Fundamental investigations of cutting of silicon for photovoltaic applications

Crystalline silicon (Si) wafers used as substrates in the semiconductor and photovoltaic (PV) industries are traditionally manufactured using a multi-wire slurry sawing (MWSS) technique. Due to its high productivity potential, the fixed abrasive diamond wire sawing (DWS) technique is of considerable interest to Si wafer producers. Although both sawing techniques are currently used in the industry, a fundamental understanding of the underlying process is still lacking, particularly for diamond wire sawing. Consequently, optimization of the wire sawing process is carried out largely based on experience and trial and error. This thesis aims to develop a systematic fundamental understanding of diamond wire sawing of Si materials used for PV applications. First of all, a comparative analysis of the characteristics of silicon wafers cut by slurry and fixed abrasive diamond wire sawing is presented. The analysis results indicate that fixed abrasive diamond wire sawing may be a viable alternative to slurry wire sawing. Modeling and experimental studies of single grit diamond scribing of Si are proposed to shed light on the basic cutting mechanisms. Although Si is brittle at room temperature, it is possible to properly control the cutting conditions to obtain a completely ductile mode of material removal. The effects of material anisotropy, abrasive grit shape, friction condition and external hydrostatic pressure on the ductile-to-brittle mode transition in cutting of single crystal Si (sc-Si) are systematically investigated. Multicrystalline Si (mc-Si) based solar cells take up the majority of the global PV market. Hard inclusions (Silicon carbide and Silicon nitride) in multicrystalline Si (mc-Si) ingots may cause wire breakage and negatively impact the process, surface/subsurface morphology and mechanical properties of the resulting wafer. Their effects are experimentally studied through the single grit diamond scribing on the mc-Si sample with high density of inclusions. Finally, it is identified that there is a correlation between the high dislocation density and the increase of fracture toughness in mc-Si. The increase in fracture toughness leads to greater capability of ductile mode of cutting and higher specific scribing energy in the brittle fracture regime. Results of these fundamental investigations are expected to generate useful knowledge for optimizing the diamond wire sawing process in order to achieve high productivity and minimum surface/subsurface damage.PhDCommittee Chair: Shreyes Melkote; Committee Member: Jianjun Shi; Committee Member: Naresh Thadhani; Committee Member: Steven Danyluk; Committee Member: Steven Lian

Nanoparticle Engineering for Chemical-Mechanical Planarization

Increasing reliance on electronic devices demands products with high performance and efficiency. Such devices can be realized through the advent of nanoparticle technology. This book explains the physicochemical properties of nanoparticles according to each step in the chemical mechanical planarization (CMP) process, including dielectric CMP, shallow trend isolation CMP, metal CMP, poly isolation CMP, and noble metal CMP. The authors provide a detailed guide to nanoparticle engineering of novel CMP slurry for next-generation nanoscale devices below the 60nm design rule. This comprehensive text also presents design techniques using polymeric additives to improve CMP performance

SURFACE CHARACTERIZATION AND TRIBOLOGY IN FLAT LAPPING OF METALS

Lapping is a loose abrasive process employed to remove very small quantities of materials leading to a good surface finish. This research makes several investigations on the lapping process, both qualitative and quantitative. Lapping has been in existence for several decades and yet remains more of an art rather than a science. The principal objective is to create a scientific basis to the study of lapping common metals with common abrasives. The important goals are to study friction, material removal rate, roughness, surface characterization, redox chemistry, burn, and microvoids during flat lapping of aluminum 2024, 304 stainless steel, and 1018 steel. The effects of different abrasives: garnet, silicon carbide, and white aluminum oxide were studied experimentally while lapping aluminum 2024, 304 stainless steel, and 1018 steel.In addition, the area of lapped parts, unfinished zones, and scratched zones were determined using image analysis. Although the aim of lapping is to improve surface finish, sometimes parts are rejected after lapping because of burn, friction, incomplete lapping, scratches, microvoids, and wear. Scratches may be caused by excessive load, low supply of abrasive slurry, or high friction and burn may be caused by excessive load. Uneven distribution of load occurs when the lapping table is not flat, but rather concave or convex in shape. The factors that cause burn, scratches, and incomplete lapping should be minimized.A new method is proposed for calculation of frictional force during lapping using the current consumed in the process. The effects of different abrasives on material removal rate and surface finish on three different types of work materials were evaluated quantitatively. It was found that silicon carbide and white aluminum oxide abrasives removed more material per minute than garnet. Furthermore, from geometric and Energy Dispersive Spectroscopy (EDS) analysis obtained using a Scanning Electron Microscope (SEM), it was confirmed that some abrasives became embedded into the lapped metal substrates. No burn was observed on the lapped samples. Scratches and unfinished lapped parts were observed primarily in 304 stainless steel. There were little or no scratches found on lapped Al 2024 and 1018 steel.Based on the net cell reaction potentials using the Nernst equation, the possible reactions during the lapping process are reactions between magnesium and its hydroxides and white aluminum oxide abrasive. Also, SiO2 from SiC abrasives oxidized Al, Mn, Mg, and Ti in Al 2024 as well as Mn in 304 stainless steel, and Al and Mn in 1018 steel. Analysis of Variance (ANOVA) was performed using Statistical Analysis Software (SASTM 9.1) in order to determine the effects of each variable. ANOVA results revealed that the main effects of abrasive types, size of abrasives, and type of work material had statistically significant influence on material removal rate and surface finish

PRECISION POLISHING DYNAMICS: THE INFLUENCE OF PROCESS VIBRATIONS ON POLISHING RESULTS

The optical pitch polishing process has been used over 300 years to obtain high quality optical surface finish with little subsurface damage. A pitch tool consists of a metal platen coated with a layer of polishing pitch whereby pitch is a highly viscoelastic material. In polishing the workpiece is rubbed against the tool while abrasive slurry is supplied in between them. During polishing the workpiece is subjected to process vibrations, whereby be these vibrations are generated by the machine itself due to moving parts, or that are transmitted from the shop floor through the machine to the workpiece. To date, little is available in the public domain regarding the role of process induced vibrations on polishing outcomes. This research investigates such vibrations, how they transfer through the pitch layer on the tool, and ultimately how they affect the material removal rates and surface finishes obtainable on fused silica workpieces. Fundamental understandings with respect to the process vibration will reduce the heuristic nature of pitch polishing and generate deterministic polishing outcomes. Key findings include the following. The pitch selection has little influence on the magnitude or range of process vibrations transmitted through the tool to the workpiece in the 1 Hz to 16 kHz range. Within the same frequency bandwidth the recorded process vibrations are in the range of 0.2 to 10 nm and the main factors found to affect their magnitude include; the polishing machine itself, process speeds, and the use of passive damping materials in the tool construction. Material removal rates and surface finishes obtained on fused silica workpieces were found to be sensitive to the extent of the process vibrations. Up to 30% changes in the material removal rates were observed with increasing vibrational magnitudes. The higher level vibrations were also found to have a negative impact on the finishes obtained in the lower spatial domains. Additional testing on a specifically made test-bed demonstrated a linear correlation between the material removal rates and the vibrational power input. This relationship was further explored by adding external vibrational sources to an existing machine, and as expected the increased vibrational power resulted in 80% higher material removal rates. The results from this experimental work facilitated Dr. Keanini’s development of a vibrational based material removal model. Additional polishing tests combined with surface topography analysis of both hard and soft pitch tools demonstrated the robustness of the proposed model to accommodate the influence of different pitch grades. The summary in general is that in pitch polishing the process vibrations are important to monitor and control for process optimization

PRECISION POLISHING DYNAMICS: THE INFLUENCE OF PROCESS VIBRATIONS ON POLISHING RESULTS

The optical pitch polishing process has been used over 300 years to obtain high quality optical surface finish with little subsurface damage. A pitch tool consists of a metal platen coated with a layer of polishing pitch whereby pitch is a highly viscoelastic material. In polishing the workpiece is rubbed against the tool while abrasive slurry is supplied in between them. During polishing the workpiece is subjected to process vibrations, whereby be these vibrations are generated by the machine itself due to moving parts, or that are transmitted from the shop floor through the machine to the workpiece. To date, little is available in the public domain regarding the role of process induced vibrations on polishing outcomes. This research investigates such vibrations, how they transfer through the pitch layer on the tool, and ultimately how they affect the material removal rates and surface finishes obtainable on fused silica workpieces. Fundamental understandings with respect to the process vibration will reduce the heuristic nature of pitch polishing and generate deterministic polishing outcomes. Key findings include the following. The pitch selection has little influence on the magnitude or range of process vibrations transmitted through the tool to the workpiece in the 1 Hz to 16 kHz range. Within the same frequency bandwidth the recorded process vibrations are in the range of 0.2 to 10 nm and the main factors found to affect their magnitude include; the polishing machine itself, process speeds, and the use of passive damping materials in the tool construction. Material removal rates and surface finishes obtained on fused silica workpieces were found to be sensitive to the extent of the process vibrations. Up to 30% changes in the material removal rates were observed with increasing vibrational magnitudes. The higher level vibrations were also found to have a negative impact on the finishes obtained in the lower spatial domains. Additional testing on a specifically made test-bed demonstrated a linear correlation between the material removal rates and the vibrational power input. This relationship was further explored by adding external vibrational sources to an existing machine, and as expected the increased vibrational power resulted in 80% higher material removal rates. The results from this experimental work facilitated Dr. Keanini’s development of a vibrational based material removal model. Additional polishing tests combined with surface topography analysis of both hard and soft pitch tools demonstrated the robustness of the proposed model to accommodate the influence of different pitch grades. The summary in general is that in pitch polishing the process vibrations are important to monitor and control for process optimization

Development Of Titanium Nitride/molybdenum Disulphide Composite Tribological Coatings For Cryocoolers

Hydrogen is a clean and sustainable form of carrier of energy that can be used in mobile and stationary applications. At present hydrogen is produced mostly from fossil sources. Solar photoelectrochemical processes are being developed for hydrogen production. Storing hydrogen can be done in three main ways: in compressed form, liquid form and by chemical bonding. Near term spaceport operations are one of the prominent applications for usage of large quantities of liquid hydrogen as a cryogenic propellant. Efficient storage and transfer of liquid hydrogen is essential for reducing the launch costs. A Two Stage Reverse Turbo Brayton Cycle (RTBC) CryoCooler is being developed at University of Central Florida. The cryocooler will be used for storage and transport of hydrogen in spaceport and space vehicle application. One part in development of the cryocooler is to reduce the friction and wear between mating parts thus increasing its efficiency. Tribological coatings having extremely high hardness, ultra-low coefficient of friction, and high durability at temperatures lower than 60 K are being developed to reduce friction and wear between the mating parts of the cryocooler thus improving its efficiency. Nitrides of high-melting-point metals (e.g. TiN, ZrN) and diamond-like-carbon (DLC) are potential candidates for cryogenic applications as these coatings have shown good friction behavior and wear resistance at cryogenic temperatures. These coatings are known to have coefficient of friction less than 0.1 at room temperature. However, cryogenic environment leads to increase in the coefficient of friction. It is expected that a composite consisting of a base layer of a hard coating covered with layer having an ultra-low coefficient of friction would provide better performance. Extremely hard and extremely low friction coatings of titanium nitride, molybdenum disulphide, TiN/MoS2 bilayer coatings, DLC and DLC/MoS2 bilayer coatings have been chosen for this application. TiN film was deposited by reactive DC magnetron sputtering system from a titanium target and MoS2 film was deposited by RF magnetron sputtering using a MoS2 target. Microwave assisted chemical vapor deposition (MWCVD) technique was used for preparation of DLC coatings. These composite coatings contain a solid lubricating phase and a hard ceramic matrix phase as distinctly segregated phases. These are envisioned as having the desired combination of lubricity and structural integrity. Extremely hard coatings of TiN and DLC were chosen to provide good wear resistance and MoS2 was chosen as the lubricating phase as it provides excellent solid lubricating properties due to its lamellar crystal structure. This thesis presents preparation; characterization (SEM and XRD), microhardness and tribological measurements carried out on TiN and TiN/MoS2 coatings on aluminum and glass substrate at room temperature. It also presents initial development in preparation of DLC coatings

Electricity from photovoltaic solar cells: Flat-Plate Solar Array Project final Report. Volume III: Silicon sheet: wafers and ribbons

The Flat-Plate Solar Array (FSA) Project, funded by the U.S. Government and managed by the Jet Propulsion Laboratory, was formed in 1975 to develop the module/array technology needed to attain widespread terrestrial use of photovoltaics by 1985. To accomplish this, the FSA Project established and managed an Industry, University, and Federal Government Team to perform the needed research and development. The primary objective of the Silicon Sheet Task of the FSA Project was the development of one or more low-cost technologies for producing silicon sheet suitable for processing into cost-eompetitive solar cells. Silicon sheet refers to high-purity crystalline silicon of size and thickness for fabrication into solar cells. The Task effort began with state-of-the-art sheet technologies and then solicited and supported any new silicon sheet alternatives that had the potential to achieve the Project goals. A total of 48 contracts were awarded that covered work in the areas of ingot growth and casting, wafering, ribbon growth, other sheet technologies, and programs of supportive research. Periodic reviews of each sheet technology were held, assessing the technical progress and the long-range potential. Technologies that failed to achieve their promise, or seemed to have lower probabilities for success in comparison with others, were dropped. A series of workshops was initiated to assess the state of the art, to provide insights into problems remaining to be addressed, and to support technology transfer. The Task made and fostered significant improvements in silicon sheet including processing of both ingot and ribbon technologies. An additional important outcome was the vastly improved understanding of the characteristics associated with high-quality sheet, and the control of the parameters required for higher efficiency solar cells. Although significant sheet cost reductions were made, the technology advancements required to meet the Task cost goals were not achieved. This FSA Final Report (JPL Publication 86-31, 5101-289, DOE/JPL 1012-125, October 1986) is composed of eight volumes, consisting of an Executive Summary and seven technology reports: Volume I: Executive Summary. Volume II: Silicon Material. Volume III: Silicon Sheet: Wafers and Ribbons Volume IV: High-Efficiency Solar Celis. Volume V: Process Development. Volume VI: Engineering Sciences and Reliability. Volume VII: Module Encapsulation. Volume VIII: Project Analysis and Integration. Two supplemental reports included in the final report package are: FSA Project: 10 Years of Progress, JPL Document 400-279. 5101-279, October 1985. Summary of FSA Project Documentation: Abstracts of Published Documents, 1975 to 1986, JPL Publication 82-79 (Revision 1),5101-221, DOE/JPL-1 012-76, September 1986

Micron Diamond Processing of Advanced Ceramics

Grinding is one of the most complex manufacturing processes in industry and understanding its physics is difficult due to the stochastic nature of the process. In this thesis, the influence of the abrasive grit’s shape and size on the grinding process is considered. A number of parameters are investigated to set a classification of the abrasives based on the grit’s shape and size. These parameters are determined according to image analyses of a large number of abrasive grits. Based on this investigation, the shapes of the abrasive grits could be classified into 21 groups. Typical grit shapes will fall into only few categories dominating the shape population. These dominant shapes are ellipsoid, sphere, quadrilateral frustrum, quadrilateral pyramid and tetrahedron pyramid. After the abrasives are assessed, a test rig for multiple grit scratching and wire saw cutting rig were developped and a series of multiple grit grinding tests are performed. For this purpose, series of scratching tests have been conducted with five different diamond abrasives. The cutting forces and the acoustic emission were used to characterize the grinding mechanism during this experiment. The machining performances of the abrasive grits are evaluated in consideration of the effect of different grit shapes on the grinding process outputs including force and acoustic emission. The experimental results show a high influence of the proportion of different grit shapes on grinding force: abrasive grits with rounded shape imply high cutting forces, while grits with pyramidal shape generate low cutting forces. Furthermore, based on the proportion of the dominant shapes in an abrasive sample a force model of the cutting force and the shape proportion of the abrasives was established. The force model and the experimental results emphasised the importance of taking into consideration the abrasive’s shape as a significant parameter that influences the grinding process. The online grinding surface creation monitoring was carried out by processing the acoustic emission signals. The acoustic emission signals are analysed in both the time and frequency domains. The results show that the signal feature extraction in the frequency domain gives excellent indication in correlation to the surface creation with different abrasive geometrical characteristics

Study of Micro-grinding of Glass using On-machine Fabricated Polycrystalline Diamond (PCD) by Micro-EDM

Ph.DDOCTOR OF PHILOSOPH