Papers by Keyword: Gettering

Abstract: This presentation focuses on semiconductor wafer cleaning technology, one of the most critical technologies in semiconductor device manufacturing for obtaining high yield and reliability, and discusses the past, present, and future of the technology. Emphasis is placed on the review of contamination control and cleaning technologies in the early days since the invention of the transistor. To celebrate the 30+1-year anniversary of the UCPSS, a review will be given of both the first conference held in Leuven in 1992 and the second one held in Bruges in 1994. There will be more research challenges and business opportunities in environmentally benign, innovative damage-free wafer cleaning and surface preparation technologies for future applications.

Abstract: In this work we use Saw Damage Gettering (SDG) in combination with emitter formation to improve the minority carrier lifetime of highly contaminated multi-crystalline silicon wafers. This process is applied to wafers from the bottom of ingots, commonly referred to as the “red zone”, which are currently discarded since their high concentration of impurities limits the efficiency of solar cells produced therefrom. SDG is a potentially simple technique designed to upgrade these wafers. In this technique, bulk impurities are dissolved via annealing. The wafers are then cooled which generates a super-saturation of impurities in solution. The system then relaxes through the formation of precipitates in the saw damaged region. SDG is shown to be enhanced when using a temperature dependent cooling rate which maximizes the flux of impurities to the saw damaged regions. In addition, these benefits were observed even after an additional gettering process occurring during an emitter formation procedure. The SDG annealing conditions required to achieve the maximum lifetime were altered by the introduction of the emitter formation process. The enhancement generated by the SDG process may be sufficient to enable red-zone wafers to be processed is the same manner as higher quality no-red zone wafer wafers without adversely affecting the resultant cell efficiency. Due to its simplicity, it is expected that SDG can easily be incorporated into current production methods.

Abstract: Multicrystalline silicon (mc-Si) substrates are widely used for photovoltaic cells. The minority carrier lifetime in mc-Si is affected by recombination associated with metallic impurities in many forms, such as point-like defects, precipitates and bound to or precipitated at structural defects such as dislocations. We have studied the effect of low temperature annealing on the lifetime and bulk iron concentration in as-received mc-Si wafers from different locations within a block. Lifetime measurements are made using a temporary iodine-ethanol surface passivation technique to minimize the occurrence of bulk hydrogenation which often occurs from dielectric films. In good wafers from the middle of the block the lifetime is reduced by annealing at 400 °C and 500 °C in a way which does not correlate with changes in bulk iron concentration. Lifetime improvements occur in relatively poor samples from the top and bottom of the block annealed at 300 °C, and also in samples from the bottom annealed at 400 °C. The improvement in bottom wafers correlates with iron loss from the bulk. Our work shows that under some conditions the lifetime in relatively poor as-grown wafers can be improved by low temperature internal gettering.

Abstract: First-principles quantum-chemical simulations are combined with TCAD device modelling to examine the impact of the intrinsic stacking faults and Σ5-(001) twist grain-boundaries on the performance of solar cell efficiency. We find from the combination of these computational methods, the optical properties of ideal stacking faults are similar to those of pure Si, whereas the optimised grainboundaryleads to a clear change in the real and imaginary parts of refractive index, increasing the solar-cell current density, and thus the solar cell efficiency. The impact at a device level is dependent upon the areal density of such material. So far as the optically absorption and carrier generation is concerned, segregation of diffusing iron at these planar defects has a negligible impact on device characteristics, but non-radiative recombination processes and carrier traps due to iron are expected to significantly affect efficiency in these regions.

Abstract: Ultra-shallow junctions (USJs) were formed by low-energy As ion implantation with the subsequent furnace annealing. It was found that the significant amount of oxygen is redistributed from the silicon bulk to the arsenic-implanted region. We present the effect of oxygen gettering at the creation of arsenic-doped USJs using the marker layer created by ion implantation of ¹⁸O isotope.

Abstract: In multicrystalline silicon for photovoltaic applications, high concentrations of iron are usually found, which deteriorate material performance. Due to the limited solubility of iron in silicon, only a small fraction of the total iron concentration is present as interstitial solute atoms while the vast majority is present as iron silicide precipates. The concentration of iron interstitials can be effectively reduced during phosphorus diffusion gettering (PDG), but this strongly depends on the size and density of iron precipitates, which partly dissolve during high-temperature processing. The distribution of precipitated iron varies along the height of a mc-Si ingot and is not significantly reduced during standard PDG steps. However, the removal of both iron interstitials and precipitates can be enhanced by controlling their kinetics through carefully engineered time-temperature profiles, guided by simulations.

Abstract: The removal of dissolved iron from the wafer bulk is important for the performance of p-type multicrystalline silicon solar cells. In this paper we review some recent progress in understanding both external and internal gettering of iron. Internal gettering at grain boundaries and dislocations occurs naturally during ingot cooling, and can also be driven further during cell processing, especially by moderate temperature anneals (usually below 700 °C). Internal gettering at intra-grain defects plays key a role during such precipitation annealing. External gettering to phosphorus diffused regions is crucial in reducing the dissolved iron concentration during cell processing, although its effectiveness depends strongly on the diffusion temperature and profile. Gettering of Fe by boron and aluminum diffusions is also found to be very effective under certain conditions.

Abstract: We report on a novel method of low pressure chemical vapor deposition of polycrystalline silicon layers used for external gettering in silicon substrate for semiconductor applications. The proposed method allowed us to produce layers of polycrystalline silicon with pre-determined residual stress. The method is based on the deposition of a multilayer system formed by two layers. The first layer is intentionally designed to have tensile stress while the second layer has compressive stress. Opposite sign of the residual stresses of the individual layers enables to pre-determine the residual stress of the gettering stack. We used scanning electron microscopy for structural characterization of the layers and intentional contamination for demonstration of the gettering properties. Residual stress of the layers was calculated from the wafer curvature.

Abstract: In this paper, we test proximity gettering layers obtained by carbon or silicon implantation for their efficiency in molybdenum and tungsten gettering. DLTS was used to measure the impurity concentration in the solid solution and so to evaluate gettering efficiency. It was found that carbon implantation is effective in capturing these impurities, whereas silicon implantation is not. Extended defects seem not to play an important role in gettering these impurities. In addition, gettering was found to be most effective at high impurity concentration.

Abstract: Diffusion of transition metals in 4H-SiC has been investigated by secondary ion mass spectroscopy using epilayers and substrates implanted with titanium (Ti), chromium (Cr), iron (Fe), or nickel (Ni). In the epilayers, Cr, Fe, and Ni atoms have diffused by argon (Ar) annealing at 1780°C for 30 min. In n⁺ substrates, the diffusivity of the metals is smaller than that in the epilayers, and only Ni has diffused by the annealing. By the Ar or helium implantation following the implantation of transition metals, diffusion of transition metals can be successfully suppressed.