Multilayer electret activated by direct contact silicon electrode.

Electrets used in microelectromechanical systems (MEMS) devices are often formed by corona charging, where ionized gases are generated in an electric field to introduce a charge to the electret surface. The purpose of this study was to investigate a new technique for creating an electret from a plasma enhanced chemical vapor deposition (PECVD) multilayer film of SiO2/Si3N4/SiO2 using a direct contact electrode of silicon. The electret formation takes advantage of deep traps in silicon nitride, which are known to develop from hydrogen interactions with silicon dangling bonds and, in some stoichiometries, nitrogen dangling bonds. The electret activation process has been optimized for maximum effective surface voltage (ESV). The deposition and activation process for the electret has the additional benefit of using commercially available equipment present in many microelectronic fabrication facilities. Standardized processes for depositing the PECVD film stack and activating the electret with a wafer level bonder have been developed.Using this new process, electret films have been produced with positive and negative effective surface voltages in excess of +/‐194.0 V. Extrapolated lifetimes, based on thermal decay studies, are calculated to be 57 years and 23 years for positive and negative electrets respectively if they are maintained in moderate to low humidity environments below 125°C. Activation energy levels in positive and negative electrets are 1.4 eV and 1.2 eV respectively. This new electret multilayer film stack and direct charging method produced thin film electrets with a half‐life 5 times greater than that reported in literature by other groups using PECVD multilayer electrets [1, 2]. A new application was investigated to see how an electret may benefit semiconductor‐liquid interactions. The PECVD electret was used to apply a gate bias to the back side of a double side polished silicon wafer to determine the effect of gate bias on the etch rates of an anisotropic silicon etch in 25% wt. tetramethylammonium hydroxide (TMAH). Our results show that the positively charged electret produced a statistically significant increase in etch rate, when compared to neutral and negatively charged electrets, as the silicon‐TMAH interface approached the depletion region produced by the electret. The mean values of the silicon etch rate were evaluated for the last hour of etching with samples categorized by electret potentials as positive, negative or neutral. The positive potential electret had a mean etch rate of 12.0 um/hr for silicon as compared to 8.8 um/hr and 8.6 um/hr for negatively and neutrally charge electrets respectively. The one way Analysis Of Variance (ANOVA) of the silicon etch rates between the neutral (control) PECVD film and the positive electret had a P value of 0.009 and falls within the 1% significance level, showing that it is very likely that the positive electret film has an effect on the final etch rate of the silicon under null hypothesis testing

Metal surface contamination in c-Si solar cell processing

Fe und Cu wurden als Schlüsselspezies für die Betrachtung von Oberflächenkontamination in der Prozessierung von c-Si-Solarzellen identifiziert. Studien mit gezielt aufgebrachten Metallkonzentrationen vor verschiedenen Passivierungs- und Diffusionsprozessschritten ergaben relativ hohe kritische Werte für Cu, außer bei thermischer Oxidation. Niedrige Werte wurden für beide Elemente vor Hochtemperaturschritten im n-Typ-Hocheffizienzprozess beobachtet, wobei sich die B-Diffusion als etwas weniger empfindlich darstellte. Temporäre Gettereffekte für Fe (in p-Typ-Si) und Cu (in n-Typ-Si) wurden beobachtet. Es zeigte sich, dass As-Cut-Wafer, unabhängig von der Sägeart (SiC-slurry oder Diamantdraht) sehr hohe Metallverunreinigungen (im Bereich 1*1011 – 5*1014 cm-2) in den Prozess einbringen. Das alkalische Ätzen verringert diese Menge kaum, was hohe Anforderungen an die anschließende Reinigung ergibt. Die Optimierung von HF/O3-Reinigungslösung für diese Flächen ergab beste Reinigungsergebnisse bei niedrigen HF-Konzentrationen, abhängig vom alkalischen Ätzschritt und anschließendem HCl/HF-Dip

Surface processing by RFI PECVD and RFI PSII

An RFI plasma enhanced chemical vapor deposition (PECVD) system and a large-scale RF plasma source immersion ion implantation (PSII) system were designed and built to study two forms of 3-D surface processing, PECVD and PSII. Using the RFI PECVD system, Ti-6Al-4V substrates were coated with diamond-like carbon films with excellent tribological and optical properties. as an innovation, variable angle spectroscopic ellipsometry (VASE) was successfully applied for non-destructive, 3-D, large-area tribological coatings quality investigation.;Based on the experience with the RFI PECVD system, a large-scale RFICP source was designed and built for the PSIL Langmuir probe and optical emission spectroscopy studies indicated that the RFI source produced stable, uniform, and clean plasma. MAGIC code was for the first time used to model PSII process, addressing different target geometries and boundaries, materials, plasma parameters, illustrated sheath formation and evolution, field distribution, ion and electron trajectories, ion incident angles, and dose distributions, which are critical for PSII design and understanding.;The RF PSII system was developed into a versatile large-area, uniform, 3-D surface processing apparatus, capable of PSII, PVD, PECVD, and in situ surface cleaning and interface properties modification, for multilayer, multi-step, and high performance surface engineering. Using the RFI PSII system, for the first time, PSII was studied as a mask-based surface layer conversion technique, for pattern writing by implantation as an alternative to current deposition-based and ink-based direct write technologies. It operates at low substrate temperature, keeps the original surface finish and dimensions, and avoids adhesion problem. A different operating mode of the RF source was discovered to perform biased sputtering of high purity quartz, which turned the RFI PSII system into a novel integrated RF PSII/PVD system for large-area, uniform, nitrogen-doped, and hydrogen-free SiO2 films deposition at low substrate temperatures. Nitrogen-doped SiO2 films with excellent optical properties were deposited on semiconductor, metal, and polymer substrates with excellent adhesion. Ellipsometry was used again for non-destructive SiO2 coatings investigation. FEL test electrodes processed by PSII/PVD showed suppressed field emission. A group of transition metals and an FEL test electrode were also implanted by nitrogen using the PSII mode and analyzed

Analysis of cell to module losses and UV radiation hardness for passivated emitter and rear cells and modules

This work presents an experimental analysis and analytical modeling of cell to module losses for passivated emitter and rear cells (PERC), which enables to build a PERC solar module with a record efficiency of 20.2%. Further, it examines the ultraviolet radiation hardness of solar modules employing crystalline silicon (c-Si) solar cells featuring dielectric passivation layers. Passivated emitter and rear cells are on the transition to mass production and expected to become the dominating c-Si solar cell technology in terms of market share in the next few years. Thus, it is of major importance to implement these high efficiency PERC into high efficiency solar modules. When transferring solar cells into a solar module additional recombination, optical, and resistive losses reduce the power of the solar module compared to the power of the solar cell, termed cell to module losses. In this work we study the individual recombination, optical, and resistive characteristics of various cell and module test samples. Based on our experimental results we develop an analytical model that allows to simulate the cell to module losses and reproduces the measurement results of test modules within the measurement uncertainty. We show that a reduction of the cell to module losses requires an adaptation of both, the solar cell as well as the solar module components. We employ the analytical model to improve the cell's front metalization, cell interconnection, light harvesting and cell spacing to reduce the cell to module losses for passivated emitter and rear cells and build an industrial like 60-cell sized solar module with a record power conversion efficiency of 20.2%. Besides the efficiency, the long-term reliability of solar modules is crucial and a performance degradation of new promising technologies can impair their importance for the industry. The application of new metalization pastes that enable to contact lowly doped emitters, increases the spectral response of a PERC in the UV wavelength range. This requires the application of new encapsulation materials with enhanced UV transmittance for PERC solar modules. We report on the UV radiation hardness of solar modules featuring PERC with various silicone nitride passivation layers and employing different encapsulation polymers. Our results reveal that employing polymers with increased UV transparency results in a solar module power loss of 14%. We show that the degradation in module power is due to a reduction of the module's open circuit voltage. This loss is related to an increased charge carrier recombination in the cell, which we ascribe to a degradation of the amorphous silicon nitride (SiN) surface passivation. We develop a novel analytical model to describe the effect of high energetic photons on the solar module performance with a critical energy to deteriorate the surface passivation

Evaluation of Cost-Effective Technologies for Highly Efficient Silicon-Based Solar Cells

The work underscores the feasibility of highly efficient silicon solar cell structures manufactured with high throughput machines. The main challenge consists in the implementation of more performant structures of intrinsic higher complexity. These structures are meant to be fabricated within similar constraints on time and on cost as for conventional devices of industrial manufacture. Achievements in this direction foster larger economical deployment of this kind of renewable energy. The structure chosen for the implementation of "high efficiency" concepts requires an advanced machining of the silicon substrate. Etching techniques, dielectric coating preparations, and metal to semiconductor contact formation were evaluated for their integration in a complete silicon solar cell fabrication sequence. These sequences were later tested to gain knowledge regarding the effects of the aforementioned advanced processing. Prototypes were created using the fabrication sequence. They were analysed to acquire a further understanding of the advanced processing influences. Statistical interpretation of the data obtained was used to support physical interpretation of the observed phenomena. For one particular implementation of a surface passivation (floating junction passivation) an attempt of modelling aiming to unveil its specific dynamics is reported. A resulting solar cell concept compatible with high throughput equipment achieved a certified efficiency of 18% (VOC = 635 mV, JSC = 37.3 mA/cm2, FF = 76 %) on substrate with a reduced thickness (120 µm). The resulting sequence prepares the back surface with a thermal oxidation for passivation purposes and with a laser technique to locally contact the bulk. The thickness is compatible with the intent of cost reduction through the decrease in material consumption. Other approaches performed on the same substrate achieved 17.1% efficiency (VOC = 617 mV, JSC = 36.1 mA/cm2, FF = 75 %). In this case the passivation was achieved with deposited dielectric layers on the back surface. Furthermore, the investigated dynamics of the floating junction passivation allows for better insight into its traits. The proposed solution has an interesting ratio of efficiency versus costs. The general consequence is a higher market appeal of this particular renewable source on the energy market. The study on the floating junction passivation allows a better exploitation of this particular implementation towards more performant silicon solar cells