Performance and modeling of paired polishing process

Paired polishing process (PPP) is a variant of the chemical mechanical polishing process which facilitates defect mitigation via minimization of maximum force as well as effective planarization via profile driven determination of force gradient. The present embodiment of PPP machine employs two polishing wheels, radially spanning the wafer surface on a counter-gimbaled base. The PPP machine is deployed to experimentally investigate the role of the process parameters on the surface roughness evolution, and the effective material removal rate. Two sets of copper and aluminum blanket layers were polished under a range of applied down force, polishing wheel speed and transverse feed rate to examine the scalability of the process parameters for different material constants. The experimental measurements along with the topological details of the polishing pad have been utilized to develop a mechanistic model of the process. The model employs the soft wheel-workpiece macroscopic contact, the polishing wheel roughness and its amplification to the local contact pressure, the kinematics of abrasive grits at the local scale, and the collective contribution of these individual micro-events to induce an effective material removal rate at the macroscale. The model shows the dependence of the material removal on the ratio of wheel rotational to feed speed for the PPP process, in a form of an asymptote that is scaled by the surface hardness of each material. The PPP machine exploits this insight and utilizes an oblique grinding technique that obviates the traditional trade-off between MRR and planarization efficiency

Theoretical and experimental investigations on conformal polishing of microstructured surfaces

Microstructured surfaces play a pivotal role in various fields, notably in lighting, diffuser devices, and imaging systems. The performance of these components is intricately related to the accuracy of their shapes and the quality of their surfaces. Although current precision machining technologies are capable of achieving conformal shapes, the post-machining surface quality often remains uncertain. To appropriately address this challenge, this paper introduces a novel conformal polishing methodology, specifically designed to enhance the surface quality of microstructured surfaces while maintaining their shape accuracy. As part of the investigations, specialized tools, namely the damping tool and profiling damping tool, are methodically developed for polishing rectangular and cylindrical surfaces. A shape evolution model is established based on the simulation of individual microstructures, incorporating the concept of finite-slip on the microstructured surface. The findings reveal that principal stresses and velocities experience abrupt variations at the convex and concave corners of rectangular surfaces as well as at the ends of cylindrical surfaces. The numerically predicted surface shape errors after polishing demonstrate reasonably good agreement with experimental results such that their discrepancies are less than 1 μm. Additionally, this method is able to successfully eradicate pre-machining imperfections such as residual tool marks and burrs on the microstructured surfaces. The arithmetic roughness (Ra) of the rectangular surface is measured to be an impressively low 0.4 nm, whereas the cylindrical surface exhibits Ra = 6.2 nm. These results clearly emphasize the effectiveness of the conformal polishing method in achieving high-quality surface finishes

Precision Machining

The work included in this book focuses on precision machining and grinding processes, including milling, laser machining and polishing on various materials for high-end applications. These processes are in the forefront of contemporary technology, with significant industrial applications. Their importance is also made clear by the important works that are included in the research that is presented in the book. Some important aspects of these processes are investigated, and process parameters are optimized. This is performed in the presented works with significant experimental and modelling work, incorporating modern tools of analysis and measurements

A novel fluorescence based method of assessing subsurface damage in optical materials

Lapping and polishing are loose abrasive finishing processes that have been used to achieve critical surface parameters in optical materials for centuries. These processes remove material from the surface through a variety of mechanical and chemical interactions. These interactions influence not only the surface of the finished material, but also the subsurface, the region immediately beneath the surface. These processes may induce a damaged layer of cracks, voids and stressed material below the surface. This subsurface damage (SSD) can create optical aberrations due to diffraction, propagate to the surface, and reduce the laser induced damage threshold of the material. It is difficult to detect SSD, as these defects lie beneath the surface. Methods have been developed to detect SSD, but they can have notable limitations regarding sample size and material, preparation time, or they can be destructive in nature. The author tested a non-destructive method for assessing SSD that consisted of tagging the abrasive slurries used in loose abrasive finishing with quantum dots (nano-sized fluorescent particles). Subsequent detection of fluorescence on the processed surface is hypothesized to indicate SSD. Quantum dots present during the lapping process were retained in the glass sample through subsequent polishing and cleaning processes. The quantum dots were successfully imaged by both wide field and confocal fluorescence microscopy techniques. The detected fluorescence highlighted defects that were not observable with optical or interferometric microscopy. Analysis indicates that most dots are firmly embedded in the surface, however examination of confocal fluorescence scans beneath the surface did show incidences of quantum dots at depths up to 10 µm beneath the surface. The incidence of these deep features was less than 20% of the sites examined. Etching of the samples exhibiting fluorescence confirmed the presence of SSD and provided a conservative SSD depth estimate of 10 µm. These etching results confirm the hypothesis that quantum dots can tag SSD. Further testing demonstrated that for quantum dots to be embedded in the surface they must experience the dynamics of the lapping process, and that quantum dots can only tag brittle fracture sites. Quantum dots that were introduced to YAG samples during loose abrasive finishing were only retained on the surface and at levels consistent with simple exposure to quantum dots prior to cleaning, possibly highlighting surface defects that were not apparent with conventional microscopy. Subsequent etching of the YAG samples showed low levels of fracture in the subsurface region, indicating few suitable defects to house the quantum dots. In addition to the research above, an instrument was design and built to measure the axial and torque loads during loose abrasive finishing. Experiments with this measurement head showed expected increases in material removal rate and surface roughness with increased axial load. Results from these tests were also used to corroborate SSD depth estimates from glass samples finished with quantum dot laden slurries

Electro-kinetically enhanced nano-metric material removal

This project is a fundamental proof of concept to look at the feasibility of using field activated abrasive particles to achieve material removal on a substrate. There are a few different goals for this project. The first goal is to prove through visualization that particle movement can be influenced and controlled by changes in electric field. The second goal is to fundamentally prove that particles controlled by electric field can remove material from a substrate. Third, it should be shown that changes in electric field can control the amount of material being removed in a given amount of time. A mathematical model will be presented which predicts metallic material removal rates based on changes in electric field strength. In this project, a technique combining concepts from electrokinetics, electrochemical mechanical planarization, and contact mechanics is proposed, aiming at enhancing planarization performance. By introducing an AC electric field with a DC offset, we try to achieve not only a better control of metallic material removal but also more flexible manipulation of the dynamic behaviour of abrasive particles. The presence of electric field will lead to electrokinetic phenomena including electroosmotic flow of an electrolyte solution and electrophoretic motion of abrasive particles. As a result, we aim to improve both the mechanical performance of planarization that is largely determined by the polishing parameters (e.g. down pressure, rotation speed, pads, and types of abrasives) and the chemical performance of planarization that is governed by selective and collective reactions of different chemical ingrediants of the slurry with the sample surface. The aim is also to understand and improve the interactions of abrasive particles with the sample.M.S.Committee Chair: Danyluk, Steven; Committee Member: Butler, David; Committee Member: Hesketh, Peter; Committee Member: Yoda, Minam

Validation of a Sensor System Solution for Process Monitoring in Robot Assisted Polishing

It was validated the possible correlation between hidden process variables (AE, force, power consumption) and the vital quality characteristic (roughness) for robot assisted flat polishing with diamond paste.\nIt was also validated the possibility of in-line roughness measurements using a scattered light instrument for this proces

In-Situ Characterization of Burr Formation in Finish Machining of Inconel 718

One of the undesirable byproducts that occur during the machining process is the development of burrs, which are defined as rough excess material that forms around the geometric discontinuities of a part. Burrs are especially problematic because they have negative impacts across the triple bottom line: economic, environmental, societal. For one, they are expensive to remove because the deburring process is entirely manual and requires skill. Further, burr material is typically discarded which is adding to the already mounting waste generated from machining such as in coolant and chip disposal. Lastly, there are many societal implications, such as operator injury during assembly and the failure of parts in service because of leftover burrs that turned into stress concentrations. Therefore, optimizing the machining process to minimize burrs and promote sustainable manufacturing is a central challenge for manufacturers today. However, the burr formation mechanism is complex, and research on the phenomenon is scarce. The current state of the art focuses almost exclusively on drilling and micro-milling processes, with very little work investigating burr formation in the conventional machining processes of turning and milling. Research as it pertains to materials that are difficult-to-machine like nickel and titanium-based superalloys is even less common, as most of the literature focuses on softer materials like aluminum and steel alloys. Superalloys are especially crucial to the aerospace industry, comprising most of the components in jet engines. Thus, the objective of this study was to characterize burr formation for nickel-based superalloy Inconel 718 using a custom-built in-situ testbed capable of ultra-high-speed imaging in orthogonal cuts. Experiments were carried out to measure the variation in burr development with respect to several cutting parameters: uncut chip thickness, tool-wear, and cutting speed. Firstly, the exit and side burr geometry were measured after each machining trial for a variety of different metrics. Results showed that all cutting parameters have an influence on the burr geometry, although not every cutting parameter had statistical significance on certain burr metrics. For instance, it was found that side burrs were much more sensitive to tool-wear than exit burrs. Then, by combining digital image correlation (DIC) with a physics-based model, the flow stress was calculated during exit burr formation and results revealed that the stress at the exit burr root was approximately equal to the flow stress. Finally, this study investigates the fracture phenomenon during exit burr formation—it was found that besides the requirement of high strain rate and depth of cut, negative exit burrs, there is a microstructural size effect, which had not been reported by prior work