Inhomogeneous Distribution Of Defects Research Articles

Precipitation Behaviors of Rapid Thermal Annealing Treated Silicon Wafers under Various Thermal Cycles

For decades, oxygen precipitates in Cz silicon wafers have been a subject of defect engineering to optimize device yields and performance at both current high volume manufacturing and future generation nodes. With the ability of precipitates capturing metallic impurities, a well controlled built-in intrinsic gettering (IG) provides a self-insurance protecting fabs from suffering unexpected yield loss caused by metal contamination outside the control limit [1]. Such IG sinks traditionally are created, either by RTP or conventional furnace heat treatments, quite deep in the bulk, resulting in a wide denuded zone (DZ). This scheme has worked perfect for most devices with monolithic integration as fast diffusing metals travel the distance equal to the DZ depth quite easy. However, the industry is increasingly adopting 3-D integration, stacking multiple chips vertically using through Si via (TSV) techniques. Multichip stacking requires each die to be thinned substantially by back grinding down to 50um or even <30um, depending on the number of stacked dies. This results in either complete or major loss of IG sinks along with the bulk removal of the wafer. In this respect, IG with a shallower DZ is desired so that a sufficient amount of IG sinks are retained even after extensive thinning. On the other hand, the effect of oxygen precipitation on the yield strength is increasingly receiving attention as photolithography overlay is becoming more sensitive to local deformation [2]. A precisely controlled high density of precipitates would be desirable in this regard.In this report, we demonstrate that a 2-step Rapid Thermal Annealing (RTA) treatment in NH3 and Ar gas is capable of producing a high density of precipitates in an extremely uniform profile in the 300mm silicon wafers under CMOS-like thermal cycles. The RTA treatment is a special variant of MDZ® that has been widely adopted in the industry for >10 years [1, 3]. Figure 1 shows typical depth profiles of precipitates in the high oxygen material after through a CMOS thermal simulation and after a subsequent extra growth cycle. Adding the extra growth cycle was necessary for the sake of ensuring all the existing precipitates to be detectable as the metrology tool (laser scattering tomography) would otherwise miss those sub-threshold size in the count, especially in the case of the lower oxygen concentration material. Quite a uniform density of precipitates is formed at the depth of around 7 microns and extended to the entire thickness of the wafers with the maximum density around 8x109/cm3 and 7x109/cm3 in the cases of higher and lower oxygen concentration materials, respectively. It can also be seen that in the case of higher oxygen material the full cycle of the D simulation has effectively grown the precipitates so about 70% of the precipitate distribution was detectable without going through a growth cycle (the D2 curve). These results are pretty consistent with the previous work reported by co-authors, where the uniform density of precipitates is explained as a result of the nitride layer blocking vacancy sinking to the wafer surface [4]. The precipitation sites are created by vacancy aggregation in the course of fast post-RTA cooling. The radial distribution of oxygen precipitates is remarkably uniform, as shown in Fig. 2 (a) and (c). In comparison, the as-grown wafers (b and d) resulted in a large variation, especially in the high oxygen material, which is due to the inhomogeneous distribution of point defects formed during crystal growth. The result clearly demonstrates that the 2-step RTA eliminated the crystal originated variability, very effectively.In summary, adding a 2-step RTA treatment produces a high density precipitates near surface, uniformly both vertically and laterally, guaranteeing a <20um thick layer of IG sinks to be retained even after through wafer thinning to 30um.

Mechanical Properties of Commercial Carbon Fibers Using a Single Filament Tensile Test

In this study, mechanical properties of commercial carbon fibers were evaluated using a single filament tensile test with various fiber gauge lengths. Tensile strength increased significantly with a decreasing length of the test specimens possibly due to small defect sites. The compliance method provided more accurate moduli of the carbon fibers, removing system errors during the single filament tensile test. The Weibull modulus revealed that shorter specimens had an inhomogeneous defect distribution, leading to a higher tensile strength and its standard deviation. X-ray diffractograms of carbon fibers showed a similar crystallinity and orientation in spite of significant differences in the fiber modulus and strength, indicating that crystalline structure of the commercial carbon fibers used in the study was not attributable to the difference in their tensile properties. †(Received January 15, 2013)

Influence of defects on extreme ultraviolet laser ablation of LiF

We study the influence of defects (neutral Li${}^{0}$ and F${}^{0}$) on the extreme ultraviolet (XUV) laser-induced ablation of 10-nm-thick LiF films. Our method combines a molecular-dynamics scheme for LiF and a set of equations describing the temporal evolution of the conduction-electron density and temperature. We find that defects may decrease the ablation threshold by a factor of around 4. This is caused by the lattice destabilization and the tensile pressure the defects induce upon creation. Metallic colloids form due to the high mobility of Li${}^{0}$ in the heated crystal. Inhomogeneous defect distributions are shown to be even more effective for crack formation and ablation. In the extreme case, when the thermal heating induced by the laser is negligible and only the effect of defect formation is considered (``cold ablation''), LiF spalls in the solid state.

Analysis of Inhomogeneous Dislocation Distribution in Multicrystalline Si

Grain boundaries and dislocations are major crystallographic defects in multicrystalline Si materials for solar cells. Heavily dislocated grains are detrimental to the photovoltaic performance. This paper attempts to clarify the origin of inhomogeneous defect distribution in multicrystalline Si. The impacts of crystal orientation and grain boundary were investigated. The crystal orientation gives an important geometrical effect in the possibility of initiating slip in a grain when subjected to stress. The presence of grain boundary can also affect dislocation distribution depending on boundary character.

A prediction scheme of the static fracture strength of MEMS structures based on the characterization of damage distribution on a processed surface

This paper presents a scheme for predicting the strength of MEMS structures patterned into arbitrary shapes by deep reactive ion etching of a silicon wafer. The scheme is based on the inhomogeneous defect distribution on the etched surfaces. Single-crystal silicon specimens with different shapes were subjected to four-point bending tests with monotonically increasing load. The distributions of the fracture strength were described using two-parameter Weibull statistics, where the two parameters are defined as functions of the etching depth representing the inhomogeneity of the damage on the etched surface in the direction perpendicular to the wafer. In order to estimate the distribution of the local strength determined by the local level of damage, the etched surfaces of specimens without a notch were tested in three different bending directions corresponding to three different stress distributions, and three different strength levels were given due to surface roughness conditions caused by the non-uniformity of the etching process. The estimated values of the parameters were used to estimate the fracture strength of four types of notched specimens with different notch tip radii. The results of comparison between the distributions of predicted strengths and experimental data showed that the fracture strength of arbitrarily shaped structures is predictable on the basis of the information obtained from specimens without notch, by taking into account the characteristics of etched surface, i.e. the inhomogeneous damage.

Cast defects induced sample-size dependency on compressive strength and fracture toughness of Mg–Cu–Ag–Gd bulk metallic glass

In this work, Mg59.5Cu22.9Ag6.6Gd11 bulk metallic glass (BMG) is selected to investigate the specimen-size dependency of the compressive fracture strength and notch toughness. As the diameter of cylindrical sample increases, apparent fracture strength is reduced by 25% from ∼950MPa at 1-mm down to ∼710MPa at 10-mm sample, which is associated with the population and size of as-cast porosity in the BMG rods as indicated by X-ray computed tomography (CT). Meanwhile, it is inappropriate to describe the size dependence of fracture strength using Weibull or Gumbel statistics owing to the inhomogeneous distribution of cast defects in different sized rods. Similar to the compressive fracture strength, cast defects in Mg59.5Cu22.9Ag6.6Gd11 BMG also yield specimen-size dependency of the measured notch toughness, when the plate-specimen thickness varies within the range of 1–8mm. With three-parameter Weibull analysis, threshold toughness Kμ of as-cast Mg59.5Cu22.9Ag6.6Gd11 BMG can be estimated, which represents a minimum toughness value below which the cracks arrest. Using finite element analysis, it is confirmed that the specimen-thickness dependency of notch toughness does not result from the transition from plane-stress to plane-strain in the specimen, but from the extrinsic flaws.

Confocal spectroscopy of InGaN LED structures

Photoluminescence of InGaN structures for green light-emitting diodes (LEDs) with multiple quantum wells as an active medium was studied with spatial and spectral resolution using confocal microscopy. Bright spots of ∼200 nm diameter were observed. Emission from these bright areas was up to 8 times more intense than from the rest of the sample surface and the band peak position in these areas was blueshifted with respect to the band position in the background surface of lower photoluminescence intensity. The data on emission properties in bright and dark areas and the dependence of these properties on the excitation power density were interpreted by assuming inhomogeneous distribution of defects acting as nonradiative recombination centres.

Scanning transmission electron microscope observations of defects in as-grown and pre-strained Mo alloy fibers

Compression testing of micro-pillars has recently been of great interest to the small-scale mechanics community. Previous compression tests on single crystal Mo alloy micro-pillars produced by directional solidification of eutectic alloys showed that as-grown pillars yield at strengths close to the theoretical strength while pre-strained pillars yield at considerably lower stresses. In addition, the flow behavior changes from stochastic to deterministic with increasing pre-strain. In order to gain a microstructural insight into this behavior, an aberration corrected scanning transmission electron microscope was used to study the defect structures in as-grown and pre-strained single crystal Mo alloy fibers. The as-grown fibers were found to be defect free over large lengths while the highly pre-strained (16%) fibers had high defect densities that were uniform throughout. Interestingly, the fibers with intermediate pre-strain (4%) exhibited an inhomogeneous defect distribution. The observed defect structures and their distributions are correlated with the previously reported stress–strain behavior. Some of the mechanistic interpretations of Bei et al. are examined in the light of new microstructural observations.

Localization of plastic deformation of quenched structural steel

Beginning with the study of Luders bands, researchers have paid enormous attention to plastic deformation processes [1, 2]. At the micro level, the goal of such research is to find a unique relation between the parameters of the dislocation structure and the deformation of metal [3]. The evolution of the dislocation structure on deformation does not reduce to simple increase in dislocation density but is a very complex process in which spatially inhomogeneous defect distributions are created. At high degrees of deformation, when the inhomogeneity of the plastic deformation is most pronounced, nondislocational deformation channels may appear [4]. In intense plastic deformation of 08 E 2C steel (ferritic‐pearlitic state) by drawing at room temperature, extended regions with ultradisperse structure—deformation channels—may be formed, as shown in [5‐7]. Such regions are seen as mottled contrast on dark-field images in matrix reflexes. Electron-microdiffraction patterns of the surfaces of such regions have a quasiannular structure as a rule. The deformation channels are sites of localization of the deformation. The length of such regions is tens of microns; their width is 1 μ m. With increase in the degree of deformation, the mean dimensions of the deformation channels increase [5‐7].

On the nucleation, coalescence, and overgrowth of HVPE GaN on misoriented sapphire substrates and the origin of pinholes

The nucleation of HVPE GaN on misoriented sapphire substrates and the transition from the nucleation layer to an epitaxial film were investigated. After a KOH/NaOH eutectic etch of the approximately 45 μ m thick GaN layer, grown on sapphire using a low temperature nucleation, high temperature epitaxy process, the cross-sections revealed columnar structures, up to roughly 1 μ m above the sapphire substrate. Photoetching of the thick GaN layers revealed inhomogeneous defect distributions along the cross-sections, which appeared to be related to the numerous pinholes originating at the GaN/sapphire interface. We present a model explaining the formation of pinholes by the coalescence of the GaN nuclei during the epitaxial overgrowth.

Electron dynamics in GaN wafers with an inhomogeneous distribution of defects in the depth direction

The dynamics of photoexcited electrons in a GaN wafer with an inhomogeneous dislocation density distribution in the depth direction was measured by the near-field heterodyne transient grating method. We investigated the dependence of the interfaces with different dislocation densities, the dependence on the pump wavelength for a sample with a decaying distribution of dislocation density, which gives information on the depth dependence on the carrier dynamics. In the latter experiment, the observed depth region was continuously changed by tuning the wavelength around the band gap of GaN. The responses of the electron dynamics can be fitted with the sum of two exponential decays for all the experiments and only the slower component decayed faster as the dislocation density increased. It was concluded that the faster component corresponded to the trap process of electrons by deep states due to inherent defects in the GaN wafer and the slower component corresponded to the recombination of the trapped electrons at the defect states by dislocation defects.

Structure, chemistry and electrical properties of extended defects in crystalline silicon for photovoltaics

AbstractThe electronic properties of present‐day multicrystalline silicon (mc‐Si) materials for photovoltaic applications are strongly influenced by point defects, their mutual interaction and their interaction with dislocations and grain boundaries. This paper presents results from fundamental investigations of metal impurity interaction with extended defects, namely a small‐angle grain boundary and bulk microdefects. It is shown that the distribution of copper silicide precipitates closely follows the density of bulk microdefects indicating the underlying physics of ‘good’ and ‘bad’ grains frequently observed in mc‐Si. Co‐precipitation of copper and nickel in the same samples leads to virtually the same distribution of multimetal silicide precipitates which according to light‐beam induced current measurements show the same recombination activity as single‐metal silicide particles. Transmission electron microscopy is used to show that for copper‐rich and nickel‐rich conditions two types of silicides co‐exist, i.e. Cu3Si precipitates containing a small amount of nickel and NiSi2 precipitates containing some copper. Finally, phosphorus‐diffusion gettering (PDG) is discussed as the main gettering process used in presentday silicon photovoltaics. Special emphasis is put on the effect of extended defects and their interaction with metal impurities on PDG kinetics. It is shown that different limiting processes will be simultaneously operative in mc‐Si as a result of inhomogeneous bulk defect distributions (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Depth-dependent critical behavior inV2H

Using x-ray diffuse scattering, we investigate the critical behavior of an order-disorder phase transition in a defective ``skin layer'' of ${\text{V}}_{2}\text{H}$. In the skin layer, there exist walls of dislocation lines oriented normal to the surface. The density of dislocation lines within a wall decreases continuously with depth. We find that, because of this inhomogeneous distribution of defects, the transition effectively occurs at a depth-dependent local critical temperature. A depth-dependent scaling law is proposed to describe the corresponding critical ordering behavior.

Heterogeneously doped nanocrystalline ceria films by grain boundary diffusion: Impact on transport properties

Heterogeneous doping of nanocrystalline ceria films, by controlled in-diffusion along grain boundaries, is explored as a means for modifying the space charge potential and the inhomogeneous distribution of defects in the space charge layer known to control the electrical properties of nanocrystalline electroceramics. Nanocrystalline cerium oxide thin films were grown by pulsed laser deposition and modified by a novel doping technique. Thin diffusion sources were deposited and cations such as Ni2+ and Gd3+ were in-diffused at temperatures of 700–800 °C along columnar grain boundaries normal to the surface; the resulting diffusion profiles were examined by Time-of-Flight SIMS. The properties of these modified films were compared with as-deposited samples, with in-diffusion resulting in decreased electrical conductivity. It is proposed that the variation in conductivity results from a redistribution of charge carriers in the space charge layers due to a change in the space charge potential.

Homogeneity Improvements in the Dielectric Characteristics of HfSiON Films by Nitridation

The correlation between conduction and lifetime in HfSiON films has been characterized with reference to the inhomogeneous distribution of defects. The electrical effects that are induced by the nitrogen have been also investigated. The temperature dependence of the leakage current revealed that the inclusion of nitrogen atoms transformed the defect levels, leading to a reduction in leakage current and improved breakdown lifetime. Moreover, it was also found that nitridation leads to a suppression of the dispersion of defects. The characteristics of oxygen vacancies were demonstrated in terms of nitridation effects.

Proton Damage in Amorphous Silicon/Crystalline Silicon Heterojunction Solar Cells: Measurement and Simulation

The applicability of amorphous silicon/crystalline silicon (a-Si:H/c-Si) heterojunction solar cells for their use in space environment was tested. The cells were subject to proton irradiation at energies between 0.8 and . The optical and electrical parameters were measured. The diffusion lengths, , were deduced from the internal quantum efficiency. A plot of vs the applied doses yielded the damage constant. This procedure was repeated for the energy range of . The damage constants were lower or comparable to those of diffused solar cells. As in the case of conventional solar cells a maximum was found for an energy of . The spectral response calculated by means of the AFORSHET simulation program was fitted to the experimental data, using an inhomogeneous defect distribution within the crystalline silicon substrate after irradiation. The defect density used as a fit parameter was proportional to the implanted proton dose.

Mechanical properties and defect chemistry

A brief overview of changes in mechanical properties of solids driven by the chemistry of point defects is given. Two major types of effects are considered: direct effects caused by non-interacting point defects and collective effects induced by interacting point defects. The first group includes (1) changes in the linear dimensions of a solid in response to a change in defect concentration and (2) stress induced due to an inhomogeneous distribution of point defects, a so-called chemical stress. The second group includes (1) defect order–disorder transitions accompanied by self-strain and (2) deviations from linear elastic behavior due to the dissociation/association of point defects. All of the above become important if the concentration of point defects is very high (above 10 21 cm − 3 ). These effects may lead to significant anomalies in mechanical properties: spontaneous stress and strain and changes in elastic constants. These effects may significantly affect the application of materials with a large concentration of point defects.

Development of a Simple Test Method for Torsional Strength of MEMS Micromirror

MEMS micromirrors have been used in optical switches and scanning devices. In these kinds of applications, the beams supporting the micromirror are twisted and deformed to a large extent. Consequently, these single crystal silicon beams have failed catastrophically from brittle fractures. In this paper, a simple fracture test method for the torsional strength of MEMS micromirror has been proposed. This method can be realized through the improvement of the previously proposed bending-torsion combined loading test with respect to the specimen dimensions and loading configuration. The developed method was applied to the actual specimens and it was obtained that shape parameter is 4.40 and scale parameter is 1 575 MPa. From the observation of fracture initiation point, it was found that the torsional fracture has taken place competitively at the sidewall subjected to high stress and the notching region involving high etching damage. Therefore, the proposed simple torsional test realizes the evaluation of the torsional strength of MEMS micromirror, which is governed by inhomogeneous defect and stress distributions.

Reconciliation of luminescence and Hall measurements on the ternary semiconductor CuGaSe2

Previous Hall and photoluminescence investigations on CuGaSe2 yielded conflicting results: defect depths determined by luminescence are lower than the ionization energy found in electrical measurements. In this contribution, we present time and spatially resolved cathodoluminescence measurements that necessitate a new interpretation of the luminescence data, leading to a consistent model for luminescence and Hall data with three acceptors with ionization energies of 60, 100, and 150meV. Luminescence decay times are long, in the range of 100ns, indicating strong capture. Luminescence is spatially inhomogeneous, indicating inhomogeneous distribution of defects.

Positron annihilation of defects in silicon deformed at different temperatures

Positron trapping in silicon deformed under high-stress conditions at room temperature is compared to that in Si plastically deformed at higher temperatures. The specific features of positron trapping in silicon plastically deformed at room temperature are related to the dislocation core structure and the inhomogeneous distribution of defects. After high-temperature deformation, positrons are trapped in rather large vacancy clusters and dislocations acting as combined positron traps.