscope (SEM) images of the samples support the proposed process.     

UV desorption and photochemistry of dimethylgold hexafluoroacetylacetonate adsorbed on a quartz substrate

The photodesorption and photodecomposition pathways of dimethylgold hexafluoroacetylacetonate, DMG (hfac), adsorbed on a cooled quartz substrate is reported for 222-nm KrCl excimer laser radiation. The time-of-flight (TOF) of neutral photoproducts, desorbed from the surface of the gold film formed during the experiment, were analyzed under collisionless conditions by a differentially-pumped mass spectrometer. Extensive dissociation of adsorbed DMG (hfac) into DMG and the hfac ligand was observed. The ligand was found to recombine with a CH₃ radical on the surface. Translational energy distributions for the detected species were obtained by deconvoluting the TOF curves into a self-consistent set of Maxwell-Boltzmann distributions for the desorbed parent molecule, laser-induced decomposition products, and surface recombination reaction products. The implications of these results for the mechanistic details of the low-pressure, laser-assisted organometallic deposition of DMG(hfac) are discussed.ONT/NRL Research Associate (Nov. 1987-Oct. 1988)NRC/NRL Cooperative Research Associate     

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.     

XeCl laser controlled chemical etching of aluminum in chlorine gas

The 308 nm XeCl laser assisted etching process of thin Al metal films on Si substrate in Cl₂ gas was investigated. Etch rates were measured versus the laser fluence on the sample, the laser repetition rate, the Cl₂ pressure and the sample temperature. Irradiation experiments under vacuum of films which were previously exposed to Cl₂, and laser assisted etching in rare gases, nitrogen and air mixtures with Cl₂ were also performed to elucidate the mechanism of the etching process. The surface morphology was investigated by scanning electron microscopy. The results show that a) Etch rates of up to about 1.5 m per pulse are obtained which are strongly dependent on the Cl₂ pressure and sample temperature. b) The etching mechanism is essentially a chemical chlorination of the Al in between the laser pulses which is followed by photo-ablation of the reaction products, c) AlCl₃ evaporation and redeposition processes can explain the observed results. d) The Al films can be etched fully and cleanly without damage to the smooth Si substrate. e) Etching through adjacent or imaged mask on the Al film yielded relatively smooth and well defined Al walls with structures of the order of 1 m.     

Controlling energy coupling and particle ejection from aluminum surfaces irradiated with ultrashort laser pulses

Hydrodynamic simulations are used to evaluate the potential of ultrashort laser pulses to localize energy at metallic surfaces, in our case aluminum. The emphasis is put on the dynamic sequence of laser energy deposition steps during the electron-ion nonequilibrium stage and the subsequent matter transformation phases. The simulations indicate correlated optical and thermodynamical states associated to specific electronic collisional mechanisms. The timescales of energy deposition deliver a guideline for using relevant relaxation times to improve the energy coupling into the material. We focus on a class of pump-probe experiments which investigate energy storage and particle emission from solids under ultrafast laser irradiation. Moreover, we have used our model to explain the experimentally observed optimization of energy coupling by tailoring temporal laser intensity envelopes and its subsequent influence on the ablation rate and on the composition of ablation products. Potential control for nanoparticle generation is discussed.     

High-rate laser-direct-write dry etching of titanium

Titanium surfaces can be etched spatially selective in a chlorine atmosphere under 488 nm cw Ar⁺-laser irradiation focused to 3 m with well-controlled etch depth and high etch rate. By scanning the substrate, patterns can be generated by laser direct writing with high scan speed. The dependence of the etch rate on various parameters, such as laser power, scan speed and chlorine pressure, is described, and the impact on three-dimensional structuring of titanium is discussed.     

Laser-induced chemistry-basic nonlinear processes and applications

Many methods and achievements in chemistry are based on using the interactive of light with atoms and molecules. It is sufficient to mention photochemistry, flashphotolysis, spectrochemistry and others. The advent of laser amplified the connection between chemistry and light. Today laser light has become a very versatile and effective tool, first, to study the dynamics of chemical reactions, secondly, to stimulate chemical reactions and finally, to analyze substance. The unique properties of laser light (high power, monochromaticity, short duration, directivity and temporal coherence) provide quite new instrumental possibilities in all these problems.     

Ar ion laser assisted chemical etching of via holes in tungsten sheets in air

Ar ion laser assisted chemical etching of 150 m thick annealed tungsten sheets in air is reported. The material removal mechanism involves local heating by the laser to temperatures in the range of 1000–1500 °C that causes rapid oxidation of the W to WO₃ which volatilizes readily. Holes with straight walls and slightly enlarged entrances near the surface were drilled with etch rates as high as 11.5 m/s at 13.8 W, and a minimum hole diameter of 21 m at 8.1 W. The diameters of the holes and the etch rates were measured and found to increase as a function of the laser power. It was found that by increasing the laser power above 11–12 W, no change was observed in the hole diameters which remained constant at about 31 m, whereas the etch rates continued to increase even faster than at low powers. Distinct adjacent holes of 25 m diameter could be drilled with their centers separated by as little as 60 m. This is therefore also the etching resolution in the present study.     

Laser-stimulated field desorption of molecular ions from a tungsten emitter

We report here on a experimental observation of photon-stimulated field emission of molecular anthracene ions from the surface of a layer adsorbed on a tungsten field-emitter tip. When the tip is irradiated with laser pulses 249, 308, and 400 nm in wavelength falling within the absorption bands of anthracene, the stimulated ion signal is proportional to the pulse fluence. The efficiency of the process decreases with the increasing laser wavelength. Photon stimulation is believed to be due to the resonance excitation of the anthracene molecules, followed by the field ionization of the excited molecules.     

Phenomenology of picosecond heating and evaporation of silicon surfaces coated with SiO2 layers

Picosecond time-resolved reflectivity measurements on bare silicon surfaces and silicon surfaces with oxide layers reveal very fast heat diffusion and material evaporation on subnanosecond time scales. With a thick oxide layer resolidification of a molten silicon surface can take place in a few hundred picoseconds. At high laser fluences, vaporization processes take only a couple of 100 ps.     

Laser-induced high-quality etching of fused silica using a novel aqueous medium

Laser-induced backside wet etching of fused-silica plates using an aqueous solution of naphthalene-1,3,6-trisulfonic acid trisodium salt (Np(SO₃Na)₃) is reported. A KrF excimer laser was employed as a light source. The etch rate varied greatly with the concentration of the solution and the laser fluence. For lower concentration solutions, the etch rate increased linearly with laser fluence. For highly concentrated solutions, however, the etch rate increased abruptly at higher fluence. Well-defined line-and-space and grid micropatterns were fabricated using a low etch rate. The etched surface was as flat as the surface of the virgin plates and the etched pattern was free of debris and microcracks. The formation and propagation of shockwaves and bubbles in the solution during the etch process were monitored. High pressure, as well as the high temperature generated by the photothermal process, plays a key role in the etching process. Received: 8 April 2002 / Accepted: 12 April 2002 / Published online: 19 July 2002     

Laser ablation and desorption from calcite from ultraviolet to mid-infrared wavelengths

3 ) at wavelengths ranging from the ultraviolet to the mid-infrared, including a Nd:YAG laser operated at the fundamental, second and third harmonics, and a tunable infrared free-electron laser (wavelength range 2.5–8 μm). The threshold for ablation and the topography of the irradiated spot were characterized by scanning electron microscopy. A clear indication of two distinct excitation mechanisms was observed, namely, cracks and fractures followed by exfoliation at ultraviolet to near-infrared wavelengths, in contrast to evaporative holes and scattered droplets in the mid-infrared. Plume emission/absorption spectroscopy, plume transmission and photoacoustic beam deflection were used to characterize the ablation plasma. The composition of atoms, molecules, or particles in the ablation plumes also has a distinctive variation as a function of the wavelength. The excitation mechanisms leading to ablation appear to be defect activation at ultraviolet to near-infrared wavelengths, molecular impurity absorption and resonant vibrational absorption of the calcite at mid-infrared. Received: 5 December 1996/Accepted: 6 January 1997     

Energy distribution of ions produced by excimer-laser ablation of solid and molten targets

2 . The measurements reveal components with different charge-to-mass ratio and distinct components with the same charge/mass ratio. The most probable kinetic energy has values of several tens of eV for singly charged ions, and is larger by a factor exceeding 2 for doubly charged ions. The role of target material and state, solid or liquid, laser photon energy and fluence has been investigated and is discussed in comparison to the findings of previous investigations. An estimate of the electrostatic plasma potential in PLA, based on electron loss rate arguments, is presented to account for the high ion energies observed. Received: 9 March 1998 / Accepted: 27 November 1998 / Published online: 24 February 1999     

Non-thermal laser-induced desorption of metal atoms with bimodal kinetic energy distribution

Laser-induced desorption of metal atoms at low rate has been studied for pulsed excitation with wavelengths of = 266, 355, 532 and 1064 nm. For this purpose sodium adsorbed on quartz served as a model system. The detached Na atoms were photo-ionized with the light of a second laser operating at = 193 nm and their kinetic energy distribution was determined by time-of-flight measurements. For = 1064 nm a distribution typical of thermal bond breaking is observed. If desorption, however, is stimulated with light of = 266 or 532 nm, the kinetic energy distribution is non-thermal with a single maximum atE _kin = 0.16 ± 0.02 eV. For = 355 nm the non-thermal distribution is even bimodal with maxima appearing atE _kin = 0.16 ± 0.02 and 0.33 ± 0.02 eV. These values of the kinetic energies actually remain constant under variation of all experimental parameters. They appear to reflect the electronic and geometric properties of different binding sites from which the atoms are detached and thus constitute fingerprints of the metal surface. The non-thermal desorption mechanism is discussed in the framework of the Menzel-Gomer-Redhead scenario. The transition from non-thermal to thermal desorption at large fluentes of the laser light could also be identified.