Depth Profile Analysis of Metals Gettered By Bulk Micro-Defects (BMDs) in Silicon Substrates

This paper reports on a new type of through-silicon via (TSV) defect, silicon fin defect, which was found after TSV deep-reactive-ion-etching (DRIE) process for TSV integration with front-end-of-line (FEOL) devices. One possible root cause for this defect is that the bulk micro defect (BMD) in silicon substrate serves as a micro-mask during etching and results in silicon fin defects at TSV bottom. These defects have to be eliminated as they are killer TSV defects for several reasons: (1) could serve as a weak point for isolation liner deposition; (2) could be a weak point for barrier/seed layer deposition; and (3) may cause mechanical failures during TSV backside reveal. Previously, silicon fin defects were removed by switching to a non-BMD silicon substrate for interposer application. However, for TSV integration with FEOL devices, the BMD layer serves as an intrinsic gettering layer for devices, therefore, it cannot be removed from the silicon substrate, which makes it challenging to get rid of silicon fin defects. In order to establish a non-destructive in-line detection method of the fin defects, scanning electron microscope (SEM) automatic process inspection (API) was set up to image the fin defects at the bottom of the trench. A special working point with high depth of focus (DoF) and contrast was created to obtain good top-down SEM imaging of the defects at the bottom of this high-aspect-ratio (HAR) structure. Three types of silicon substrates (A, B, and C) were used for this study to investigate the potential root cause. SEM API results show defect rates of 20%, 3.3% and 0% for substrates A, B, and C, respectively. This is in good agreement with both BMD simulation results and benchmarking data in which substrates A, B, and C had normalized BMD densities of 11.7, 5.74, and 1 cm-3, respectively, with a comparable BMD size of 80~90 nm and a denuded zone (DNZ) depth of 10~15 μm. The correlation between BMD density in a silicon substrate and silicon fin defect rate indicates that BMD is a key root cause for silicon fin defects. To eliminate silicon fin defects, an optimized DRIE process has been developed. On the same type of substrate, the DRIE process with a typical voltage bias results in a defect rate of 6.7%, while no silicon fin defect was detected out of 200 TSVs with a polynomial bias ramp to relatively higher final voltage bias during the last 15 μm etch. The hypothesis is that higher voltage bias is able to sputter away BMD and shows potential to get rid of the silicon fin defects at the TSV bottom. In summary, a capable inspection method, a preferred silicon substrate with BMD spec range, and a promising way for DRIE process optimization to eliminate the silicon fin defect at the TSV bottom have been identified and developed in this work. Detailed results and analysis, particularly the fin defect images, statistical inspection results, BMD benchmarking data, simulation results, and TSV profile with optimized process will be discussed in the paper.

The effects of two‐step rapid thermal processing (RTP) sequentially in different ambients on the formation of denuded zone (DZ) and oxygen precipitation in the Czochralski (CZ) silicon wafers have been investigated. With the first‐step RTP in Ar ambient, no obvious DZ but a high density of bulk micro‐defects (BMDs) were formed in the sample with the second‐step RTP in N2 ambient, while the BMD density in the sample with the second‐step RTP in O2 ambient was remarkably low. With the first‐step RTP in N2 ambient, a high density of BMDs and a width of DZ could be formed in either of the samples with the second‐step RTP in Ar or O2 ambient, but the DZ width in the two samples differed to a certain extent. With the first‐step RTP in O2 ambient, the two samples with the second‐step RTP in Ar and N2 ambients respectively possessed reasonably comparable DZ width and BMD density. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of internal gettering of copper in the vertical direction of p-type silicon wafer

The formation of a denuded zone (DZ) by conventional furnace annealing (CFA) and rapid thermal annealing (RTA) based denudation processing is investigated and the gettering of copper (Cu) atoms in germanium co-doped heavily phosphorus-doped Czochralski (GHPCZ) silicon wafers is evaluated. It is suggested that both a good quality defect-free DZ with a suitable width in the sub-surface area and a high density bulk micro-defect (BMD) region could be formed in heavily phosphorus-doped Czochralski (HPCZ) silicon and GHPCZ silicon wafers. This is ascribed to the formation of phosphorus-vacancy (P-V) related complexes and germanium-vacancy (GeV) related complexes. Compared with HPCZ silicon, the DZ width is wider in the GHPCZ silicon sample with CFA-based denudation processing but narrower in the one with two-step RTA pretreatments. These phenomena are ascribed to the enhancing effect of germanium on oxygen out-diffusion movement and oxygen precipitate nucleation, respectively. Furthermore, fairly clean DZs near the surface remain in both the HPCZ and GHPCZ silicon wafers after Cu in-diffusion, except for the HPCZ silicon wafer which underwent denudation processing with a CFA pretreatment, suggesting that germanium doping could improve the gettering of Cu contamination.

Woongjin Energy Co., Ltd., Daejeon 305-509, Korea(Received November 13, 2014)(Revised November 27, 2014)(Accepted December 5, 2014)Abstract Recent studies have shown methods of improving solar cell efficiency. Especially on single crystalline siliconwafer which is high-efficiency solar cell material that has been widely studied. Interstitial oxygen (Oi) is the main impurityin the Czochralski (Cz) growing method, and excess of this can form precipitates during cell fabrication. We havedemonstrated the effect of Oi impurity and oxygen precipitation concentration of the wafer on Cz-silicon solar cell efficiency.The result showed a decrease in cell efficiency as Oi and oxygen precipitation increase. Moreover, we have found that thecritical point of [Oi] to bring higher cell efficiency is at 14.5 ppma in non-existent Bulk Micro Defect (BMD).Key wordsSolar cells, Efficiency, Czochralski silicon, Oxygen precipitation, BMD

The properties of nitrogen-doped and hydrogen-annealed Czochralski-grown silicon (NHA-CZ-Si) wafers were investigated in this study. The quality of the subsurface was investigated by monitoring the generation lifetime of minority carriers, as measured by the capacitance-time measurements of a metal oxide silicon capacitor (MOS C-t). The intrinsic gettering (IG) ability was investigated by determining the nickel concentration on the surface and in the subsurface as measured by graphite furnace atomic absorption spectrometry (GFAAS) after the wafer was deliberately contaminated with nickel. From the results obtained, the generation lifetimes of these NHA-CZ-Si wafers were determined to be almost the same as, or a little longer than those of epitaxial wafers, and the IG ability was proportional to the total volume of oxygen precipitates [i.e., bulk micro defects (BMDs)], which was influenced by the oxygen and nitrogen concentrations in the wafers. Therefore, it is suggested that the subsurface of the NHA-CZ-Si wafers is of good quality and the IG capacity is controllable by the nitrogen and oxygen concentrations in the wafers.

Correlation between degradation of cathode performance and chromium concentration in (La,Sr)MnO3 cathode

The intrinsic gettering (IG) effects in a germanium-doped Czochralski (GCz) silicon wafer have been investigated through a processing simulation of dynamic random access memory making and an evaluation on IG capability for copper contamination. It has been suggested that both the good quality defect-free denuded zones (DZs) and the high-density bulk microdefect (BMD) regions could be generated in GCz silicon wafer during device fabrication. Meanwhile, it was also indicated that the tiny oxygen precipitates were hardly presented in DZs of silicon wafer with the germanium doping. Furthermore, it was found in GCz silicon wafer that the BMDs were higher in density but smaller in size in contrast to that in conventional Cz silicon wafer. Promoted IG capability for metallic contamination was therefore induced in the germanium-doped Cz silicon wafer. A mechanism of the germanium doping on oxygen precipitation in Cz silicon was discussed, which was based on the hypothesis of germanium-related complexes.

The 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.

Gettering 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.

Assessment of Trace Metal Contaminants and Consumer Preference in Tomato Varieties Produced in Two Mining Communities of Nigeria

ABSTRACTThis study probes heavy metals (HMs) concentration in groundwater, soil, vegetables, chicken eggs, and buffalo milk samples collected from different land-use types (LUT) with special emphasis on human health risk via their consumption. Our results depicted that HMs (Ni, Cr, Pb, and Cd) concentration in groundwater of all LUT; Cd concentration in agricultural soil; Ni, Cr, Mn, Cd, and Pb concentration in buffalo milk; and Ni, Cd concentration in chicken eggs of all LUT surpassed the recommended permissible limits. While, on the other hand, Cr concentration in industrial and Pb concentration in agricultural LUT also exceeded permissible limits in the case of chicken egg samples. The concentration of Cr, Pb, and Cd in most of the vegetable samples of different LUT also crossed permissible limits. The accumulation factor for selected HMs followed trends for different LUT as Industrial > Agricultural > Residential, showing the transfer of risk from soil to vegetables. Our results for principle component analysis unravel that, unlike residential, industrial, and agricultural, LUT were highly affected from metals contaminations when different environmental matrices were studied. Health risk index (HRI) was chronicled >1 for Cd in groundwater of industrial and residential sites, in the eggs found in the industrial site, and for Pb in groundwater of industrial and agricultural sites due to higher daily intake of metal, while all other HMs revealed HRI < 1 in all LUT.

Yttria-stabilized zirconia (YSZ) is the most common and the most reliable electrolyte material for solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs), collectively referred to as solid oxide cells (SOCs). Typical SOCs for practical applications are of the fuel-electrode-supported cell variety because these type of cells could be operated under high current densities. Here, the YSZ electrolyte is co-sintered with porous NiO-YSZ support at temperatures as high as 1400 °C to obtain dense electrolyte layer on the anode. In this process, NiO is expected to diffuse into the YSZ electrolyte. The influence of dissolved NiO into YSZ electrolyte has been investigated in AIST [1-3]. For the YSZ electrolyte with intentionally dissolved NiO, the phase transformation from cubic phase to tetragonal phase is accelerated in the lower oxygen potential range where NiO is reduced to Ni metal in the YSZ electrolyte, consequently, degradation in the conductivity of YSZ electrolyte occurs. In this study, the influence of co-sintering process of YSZ electrolyte with the NiO-YSZ support, i.e. NiO diffusion into YSZ electrolyte under co-sintering process, and its influence on the oxide ion diffusivity of YSZ electrolyte are examined.YSZ electrolyte layers were prepared on the NiO-YSZ substrates by screen-printing method or dip-coating method, and then co-sintered at high temperatures between 1370 °C to 1550 °C. The co-sintered YSZ/NiO-YSZ half-cells were reduced under dry H2 flow for 5 to 300 hours. The crystal structure and Ni distribution of the YSZ electrolyte layer were analyzed using Raman spectroscopy and high-spatial-resolution SIMS (secondary ion mass spectrometry), respectively. Lastly, the oxide ion diffusivity of the YSZ electrolyte layer was obtained using the oxygen isotope exchange method and high-spatial-resolution SIMS technique.By co-sintering of YSZ electrolyte and NiO-YSZ support, NiO uniformly diffused into YSZ electrolyte layer, and the amount of dissolved Ni (NiO) increased with increasing sintering temperature. When the half-cell was reduced at 900 °C, a phase transformation of the YSZ electrolyte was clearly observed, moreover, the oxide ion diffusivity of the YSZ electrolyte also decreased. By reduction of the half-cell at 700 °C, such phase transformation could not be observed, however, the oxide ion diffusivity is slightly but clearly decreased. In the presentation, the mechanisms of phase transformation and conductivity degradation will be discussed. H. Yokokawa et. al., Fuel Cells, 19 (2019) 311–339.T. Ishiyama, H. Kishimoto, K. D. Bagarinao, K. Yamaji, T. Horita, H. Yokokawa, ECS Transactions, 78 (2017) 321–326.T. Shimonosono, H. Kishimoto, M.E. Brito, K. Yamaji, T. Horita, H. Yokokawa, Solid State Ionics,225, 69 (2012).

The feasibility of using piezoelectric drop‐on‐demand inkjet printing technology to produce trace metal contamination standards on silicon wafers has been demonstrated. Prototype standards of Fe and Cu contamination with surface concentrations between 1013 atoms/cm2 and 1015 atoms/cm2 have been produced using a piezoelectric inkjet printer. Preliminary secondary ion mass spectrometry (SIMS) measurements show that secondary ion intensities are linear with respect to concentration. The success of these preliminary experiments has prompted further work that is currently underway to prepare additional elements at concentrations ranging from 107 atoms/cm2 to 1015 atoms/cm2 for characterization by both SIMS and total reflection x‐ray fluorescence (TXRF).

Sb surface segregation and doping during Si(100) molecular beam epitaxy at low temperature have been studied by depth profiling with secondary ion mass spectrometry, x-ray photoelectron spectroscopy, and conductivity measurements. We find that at a growth temperature near 320 °C complete donor activation and doping densities near 5×1020 cm−3 are obtainable. For temperatures between 320–500 °C, measurements point to the existence of distinct surface segregation regimes. For dilute surface and bulk Sb concentrations the measurements reveal a region where the surface segregation is constant and bulk and surface concentrations are linearly related. For bulk concentrations near the solid solubility limit, the surface segregation increases rapidly with increasing bulk concentration for temperatures between 350–450 °C. Finally, for Sb-saturated surfaces the surface segregation actually decreases, and doping levels exceeding the solid solubility limit are possible.