Thin Damaged Layer Research Articles

An approximate formula for the Rayleigh wave velocity in the machined surface with residual stress

The present paper is concerned with the Rayleigh wave propagation in the machined surface with a thin damaged layer with residual stress. We assume that the layer and the half-space are bonded perfectly to each other. Biot’s theory of small deformations influenced by initial stress forms the basis for this study. With the help of the effective boundary condition method, a third-order approximate secular equation of Rayleigh waves is established for the case that the layer and half-space are both orthotropic. In addition, an explicit third-order approximate formula for the Rayleigh wave velocity is derived from the secular equation. By considering a sample of silicon wafer by fine grinding with fine abrasive grains, the accuracy of the approximate formula has been verified. This study is meant to serve as the mathematical foundation for non-destructive testing of residual stress in the practical applications.

An empirical equation for prediction of silicon wafer deformation

A thin subsurface damage layer is often formed in the ground layer of a silicon wafer. Compressive stresses exist in the damage layer of the wafer and cause the wafer to deflect when it is unloaded from the vacuum chuck. The residual stresses play an important role in evaluating the machining quality of a wafer. A concise yet accurate stress-deflection relationship is desired so that the residual stresses can be calculated from the wafer deflection. The theoretical equation based on minimizing the total strain energy is often used. However, the anisotropic effects of silicon are neglected and would incur errors when it is applied on silicon wafers. This study establishes an empirical equation via a finite element (FE) model. Both equations are verified in experiments. It is found that FE model is more accurate than the theoretical equation.

Thermal conductivity reduction of tungsten plasma facing material due to helium plasma irradiation in PISCES using the improved 3-omega method

The near-surface region of plasma facing material (PFM) plays an important role in thermal management of fusion reactors. In this work, we measured thermal conductivity of tungsten (W) surface layers damaged by He plasma in PISCES at UCSD. We studied the damage effect on both bulk, and thin film, W. We observed that the surface morphology of both bulk and thin film was altered after exposure to He plasma with the fluence of 1 × 1026 m−2 (bulk) and 2 × 1024 m−2 (thin film). Transmission electron microscopy (TEM) analysis reveals that the depth of the irradiation damaged layer was approximately 20 nm on the bulk W exposed to He plasma at 773 K for 2000 s. In order to measure the thermal conductivity of this exceedingly thin damaged layer in the bulk W, we adopted the well-established ‘3-omega’ method and employed novel nanofabrication techniques to improve the measurement sensitivity. For the damaged W thin film sample, we measured the reduction in electrical conductivity and used the Wiedemann-Franz (W-F) law to extract the thermal conductivity. Results from both measurements show that thermal conductivity in the damaged layers was reduced by at least ∼80% compared to that of undamaged W. This large reduction in thermal conductivity can be attributed to the scattering of electrons, the dominant heat carriers in W, caused by defects introduced by He plasma irradiation.

Orientation-dependent plasma etch rates of single crystal silicon for dry etcher parts

Abstract We have evaluated the orientation dependence of plasma etch rates for single crystal silicon using SF6/Ar and CF4/Ar/O2 chemical gases in an inductively coupled plasma dry etcher. Etch rates for different crystal orientations having various surface conditions were measured. The results indicate that the <111> crystal lattice orientation is etched at a lower rate than the <100> orientation. The dependence of the silicon etch rate on the crystal orientation is derived from differences in the local neighborhood of the exposed surface atoms, such as number of neighbors, dangling bond, and surface bond. Finishing the silicon parts with mirror-finish polishing results in surfaces with a very thin subsurface damage layer that exhibits a lower plasma etch rate. This compares to the much thicker damage layer caused by abrasive machining of the surface during grinding steps, and to the portion of this that remains after polishing.

Laser etch back process to fabricate highly efficient selective emitter c-Si solar cells

We developed a novel cost effective process scheme for the fabrication of highly efficient selective emitter solar cells, which uses a laser doping method combined with an etch back process. The laser doping process using a 150ns pulse width green (532nm) laser effectively controls the doping profiles to form a selective emitter. However, laser damage was created on the laser-doped surface and eventually the performances and stabilities of laser-doped cells were degraded due to this damage. Using a transmission electron microscope (TEM), the damage was examined and found to have a thickness of 40nm of amorphous silicon. This thin damage layer was effectively removed in an acid mixture solution. The combined process of laser doping and etch back is called the laser etch back process. After removal of this thin damage layer, the cell efficiencies were significantly improved up to 19.17%.

Measurement of thermal conductivity in proton irradiated silicon

We investigate the influence of proton irradiation on thermal conductivity in single crystal silicon. We apply a laser based modulated thermoreflectance technique to measure the change in conductivity of the thin layer damaged by proton irradiation. Unlike time domain thermoreflectance techniques that require application of a metal film, we perform our spatial domain measurement on uncoated samples. This provides greater sensitivity to the change in conductivity of the thin damaged layer. Using sample temperature as a parameter provides a means to deduce the primary defect structures that limit thermal transport. We find that under high temperature irradiation the degradation of thermal conductivity is caused primarily by extended defects.

Electron Spin Resonance Probing of Defects in Si Foils Fabricated by the SLIM-Cut Method

Silicon thin (< 100μm) foils for photovoltaic applications have been fabricated using a novel low-temperature (< 150°C) lift- off process with the aim of reducing production costs. The quality of the resulting material has been assessed by electron spin resonance (ESR) spectroscopy in terms of occurring paramagnetic defects. For reasons of comparison, foils are prepared from (100) and (111) oriented silicon wafers, for which similar results were obtained. In previous work on the as-cleaved foils, hole traps were detected which could be associated with D-line defects (Si dangling bonds in disordered environment). These traps disappear after vacuum annealing (606°C), but at the same time new specific defects associated with Si/SiO2 interfaces are detected, in particular, Pb0 and Pb centers in the case of (100) and (111) Si foils, respectively. Silicon etch back experiments indicated all signals to be located only in a top layer (< 4.4μ m thick) on the crack side of the foil. The data show that the SLIM-Cut fabrication method may result in high quality Si material after etching off a thin damaged surface layer.

The origin of a peak in the reststrahlen region of SiC

A peak in the reststrahlen region of SiC is analyzed in order to establish the origin of this peak. The peak can be associated with a thin damaged layer on the SiC wafers, and a relation is found between surface roughness and the height of this peak, by modeling the damaged layer as an additional layer when simulating the reflectivity from the wafers.

Effect of surface roughness on the temperature of a silicon wafer heated by incoherent radiation

The effect of surface roughness of a silicon wafer on its temperature on heating by incoherent radiation is studied. The temperature-time diagrams of heating and cooling are obtained experimentally for double-side- and single-side-polished wafers at different arrangements of the rough surface with respect to the radiation source. The steady-state temperatures of such wafers under identical conditions of heating from 100 to 250°C are compared. It is established that the highest steady-state temperatures are common to the wafers presenting their rough side to the radiation source, the lowest steady-state temperatures to the double-side- polished wafers, and the intermediate temperatures to the wafers presenting their polished side to the radiation source. An optical model of the rough surface is suggested. In this model, the rough surface is represented by a negligibly thin damaged layer characterized by its own optical parameters. In the model, an optical parameter is introduced to characterize the degree of roughness of the illuminated surface. The dependence of the absorptivity of the wafer on this parameter and on the arrangement of the rough surface with respect to the radiation source is treated theoretically. The model provides a qualitative interpretation of the experimental data.

Molecular Depth Profiling of Sucrose Films: A Comparative Study of C60n+ Ions and Traditional Cs+ and O2+ Ions

Depth profiling of sucrose thin films was investigated with time-of-flight secondary ion mass spectrometry (TOF-SIMS) using 10 keV C(60)(+), 20 keV C(60)(2+), and 30 keV C(60)(3+), and 250, 500, and 1000 eV Cs(+) and O(2)(+) as sputtering ions. With C(60)(n+) ions, the molecular ion signal initially decreases and reaches a steady state that is about 38-51% of its original intensity, depending on the energy of the C(60)(n+) ions. In contrast, with Cs(+) and O(2)(+) sputtering, molecular ion signals decrease quickly to the noise level, even using very low-energy (250 eV) ions. In addition, the measured width of the sucrose/Si interface is much narrower using C(60)(+) ions than that using Cs(+) or O(2)(+) ions. To understand the mechanisms of sputtering-induced damage by these ions, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) were used to characterize the bottoms of these sputter craters. XPS data show very little chemical change in the C(60)(+) sputter crater, while considerable amorphous carbon was found in the O(2)(+) and Cs(+) sputter craters, indicating extensive decomposition of the sucrose molecules. AFM images show a very flat bottom in the C(60)(+) sputter crater, while the bottoms of the Cs(+) and O(2)(+) sputter craters are significantly rougher. We used the sputtering model developed by Wucher and co-workers to quantitatively analyze our C(60)(1-3+) data. The results show that low energy C(60)(+) ions generate a relatively thin damage layer with a high molecular ion signal, suggesting that low energy C(60)(+) may be the optimal choice for molecular depth profiling of sucrose films.

Damaged Layer Analysis for AFM-Based Mechanical Modifications on (100) Si Surface

Micro/Nanofabrication of silicon substrate based on the atomic force microscope (AFM) followed by wet chemical etching was demonstrated. A specially designed cantilever with a diamond tip, allowing the formation of damaged layer on silicon substrate by a simple scratching process, has been applied instead of conventional Si or Si3N4-based micro cantilever for scanning. A thin damaged layer forms in the substrate at the diamond tip-sample junction along scanning path of the tip, which was found to be a low crystallized amorphous silicon layer. Hence these sequential processes, called tribo nanolithography, TNL, can fabricate 2D or 3D micro structures in nanometer range. The developed TNL tools show outstanding machinability against single crystal silicon wafer. Hence, they are expected to have a possibility for industrial applications as a micro-to-nano machining tool. According to our results, it has been clearly known that the damaged layer withstands against aqueous potassium hydroxide solution, while it dissolves in diluted hydro fluoric (DHF) solution.

Influence of a structurally damaged surface layer of the hexagonal crystal on the dispersion and attenuation of Rayleigh surface acoustic waves

Analytical expressions for the dispersion of the phase velocity and the inverse attenuation length of Rayleigh waves are derived with allowance made for a thin (as compared to the length of the surface wave) isotropic damaged surface layer that is contiguous with vacuum and located on the surface of a hexagonal crystal with the sixfold axis perpendicular to the surface. It is demonstrated that, in the limit of long wavelengths (as compared to the characteristic inhomogeneity size), which is of greatest interest for experimenters, the change in the dispersion of the phase velocity of Rayleigh waves is proportional to the second power of the frequency, whereas the inverse attenuation length of Rayleigh waves is proportional to the fifth power of the frequency. The inverse attenuation length of the Rayleigh wave is calculated numerically. The calculation method previously proposed by one of the authors (Kosachev, 1998) is generalized to the case of an isotropic damaged layer on an anisotropic (hexagonal) substrate.

Characteristics of mask layer on (100) silicon induced by tribo-nanolithography with diamond tip cantilevers based on AFM

Abstract Nanoscale fabrication of silicon substrate based on the use of atomic force microscopy (AFM) followed by chemical etching was demonstrated. A specially designed cantilever with a diamond tip, allowing the formation of damaged layer on silicon substrate by a simple scratching process, has been applied instead of conventional silicon cantilever for scanning. A thin damaged layer forms in the substrate at the diamond tip–sample junction along scanning path of the tip, which was found to be a low crystallized amorphous silicon layer. Hence, these sequential processes, called tribo-nanolithography, TNL, can fabricate 2D or 3D microstructures in nanometer range. Diamond tip fabrication processes for TNL follow the micro-patterning, wet chemical etching and CVD based on MEMS processes. The developed TNL tools show outstanding machinability against single crystal silicon wafer. Hence, they are expected to have a possibility for industrial applications as a micro-to-nano-machining tool. In our previous work, is has been clearly known that the damaged layer withstands against aqueous potassium hydroxide solution, while it dissolves in diluted hydro fluoric solution. This study demonstrates the effect of various machining parameters on mask layer, followed by wet chemical etching in potassium hydroxide and hydro fluoric solution.

Study on Structure Transformation of Si Wafer in Grinding Process

In this paper, the surface and subsurface of silicon wafers ground by different wheels have been studied. In the conventional grinding with diamond wheels, it is shown from the top that the subsurface of wafer consists of amorphous Si, followed by a thin damaged layer, strained crystal with a large compressive residue stress, and then the bulk material in single crystal. In a severe condition which causes grinding burn, part of amorphous Si is re-crystallized to form a poly-crystal Si, and part of amorphous Si possibly reacts with oxygen to form SiO2. This phenomenon becomes more pronounced in the backgrinding process with a fine grit diamond wheel when the conditions are improperly selected. In order to obtain a defect-free crystal Si structure in grinding, authors have proposed a new chemo-mechanical grinding (CMG) process which enables to remove Si from wafer but with no structure transformation induced to its surface.

Diamond Tip Cantilever for Micro/Nano Machining Based on AFM

Nano-scale fabrication of silicon substrate based on the use of atomic force microscopy (AFM) was demonstrated. A specially designed cantilever with diamond tip allows the formation of damaged layer on silicon substrate by a simple scratching process. A thin damaged layer forms in the substrate along scanning path of the tip. The damaged layer withstands against wet chemical etching in aqueous KOH solution. Diamond tip acts as a patterning tool like mask film for lithography process. Hence these sequential processes, called tribo-nanolithography, TNL, can fabricate 2D or 3D micro structures in nanometer range. This study demonstrates the fabrication processes of the micro cantilever and diamond tip as a tool for TNL. The developed TNL tools show outstanding machinability against single crystal silicon wafer. Hence, they are expected to have a possibility for industrial applications as a micro-to-nano machining tool.

Raman spectroscopy of porous and bulk GaP subjected to MeV-ion implantation and annealing

Porous layers on (100)-oriented n-type liquid encapsulated Czochralski grown GaP crystals were fabricated by electrochemical etching in a H2SO4 aqueous solution and analyzed by scanning electron microscopy. C+12 ions were introduced at room temperature by 3 MeV energy implantation into porous and bulk samples at two ion doses of 1014 and 1015 cm−2. The prepared samples were annealed in the temperature range between 200 and 600 °C applying rapid thermal annealing (RTA) technique. A comparative micro-Raman study was carried out on the porous and bulk substances. Porosity was found to lead to the violation of the selection rules and to remarkable changes in the optical properties. Additionally, Fröhlich-type modes were observed in the Raman spectra of the porous layers. High energy implantation produces a thin high damaged layer, buried at the depth of the mean projected range. Implantation does not result in a drastic damage of the samples and they undergo a fast recovery after RTA. After this treatment a semi-insulating GaP layer is created, which is thermally stable up to 600 °C.

A damage-reduced process revealed by photoluminescence in photoelectrochemical etching GaN

Photoelectrochemical (PEC) etching technique has been proven to be an effective method to etch GaN. Despite its success, investigations on etching-induced damage are still scare. In this work, the damage induced by PEC etching of GaN in KOH electrolyte was studied. Photoluminescence (PL) spectroscopy was used to explore the origin of etching-induced damaged layer. From the variable temperature PL measurements, the origin of etching-induced damage was attributed to be the defect complex of VGa-ON (gallium vacancy bonds to oxygeon on nitrogen antisite). With determination of the defect origin, the electronic transition in the etch damage-related yellow luminescence (YL) band was suggested to be deep donor-like state to shallow-acceptor transition. In addition, a post-treatment method with boiled KOH chemical etching was developed to remove the thin damaged layer. In this method, crystallographic etching characteristics of boiled KOH was observed to assist in the formation of smooth sidewall facets. As revealed by the reduction of yellow luminescence, we propose this novel technique as a near damage-free etching method.

A Damage-Reduced Process Revealed by Photoluminescence in Photoelectrochemical Etching GaN

AbstractPhotoelectrochemical (PEC) etching technique has been proven to be an effective method to etch GaN. Despite its success, investigations on etching-induced damage are still scare. In this work, the damage induced by PEC etching of GaN in KOH electrolyte was studied. Photoluminescence (PL) spectroscopy was used to explore the origin of etching-induced damaged layer. From the variable temperature PL measurements, the origin of etching-induced damage was attributed to be the defect complex of VGa-ON (gallium vacancy bonds to oxygeon on nitrogen antisite). With determination of the defect origin, the electronic transition in the etch damage-related yellow luminescence (YL) band was suggested to be deep donor-like state to shallow-acceptor transition. In addition, a post-treatment method with boiled KOH chemical etching was developed to remove the thin damaged layer. In this method, crystallographic etching characteristics of boiled KOH was observed to assist in the formation of smooth sidewall facets. As revealed by the reduction of yellow luminescence, we propose this novel technique as a near damage-free etching method.

Initiation And Growth Behaviours Of A FatigueCrack In Shot-peened Steel

In this paper, rotating bending fatigue tests are carried out in order to clarify the difference of initiation and growth behavior of a crack in shot-peened carbon steel specimens and not-peened ones. The material used is a quenched and tempered carbon steel. The difference between the two is clarified through the successive observations of the specimen surface by the replication technique and the measurements of the residual stress during the fatigue test. The effect of shot-peening on the growth behavior of a crack is discussed based on the fact that it is classified into three parts : the effect in thin damaged layer due to shot-peening, the effect in work-hardened layer with residual stress and the effect in non-work-hardened layer.

Sputtering of alkali halides studied by a mass selected time of flight spectroscopy

Abstract Energy distributions of alkali and halogen atoms sputtered with a low-energy electron beam have been measured for NaCl, CsI and KBr single crystals. Energy spectra for alkali atoms can be described by the Maxwell-Boltzmann distribution. In case of halogen atoms, for some crystals, an additional, non-thermal component appears. The behaviour of this component was investigated as a function of the target temperature, the beam energy, its current density and the ejection angle. The results can be described by creation of focused replacement sequences and H-centre migration model. The ejection of non-thermal halogen atoms from the (100) surface of KBr single crystal is strongly forward peaked along the normal. This effect is caused by a thin damage layer appearing on the surface due to a strong nonstoichiometry of the erosion process itself.