Bulk Microdefect Density Research Articles

Influence of rapid thermal process on intrinsic gettering in fast neutron irradiated Czochralski silicon

A rapid thermal process (RTP) was first introduced into the intrinsic gettering (IG) processes of fast neutron irradiated Czochralski (CZ) silicon. The effect of RTP conditions on bulk microdefects (BMDs) and denuded zone (DZ) was investigated. Fourier transform infrared absorption spectrometer (FTIR) was used to measure the concentration of interstitial oxygen ([O i]). Bulk microdefects were observed by optical microscope. The results show that, according to the variation of [O i], it is found that RTP doesn't change the processes of oxygen precipitation in fast neutron irradiated Czochralski silicon. Perfect denuded zone, dense oxygen precipitates and defects form in the bulk of irradiated samples. With increasing temperature of RTP, the width of denuded zone decreases. Increasing RTP cooling rate, the density of Bulk microdefects increases. DZ forms in the sample that annealed in nitrogen atmosphere.

Standardization of Characterization of Bulk Microdefects and Denuded Zones in Annealed CZ Si

The requirement to standardize measurement methods for BMD (Bulk Micro Defect) density and DZ (Denuded Zone) CZ silicon has lead to the establishment of a SEMI standard for annealed CZ silicon wafers. Therefore, it was decided that we should aim at standardizing the preferential-etching and 90-degrees laser- scattering tomography techniques as a collaborative work between JEITA (Japan Electronics and Information Technology Industries Association) and JSPS (Japan Society of Promotion of Science) 145th Committee. In this work, we carried out a set of round robin tests and examined whether we could jointly standardize both the preferential-etching and the 90 degrees laser-scattering methods. This resulted in a standardized measurement protocol for BMD density and DZ width, which has become known as the JEITA standard EM 3508 [1].

Effect of carbon doping on oxygen precipitation behavior in internal gettering processing for Czochralski silicon

The effect of carbon doping on oxygen precipitation behavior in internal gettering processing for Czochralski silicon (Cz-Si) has been investigated. Through high–low–high anneals in argon ambient, it has been shown that both good-quality denuded zone and high-density bulk microdefects (BMDs) could be formed in high carbon content (H[C]) Cz-Si wafer. Meanwhile, the density of BMDs in H[C] Cz-Si crystal was a little higher compared with that in the Cz-Si crystals with lower carbon content or without carbon doping. The concentration of substitutional carbon almost remained during the whole thermal cycles. Moreover, the [C i–O i]C(3) centers with low concentration were detected by infrared spectroscopy. Weak effect of carbon on oxygen precipitation in H[C] Cz-Si during internal gettering processing is considered. Based on the experimental results, H[C] Cz-Si crystal is suggested to be one of silicon materials used for larger scale integrated circuit fabrication.

Combination of optical measurement and precipitation theory to overcome the obstacles of detection limits

Ham's theory was applied in order to become independent of detection limits for oxide precipitates and to quantify the phenomenon of oxygen loss to invisible BMDs during thermal treatments. The density of detectable bulk micro-defects (BMDs) depends on the size distribution of grown-in nuclei and the ramp rate, temperature, and duration of the thermal treatment applied. There is no correlation to the invisible BMDs. During conventional annealing, the density of the invisible BMDs decreases exponentially with increasing radius of precipitates at a nearly constant loss of interstitial oxygen. Only if the calculated radius exceeds 70 nm, a 100% loss of interstitial oxygen to BMDs detectable by scanning infrared microscopy (SIRM) seems to be possible. After RTA processing at 1230 °C, a period of 1 h at 1000 °C would be necessary for the growing oxide precipitates to reach a saturated density detectable by SIRM, but there remains a very high density of invisible BMDs consuming interstitial oxygen. In N-doped silicon, the vast majority of BMDs is detectable by SIRM, cleave and etch, and infrared light scattering tomography after thermal processing.

Rapid-thermal-processing-based intrinsic gettering for nitrogen-doped Czochralski silicon

In order to optimize the intrinsic gettering (IG) process based on the rapid thermal processing (RTP) for nitrogen-doped Czochralski (NCZ) silicon wafer, the effects of RTP and the following nucleation anneal on oxygen precipitation in NCZ silicon and conventional CZ silicon wafers have been comparatively investigated. It was found that, for NCZ silicon, the RTP at enough high temperature (e.g., 1250°C) was necessary for generation of high density of bulk microdefects (BMDs) in the subsequent anneals despite the nitrogen enhancement effect on oxygen precipitation. With enough high concentration of vacancies introduced by the RTP at 1250°C, for the conventional CZ silicon wafer, the vacancy enhancement effect on the nucleation of oxygen precipitates was most significant at 800°C; while for NCZ silicon wafer, the vacancies and nitrogen atoms coacted most remarkably to nucleate the oxygen precipitates during the ramping anneal from 800to1000°C. It was further found that the doped nitrogen is superior to the vacancy to enhance the nucleation of oxygen precipitates at temperatures of 900–1000°C. Moreover, in order to maximize the density of BMDs, the RTP-based IG process for NCZ silicon wafer should be different from that for the conventional CZ silicon wafer.

Formation of a denuded zone in nitrogen-doped Czochralski silicon wafer treated by ramping anneals

The formation of a denuded zone (DZ) and the bulk-microdefects (BMDs) region within conventional and nitrogen-doped Czochralski (NCZ) silicon subjected to ramping anneals has been investigated. It was found that the terminal temperature of the ramping anneal should be high enough to create a DZ while the starting temperature should be low enough to generate desirable high density of BMDs. Comparatively speaking, with the same heat treatment, the NCZ silicon possesses higher density of BMDs and a narrower DZ than CZ silicon. Moreover, for NCZ silicon, the ramping anneal can initiate at relatively higher temperature to generate an appropriately high density of BMDs. Of importance is that the ramping anneal with a final isothermal anneal at an elevated temperature such as 1150 °C can effectively create a substantial DZ, where there were no defects generated in the subsequent heat treatment significantly enabling oxygen precipitation.

Intrinsic gettering in germanium-doped Czochralski crystal silicon crystals

The intrinsic gettering (IG) of germanium-doped Czochralski (GCZ) silicon with different concentrations of germanium has been investigated in this paper. The conventional Czochralski (CZ) and the GCZ silicon samples were annealed using a one-step high temperature process followed by a sequence of low–high temperature annealing cycles. It was found that the good defect-free denude zones in the near surface of the GCZ silicon could be achieved using simply a one-step high temperature annealing process. Furthermore, the density of bulk microdefects as IG sites was higher than that in the CZ silicon, as a result of germanium enhancing oxygen precipitation during three-step annealing. Meanwhile, the experimental results showed that germanium also enhanced the out-diffusion of oxygen. Furthermore, it is believed that germanium doping can increase the ability of IG in CZ silicon wafers.

A Study of Metal Impurities Behavior due to Difference in Isolation Structure on ULSI Devices

We have studied the behavior of metal impurities such as Cu and Ni with different isolation techniques in ULSI devices. We have found by using convergent beam electron diffraction (CBED) that the stress in a shallow trench isolation (STI) structure is larger than that of a local-oxidation-of-silicon (LOCOS) structure. With LOCOS, the highly doped substrate efficiently gettered Cu. However, with STI, a small amount of Cu diffused to the device area because of the high stress. On the other hand, high density of bulk micro defect (BMD) generated with the thermal processing of STI effectively getters Ni impurities. With the LOCOS-based wafer, a large amount of Ni diffuses to the device region because of the low BMD density.

Behavior of Thermally Induced Defects in Heavily Boron-Doped Silicon Crystals

The effect of the first-step heat treatment temperature on the bulk microdefect (BMD) and oxidation-induced stacking fault (OiSF) formation in two kinds of heavily boron-doped silicon wafers, with and without an OiSF ring area, were investigated by comparing it with that in lightly boron-doped wafers. The BMD density was observed to be higher in the heavily doped silicon than in the lightly doped one at the same oxygen concentration. Unlike in the lightly doron-doped silicon, in the heavily boron-doped silicon, OiSFs were formed over the entire wafer surface regardless of the OiSF ring position when the first-step heat treatment was carried out at 900°C.

Experimental Study of the Degradation of Mechanical Strength of Silicon Wafers Caused by Large Scale Integration Processes

The historical change of mechanical strength of silicon wafer during the large scale integration (LSI) fabrication process has been studied. It was confirmed that the major factor which determines the strength of the bulk crystal is the bulk microdefect (BMD) density in the metal oxide semiconductor field effect transistor fabrication process with local oxidation of silicon (LOGOS) isolation. Unfortunately, the LOGOS fabrication process increases the BMD density and reduces the wafer strength. Further, this study revealed that the ion implantation process induces the most serious degradation of the mechanical strength in the surface region of the Si substrate in the LSI fabrication processes. Whether or not this deterioration in strength is recovered depends on the status of ion implantation and the method of subsequent annealing. The deterioration of strength was caused by ion implantation due to the effect of interstitial silicon atoms and their clusters in a low dose situation (<1014 cm−2), and the amorphous layer or surface roughness in a high dose condition (>1015 cm−2). In order to reduce the crystal defects or wafer‐slips, attention should be paid to the procedural steps which cause reduction in the mechanical strength of silicon crystals while designing the LSI fabrication process.

An Investigation of the Limit of Detection and the Scattering Dependence of the Optical Precipitate Profiler (OPP)

AbstractThe Optical Precipitate Profiler (OPP) and similar instruments are establishing themselves as routine techniques for measuring bulk micro-defect density (BMD) and denuded zone depths in semiconductor wafers. This task has traditionally been done by means of the cleave-and-etch technique in which defects are counted on a cleaved edge of an etched wafer using an optical microscope. The OPP makes this task faster, simpler and more accurate. In addition it can perform the measurement on a whole wafer.We have used the OPP to measure defect density and scattering intensities from a set of silicon wafers with different precipitate sizes. The wafers were all given the same nucleation heat treatment but the precipitates were then grown at 1000°C for 1, 2, 4, 8, 16 and 32 hours. The size distributions and densities of the precipitates in each wafer were determined by Transmission Electron Microscopy (TEM). In a second experiment wafers with precipitate densities varying over four orders of magnitude as determined by cleaveand-etch were also measured by the OPP.From these experiments we have estimated that the OPP can detect defects with an equivalent radius of 16 ± 8 nm and larger. We have also found that the scattering voltage (strength) depends on the precipitate radius, r3, as approximately r3 and that there is an upper number density limit above which the OPP is unable to measure the precipitate density accurately. For our setup this upper density limit was about 1.5 - 2.0 × 1010 defects/cm3 as measured by cleave-and-etch.

The Effect of Hydrogen Annealing on Oxygen Precipitation Behavior and Gate Oxide Integrity in Czochralski Si Wafers

We investigated the effect of hydrogen annealing in the temperature range from 850 to 1200°C on oxygen precipitation and gate oxide integrity in Czochralski Si wafers. The bulk microdefect density of dynamic random access memory thermal simulation wafers showed a strong dependence on the ramp‐up rate conditions of hydrogen annealing. The gate oxide integrity improved after hydrogen annealing at temperatures above 1000°C. However, for better stability of the gate oxide integrity after the heat‐treatment, higher temperature annealing is necessary. We observed that the effect of hydrogen annealing was limited to the near surface, because the gate oxide integrity of hydrogen‐annealed wafers degraded to that for nonannealed wafers after repolishing.

The scanning infra-red microscope (SIRM): Evaluating a new tool for mapping sub-100nm defects in semiconductors

The manufacturing yield and reliability required of modern silicon micro-electronic devices has led to a need for wafer and device manufacturing processes which are immune to metallic contamination. To this end, various ‘gettering’ mechanisms have been developed in which unwanted metallic impurities are ‘gettered’ to regions of the wafer away from the active device layer. ‘Internal (or intrinsic) gettering’ is a method in which silicon-oxide defects are produced in the bulk of the wafer, with a near-surface defect-free zone (denuded zone). In the event ofmetallic contamination during high temperature device manufacturing steps, these bulk microdefects (BMDs) act as heterogeneous sites for metal-silicide precipitation during the cooling, and prevent precipitation of the metals in the device layer. For this reason it is essential to monitor the spatial distribution and density of bulk microdefects during device manufacturing.The standard method by which the density is measured is termed cleave and etch.

The fractal approach for the evaluation of microdefects in silicon

The fractal properties of the cluster of microdefects in the subsurface damaged layer of a silicon monocrystal have been experimentally investigated. The experiments were carried out with common silicon substrates with artificial heterogeneity introduced by ion implantation. It was discovered that thermal annealing changes the fractal dimension of the cluster under investigation, The connection between the value of the fractal dimension and the bulk microdefect density in the cluster was established, For the first time it was demonstrated, based upon the property of selfsimilarity, that the fractal dimension may be evaluated by means of the radial distribution of values of condenser photo-EMF signals, a measurement that greatly simplifies the investigation of fractal objects in general.

Anomalous depth distributions of bulk microdefects in heat-treated Czochralski silicon wafers due to nonequilibrium self-interstitials

Anomalous depth distributions of bulk microdefects (BMDs) are observed in Czochralski silicon wafers subjected to two-step annealing [(550–700 °C)×t1+(850–950 °C)×t2, where t1 and t2=1–100 h]. The number density of BMDs near the surface is smaller than that in the bulk when t1 is short, and is larger when t1 is long. The anomalous distribution extends deeper than 100 μm from wafer surfaces and cannot be explained by the behavior of interstitial oxygen atoms. Distributions are examined under various annealing conditions, such as annealing temperature, rate of temperature ramping, ambient atmosphere, and initial oxygen concentration. The anomalous distributions are found to be formed in the early stage of second-step annealing only when the annealing starts with a rapid temperature rise. A formation model of anomalous distributions is proposed based on the following assumptions: (1) self-interstitials exist in the thermal equilibrium state, (2) wafer surfaces are a permanent source and sink of self-interstitials, (3) growing oxygen precipitates produce self-interstitials, and (4) self-interstitial undersaturation enhances stable growth of precipitate nuclei, and supersaturation suppresses stable growth. The nonequilibrium self-interstitial concentration induced in the bulk after the rapid temperature rise is responsible for the anomalous distributions. All the experimental characteristics are reasonably explained by the model. The formation process of the anomalous distributions is detected by three-step annealing experiments. Basic properties of self-interstitials in silicon are extracted from experimental results combined with the model. The activation energy for migration is about 2.5 eV. The diffusion coefficient is about 10−6 cm2 s−1 at 900 °C. The thermal equilibrium concentration is estimated as about 1012 cm−3 at 1000 °C. These results are close to recent experimental estimates utilizing impurity diffusion in floating zone silicon.

Gettering of Interstitial Iron in P/P+ Epitaxial Silicon Wafers

AbstractGettering of interstitial iron in p/p+ epitaxial silicon wafers during an integrated circuit device simulation annealing was examined. The results from the deep level transient spectroscopy and optical microscope examination suggest no correlation between the interstitial iron concentration in the epitaxial layer and the bulk microdefect density. An electron microscopic analysis revealed that gettering of interstitial iron took place at an oxide polyhedral precipitate located at the center of a bulk stacking fault. By comparing the results of iron implanted to non-implanted samples, it is concluded that iron gettering occurs via a phase transformation of Si02 to an α FeSi2 iron disilicide. In this study, it is suggested that gettering of iron atoms is fundamentally different from that of other transition metals where the silicide formation occurs predominantly along the Frank partial dislocations of the bulk stacking fault.

Effects of Various Pre‐Intrinsic and Phosphorus Diffusion Gettering Treatments Upon Quality of Czochralski Silicon Wafer Surface during a Simulated 4 Megabit Dynamic Random Access Memory Process

Effects of various preintrinsic and phosphorus diffusion gettering treatments upon quality of near‐surface region in Czochralski silicon wafers are studied during a simulated 4 Mb dynamic random access memory process. Denuded zone depth and bulk microdefect density are determined by synchrotron radiation section topography. Minority carrier lifetime and junction characteristics from test device structures were measured to determine overall gettering efficiency. A two‐step thermal anneal cycle before actual device processing resulted in formation of precipitates, dislocations, stacking faults, and a well‐defined denuded zone in samples with medium oxygen content only when a low‐temperature cycle at 775°C was followed by a high‐temperature one at 1150°C. The longest minority carrier lifetimes are observed in samples where very few or no defects are visible in the bulk of the wafer in the section topographs. Phosphorus gettering, however, was found to be effective for improving both minority carrier lifetime and junction properties.

Effectiveness of CVD thin Film Backside Gettering and Its Interaction with Intrinsic Gettering

AbstractThe effectiveness of CVD thin film backside gettering on n-type CZ silicon wafers for CMOS technology has been investigated using optical techniques for bulk microdefect analyses and transmission electron microscopy for interfacial structure study. The deposition of LPCVD polysilicon (500-2000 nm), silicon nitride (150 nm), or poly/nitride films on the backside of Si wafers was found to enhance the bulk precipitation. Bulk microdefect density increased as the thickness of polysilicon increased. At the polysilicon/silicon interface, no extended line defects from polysilicon were observed. Based on the results of minority carrier lifetime and oxide breakdown measurements, the best gettering efficiency was given by 1300 nm polysilicon backside gettering scheme.