Laser Scattering Tomography Research Articles

Characterizing and modeling the evolution of silicon oxide precipitates during thermal cycles

The effects of oxidizing annealing cycles on Si high-quality wafers (as used to monitor IC production tools or SOI applications) were studied using different techniques (nano-scale observations and chemical analysis in Transmission Electron Microscope (TEM), but also large scale measurements, such as Laser Scattering Tomography (LST) and chemical etching (CE), thus coupling local analyses with global measurements of defects density and size) to determine their characteristics. Interstitial oxygen loss during cycles was also measured using Fourier Transform Infra-Red spectroscopy (FTIR).It appeared that each cycle is composed of a defect nucleation stage and a growth stage, and that the distribution of these defects (mainly oxide precipitates) along wafer radius essentially depends on the initial point defect density. The evolution of these precipitates was described by a diffusion-and-growth model derived from Ham's model and taking into account various phenomena: oxygen loss, point-defect distribution and cycle effects (up/down ramp and high-temperature plateau). Extensions and limits of the model are also discussed.

Determination of silicon oxide precipitate stoichiometry using global and local techniques

The effects of high temperature annealing in an oxidizing atmosphere on defect evolution have been studied using global and local techniques. Modeling the evolution of silicon oxide precipitates requires knowledge of the precipitate stoichiometry. In this study we followed precipitate size evolution during the annealing process. The stoichiometry was determined using two different methods: first, results were obtained using Laser Scattering Tomography (LST) technique to determine the size and density of precipitates and compared with model predictions, considering both SiO and SiO2 cases. Then, the precipitate composition and stoichiometry were measured directly, using Electron Dispersive X-ray Spectroscopy (EDX) in an Transmission Electron Microscope (TEM). It is shown that the most probable stoichiometry is SiO and both methods provide similar results.

Delineation of Microdefects in Silicon Substrates by Chromium-Free Preferential Etching Solutions and Laser Scattering Tomography

Chromium-free preferential etching techniques in combination with light optical microscopy were compared with the non-destructive Laser Scattering Tomography (LST) for the evaluation of crystal defect densities in Czochralski substrates grown under different conditions. Dichromate containing etching solutions (original Secco etch and dilute Secco etch) were included into the study as reference. The chromium-free etching solutions with high etch rates comprised mixtures of nitric, hydrofluoric and acetic acid with water (JEITA 1, MEMC). Those with low etch rates consisted of mixtures of nitric and acetic or propanoic acid with hydrogen peroxide which form peracetic or perpropanoic acid (Organic Peracid Etches). OPE solutions provide improved discrimination of different types of defects and work also on highly doped substrates. As a general result, it turned out that the defect densities determined by the preferential etching solutions applied were significantly higher than those evaluated by LST. Relatively close to the LST defect densities are those determined by original Secco etch for larger etch pits.

Characterization of Structural Defects in Silicon and SOI Wafers by Means of Laser Scattering Tomography

The ongoing quality improvement of Silicon-On-Insulator wafers motivates further developments of Laser Scattering Tomography (LST). LST coupled with advanced image processing and simulations enables to have highly reproducible quantitative information on small oxygen precipitates and extended defects, such as dislocations. The growth kinetics of individual defects is discussed. Theoretical predictions of dislocation light scattering dependence on polarization have been confirmed by the determination of dislocation Burgers vector and lines from LST measurements.

Characterization of Structural Defects in Silicon and SOI Wafers by Means of Laser Scattering Tomography

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Inclusions in LEC-grown Si GaAs single crystals

The relationship between surface flaws and three-dimensional defects in LEC-grown SI GaAs was investigated by laser scattering tomography (LST), Surfscan mappings and analytical REM. Occasionally found singular scatterers (SSs)—the general features of which are clearly different from decoration and matrix As-precipitates—were analysed in depth in relation to the crystal growth conditions. It is shown that these defects cause outsized light scattering defects on the surface of finally polished wafers known as area counts . It has been concluded that the reason for these area counts are particles of foreign phases captured by the moving solid/liquid interface. A reasonable agreement between experimental results and the conclusions of the “capture theory” of C hernov et al. has been ascertained. From this understanding of area counts measures were derived to surely avoid inclusions during LEC growth of GaAs single crystals.

Performance of semi-insulating GaAs-based radiation detectors: Role of key physical parameters of base materials

In this work, the requirements of detector-grade semiconductor materials for radiation detectors, applicable in X-ray digital radiology, are identified. The study includes 12 various bulk semi-insulating (SI) GaAs single crystals grown by LEC and VGF methods, undoped and Cr-doped, obtained from 8 different suppliers. Conductivity, Hall, glow discharge mass spectrometry (GDMS), etch pit density (EPD), scanning electron beam induced current (S-EBIC), X-ray and laser scattering tomography (LST) techniques are used for the bulk SI GaAs material evaluation. The radiation detectors fabricated on these SI GaAs single crystals have been characterized by capacitance methods and their performances have been evaluated from detected pulse height spectra of 57Co. The correlation between the physical characteristics of the base materials and the performance of the detectors is demonstrated and discussed. Key detector-grade SI GaAs parameters, useful for material evaluation, are identified.

Laser Scattering Tomography on Magnetic CZ-Si for Semiconductor Process Optimization

Laser scattering tomography (LST) and band-to-band photoluminescence (PL) are applied for supporting a MEMS process optimization. Process wafers are based on magnetic CZ grown silicon material. LST allows the characterization of number-size distributions of oxygen precipitates in various stages of the process flow. Precipitation is shown to be affected by the design of high-temperature anneal post initial oxidation. PL gives useful information on relative concentration level and radial distribution of recombination centers within process wafers. The initial oxidation leads to significant reduction of recombination centers. The combined LST/PL information enables valuable conclusions towards process optimization.

Investigation of Defects in the Edge Region of Multicrystalline Solar Silicon Ingots

The structural and electrical properties of as grown multicrystalline (mc) solar silicon have been characterized with special emphasis on the ingot's edge regions. For this purpose a vertical cross section of an mc-Si Bridgman ingot was investigated by Fourier transform infrared spectroscopy (FTIR), laser scattering tomography (LST), lateral photovoltage scanning (LPS), infrared microscopy and microwave detected photoconductivity decay (&PCD). Images of the distribution of dislocations, grain boundaries, precipitates, impurities (O, N, C) and the minority charge carrier lifetime were obtained, partly differentiating the defects by their electrical activity. In particular the LPS method displays dopant striations indicating the shape of the phase boundary. Deviations of the phase boundary from a slightly convex shape in the middle of the ingot to a concave one in the vicinity of the side walls could be observed. The existence of an horizontal temperature gradient deduced from this shape is the reason for convection in the melt. The influence on the concentration profiles of interstitial oxygen and the correlation with the minority charge carrier lifetime are discussed.

Photoluminescence characterization of nano‐size defects in sub‐surface region of silicon wafers

AbstractThe sub‐surface defects in annealed wafers with an oxide layer are characterized by ultraviolet‐excited photoluminescence spectroscopy and mapping. Two types of defects are detected as a reduction of band‐edge emission in the region determined to be defect‐free by laser scattering tomography (LST): an interface gap state between silicon and the oxide layer, and a nitrogen‐related defect (which is below the detection limit of LST). Copyright © 2005 John Wiley & Sons, Ltd.

Evaluation of small scattering defects densities by laser scattering tomography: application to levitated glasses

A specific experimental Laser Scattering Tomography (LST) acquisition procedure is presented. It is adapted to the characterization of materials containing scattering defects ranging from 1 to 10 2 /mm 3 . The technique makes it possible to obtain good resolution within a volume chosen to contain a statistically significant defect density. This method is used to show that the gas levitation technique makes it possible to significantly decrease scattering defects in glasses. In parallel, individual study of defects in such glasses is also presented.

Laser Scattering Tomography for Crystal Characterization: Quantitative Approaches

Laser Scattering Tomography (LST) is used to investigate submicroscopic precipitates and the patterns of decorated defects in crystals. Experimental conditions to achieve quantitative results are described. Macroscopic volumes of the samples can be investigated inexpensively without focus degradation and with microscopic resolution by a technique called depth integration. Relative volumes of scatterers and statistical relevant distributions of relative precipitate volumes are determined by evaluation of the intensities in the LST images. The 3-dimensional arrangement of precipitates can be studied in stereoscopic images and by spatial computer reconstruction. Planes inside the crystals, preferably populated by decorated dislocations, can be revealed and evaluated for their orientation dependencies.

Laser Scattering Tomography for Crystal Characterization: Quantitative Approaches

Laser Scattering Tomography (LST) is used to investigate submicroscopic precipitates and the patterns of decorated defects in crystals. Experimental conditions to achieve quantitative results are described. Macroscopic volumes of the samples can be investigated inexpensively without focus degradation and with microscopic resolution by a technique called depth integration. Relative volumes of scatterers and statistical relevant distributions of relative precipitate volumes are determined by evaluation of the intensities in the LST images. The 3-dimensional arrangement of precipitates can be studied in stereoscopic images and by spatial computer reconstruction. Planes inside the crystals, preferably populated by decorated dislocations, can be revealed and evaluated for their orientation dependencies.

Quantitative approaches in laser scattering tomography

Laser scattering tomography (LST) is extended to quantitative measurements and to macroscopic fields. A technique is described to image patterns of submicroscopic scatterers decorating defects in depths up to a few mm without focus degradation. It is especially useful to reveal the arrangement of precipitates of low concentration and assures the proportionality between the measured intensity of the scatterer and its scattering cross-section. The relative volumes of the submicroscopic precipitates are determined from the measured intensities. The extended LST technique is demonstrated for GaAs crystals grown by the vapour pressure controlled Czochralski (VCz) technique. Statistically relevant histograms of the volumes of submicroscopic As precipitates are presented. By Fourier analysis planes preferably populated by decorated dislocations can be evaluated for their orientation. Dependencies along macroscopic dimensions of the samples can be measured, e.g. along a wafer radius.

Precipitate engineering in GaAs studied by laser scattering tomography

Annealing experiments were carried out with semi-insulating LEC GaAs. The volumes of the precipitates were determined by an advanced technique of laser scattering tomography (LST) qualified for quantitative volume measurements and their statistical relevant evaluation. The dependence of the volume distributions of precipitates on annealing conditions is presented. The dependence of the total volume of the precipitate ensemble on hold temperature and time is shown. The experimental results cannot be explained completely in the framework of Ostwald ripening of the As precipitates. A behaviour not described earlier has been observed: at cooling inside the assumed homogeneity range of GaAs the growth of As precipitates followed by their dissolution.

Laser scattering experiments in VCz GaAs

The method of laser scattering tomography (LST) was adapted to investigate scatterers in semi-insulating GaAs grown by the vapour pressure controlled Czochralski (VCz) technique. The LST method was extended to evaluate quantitatively the scattering for macroscopic fields. The LST images of as-grown VCz GaAs crystals showed a reduced number of scattering particles in comparison with conventional LEC crystals. The dependency of the total scattered intensities along a wafer radius and intensity histograms are discussed.

Analysis of grown-in defects in Czochralski Si

Octahedral void defects are widely recognized to be a cause of crystal-originated particles, flow-pattern defects, laser-scattering tomography defects, and oxide defects. Their structure has been characterized in detail and the dependence of their generation on Si growth conditions has been extensively examined. Void defects have been a major theme in the Si micro-device industry for several years and many unsolved problems still remain. This paper reviews this important field, and also presents some recent experimental results on void defects that we have obtained.

Mathematical modeling of grown-in defects formation in Czochralski silicon

On the basis of a mathematical model proposed previously [S. Kobayashi, J. Crystal Growth 174 (1997) 163], oxygen precipitation in growing Czochralski (CZ) crystals is analyzed, and the calculated results are compared with experimental data of grown-in defects observed with laser scattering tomography (LST). The behavior of oxygen precipitates is qualitatively very similar to that of grown-in defects, but there are quantitative discrepancies between the calculation and the observation. The formation process of grown-in defects is discussed.

The Direct Observation of Grown‐in Laser Scattering Tomography Defects in Czochralski Silicon

For the first time as‐grown laser scattering tomography defects (LSTD), twins, was observed directly in the Si wafer with transmission electron microscopy (TEM). The shape of the grown‐in LSTDs, estimated from the TEM images, were octahedral and tetrahedral, with sidewalls lying in the {111} plane. The size of grown‐in LSTDs are approximately 100 nm. The oxygen concentration inside the grown‐in LSTD was close to the detection limit of TEM‐EDX.

Study on microscopic defects in Fe-Doped InP single crystals

Effect of crystal growth conditions on the density of microscopic defects, observed on as-polished Fe-doped InP LEC single crystal wafers, has been investigated by an interference contrast microscope and a laser scattering tomography (LST) system. It was found that crystal rotation speeds affect the density of microscopic defects. In addition, the density depends on the duration for which the InP melt is held in molten state before crystal growth. On the other hand, it was found that the H/sub 2/O concentration in B/sub 2/O/sub 3/ has no correlation with the generation of microscopic defects. The relationship between the microscopic defects and the stoichiometry of grown crystals was investigated by coulometric titration analysis for the indium concentration. The density of microscopic defects is reduced as the indium concentration decreased. From the present results, it is speculated that the origin of microscopic defects is indium or indium oxide.