Convergent Polishing: A Simple, Rapid, Full Aperture Polishing Process of High Quality Optical Flats &amp;amp; Spheres

Suratwala, Tayyab; Steele, R.; Feit, Michael D.; Dylla‐Spears, Rebecca; Desjardin, Richard; Mason, D.; Geraghty, Paul B.; Miller, Phil; Shen, Nan

doi:10.3791/51965

Cited by 11 publications

(6 citation statements)

References 12 publications

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“…As‐fabricated glass samples (50 mm diameter × 10 mm thick) of fused silica (Corning 7980, Corning NY), borosilicate glass (BK7, Schott Glass Technologies, Duryea, PA), and phosphate glass (LHG‐8 laser glass (Hoya Corporation USA, Fremont, CA) were used for AFM nanoscratching. These parts were polished using the Convergent Polishing method using a colloidal silica polishing slurry (Ultra‐Sol ® S27, Eminess Technologies, Scotsdale, AZ) at a 0.6 mL/min slurry feed rate (single pass) for 8 h with 0.3 psi applied pressure, and matched 20 rpm optic and lap rotation rate. Two additional fused silica samples were polished differently using cerium oxide (Stabilized Hastilite PO) at different polishing removal rates by increasing or decreasing the optic and lap rotation rate by 2× (i.e., either 40 or 10 rpm).…”

Section: Methodsmentioning

confidence: 99%

Nanoscratching of Optical Glass Surfaces Near the Elastic–Plastic Load Boundary to Mimic the Mechanics of Polishing Particles

et al. 2016

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The nanomechanical deformations on glass surfaces near the elastic-plastic load boundary have been measured on various glasses by nanoscratching using an atomic force microscope (AFM) to mimic the mechanical interactions of polishing particles during optical polishing. Nanoscratches were created in air and aqueous environments using a 150-nm radius diamond-coated tip on polished fused silica, borosilicate, and phosphate glass surfaces; the topology of the nanoscratches were then characterized by AFM. Using load ranges expected on slurry particles during glass polishing (0.05-200 lN), plastic-type scratches were observed with depths in the nm range. Nanoscratching in air generally showed deeper & narrower scratches with more pileup compared to nanoscratching in water, especially on fused silica glass. The critical load needed to observe plastic deformation was determined to range from 0.2-1.2 lN for the three glasses. For phosphate glass, the load dependence of the removal depth was consistent with that expected from Hertzian mechanics. However, for fused silica and borosilicate glass in this load range, the deformation depth showed a weak dependence with load. Using a sub-T g annealing technique, material relaxation was observed on the nanoscratches, suggesting that a significant fraction of the deformation was due to densification on fused silica and borosilicate glass. Repeated nanoscratching at the same location was utilized for determining the effective incremental plastic removal depth. The incremental removal depth decreased with increase in number of passes, stabilizing after~10 passes. In water, the removal depths were determined as 0.3-0.55 nm/pass for fused silica, 0.85 nm/pass for borosilicate glass, and 2.4 nm/pass for phosphate glass. The combined nanoscratching results were utilized to define the composite removal function (i.e., removal depth) for a single polishing particle as a function of load, spanning the chemical to the plastic removal regimes. This removal function serves as an important set of parameters in understanding material removal during polishing and the resulting workpiece surface roughness.

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Section: Methodsmentioning

confidence: 99%

Nanoscratching of Optical Glass Surfaces Near the Elastic–Plastic Load Boundary to Mimic the Mechanics of Polishing Particles

et al. 2016

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“…15,17 This pad polishing technique allows for finishing flat and spherical glass optics in which a workpiece, independent of its initial shape (i.e., its surface figure), will converge to a final surface figure with excellent surface quality under a fixed, unchanging set of polishing parameters in a single polishing iteration. Cost reductions can be realized with short process times (i.e., high removal rates) and few process iterations and metrology cycles.…”

Section: (4) Strategy For Optimized Polishing: Simultaneous High Remomentioning

confidence: 99%

“…[14][15][16][17] At the microscopic scale, the same parameters that affect material removal rate are also expected to influence the surface roughness. [14][15][16][17] At the microscopic scale, the same parameters that affect material removal rate are also expected to influence the surface roughness.…”

Section: Introductionmentioning

confidence: 99%

“…In our previous studies, the factors that determine material removal at macroscopic scale lengths were comprehensively evaluated. [14][15][16][17] At the microscopic scale, the same parameters that affect material removal rate are also expected to influence the surface roughness. A schematic illustration of the polishing interface, outlining key parameters that influence material removal and workpiece roughness at microscopic scale lengths, is shown in Fig.…”

Section: Introductionmentioning

confidence: 99%

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Mechanism and Simulation of Removal Rate and Surface Roughness During Optical Polishing of Glasses

et al. 2016

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Glass optics with ultra‐low roughness surfaces (<2 Å rms) are strongly desired for high‐end optical applications (e.g., lasers, spectroscopy, etc.). The complex microscopic interactions that occur between slurry particles and the glass workpiece during optical polishing ultimately determine the removal rate and resulting surface roughness of the workpiece. In this study, a comprehensive set of 100 mm diameter glass samples (fused silica, phosphate, and borosilicate) were polished using various slurry particle size distributions (PSD), slurry concentrations, and pad treatments. The removal rate and surface roughness of the glasses were characterized using white light interferometry and atomic force microscopy. The material removal mechanism for a given slurry particle is proposed to occur via nano‐plastic deformation (plastic removal) or via chemical reaction (molecular removal) depending on the slurry particle load on the glass surface. Using an expanded Hertzian contact model, called the Ensemble Hertzian Multi‐gap (EHMG) model, a platform has been developed to understand the microscopic interface interactions and to predict trends of the removal rate and surface roughness for a variety of polishing parameters. The EHMG model is based on multiple Hertzian contacts of slurry particles at the workpiece–pad interface in which the pad deflection and the effective interface gap at each pad asperity height are determined. Using this, the interface contact area and each particle's penetration, load, and contact zone are determined which are used to calculate the material removal rate and simulate the surface roughness. Each of the key polishing variables investigated is shown to affect the material removal rate, whose changes are dominated by very different microscopic interactions. Slurry PSD impacts the load per particle distribution and the fraction of particles removing material by plastic removal. The slurry concentration impacts the areal number density of particles and fraction of load on particles versus pad. The pad topography impacts the fraction of pad area making contact with the workpiece. The glass composition predominantly impacts the depth of plastic removal. Also, the results show that the dominant factor controlling surface roughness is the slurry PSD followed by the glass material's removal function and the pad topography. The model compares well with the experimental data over a variety of polishing conditions for both removal rate and roughness and can be extended to provide insights and strategies to develop practical, economic processes for obtaining ultra‐low roughness surfaces while simultaneously maintaining high material removal rates.

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“…A systematic matrix of polishing experiments using a diverse set of workpiece materials (each 50‐60 mm in diameter) and a variety polishing slurries (30‐80 nm size particles) was conducted. All of the polishing was conducted using the same loading and kinematic conditions (1.1 psi applied pressure, 20 rpm lap rotation, 20 rpm workpiece rotation, lap‐workpiece center distance of 75 mm, 12 rpm stroke, and 50 mm stroke distance) on a diamond conditioned polyurethane polishing pad (MHN 50 mil thick with 1 cm spaced grooves) using the Convergent Polishing method . Diamond conditioning and ultrasonic pad treatment was performed between each slurry‐workpiece combination (see details in References ).…”

Section: Methodsmentioning

confidence: 99%

Influence of partial charge on the material removal rate during chemical polishing

et al. 2018

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A partial charge‐based chemical polishing model has been developed, which can serve as metric for describing the relative polishing material removal rate for different combinations of slurries and workpieces. A series of controlled polishing experiments utilizing a variety of colloidal polishing slurries (SiO2, CeO2, ZrO2, MgO, Sb2O5) and optical materials [single crystals of Al2O3 (sapphire), SiC, Y3Al5O12 (YAG), CaF2, and LiB3O5 (LBO); a SiO2‐Al2O3‐P2O5‐Li2O glass ceramic (Zerodur); and glasses of SiO2:TiO2 (ULE), SiO2 (fused silica), and P2O5‐Al2O3‐K2O‐BaO (Phosphate)] was performed and its material removal rate was measured. As previously proposed by Cook (J Non‐Cryst Solids. 1990;120:152), for many polishing systems, the removal rate is governed by a series of chemical reactions which include the formation of a surface hydroxide, followed by condensation of that hydroxyl moiety with the polishing particle, and a subsequent hydrolysis reaction. The rate of condensation can often be the rate limiting step, thus it can determine the polishing material removal rate. By largely keeping the numerous other factors that influence material removal rate fixed (such as due to particle size distributions, interface interactions, pad topography, kinematics, and applied pressure), the material removal rate is shown to scale exponentially with the partial charge difference (δwp‐s) between the workpiece and polishing slurry particle for many of the slurry‐workpiece combinations indicating that condensation rate is the rate limiting step. The partial charge (δ) describes the equilibrium distribution of electron density between chemically bonded atoms and is related to the electronegativity of the atoms chemically bonded to one another. This partial charge model also explains the age‐old experimental finding of why cerium oxide is the most effective polishing slurry for chemical removal of many workpieces. Some of the slurry‐workpiece combinations that did not follow the partial charge dependence offer insight to other removal mechanisms or rate limiting reaction pathways.

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Convergent Polishing: A Simple, Rapid, Full Aperture Polishing Process of High Quality Optical Flats & Spheres

Cited by 11 publications

References 12 publications

Nanoscratching of Optical Glass Surfaces Near the Elastic–Plastic Load Boundary to Mimic the Mechanics of Polishing Particles

Nanoscratching of Optical Glass Surfaces Near the Elastic–Plastic Load Boundary to Mimic the Mechanics of Polishing Particles

Mechanism and Simulation of Removal Rate and Surface Roughness During Optical Polishing of Glasses

Influence of partial charge on the material removal rate during chemical polishing

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