Harvey M. Pollicove

Fixed abrasive flat lapping with 3M-3M Trizactl diamond tile abrasive pads

Lapping is an important dimensioning and finishing technology that is used in many different industries including metal, electronic, and optical component fabrication, as well as silicon wafer production. In each of these applications lapping is used to produce flat substrates of a controlled thickness, flatness, and surface roughness. Lapping can be performed in either a single -sided or double -sided operation! Conventional lapping technology can be divided into two basic categories: loose abrasive grinding (slurry lapping) or fixed abrasive lapping. In slurry lapping the abrasive is in the form of an aqueous slurry of abrasive minerals (typically alumina or silicon carbide) and the lapping surface is the machine platen (typically cast iron).1 Conventional fixed abrasive lapping is also called pellet lapping. In this process the abrasive (typically diamond) is incorporated into small pellets (metal, vitreous, or resin bond) which are attached to cast iron machine platens. The lapping surface during pellet lapping is formed by the top surfaces of all of the pellets. Here we report on a structured abrasive lapping technology developed by 3M. The structured abrasive pad consists of an organic (polymeric binder) - inorganic (abrasive mineral, i.e., diamond) composite and is used with coolant. Typical lapping coolants including deionized water are used as lubricants without the addition of any free abrasive mineral. Table 1 shows a comparison between conventional loose abrasive, pellet, and 3M fixed abrasive lapping technology.

Metrology challenges of thin optical wafers for high finesse etalons

We present a number of measurement methods that we use in the fabrication of solid LiNbO3 etalons. Several unique technical features, such as high finesse, large aperture/high aspect ratio, and tunability, require careful monitoring of quality parameters throughout the production process. We point out the critical issues and how we monitor them.

Extending the range of interferometry though subaperture stitching

Subaperture stitching is a well-known technique for extending the effective aperture or dynamic range of phase measuring interferometers. Stitching has been routinely applied to extend the spatial coverage of plano interferometers. Due to the presence of additional mechanical and optical degrees of freedom, it has been much more difficult to apply stitching techniques to spherical and aspherical interferometric testing. Particular care must be taken in mechanical alignment, motion control, and mathematical processing of subaperture phase data in order to obtain an accurate reconstruction of the full-aperture phase map. We have designed and developed an interferometer workstation specifically to perform high-accuracy subaperture stitching of spherical and flat surfaces up to 200 mm (8) in diameter. The workstation combines a commercial 100 mm (4) Fizeau interferometer, a 6-axis stage, and a software package that automates the entire stitching process. Automation of the measurement design, motion control, phase data acquisition, and data analysis process allows complex stitched measurements to be made in a shop-floor production environment by optical technicians of modest skill. Mathematical techniques were developed to compensate for several classes of systematic and random errors inherent in the measurement process, e.g. motion errors, magnification error, viewing system distortion, and reference wave error. This allows each measurement to be self-calibrated. Basic system capability is demonstrated by comparing a conventional full-aperture phase measurement of a surface to a measurement reconstructed from stitched subapertures.

In-process surface measurement of replication material during UV curing

UV curing plays an important role in the optical shop for cementing optics or for the fabrication of replicated optics. Material shrinkage is a common but unwanted effect during UV curing, because it causes stress and form modifications. In order to minimize these effects it is useful to measure shrinkage and surface changes that occur during the UV curing process. For replicated optics, shrinkage and form modifications are usually measured by comparing the replica to the mould geometry after the curing process has finished. In this paper, a measuring method is presented, which enables the observation of shrinkage and changes of the surface shape in situ. The surface of UV hardening material, in this case a commercially available optical adhesive, is monitored interferometrically during UV curing. This enables observing the material effects that cause stress and form modifications in real time.

Novel method for computing reference wave error in optical surface metrology

Despite advances in various metrology tools, interferometry remains the method of choice for measurements of optical surfaces. Fizeau interferometers can achieve precisions of X/100 PV (and better) with proper environmental control. The quality of the reference surface, however, usually limits the uncalibrated accuracy to merely X/10 PV or so. Various methods have been developed for absolute (unbiased) surface testing, including the N-position, 3- flat, 2-sphere, and random average tests. The basic principle of these tests is that the reference wave error remains invariant when the part is moved. These tests as a rule require multiple parts and/or measurements at different positions. Sub-aperture stitching requires measurements at multiple positions, and thus in principle can measure reference wave error. QEDs stitching algorithm exploits this possibility to produce a measurement of the reference surface along with the stitched full-aperture phase. The precision mechanics of QEDs stitching workstation make it an excellent platform for performing conventional reference wave calibrations as well. Results obtained from the QED stitching algorithm are compared with other calibration methods performed on the same workstation. The mean results and uncertainties of the various methods are evaluated, and limitations discussed.

New polishing technology for large mirrors and lenses

Summary The PrecessionsTM process, and the 200mm capacity All machine which embodies it, are outlined in the accompanying paper New method to control form and texture on industrially-sized lenses. In this paper, we summarise the development of the process to address larger optical components. The first machines constructed for this application are of 600mm capacity. A conceptual design of a 1-2m class machine of bridge- configuration has also been produced. Unlike the All machine, the work-piece on these larger machines lies in a horizontal plane i.e. with the rotation axis of the work-piece spindle vertical. This configuration is preferred because it allows the use of a hydrostatic support-system for the work-piece; important for large optics in general, and particularly so for light-weight optics. The machine can also provide a clear vertical path above the work-piece for optical testing, or for access by lifting gear. It is interesting to consider how the PrecessionsTM process can be scaled. As an example, consider first that the type of surface-form remains the same; it might, for example, be an f/2 parabola. Consider then that the work-piece diameter, the bonnet-diameter, the polishing spot-sizes used, and the tool-path on the surface, are all stretched by a factor of two. The area of the polishing spot and that of the work-piece have both increased by a factor of 4, but the number of convolutions of the spiral tool-path remains the same. Since the spot instantaneously addresses the same fraction of the overall work-piece surface-area, the terms all cancels in relation to cycle-time. However, for the same tool-rpm, the larger bonnet delivers a proportionally higher surface-speed, because the polishing action is further from the axis of rotation. Therefore, the volumetric removal rate (proportional to contact-area times surface-speed) tends to vary as the cube rather than the square of the scale-factor. As a result, the larger bonnet working over the larger work-piece with the same polishing pressure, will remove twice the depth of material in the same time. The above scaling could, in principle, reduce cycle times on larger parts. In practice, we have found that excessive removal-rates lead to impractically short dwell-times, especially near the centre of a spiral tool-path. Such scaling also demands increased traverse-speeds and accelerations from the machine, and this rapidly becomes a limitation. Moreover, as the process converges on final form, smaller depths of material need to be removed. In practice, we have found it necessary to moderate the process by diluting the slurry and by moving from a polyurethane tool-surface to a material that delivers a lower-removal rate, such as Multitex. We are also developing bonnets that are compatible with operation at a reduced polishing pressure, in order to give additional control for fine-removal. The current status of the process development is as follows. A form error of -80nm peak-to-valley has been achieved aspherising a 100mm diameter part. This work revealed some subtle but significant limitations in the optimisation code, mostly concerned with convergence and residual ripples. Recent work has focussed on this aspect, and the revised code is now ready for polishing trials. A 150mm diameter part is currently in-process, and work is about to commence on a 300mm fused silica mirror for a space application.

Compensation technology of glass molding accuracy

To realize high-precision glass molding, some themes are yet to be settled, including performance of molding machine, molding conditions, material and accuracy of molds. To get a desired product profile with predetermined accuracy, molds must be manufactured according to specified accuracy. The present molding machine has a high transferability of fine profile, but causes minor difference in profiles by thermal expansion and contraction of mold and glass material as it involves a heating process. We clarify the results of experiment and verification of the two methods , in consideration of a heat expansion difference, molds is manufactured beforehand and feedback of geometrical error of prototype mold product to mold profile.

Cleaning of parts for precision-optic and glass substrates before coating

The state of the art cleanliness of substrate surfaces prior to a coating process is decisive for its success. Any contamination of the surface affects the adhesion of the coating and leads to defects. The degree of cleanliness of the surface can not be expressed in numerical terms and can only be demonstrated by the use of suitable aids. The objective sought is freedom from residues and freedom from particles, as perfectly as possible. Uniform, reproducible quality is indispensable, even when the products supplied from the preceding stages of manufacture vary within relatively wide tolerances in terms of shape, size, nature and degree of contamination. The solution to this type of problem requires well-tried process technologies in user-friendly equipment which operates safely and economically. One well-tried cleaning procedure prior to coating is based on cleaning with aqueous solutions plus ultrasound, followed by drying. In the course of todays increasing awareness of environmental matters, processes which make use of solvents prior to coating have meanwhile disappeared completely from factories. Cleaning in aqueous solutions is carried out in accordance with precise cleaning mechanisms. Cleaning is always a multistage process, in which cleaning and rinsing stages alternate repeatedly. Modern multi-chamber cleaning plants are to be found in the optical and electronics industries and fine mechanics as well as in the high-vacuum coating area. (coated lenses and hard coating)

Effect of process parameters on surface morphology in MRF

Spot surface morphology can be considered as a footprint of the removal process in magnetorheological finishing (MRF). When properly processed, it can account for the performance of the MR fluid in polishing. Experiments were done using different conditions to vary removal rate and evaluate the resulting effect on microroughness and the overall spot surface morphology. Such experiments have been performed on two optical glasses, with several different MR fluids and a wide range of machine parameter settings. Atomic force microscope (AFM) measurements show that the surface morphology has a strong dependence on the abrasive type in the MR fluid. Interferometry measurements show that the roughness inside the spot increases with the rate of the material removal.

Characterizing optical polishing pitch

Characterization data for five experimental optical polishing pitch products were compared to those for corresponding standard commercial optical polishing pitches. The experimental pitches were tested for three physical properties: hardness, viscosity at 90°C, and softening point. A Shore A Durometerl test was used to measure hardness. Viscosity data were collected using a Stony Brook Scientific falling needle viscometer. Softening point was determined using the ASTM D3104-97 method. Results demonstrate that the softest and the hardest batches of the experimental grades of optical pitch are comparable to the industry-accepted standards, while the other grades of pitch are not. The experimental methodology followed in this research may allow opticians to rapidly compare different brands of pitch to help identify batch- to- batch differences and control pitch quality before use.