Silicon Contamination Research Articles

Diffusion mechanisms of Mo contamination in Si

Molybdenum contamination of silicon can have serious detrimental consequences for the efficiency of solar cells, raising durability concerns for novel solar cell designs that utilize $\mathrm{Mo}{\mathrm{O}}_{3}$ in contact with Si. Density functional theory simulations of Mo defects in Si revealed that Mo is preferentially accommodated in tetrahedrally coordinated interstitial sites and that the contamination may reach a sufficiently high concentration to cause a 20% relative solar cell efficiency degradation if processing steps are performed between 950 and 1300 K. The formation energy of the most energetically favored Mo defect in Si has a minimum value of 1.58 eV at the valence band maximum and a maximum of 2.10 eV at higher Fermi levels, indicating that higher Mo defect concentrations may occur in $p$-type Si than intrinsic or $n$-type Si. The diffusion processes for Mo in Si were investigated, and it was identified that interstitial diffusion dominates over a vacancy-mediated mechanism under all equilibrium conditions. Migration barriers were calculated to be 2.29 eV for charge neutral and 2.03 eV for charge +1 defects, occurring under $n$-type and $p$-type doping, respectively, indicating that Mo diffusion is faster in $p$-type Si, and hence potentially more effectively gettered than it would be in $n$-type Si.

Formation of Microparticles from Silicone Contaminants under Simulated Space Environment

Formation of Microparticles from Silicone Contaminants under Simulated Space Environment

Laser induced breakdown spectroscopy of polymer matrix composites for real-time analysis of trace surface contaminants: A review

Laser induced breakdown spectroscopy (LIBS) is a materials characterization technique that has been advanced and refined to provide in situ, real-time quality control of polymer matrix composite (PMC) surfaces. A LIBS system was designed and assembled at the NASA Langley Research Center in order to detect ultralow concentrations of silicone contamination on carbon fiber reinforced polymer (CFRP) materials. The LIBS instrument provides high sensitivity detection and nearly non-destructive inspection of PMC substrates prior to adhesive bonding. This review focuses on work conducted using LIBS as a characterization tool for surface contaminants for improved adhesive bonding and coating processes of CFRP materials. In addition, this work describes how the LIBS technique was advanced by analyzing the laser parameters, studying the laser-matter interactions, and comparing results to X-ray photoelectron spectroscopy. Discussion is also presented on LIBS instrumentation, recommendations for laser parameters and instrumentation components, LIBS technique maturity, applications, and limitations.

Identification of colloidal silica polishing induced contamination in silicon

This paper presents a multiscale characterisation approach, analysing the effect of colloidal silica polishing on crystallographic defects in multicrystalline silicon. Colloidal silica polishing for as little time as 30 min was found to significantly increase the recombination activity of all defects, as measured by electron beam induced current mapping. The impurities responsible for the room temperature contamination of defects due to colloidal silica polishing were Cu and Ni, as measured by atom probe tomography.

Correlation of Trace Silicone Contamination Analyses on Epoxy Composites Using X-ray Photoelectron Spectroscopy (XPS) and Laser-Induced Breakdown Spectroscopy (LIBS).

Surface treatment and surface characterization techniques are critical to ensure that adherends are chemically activated and free of contaminants before adhesive bonding. Silicone contamination from mold release agents and other sources can interfere with interfacial bonding, decreasing the durability and performance of bonded composite structures. It is necessary to have tools and methods that can be used in a production environment to reliably detect low levels of contaminants in a rapid, simple, and cost-effective manner to improve bond reliability. In this work, surface characterization of carbon fiber reinforced polymer (CFRP) composites with epoxy matrix was performed using laser-induced breakdown spectroscopy (LIBS), and the results were compared with those obtained from X-ray photoelectron spectroscopy (XPS). Laser-induced breakdown spectroscopy offers many advantages over XPS in terms of ease of use, sample preparation, and real-time results. The objective of the comparison was to study the sensitivity of LIBS and to investigate the quantification of the surface species measured by LIBS. Another objective was to assess the reliability of each technique for surface contaminant characterization. The as-processed CFRP panels had trace surface silicone contamination from the fabrication process, the source of which was not investigated. The composites were laser treated at select average laser power levels, resulting in varying levels of contamination reduction. The Si atomic percentage measurements using XPS were conducted on both control and laser-ablated surfaces. The results showed an excellent correlation in Si concentration between the two techniques.

Evidence for Molybdenum‐Hydrogen Bonding in p‐Type Silicon upon Annealing under Illumination

Transition metals (TMs) are common contaminants in solar silicon produced by block casting or using lower grade polysilicon feedstock. Gettering can effectively remove certain metals, but slow diffusing TMs can remain and act as highly efficient recombination centers. One method to reduce their recombination activity is hydrogen passivation, however, this is generally ineffective in p‐type silicon, in which hydrogen and TMs are typically both in positive charge states. Recent studies have suggested that under illumination, the charge state of hydrogen in silicon can be altered, and its bonding with certain defects can be enhanced. In this work, deep level transient spectroscopy (DLTS), Laplace DLTS, and capacitance‐voltage measurements are used to study TM‐H bonding in p‐type silicon intentionally contaminated with Mo and hydrogenated via remote H plasma. It is found that heat‐treatments at moderate temperatures (<250 °C) under illumination with an LED at 850 nm result in a reduction in the concentration of electrically active Mo, with no similar reduction seen in samples annealed in the dark. The passivated fraction of Mo can be recovered after dark annealing at temperatures >200 °C. The results obtained provide evidence that upon heat‐treatments with illumination Mo‐H bonding in p‐type Si occurs. Possible mechanisms of the observed effects are discussed.

INVESTIGATION OF INFLUENCE OF TECHNOLOGICAL IMPURITIES ON THE I–V CHARACTERISTICS OF THE BIPOLAR n–p–n-TRANSISTOR

Contamination of the monocrystal silicon with technological impurities in the devices fabrication process exerts a considerable influence on the electro-physical characteristics of the bipolar n–p–n-transistors. Revelation of the causes of the labile reproducibility of the basic characteristics of the bipolar planar n–p–n-transistors is vital for the purpose of establishing the factors, determining reliability and stability of the operational parameters of the integrated circuits. There were investigated I–V characteristics of the various lots of the bipolar n–p–n-transistors, fabricated under the epitaxialplanar technology as per the similar process charts with the identical used technological materials, however, at different times. It is established that the electro-physical characteristics of the bipolar n–p–n-transistors substantially depend on the contents of the technological impurities in the substrate material. Availability of the high concentration of the generation-recombination centers, related to the metallic impurities, results both in increase of the reverse current of the collector – base junction of the transistors and the significant reduction of the breakdown voltage of the collector junction. The most probable cause of deterioration of the electro-physical parameters of the bipolar n–p–n-transistors is the material contamination with the technological impurities (such, as Fe, Cl, Ca, Cu, Zn and others) during the production process of the devices fabrication. The sources of impurity may be both the components and sub-assemblies of the technological units and the materials and reagents under usage.

Modeling electron beam parameters and plasma interface position in an anode plasma electron gun with hydrogen atmosphere

A beam glow discharge based electron gun can be applied as heater for silicon crystal growth systems in which silicon rods are pulled from melt. Impacts of high-energy charged particles cause wear and tear of the gun and generate an additional source of silicon contamination. A steady-state model for electron beam formation has been developed to model the electron gun and optimize its design. Description of the model and first simulation results are presented. It has been shown that the model can simulate dimensions of particle impact areas on the cathode and anode, but further improvements of the model are needed to correctly simulate electron trajectory distribution in the beam and the beam current dependence on the applied gas pressure.

Hafnium Impurity Defects in Silicon: A Characterization

The aim of this work is to characterize the electrical properties of the defects induced by hafnium (Hf) contamination in silicon by means of different experimental techniques. Hf is introduced in silicon by shallow ion implantation at low doses in order to match experimental conditions close to an accidental contamination; moreover annealing is performed in RTP, that allows to maintain the metallic contaminant in solution. During the annealing the most part of Hf atoms segregate at the SiO2/Si interface, increasing the density of interface states. A detectable part of Hf atoms diffuse through the silicon bulk producing generation and recombination centers and traps for holes and electrons. Eight levels are detected, six in the upper and two in the lower half of the silicon bandgap, respectively. Hf acts differently in p- or n-type substrates: it is more effective as hole trap and as a recombination center in p-type and it de-activates above an implanted dose of 3 × 1011 cm−2 only in p-type silicon. The accurate determination of Hf-induced defects even at low concentration allows to find and pinpoint possible hafnium contamination affecting electronic devices.

Surface characterization of carbon fiber reinforced polymers by picosecond laser induced breakdown spectroscopy

Adhesive bonding of composite materials requires reliable monitoring and detection of surface contaminants as part of a vigorous quality control process to assure robust and durable bonded structures. Surface treatment and effective monitoring prior to bonding are essential in order to obtain a surface which is free from contaminants that may lead to inferior bond quality. In this study, the focus is to advance the laser induced breakdown spectroscopy (LIBS) technique by using pulse energies below 100μJ (μLIBS) for the detection of low levels of silicone contaminants in carbon fiber reinforced polymer (CFRP) composites. Various CFRP surface conditions were investigated by LIBS using ∼10ps, 355nm laser pulses with pulse energies below 30μJ. Time-resolved analysis was conducted to optimize the gate delay and gate width for the detection of the C I emission line at 247.9nm to monitor the epoxy resin matrix of CFRP composites and the Si I emission line at 288.2nm for detection of silicone contaminants in CFRP. To study the surface sensitivity to silicone contamination, CFRP surfaces were coated with polydimethylsiloxane (PDMS), the active ingredient in many mold release agents. The presence of PDMS was studied by inspecting the Si I emission lines at 251.6nm and 288.2nm. The measured PDMS areal densities ranged from 0.15 to 2μg/cm2. LIBS measurements were performed before and after laser surface ablation. The results demonstrate the successful detection of PDMS thin layers on CFRP using picosecond μLIBS.

N-type high-performance multicrystalline and mono-like silicon wafers with lifetimes above 2 ms

Combined with advanced crystal growth technology and reduced dislocation densities, the higher tolerance to metal contamination of n-type silicon makes n-type cast-grown silicon a potential option for low cost high quality substrates for solar cells. Using a combination of photoconductance based lifetime testing and photoluminescence imaging, we have investigated the carrier lifetime in wafers from the bottom, middle, and top parts of a n-type high-performance multicrystalline (HPM) silicon ingot, and wafers from n-type mono-like silicon ingots after each high temperature solar cell processes, including after boron diffusion, phosphorus diffusion, and hydrogenation. Although boron diffusion leads to a degradation of the sample lifetime, phosphorus diffusion and hydrogenation is effective at recovering the lifetime in the intra-grain region and at the grain boundaries respectively. Quasi-steady-state photoconductance (QSSPC) measurements show that the arithmetic average lifetime of HPM silicon wafers and mono-like silicon wafers can reach up to 1.8 and 3.3 ms respectively for a process sequence including a boron diffusion, with corresponding implied open circuit voltage of about 720 mV. If the boron diffusion can be avoided, average lifetimes up to 3.0 and 6.6 ms can be achieved respectively, highlighting the excellent potential of n-type cast-grown materials.

The removal of metal impurities from the surface of Czochralski wafers using a porous silicon-based gettering under a gas flow HCl/O2 dry

In this paper, the gettering of Czochralski silicon (CZ) with a porous silicon layer (PSL) oxidized in the presence of hydrochloric acid (HCl) gas was well discussed. The porous silicon damage reduces the activity of the contaminants in silicon and seems to be more effective when used in combination with HCl treatments. This process combined both mechanical and chemical parts to enhance the impurity removal. The getter was evaluated by means of the Hall Effect device to measure the carrier mobility. An enhancement of a minority carrier lifetime was evaluated by the QSSPC technique. The C–V characteristics indicated a decrease of the bulk dopants concentration. In the dark I–V characteristics, we observed a reduction of the leakage current density (JL) which goes back to the important decrease of the majority carriers. However, this new process can getter unwanted impurities deep-levels and enhanced the photovoltaic properties of solar cells.

Formation of intrinsic and silicon defects in MoO3 under varied oxygen partial pressure and temperature conditions: an ab initio DFT investigation

DFT simulations predict how varied MoO3 preparation conditions could change intrinsic defect concentrations and avoid silicon contamination in photovoltaic applications.

Comprehensive examination of acid leaching behaviour of mineral phases from red mud: Recovery of Fe, Al, Ti, and Si

Red mud represents an environmental and economic liability for the alumina industry in the form of wasted raw material. Although some leaching studies have been performed, there are deficiencies in the current literature regarding the amounts of metals extracted relative to each other, and minimal information regarding silicon contamination of extracts. There is also limited knowledge of extraction efficiencies of different acids (particularly in the case of phosphoric acid) under the same experimental conditions. This study focused on the leaching behaviour of the four most extractable elements present within red mud (iron, titanium, aluminium and silicon). By varying the experimental conditions, acid concentration, and type of acid, a comprehensive dataset of leaching trends was obtained. This allowed for direct comparison of leaching efficiency for the four elements under the same conditions, which was difficult previously due to the variation of experimental conditions and red mud composition between studies. The patterns in recoveries were explained in terms of the reactivities of the mineral phases within red mud and the interaction between the different acids and the reaction surfaces. Out of the four acids studied (nitric, hydrochloric, sulfuric, and phosphoric) phosphoric and hydrochloric acids produced some of the best recoveries for iron (76–78%) and titanium (23–24%), and phosphoric acid also produced the highest recoveries for silicon (49%) and aluminium (50%). The differences observed between the acid types and reaction conditions revealed potential for development of element selective extraction methods. Additionally, explaining leaching behaviour in terms of the mineral phases present allowed easier prediction of expected leaching trends for these four elements, which made this study applicable to red muds with a wide variety of compositions.

Ga contamination in silicon by Focused Ion Beam milling: Dynamic model simulation and Atom Probe Tomography experiment

Focused Ion Beam (FIB) milling is a widely used and important technique to prepare Transmission Electron Microscopy (TEM) lamella samples. However, it unavoidably introduces contamination in the samples like implanted ions. While the conventional ion-solid simulation model, Binary Collision Approximation (BCA), is unable to describe the FIB process, a dynamic BCA model is used in this study to predict the level of Ga implantation along the substrate depth. The FIB process involves significant material transport and local composition alteration, which requires a dynamic model for simulation. To validate the dynamic BCA model's application on simulating the FIB process, atomic level composition analysis Atom Probe Tomography (APT) is performed on Ga FIB processed silicon samples. The experimental data confirm that the dynamic BCA model is capable to predict FIB induced Ga ion implantation in silicon samples.

Numerical Calculation of Optical Properties of Silicone-Contaminant Films

The performance of solar arrays, thermal-control surfaces, and optical components is severely degraded by molecular contaminants outgassed from spacecraft materials. These outgassed molecular contaminants are reportedly deposited in “droplet” configurations. The size of droplets, on the order of single to tens of micrometers, far exceeds the amplitudes of the ultraviolet and visible wavelengths, implying that these droplet configurations are highly capable of scattering the incident light. For optical components of spacecraft, their optical properties in these spectral regions are particularly affected by the presence of droplet contaminants. Silicone adhesives are commonly used for spacecraft and are a well-known contamination source. Hence, silicone contaminants were deposited on quartz substrate, and then the optical characteristics of the contaminated substrates were investigated. To obtain optical constants for silicone contamination, the hemispherical spectral transmittance of the contaminant film was calculated using the effective medium approximation theory and the multiple reflectance/transmittance model. The directional spectral transmittance was simulated using these optical constants and rigorous coupled-wave analysis numerical calculations. This is the first trial for the calculation of optical properties for droplet-configuration contaminants. This method shows potential as a possible means to simulate the directional transmittance spectrum of droplet contaminants.

Measurement of radioactive contamination in the CCD’s of the DAMIC experiment

DAMIC (Dark Matter in CCDs) is an experiment searching for dark matter particles employing fully-depleted charge-coupled devices. Using the bulk silicon which composes the detector as target, we expect to observe coherent WIMP-nucleus elastic scattering. Although located in the SNOLAB laboratory, 2 km below the surface, the CCDs are not completely free of radioactive contamination, in particular coming from radon daughters or from the detector itself. We present novel techniques for the measurement of the radioactive contamination in the bulk silicon and on the surface of DAMIC CCDs. Limits on the Uranium and Thorium contamination as well as on the cosmogenic isotope 32 Si, intrinsically present on the detector, were performed. We have obtained upper limits on the 238 TJ (232 Th) decay rate of 5 (15) kg_1 d_1 at 95% CL. Pairs of spatially correlated electron tracks expected from 32 Si-32 P and 210 Pb-210 Bi beta decays were also measured. We have found a decay rate of 80+l10 -65 kg_1 d_1 for 32 Si and an upper limit of - 35 kg-1 d-1 for 210 Pb, both at 95% CL.

In Search of an Alternative Solar Grade Silicon Production By Electrochemical Reduction of SiO2

Silicon plays an important role in fabrication of solar cells and solar chips. Metallurgical grade silicon (98 to 99 % Si) demands further purification such as zone refining [1, 2], metallothermic reduction [3] and Siemens process [4] to convert it to solar grade. A promising method, based on direct electrochemical reduction of oxides by FFC Cambridge Process [5], was adopted to form silicon from porous SiO2 pellets in molten CaCl2 and CaCl2–NaCl salt mixture [6]. The contamination of silicon powder by iron and nickel originating from stainless steel cathode was reported to disqualify the product from solar applications. SiO2 pellets, sintered at 1300oC for 4 hours, were sandwiched between pure silicon wafer plates to minimize contamination of silicon. The promising results indicated a potential alternative method of direct solar grade silicon production for expanding solar energy industry. [1] P.R. Mei, S.P.Moreira, E.Cardoso, A.D.S.Cortes, F.C.Marques, “Purification of metallurgical silicon by horizontal zone melting”, Sol. Energ. Mat. Sol., 98 (2012), 233-239. [2] P. Siffert, E.F. Krimmel, eds. Silicon: Evolution and future of a technology. Berlin,Springer (2004). [3] K. Yasuda, T.H. Okabe, “Solar-grade silicon production by metallothermic reduction”, JOM,62 (12) (2010), 94-101. [4] Siemens & Halske: BRD Patents 1.102.117 and 1.140.1594 filed 1954, issued 1956 [5] G.Z. Chen, D. J. Fray, T. W. Farthing, “Direct Electrochemical Reduction of Titanium Dioxide to Titanium in Molten Calcium Chloride”, Nature, 407 (2000), 361-364. [6] E. Ergül, İ. Karakaya, M. Erdoğan, “Electrochemical decomposition of SiO2 pellets to form silicon in molten salts” Journal of Alloys and Compounds, 509 (2011), 899-903.

Impact Assessment about UV- and AO-Irradiated Silicone Contaminants through Optical Property Measurement

Impact Assessment about UV- and AO-Irradiated Silicone Contaminants through Optical Property Measurement

Detection and Prevention of Palladium Contamination in Silicon Devices

In this work we report the results of a set of experiments carried out to assess the ability of recombination lifetime measurements for the detection of palladium contamination in silicon. Palladium is found to be a very effective recombination center, so recombination lifetime measurements are a very sensitive method to detect palladium in silicon. The surface segregation of palladium was monitored by the reduction of its recombination activity in the silicon volume. The palladium segregation at the wafer surface was checked by selective etching, and by Transmission Electron Microscopy (TEM) and Energy Dispersive X-ray (EDX) analysis.After validating recombination lifetime measurements for palladium detection, we use these measurements to define suitable approaches to the prevention of palladium contamination of silicon devices. The efficiency of a diffusion barrier layer (silicon nitride) and of decontamination by wet cleaning are tested.