Thin-film formation of Si clathrates on Si wafers

In this study, we prepared Si clathrate films (Na₈Si₄₆ and Na_xSi₁₃₆) using a single-crystalline Si substrate. Highly oriented film growth of Zintl-phase sodium silicide, which is a precursor of Si clathrate, was achieved by exposing Na vapour to Si substrates under an Ar atmosphere. Subsequent heat treatment of the NaSi film at 400 °C (3 h) under vacuum (<10⁻² Pa) resulted in a film of Si clathrates having a thickness of several micrometres. Furthermore, this technique enabled the selective growth of Na₈Si₄₆ and Na_xSi₁₃₆ using the appropriate crystalline orientation of Si substrates.     

Laser removal of copper particles from silicon wafers using UV, visible and IR radiation

Laser removal of small copper particles from silicon wafer surfaces was carried out using Nd:YAG laser radiation from near-infrared (1064 nm) through visible (532 nm) to ultraviolet (266 nm). It has been found that both 266 nm and 532 nm are successful in removing the particles from the surface whereas 1064 nm was shown to be ineffective in the removal of particles. The damage-threshold laser fluence at 266 nm was much higher than other wavelengths which provides a much wider regime for safe cleaning of the surface without causing any substrate damage. The cleaning efficiency was increased with a shorter wavelength. The effect of laser wavelength in the removal process is discussed by considering the adhesion force of the particle on the surface and the laser-induced cleaning forces for the three wavelengths. Received: 31 May 2000 / Accepted: 14 July 2000 / Published online: 20 June 2001     

Femtosecond laser micromilling of Si wafers

Femtosecond laser micromilling of silicon is investigated using a regeneratively amplified 775 nm Ti:Sapphire laser with a pulse duration of 150 fs operating at 1 kHz repetition rate. The morphological observation and topological analysis of craters fabricated by single-shot laser irradiation indicated that the material removal is thermal in nature and there are two distinct ablation regimes of low fluence and higher fluence with logarithmical relations between the ablation depth and the laser fluence. Crater patterns were categorized into four characteristic groups and their formation mechanisms were investigated. Femtosecond laser micromilling of pockets in silicon was performed. The effect of process parameters such as pulse energy, translation speed, and the number of passes on the material removal rate and the formation of cone-shaped microstructures were investigated. The results indicate that the microstructuring mechanism has a strong dependence on the polarization, the number of passes and laser fluence. The optimal laser fluence range for Si micromilling was found to be 2-8 J/cm² and the milling efficiency attains its maximum between 10 and 20 J/cm².     

Ultraviolet laser removal of small metallic particles from silicon wafers

Laser removal of small 1 μm sized copper, gold and tungsten particles from silicon wafer surfaces was carried out using ultraviolet radiation at 266 nm generated by Nd:YAG harmonic generation. Successful removal of both copper and gold particles from the surface was achieved whereas tungsten particles proved to be difficult to remove. The cleaning efficiency was increased with an increase of laser fluence. The optimum processing window for safe cleaning of the surface without any substrate damage was determined by measuring the damage threshold laser fluence on the silicon substrate and the required fluence for complete removal of the particles. The different cleaning efficiencies with particle type are discussed by considering the adhesion force of the particle on the surface and the laser-induced cleaning force for the three different particles.     

High-resolution electron microscopy of Si cones formed on Ar+-bombarded Si wafers

Conical Si projections generated on Si wafers bombarded with obliquely incident Ar⁺ ions were studied by high-resolution transmission electron microscopy. The cones were composed of an [111]-oriented bulk phase covered with a disordered thin layer, but the bulk phase was not perfectly ordered, containing an amorphous domain underneath the outermost area. Such a multi-phase structure is inexplicable in terms of ion erosion, suggesting interplay of the redeposition of sputtered Si atoms on the bombarded area with the ion-erosion process so as to promote cone evolution. The cones were also characterized by the development of web-like platelets at their acute angles, an indication of a crystal growth process involved in the surface phenomenon observed.     

Photoacoustic and photothermal radiometry spectra of implanted Si wafers

The effects of Ar⁺⁸ and O⁺⁶ ion implantation of Si were investigated by photoacoustic (PA) and photothermal radiometry (PTR) methods. The surface of Si sample was treated with Ar⁺⁸ or O⁺⁶ ions with various doses. Amplitude and phase PA spectra of Si with and without ion-implantation were measured and analyzed in the wavelength range from 800 to 1600 nm (the energy range from 0.75 to 1.55 eV) and frequency of modulation, from 1 Hz to 100 kHz.     

Infrared spectroscopy of oxide layers on technical Si wafers

Measurements of the reflectance and transmittance in the infrared part of the spectrum (400...4000cm^–1) are carried out on oxidized Si wafers. Intrinsic dielectric properties of the oxide layers prepared by various methods are derived from experimental data. The dielectric functions of the oxides can be fitted by oscillator models. The related model parameters are compared with those of other SiO₂ samples, as crystals and glasses. Optical arrangements to detect and characterize layers of thicknesses down to 3 nm are discussed. In particular, it is shown that experiments with polarized light at oblique incidence up to 80° are a powerful tool to characterize those layers.Experiments are reported to determine, in addition, extrinsic properties such as B and P atoms in the oxide layer or defects due to an ion implantation treatment.Conditions are discussed which should be applied to obtain the best quantitative analysis of the defect concentration.     

Near-IR photoluminescence from Si/Ge nanowire-grown silicon wafers: effect of HF treatment

We present the room-temperature near-infrared (NIR) photoluminescence (PL) properties of Si/Ge nanowire (NW)-grown silicon wafers which were treated by vapor of HF:HNO₃ chemical mixture. This treatment activates or enhances the PL intensity in the NIR region ranging from 1000 nm to 1800 nm. The PL consists of a silicon band-edge emission and a broad composite band which is centered at around 1400–1600 nm. The treatment modifies the wafer surface particularly at defect sites especially pits around NWs and NW surfaces by etching and oxidation of Si and Ge. This process can induce spatial confinement of carriers where band-to-band (BB) emission is the dominant property in Si-capped strained Si/Ge NW-grown wafers. Strong signals were observed at sub-band-gap energies in Ge-capped Si/Ge NW-grown wafers. It was found that NIR PL is a competitive property between the Si BB transition and deep-level emission, which is mainly attributable to Si-related defects, Ge dots and strained Ge layers. The enhancement in BB and deep-level PL is discussed in terms of strain, oxygen-related defects, dot formation and carrier-confinement effects. The results demonstrate the effectiveness of this method in enhancing and tuning NIR PL properties for possible applications.     

Tritium recovery from co-deposited layers using 193-nm laser

Recovery of tritium from co-deposited layers formed in deuterium–tritium plasma operations of the TFTR (Tokamak Fusion Test Reactor) was investigated by the use of an ArF excimer laser operating at the wavelength of 193 nm. At the laser energy density of 0.1 J/cm², a transient spike of the tritium-release rate was observed at initial irradiation. Hydrogen isotopes were released in the form of hydrogen-isotope molecules during the laser irradiation in vacuum, suggesting that tritium can be recovered readily from the released gases. In a second experiment, hydrogen (tritium) recovery from the co-deposited layers on JT-60 tiles that had experienced hydrogen-plasma operations was investigated by laser ablation with a focused beam of the excimer laser. The removal rate of the co-deposited layers was quite low when the laser energy density was smaller than the ablation threshold (1.0 J/cm²), but reached 1.1 μm/pulse at the laser energy density of 7.6 J/cm². The effective absorption coefficient in the co-deposited layers at the laser wavelength was determined to be 1.9 μm^-1. The temperature of the surface during the irradiation at the laser energy density of 0.5 J/cm² was measured on the basis of Planck’s law of radiation, and the maximum temperature during the irradiation decreased from 3570 K at the initial irradiation to 2550 K at the 1000th pulse of the irradiation. Received: 5 August 2002 / Accepted: 7 August 2002 / Published online: 28 October 2002 RID="*" ID="*"Corresponding author. Fax: +81-29/2825917, E-mail: shu@tpl.tokai.jaeri.go.jp     

Nanopolishing of silicon wafers using ultrafine-dispersed diamonds

In the present study, two new methods are proposed for the polishing of silicon wafers using ultrafine-dispersed diamonds (UDDs). The first proposed polishing method uses a polishing tool with an ultrafine abrasive material made through the electrophoretic deposition of UDDs onto a brass rod. Dry polishing tests showed that the surface roughness of the silicon wafer was reduced from R_a=107 to 4 nm after polishing for 30 min. The second method uses a new polishing pad with self-generating porosity. By polishing with the new pad in combination with the polycrystalline UDD in a water suspension, it is possible to achieve the specified surface roughness of the silicon wafer much faster than when using a conventional pad made of foamed polyurethane. The tests showed that the surface roughness of the silicon wafer was reduced from R_a=107 to 2 nm after polishing for 90 min.     

Removal of submicron particles from nickel-phosphorus surfaces by pulsed laser irradiation

Pulsed laser cleaning was demonstrated to be an efficient way for removing submicron particles from the nickel-phosphorus (NiP) surface both experimentally and theoretically. Experimentally, it is found that using KrF excimer laser with a pulse width of 23 ns the cleaning threshold is about 20 mJ / cm² for removing quartz particles from the NiP surface and laser cleaning efficiency increases rapidly with increasing laser fluence. The theoretical analysis shows that the peak cleaning force (per unit area) is larger than the adhesion force (per unit area) for submicron quartz particles on the NiP surface when it is irradiated by excimer laser with a fluence above 10 mJ / cm². Therefore, it is possible to remove submicron quartz particles from NiP surfaces by laser irradiation. The difference between the cleaning force (per unit area) and the adhesion force (per unit area) increases with increasing laser fluence, leading to a higher cleaning efficency for quartz particles on the NiP surface.     

On the non-linear dependence of photocarrier radiometry signals from Si wafers on the intensity of the laser beam

The subject of this research was the dependence of the infrared photocarrier radiometric (PCR) signal on the intensity of the exciting super-bandgap laser beam. It has been shown that the amplitude of the PCR signal is proportional to the intensity to a power β, such that 1≤β≤2. The power dependence of the amplitude is an important indicator of the photoexcited carrier recombination physics, specifically in semiconductors ranging between monopolar (β = 1) and bipolar (β = 2) limits. The study was made with laser beams of varying power and spotsize and wafers with different transport parameters. It has been found that the conventional approach using β = 1 is inadequate and inconsistent with experimental slopes of amplitude vs. power.     

Role of a‐Si:H bulk in surface passivation of c‐Si wafers

The low thermal stability of hydrogenated amorphous silicon (a‐Si:H) thin films limits their widespread use for surface passivation of c‐Si wafers on the rear side of solar cells. We show that the thermal stability of a‐Si:H surface passivation is increased significantly by a hydrogen rich a‐Si:H bulk, which acts as a hydrogen reservoir for the a‐Si:H/c‐Si interface. Based on this mechanism, an excellent lifetime of 5.1 ms (at injection level of 10¹⁵ cm^–3) is achieved after annealing at 450 °C for 10 min. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Characterization of Al/Si junctions on Si(100) wafers with chemical vapor deposition-based sulfur passivation

Chemical vapor deposition-based sulfur passivation using hydrogen sulfide is carried out on both n-type and p-type Si(100) wafers. Al contacts are fabricated on sulfur-passivated Si(100) wafers and the resultant Schottky barriers are characterized with current–voltage (I–V), capacitance–voltage (C–V) and activation-energy methods. Al/S-passivated n-type Si(100) junctions exhibit ohmic behavior with a barrier height of <0.078 eV by the I–V method and significantly lower than 0.08 eV by the activation-energy method. For Al/S-passivated p-type Si(100) junctions, the barrier height is ~0.77 eV by I–V and activation-energy methods and 1.14 eV by the C–V method. The discrepancy between C–V and other methods is explained by image force-induced barrier lowering and edge-leakage current. The I–V behavior of an Al/S-passivated p-type Si(100) junction remains largely unchanged after 300 °C annealing in air. It is also discovered that heating the S-passivated Si(100) wafer before Al deposition significantly improves the thermal stability of an Al/S-passivated n-type Si(100) junction to 500 °C.     

Radiation of caustic beams from a collapsing bullet

We show both theoretically and experimentally that a collapsing (2+1)-dimensional wave packet in a medium with cubic nonlinearity and a two-dimensional dispersion of an order higher than parabolic irradiates untrapped dispersive waves. The studies are performed for a spin-wave bullet propagating in an in-plane magnetized ferrimagnetic film. An induced uniaxial anisotropy in such a medium leads to the formation of narrow spin-wave caustic beams whose angles to the bullet's propagation direction are modified by the motion of the source.     

UV laser-induced high resolution cleaving of Si wafers for micro-nano devices and polymeric waveguide characterization

In this work we propose a method for cleaving silicon-based photonic chips by using a laser based micromachining system, consisting of a ND:YVO₄ laser emitting at 355 nm in nanosecond pulse regime and a micropositioning system. The laser makes grooved marks placed at the desired locations and directions where cleaves have to be initiated, and after several processing steps, a crack appears and propagate along the crystallographic planes of the silicon wafer. This allows cleavage of the chips automatically and with high positioning accuracy, and provides polished vertical facets with better quality than the obtained with other cleaving process, which eases the optical characterization of photonic devices.This method has been found to be particularly useful when cleaving small-sized chips, where manual cleaving is hard to perform; and also for polymeric waveguides, whose facets get damaged or even destroyed with polishing or manual cleaving processing. Influence of length of the grooved line and speed of processing is studied for a variety of silicon chips. An application for cleaving and characterizing sol-gel waveguides is presented. The total amount of light coupled is higher than when using any other procedure.     

Silicone oxidation for a‐Si:H passivation of wafers bonded to glass

A possible scenario for wafer‐based silicon photovoltaics is the processing of solar modules starting from thin silicon wafers bonded to glass. However, interactions between the adhesive used for bonding and the solar cell processing can affect the surface passivation of the bonded wafer and decrease cell performances. A method that suppresses these interactions and leads to state‐of‐the‐art a‐Si:H surface passivation is presented in this Letter. The method is based on an increase of the surface cross‐linking of a silicone adhesive by means of an O₂ plasma and it is successfully tested on three different silicones. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)