Near-surface Damage Research Articles

Effect of ion implantation on thermal shock resistance of magnesia and glass

Monocrystals of magnesia together with glass samples have been subjected to ion implantation prior to thermal shock testing in an impulse plasma of continuously varied intensity. Measurements of the separation between fragments have been used to estimate the surface temperature. Fracture and deformation characteristics of the surface layer are measured in ion implanted and unimplanted samples using optical and scanning electron microscopy. Implantation-induced near-surface damage is analysed by ion channelling using 2 MeV He + ions. Ion implantation is shown to modify the near-surface structure of magnesia samples by introducing damage, which makes crack initiation easier under thermal stresses. The fracture threshold and maximum crack density are shifted towards the lower temperature range. Ion implanted MgO crystals show a ten-fold increase in surface crack density. An increased crack density results in a decreased degree of damage characterized by the depth of crack penetration. The thermal stress resistance parameter of glass samples is increased at relatively small doses and decreased at higher doses. The results suggest that crack density and the degree of fracture damage in brittle ceramics operating under thermal shock conditions can be effectively controlled by ion implantation which provides crack initiating defects in the near-surface region.

X-ray scattering from a rough surface and damaged layer of polished wafers

Theoretical and experimental investigations were performed to show the application of X-ray crystal truncation rod scattering combined with X-ray reflectivity in the measurements of surface roughness and near-surface damage of mechanochemically polished wafers. By fitting the measured crystal truncation rod curves it has been shown that polished wafers are divided into three parts-irregular steps on the surface, a damaged thin layer beneath the surface and a perfect bulk. The results show that the root mean square of the surface roughness of mechanochemically polished Fe-doped and/or S-doped InP wafers is one to two atom layers, and that the lateral correlation length of the surface roughness is about 3000-7500 AA. The thickness of the damaged region is found to be about 1000 atom layers.

Removal of surface contamination after reactive ion etching of silicon dioxide

Reactive Ion Etching (RIE) during semiconductor device fabrication can cause surface and near-surface damage or contamination which may degrade electrical performance. We have investigated methods of removing this damage and contamination prior to further device processing. Using X-ray photoelectron spectroscopy we have analysed the surface residue remaining after a CHF 3-based RIE process used for the removal of thermal oxide on Si substrates. The dry etched wafer surface was studied before and after wet chemical cleaning or UV/O 3 exposure. Surface morphology was investigated using atomic force and scanning optical microscopy. Thickness of thermal oxides grown upon dry etched and subsequently treated wafer surfaces were measured. The results indicate the formation of a modified Si surface region underlying a residue layer formed during the RIE process. We report an investigation of the composition and oxidation behaviour as a function of depth through this near-surface region. This was performed by chemical depth profiling using a sequence of room temperature UV/O 3 oxidation and dilute HF oxide removal steps.

Review: Various methods of machining advanced ceramic materials

A collaborative programme has been initiated recently under the title ‘Machining of Advanced Engineering Materials Using Traditional and Laser Techniques’. A first step of reviewing the ‘state-of-the-art’ of the various aspects of this venture has been undertaken, examining aspects of machining Ceramic Matrix Composites (CMCs) by a range of methods, these including grinding, ultrasonics, abrasive water jet, electrical discharge and laser techniques. This paper reports on each and considers the trade-off between the material removal rate and the level of near-surface damage that these process involve.

Effect of near-surface damage on C-V measurements of Schottky barrier diodes

During a number of current silicon technological processes used for the realization of Schottky barrier diodes, the silicon surface is bombarded by energetic particles so that deep traps are being detected in the device up to a fraction of a micron from the metal-silicon interface. The effect of such traps on the parameters, usually extracted from capacitance-voltage measurements under the form 1/ C 2 − V is studied. It is shown that the traps modify the extracted doping profile at a low reverse bias and that they have a strong effect on the extracted barrier height which can be larger or lower than the value obtained in the absence of traps, depending upon the trap characteristics. Experimental results obtained on Ti−W / nSi Schottky barrier diodes confirm these considerations.

An analysis of the creep embrittlement of Inconel alloy X-750 due to its prior exposure to reduced air pressure at 1150°C

Inconel alloy X-750 test pieces exposed to a reduced air pressure at 1150°C for various times exhibited significant embrittlement during subsequent creep tests in air at 700°C. This embrittlement was characterized by a systematic reduction in time and strain to fracture and an acceleration of the minimum creep rate with increasing time of prior exposure. Earlier work has shown that the near-surface environmental damage in the form of heavily cavitated boundaries and γ-free zones are responsible for this form of embrittlement. The purpose of this investigation is to provide a comprehensive analysis of the effect of the above two forms of damage on the time and strain to fracture and the minimum creep rate. Towards this end, a three-zone creep fracture model has been developed to explain the experimental results.

Synergistic erosion of graphite due to simultaneous bombardment by C + and H +

The chemical sputtering of pyrolytic graphite due to simultaneous bombardment by C + and H + ions has been studied as a function of ion energies, relative fluxes and sample temperature. Combined bombardment by C + ions (1 and 3 keV) and 300 eV H + does not result in a significant increase in the hydrocarbon yield (i.e., Σ C i H j /H + ) over the H + -only case. On the other hand, for 100 eV H + ions, an increase of up to about ∼50% in the hydrocarbon yield was observed, as the Φ C + / Φ H + flux ratio was varied from about 0.01 to 0.05. Above this flux ratio, the surface damage caused by the energetic ions appears to saturate, and there is no further increase in hydrocarbon production. The effect of near-surface damage on hydrocarbon formation was also seen for He + +H + cobombardment.

Surface radiation damage

The bombardment of solid surfaces with ions having energy below or close to the damage threshold can lead to the deposition of layers of crystalline material or to the formation of interesting surface compound layers. The structure of the grown materials is dependent on the extent to which the energy deposited by the incident ions enables structural rearrangement to occur without causing atomic displacements. Radiation damage very close to the surface can be highly stable and is sensitive to factors such as angle of incidence and the presence of contaminants as well as the energy of the incident ions and the mass ratio of the collision partners. In the present paper, the state of knowledge of the buildup and annealing of near-surface radiation damage due to ion bombardment is reviewed with particular reference to the potential of ion beams for the growth of crystalline films at low temperatures.

Secondary electron emission spectroscopy and total electron yield measurements for the assessment of near-surface damage in diamond

In this work the sensitivity of secondary electron emission (SEE) spectroscopy in the 0–50 eV range and total electron yield (TEY) for characterising the near-surface region of diamond with varying levels of damage is examined. This is accomplished by SEE and TEY measurements of diamond surfaces subjected to ever increasing doses of 1 keV Ar ion irradiation. The irradiation gradually damages, amorphises and eventually graphitises the diamond surface thus providing a continuous spectrum of different forms of carbon ranging from crystalline sp 3 to amorphous sp 2 bonded carbon. A strong and abrupt charging effect is observed as a function of irradiation dose. This effect was correlated with changes in TEY and sample conductivity. It is found that TEY and SEE are very sensitive measures of the crystalline perfection of the near-surface region of diamond. It is suggested that SEE and TEY be used as sensitive tools for the characterisation of the near-surface region of diamond thin films.

Dry-etching techniques and chemistries for III–V semiconductors

Abstract Dry etching of III–V materials using both chlorine-based (CCl2F2, SiCl4, BCl3 and Cl2) and CH4-H2 discharges is briefly reviewed. The etch rates using chlorine-based mixtures are generally faster than those utilizing CH4-H2, but the latter gives smoother surface morphologies for indium-containing compounds. The use of microwave (2.45 GHz) electron cyclotron resonance (ECR) discharges minimizes the depth of lattice disorder resulting from dry etching, relative to conventional r.f. (13.56 MHz) discharges. Recent results on the systematics of ECR plasma etching of both indium- and gallium-based III–V semiconductors using CCl2F2-O2 and CH4-H2 mixtures will be discussed, including the determination of the maximum self-biases allowable which do not induce near-surface damage in the semiconductor. A further key issue is the prevention of changes in the surface stoichiometry of materials such as InP, where the lattice constituents may have considerably different volatilities in the particular discharge.

Plasma etching damage in GaAs studied by resonant Raman scattering

We have used resonant Raman scattering by longitudinal optical (LO) phonons to study the effect of reactive ion etching (RIE) in a CHF3 plasma on n-type GaAs. Interference effects between dipole allowed and electric-field-induced forbidden LO phonon scattering have been exploited to distinguish between impurity-induced and electric-field-induced forbidden scattering. This allowed us to analyze both RIE-induced near-surface damage and resulting changes in the space-charge electric field as a function of the self-bias voltage applied in RIE. For bias voltages beyond 200 V a well-defined increase in defect concentration and consequently a reduction in crystalline perfection was observed. The surface electric field averaged over the probing depth of the Raman experiment shows an initial decrease followed by an increase with increasing bias voltage.

SEM electron channelling patterns as a technique for the characterization of ion‐implantation damage

SUMMARYElectron channelling patterns (ECPs) formed in back‐scattered images in the scanning electron microscope (SEM) have been used occasionally to confirm surface amorphization during ion implantation. In order to place such observations on a more quantitative basis, the study reported here has explored the variation of ECP appearance with both specimen damage levels (and thus subsurface structures) and SEM accelerating voltage (i.e. sampled depth).Polished and annealed (0001) single crystal sapphire discs were implanted to various damage levels up to both subsurface and full surface amorphization. Damage levels were measured independently by Rutherford back‐scattering (RBS). Selected‐area ECPs were obtained in a Jeol‐840 electron microscope operating over the range 5–40 kV in 5‐kV steps.Progressive ECP degradation—in terms of high‐order line disappearance—was observed with increasing dose, culminating in total pattern loss when full surface amorphization occurred. However, ECP information could still be obtained from the damaged near‐surface material even when a subsurface amorphous layer was present, thus demonstrating the shallow retrieval depth of information from the ECP technique. Indeed, because the spatial distribution of damage from ion implantation is both calculable and measurable, these experiments have also allowed us, for the first time, to explore and demonstrate the shallow sample depths from which the majority of ECP contrast originates (< 150 nm in sapphire at an accelerating voltage of 35 kV), even when the beam penetration is considerable by comparison (∼ 5 μm). Furthermore, the way in which this sampled depth varies with SEM accelerating voltage is both demonstrated and shown to be a powerful diagnostic technique for studying the distribution of near‐surface structural damage.

Near-surface damage created in silicon by BF 2+ implantation

The near-surface damage in silicon induced by the bombardment of 147 keV BF 2 + has been investigated by 2 MeV He + Rutherford-backscattering spectrometry. The implantation was carried out at room temperature with the ion doses ranging from ~ 10 13 to ~ 10 16 cm −2. The radiation damage was compared with corresponding B + and F + atomic-ion implantation. A damage enhancement at the surface region of the silicon implanted with BF 2 + has been observed and it is attributed to the multiple-collision effect between molecular ions and host atoms.

7354. Near-surface damage and contamination of silicon following electron cyclotron resonance etching: A S Yapsir et al, J Vac Sci Technol, A8, 2939–2944

7354. Near-surface damage and contamination of silicon following electron cyclotron resonance etching: A S Yapsir et al, J Vac Sci Technol, A8, 2939–2944

Ion milling damage in InP and GaAs

Near-surface damage created by Ar+ ion milling in InP and GaAs was characterized by capacitance-voltage, current-voltage, photoluminescence, ion channeling, and transmission electron microscopy. We find no evidence of amorphous layer formation in either material even for Ar+ ion energies of 800 eV. Low ion energies (200 eV) create thin (≤100 Å) damaged regions which can be removed by annealing at 500 °C. Higher ion energies (≥500 eV) create more thermally stable damaged layers which actually show higher backscattering yields after 500 °C annealing. Heating to 800 °C is required to restore the near-surface crystallinity, although a layer of extended defects forms in GaAs after such a treatment. No dislocations are observed in InP after this type of annealing. The electrical characteristics of both InP and GaAs after ion milling at ≥500 eV cannot be restored by annealing, and it is necessary to remove the damaged surface by wet chemical etching. For the same Ar+ ion energies the damaged layers are deeper for InP than for GaAs after 500 eV ion milling at 45° incidence angle. Removal of ∼485 and ∼650 Å from GaAs and InP, respectively, restores the initial current-voltage characteristics of simple Schottky diodes.

Dry Etching Techniques and Chemistries for III-V Semiconductors

ABSTRACTDry etching of HI-V materials using both Cl-based (CCl2F2, SiCl4, BCl3, Cl2) and CH4/H2 discharges will be reviewed. The etch rates using chlorine-based mixtures are generally faster than those utilizing CH4/H2, but the latter gives smoother surface morphologies for In-containing compounds. The use of microwave (2.45 GHz) electron cyclotron resonance (ECR) discharges minimizes the depth of lattice disorder resulting from dry etching, relative to conventional RF (13.56 MHz) discharges. Recent results on the systematics of ECR plasma etching of both In- and Ga-based III-V semiconductors using CCl2F2/O2 and CH4/H2 mixtures will be discussed, including the determination of the maximum self-biases allowable which do not induce near-surface damage to the semiconductor. A further key issue is the prevention of changes in the surface stoichiometry of materials such as InP, where the lattice constituents may have considerably different volatilities in the particular discharge.

Photothermal deflection spectroscopy of heat-treated GaAs, InP, and InGaAsP alloys

Photothermal deflection spectroscopy is used to study the effect of heat treatment in bulk GaAs, InP, and epitaxial InGaAsP grown on InP. We show that increased sub-band-gap absorption is related to damage near the surface. We also show that from a measurement of the phase we can differentiate between purely surface and near-surface damage.

Optical properties of ion-implanted GaAs: The observation of finite-size effects in GaAs microcrystals.

We have carried out reflectivity measurements, for photon energies from 2.0 to 5.6 eV in the electronic interband regime, for a series of unannealed ion-implanted GaAs samples which had been exposed to 45-keV ${\mathrm{Be}}^{+}$ ions at various fluences up to 5\ifmmode\times\else\texttimes\fi{}${10}^{14}$ ions/${\mathrm{cm}}^{2}$. The microstructure of the near-surface implantation-induced damage layer in these samples is known (from previous Raman work) to consist of a fine-grain mixture of amorphous GaAs and GaAs microcrystals, with the characteristic microcrystal size L of the microcrystalline component decreasing with increasing fluence (L=55 A\r{} at 5\ifmmode\times\else\texttimes\fi{}${10}^{14}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$). The optical dielectric function of each sample's damage layer has been derived from the observed reflectivity spectrum by Lorentz-oscillator analysis. Then, by inverting the effective-medium approximation, we have extracted the dielectric function of the microcrystalline component (\ensuremath{\mu}-GaAs) within the damage layer.The optical properties of \ensuremath{\mu}-GaAs differ appreciably from those of the bulk crystal, the difference increasing with decreasing L. We find that the microcrystallinity-induced spectral changes are concentrated in the linewidths of the prominent interband features ${E}_{1}$, ${E}_{1}$+${\ensuremath{\Delta}}_{1}$, and ${E}_{2}$. These linewidths increase linearly and rapidly with inverse microcrystal size: ${\ensuremath{\Gamma}}_{\ensuremath{\mu}}$=${\ensuremath{\Gamma}}_{0}$+${\mathrm{AL}}^{\mathrm{\ensuremath{-}}1}$, where ${\ensuremath{\Gamma}}_{0}$ is the linewidth in the bulk crystal, ${\ensuremath{\Gamma}}_{\ensuremath{\mu}}$ is the linewidth in \ensuremath{\mu}-GaAs, and A is a constant. For the ${E}_{1}$ and ${E}_{2}$ peaks, the experimentally determined value of A is such that the finite-size broadening (${\mathrm{AL}}^{\mathrm{\ensuremath{-}}1}$) is about 0.2 eV when L=100 A\r{}. We propose a simple theory of the finite-size effects which, when combined with band-structure information for GaAs, semiquantitatively accounts for our observations. Small microcrystal size implies a short time for an excited carrier to reach, and be scattered by, the microcrystal boundary, thus limiting the excited-state lifetime and broadening the excited-state energy. An alternative uncertainty-principle argument is also given in terms of the confinement-induced k-space broadening of electron states.

Carbonaceous overlayer and near-surface damage after pure CF4 reactive ion beam etching of silicon: Exposure dose effects

Reactive ion beam etching (RIBE) is obtained from a specific ion gun, the electrostatic reflex ion source (Maxi-ERIS). It is operated with pure CF4 gas and provides a fair alternative process for selective etching of SiO2 over Si. The slow-down of the Si etch rate is related to the growth of a carbonaceous blocking overlayer. This latter is removed by a dip for 60 s in concentrated HF (50%). This wet clean-up procedure leaves a Si substrate the electrical properties of which are only slightly altered as compared to a control sample (Hg/Si contact characteristics). Auger electron spectrometry and ellipsometric data on both as-etched and wet-cleaned samples provide information about the mechanisms which dominate the CFx+/Si interaction. They are discussed with a special emphasis on the exposure close effect.

Observation of near-surface damage by phonon scattering.

We have investigated the feasibility of phonon-reflection techniques as nondestructive means to probe surface and/or near-surface damage in otherwise highly perfect crystals. An UHV liquid-helium stage, suitable for phonon-reflection measurements, was installed on a beam line of a tandem van de Graaff accelerator which was used to implant MeV ions into the substrate in order to modify the surface region in situ. Here, we report our investigation on the effects of 1-MeV ${\mathrm{Ar}}^{+}$ implantation in ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ single crystals by monitoring the reflection of terahertz (THz) phonons (50-A\r{} wavelength) from the implanted region. The results are compared to other surface techniques. Using a 15-kV ion gun on the same beam line, we have also bombarded ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ crystals coated with thin films of gold. The effects of a 7.5-keV ${\mathrm{Ar}}^{+}$ beam on this Au-${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ system are also discussed in this paper.