Czochralski Si Wafers Research Articles

Boron segregation to extended defects induced by self-ion implantation into silicon

The evolution of boron segregation to extended defects during thermal annealing was studied with secondary ion mass spectrometry and cross-sectional transmission electron microscopy. Czochralski Si wafers with a boron concentration of 3×1017 cm−3 were implanted with 50 keV Si ion for doses from 5×1013 to 2×1015 cm−2 and then annealed at 720, 820, or 870 °C in nitrogen ambient for various annealing times. The evolution of boron segregation peaks to three types of dislocation loops, end-of-range (EOR) dislocation loops, clamshell defects, and Rp (the projected range) defects, is closely related to the evolution of dislocation loops. As annealing temperature and time increase, the boron segregation peaks grow, remain stable, and then disappear together with the dislocation loops. For lower temperature annealing, the boron segregation peaks grow more slowly and reach higher peak concentrations. In addition to the boron segregation to dislocation loops, boron segregation to {311} defects was also found. The boron segregation peak to {311} defects is unstable and dissolves completely after annealing at 820 °C for 10 min. An analytic model for the boron segregation to EOR dislocation loops was developed under equilibrium condition by taking account of the average radius and area density of the EOR dislocation loops. The boron segregation energy to the EOR dislocation loops was found to be 0.75 eV. The evolution of the boron segregation peak was explained with the analytic model. The experimental boron segregation profiles can be well reproduced with the analytic model.

Studies of Boron Segregation to {311} Defects in Silicon-Implanted Silicon

Czochralski Si wafers with a boron concentration of 2.7×1017 cm-3 were implanted with 50 keV or 150 keV Si+ with doses from 5×1012 cm-2 to 5×1015 cm-2, followed by annealing at 670°C, 720°C or 820°C in nitrogen ambient. During thermal annealing, boron pile-up in the {311} defect region was observed for the first time. In higher temperature annealing, the boron pile-up forms and dissolves more quickly, but has a lower peak value. The boron pile-up forms in the region where the self-interstitial concentration exceeds 3×1017 cm-3 regardless of implant energy and dose. The boron pile-up originates from the boron segregation to {311} defects. The process of boron segregation is limited by boron diffusion. The number of boron atoms segregated to {311} defects increases with annealing time, as t1/2. After reaching its maximum, the number of segregated boron atoms falls exponentially with a characteristic decay time of 14 h at 670°C or 3 h at 720°C. Spreading resistance profiling reveals that boron atoms segregated to {311} defects are electrically inactive.

Comment on “The Effect of Hydrogen Annealing on Oxygen Precipitation Behavior and Gate Oxide Integrity in Czochralski Si Wafers” [H. Abe, I. Suzuki, and H. Koya (pp. 306–311, Vol. 144, No. 1, 1997)

not Available.

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.

Comparison of high temperature annealed Czochralski silicon wafers and epitaxial wafers

High temperature annealing of Czochralski Si wafers in Ar or hydrogen ambients reduces as-grown crystal defects close to the surface of Si wafers. This results in improved electrical properties and an oxygen denuded zone. The depth profile of the defect density and the defect size distribution is investigated by removing successive Si layers by polishing and analyzing crystal originated particles. The efficiency of dissolving crystal defects by annealing was found to depend significantly on the size distribution of the defects of the as-grown Czochralski Si wafers. The results are compared with the characteristics of epitaxial grown Si wafers. The distinctly lower defect level of epitaxial wafers is responsible for their superior performance in device processes.

Study of Near-Surface Microdefects in Czochralski-Si Wafers After a CMOS Thermal Process

Study of Near-Surface Microdefects in Czochralski-Si Wafers After a CMOS Thermal Process

Improvement of Czochralski Silicon Wafers By High‐Temperature Annealing

High‐temperature annealing in hydrogen, argon, and oxygen ambients improves the electrical performance of Czochralski Si wafers considerably. The gate oxide integrity of such wafers can approach values close to 100% yield after annealing for 1 to 2 h at 1200°C in argon and hydrogen ambient which is related to a significant reduction of near‐surface crystal defects as compared to nonannealed polished wafers. The perfection of epitaxial wafers is, however, not obtained. The high‐temperature treatment deteriorates the surface of polished wafers depending on the ambient used. A protective oxide layer grown during annealing in an oxygen ambient prevents roughening due to desorption of . A more pronounced roughening occurs by annealing in hydrogen or argon which is tightly connected to the chemical composition of the surface. A hydrogen terminated reconstructed Si (100) surface is observed after annealing in hydrogen. An oxygen‐denuded zone is formed close to the surface during high‐temperature annealing preventing oxygen precipitation in this region of the wafer. The oxygen precipitation in the bulk of the wafers depends significantly on the details of the annealing process. A precipitation behavior similar to nonannealed Si wafers can be obtained by appropriate process parameters.

Improvement of minority carrier diffusion length in Si by Al gettering

Gettering is already an integral part of fabricating integrated circuits using Si substrates. It is anticipated that this will also be true for solar cell fabrication in the near future. A readily available technique compatible with solar cell processing is gettering by the Si wafer back surface Al. Recently, available solar cell efficiency studies have shown the beneficial effects of the wafer backside Al, including that of gettering, a wafer backside field, and grain boundary and dislocation passivation. In this article, we report on experimental results which showed that Czochralski Si wafer bulk minority carrier diffusion lengths can be substantially improved by wafer backside Al treatment, which also provided an effect of protection from environmental contamination. In these experiments, only the effect of gettering is present and therefore the results constitute an unambiguous demonstration of the benefits of gettering by Al.

Minority Carrier Diffusion Length Improvement in Czochralski Silicon by Aluminum Gettering

AbstractGettering is widely used for fabricating integrated circuits using Si substrates, and has great potential for solar cell fabrications as well. Recently available solar cell efficiency studies have shown the benefits of the wafer backside Al, attributable to effects of gettering, a wafer backside field, and passivation of grain boundaries and dislocations. In this paper, we report experimental results which showed unambiguously that Czochralski Si wafer bulk minority carrier diffusion lengths can be significantly improved due to gettering of impurities by wafer backside Al, which also provided a protection from environmental contamination.

Warp of Czochralski Wafer with Back-Surface Polycrystalline Silicon Film

Warp of Czochralski (CZ) Si wafers with back-surface polycrystalline silicon (BPS) film was measured with elevation of ambient temperature to 620-680°C, using a film internal stress meter (ULVAC Corporation FIS-1000). For the first time, to the aurthors' knowledge, it was clarified that the wafers were warped because of film-induced stress occurring at the film deposition temperature. Film-induced stress is divided into temperature-dependent stress (thermal stress) and residual stress at room temperature. Thermal stress is quantified under the assumption that warp depends on the apparent linear thermal expansion coefficients of the BPS film. Residual stress was measured by means of the X-ray diffraction method.

Point-like and extended defects in Si and GaAs

A survey of crystal defects in Czochralski (CZ) Si and liquid encapsulated Czochralski (LEC) semi-insulating (SI) GaAs is given and basic concepts governing behaviour and interactions between point-like and extended defects are reviewed. The concepts, namely oxygen precipitate nucleation and precipitate-dislocation complex (PDC) formation in Si, cellular structure, non-stoichiometry, and slip dislocation formation in GaAs, are used to interpret our recent results on characterization of CZ Si and SI GaAs wafers.

Liquid silicide formation on the Si wafer free surface during Ni diffusion at 1200 °C

An investigation of nickel diffusion in silicon wafers at 1200 °C, which is above a few Ni-Si eutectic and peritectic temperatures, has been performed. Czochralski Si wafers with nickel film on the back surfaces and mechanical damages on the front surfaces were annealed in different ambients for various periods of time. The wafers annealed in vacuum showed that a liquid phase rich in nickel has been formed at their front (initially free) surfaces at the annealing temperature. The amount of the liquid-phase material increases with an increase of the annealing time. After cooling down, the solidified materials showed various morphologies: spheres, polygons, dendrites, and terraces. The thermodynamic driving force leading to this phenomenon is attributed to the Si (free) surface energy.

Modeling of luminescence phase delay for nondestructive characterization of Si wafers

We have modeled the generation, diffusion, and recombination of photoexcited electrons and holes for the case of Czochralski Si wafers having a defect-free-zone (DFZ) device layer of thickness d above a highly precipitated wafer core and having a finite surface recombination velocity, S. The incident photoexcitation source has a Gaussian power distribution and is focused to a small spot on the sample surface. When the source is sinusoidally modulated at frequency ν, the intrinsic band-edge photoluminescence (PL) emission displays modulations at the fundamental and first overtone of the modulation frequency. The PL signals at frequencies ν and 2ν are delayed in phase, with respect to the source modulation by angles φ2(ν) and φ2(2ν). We relate these phase angles to material properties such as d, S, the optical absorption coefficient α at the incident wavelength, and to the effective carrier lifetimes τ1 and τ2 in the DFZ and precipitated wafer core, respectively. We show that when τ1 and τ2 are independently measured and S≲100 cm/s, as is common for a Si surface passivated with a thermally grown oxide layer, it is possible to deduce d from a measurement of φ2(ν) or φ2(2ν).

Effects of heavy boron doping upon oxygen precipitation in Czochralski silicon

In this investigation, diffuse x-ray scattering, Bragg line profile, and transmission electron microscopy have been employed for the study of point defects and their interaction with oxygen impurities in heavily boron-doped Czochralski Si wafers during various thermal treatments between 450 and 1050 °C for time intervals from 2 to 128 h. Bragg line profile data show that (1) materials tend to become more perfect during the initial stages of thermal annealing regardless of anneal temperatures and (2) the integral width and full width at half-maximum both exhibit minima after a thermal treatment at 450 °C for 32 h while the opposite behavior is observed for a thermal treatment at 1050 °C. The diffuse x-ray scattering data have shown that (1) the nature of the predominant defects depends upon annealing temperature, time and ramping cycles; and (2) the mean cluster size ranges between 1.4 and 2.0×103 nm regardless of annealing temperature and time. Transmission electron microscopy results show (1) slower precipitation kinetics occur than in lightly doped materials, (2) virtually no precipitates have been observed, even after 128 h for annealing temperatures up to 650 °C, (3) amorphous precipitates with a {100} platelet morphology are observed after prolonged anneals at 800 °C, and (4) the appearance of complex precipitate structures have been observed at 1050 °C. These results indicate significantly different behavior from that of lightly doped silicon. Finally, using a thermodynamic and kinetic model, we attempt to explain these heavy boron doping effects on SiO2 precipitation in Czochralski Si.

Effects of 450 °C thermal annealing upon oxygen precipitation in heavily B- and Sb-doped Czochralski Si

In this study we investigated effects of 450 °C preanneal upon oxygen precipitation during subsequent low (700 °C)-medium (950 °C) temperature two-step furnace anneals in heavily B- and Sb-doped Czochralski Si wafers. Our optical microscopy and synchrotron radiation section topographic data have shown that, for heavily B-doped materials, a 450 °C anneal up to 52 h is found to reduce oxygen precipitation in two-step furnace annealing, whereas for Sb-doped wafers, oxygen precipitation rates increase monotonically with the increase in 450 °C preanneal time.

Measurement of Residual Stress in Bent Silicon Wafers by Means of Photoluminescence

The photoluminescence (PL) method was used to measure the residual stress in plastically-bent Si single crystal wafers. The bound-exciton PL line showed remarkable energy shift by the residual stress and the stresses measured from the PL shift agreed well with values of the stress deduced from the theory of plastic bending. It was found that the residual stress introduced by the plastic deformation is quite different in floating-zone (FZ) wafers from that in Czochralski (CZ) Si wafers, possibly because of the difference in the generation and multiplication processes of dislocations in the two crystals. Annealing of the bent specimen at 1000°C reduced the residual stress in the specimen, and the peak position of the bound-exciton PL line reverted to that before bending.