Don Golini

Fundamentals of Magnetorheological Fluid Utilization in High Precision Finishing

Magnetorheological finishing (MRF) is an enabling technology that may produce surface accuracy on the order of 30 nm peak to valley (p-v) and surface micro-roughness less than 10 A rms. In MRF, mechanical energy for material removal over the portion of the workpiece surface is generated by the magnetically controlled hydrodynamic flow of a magnetorheological polishing fluid. A fundamental advantage of MRF over existing technologies is that the polishing tool does not wear, since the recirculated fluid is continuously monitored and maintained. Polishing debris and heat are continuously removed. The technique requires no dedicated tooling or special setup. A unique attribute of the MRF process is its determinism that is attained through the use of a well-defined material removal function to eliminate known surface error. The efficiency of material removal and the removal process stability are the crucial factors in MRF. In turn, they are primarily dependent on MR polishing fluid stability. It is shown that the joint use of physicochemical and rheological factors along with specially developed methods of the slurry handling, pumping, and in-line monitoring and maintaining provides a level of MR slurry stability that is quite adequate for high precision finishing. Attention is given to methods of MR slurry property measurements.

Progress Update in Magnetorheological Finishing

In magnetorheological finishing (MRF), magnetically stiffened magnetorheological (MR) abrasive fluid flows through a preset converging gap that is formed by a workpiece surface and a moving rigid wall, to create precise material removal and polishing. Theoretical analysis of MRF, based on Bingham lubrication theory, illustrates that the formation of a core attached to the moving wall results in dramatically high stress on the workpiece surface. A correlation between surface stress on the workpiece and material removal is obtained. A unique attribute of the MRF process is its determinism. MRF has been successfully implemented to polish optical surfaces to very high precision. MRF reduces the surface micro roughness of optical materials to ≤ 10A. Figure errors are corrected to a fraction of a wavelength of light and subsurface damage is removed. A wide range of optical surface shapes, including aspheres, has been polished on many different materials. Other applications in precision finishing are being considered, including integrated circuits and advanced ceramics.

Deterministic Manufacturing Processes for Precision Optical Surfaces

This paper reports on Center for Optics Manufacturing (COM) indus try-academic collaborative developments that incorporate computer-aided determinist ic manufacturing technology to produce highly precise spherical and aspherical optics. CO M initiated developments in computer numerically controlled deterministic microgrinding equipment and magnetorheological finishing processes are eliminating the industry’s reliance on the specialized skills required to operate today’s costly and labor-intensive conventional manufacturing proce sses. These enabling technologies extend the manufacturing state-of-the-art and provide opti cal manufacturers with the cost effective capability to produce the most difficult optics.

Multiple Application of Magnetorheological Effect in High Precision Finishing

Magnetorheological finishing (MRF) is enabling technology that may produce surface accuracy of the order of 10 nm peak-to-valley and surface micro-roughness less than 10A. In this technique, magnetorheological fluid performs the primary function, which is deterministic material removal. There are several auxiliary devices that also function on a “smartness” of magnetorheological fluid. A non-intrusive magnetorheological valve is an integral part of the finishing machine “circulatory system” and provides the pump “stiffness” and computer control of the flow rate. To provide MR polishing fluid continuous circulation, a special means for fluid removal from the polishing wheel and returning to the delivery system has been designed. Once the fluid leaves the polishing zone, it is removed from the wheel surface by a suction cup. The cup has a magnetic system that forms a dynamic magnetorheological fluid seal in the gap between the cup and the wheel surface. Finite element analysis software has been used for magnetic systems and flow design and optimization.

Improvement of figure and finish of diamond turned surfaces with magneto-rheological finishing (MRF)

Single Point Diamond Turning (SPDT) has been a cost effective technique to achieve the required figure and roughness specification on a wide range of infrared (IR) optics. SPDT is one of the few technologies that can efficiently generate aspherical surfaces, and as recent developments such as fast-tool servos mature, “free-form” surfaces are becoming feasible as well. Optical end-user requirements for a wide range of industries are continuing to tighten, driven, for example by multi-spectral systems that require good performance at shorter wavelengths in addition to IR. In many cases, specified shape tolerances can exceed SPDT capabilities. Additionally, SPDT typically leaves “turning marks” (affecting micro-roughness) that can be detrimental to performance. In some cases, surface integrity (e.g. sub-surface damage) can also be of concern. Magneto-Rheological Finishing (MRF®) has the proven ability to simultaneously improve roughness, figure, and surface integrity in a fast and cost effective manner. MRF is a deterministic, sub-aperture polishing technology, and is typically employed as the last manufacturing step. MRF can deterministically remove from tens of nanometers to microns worth of material, while efficiently “converging” to the specified requirements. Conversely, SPDT has proven to be very effective in removing the hundreds of microns (if not mm) sometimes required to “pre-shape” an aspheric surface before its final polish. After a brief introduction of MRF, this paper will discuss how SPDT and MRF processes can complement one another very effectively. Examples of MRF results on a wide range of IR materials will be presented.

New industrial applications of magnetorheological finishing (MRF)

MRF is a deterministic, sub-aperture finishing process with system stability, high material removal rates, and a shear mode of material removal that makes it uniquely well-adapted to a variety of optical and industrial applications.

Magnetorheological Suspension-Based High Precision Finishing Technology (MRF)

In MRF the mechanical energy for material removal is generated by the hydrodynamic flow of a magnetorheological (MR) polishing suspension throughout a preset converging gap formed by the workpiece surface and a moving rigid wall. MRF reduces the surface microroughness of optical materials to < 10 A. Shape errors are corrected to a fraction of a wavelength of light and subsurface damage is removed.

Prime Silicon and Silicon-on-Insulator (SOI) Wafer Polishing With Magnetorheological Finishing (MRF)

The December 2001 [1, 2] edition of the International Technology Roadmap for Semiconductors [3] (ITRS-2001) identifies several challenges for the manufacturing of silicon and silicon-on-insulator (SOI) wafers. For silicon, edge exclusion, site flatness and nanotopography1 requirements will become extremely challenging. For SOI, requirements for the control of the top silicon layer and its associated uniformity are pushing the limits of metrology. Keeping ± 5% tolerances on thicknesses, gradually decreasing from more than 100nm to less than 20nm for partially depleted devices (let alone from 30 to 3nm for fully depleted devices) is exceeding the capabilities of traditional chemo-mechanical-polishing (CMP) processes [5]. This paper will briefly describe magnetorheological finishing (MRF) and its suitability for prime silicon and SOI wafer polishing. Particular emphasis will be placed on MRF’s ability to improve the global flatness and the total thickness variation (TTV) on prime silicon wafers, and to reduce the nominal thickness of the top silicon layer, while improving thickness uniformity on SOI wafers. The paper will also touch upon the process qualification issues associated with the tight requirements of the semiconductor industry.Copyright

Polishing high aspect ratio substrates for optics, telecommunications, and microelectronics applications using magnetorheological finishing (MRF)

Precise flatness or thickness control is extremely difficult to achieve for substrates with a high diameter to thickness ratio. MRF is able to polish high aspect ratio substrates for telecommunications and microelectronic applications to the nanometer level.

Novel approach in magnetorheological finishing(MRF) system configuration

In classical polishing techniques, each geometry of the part requires a dedicated, appropriately shaped polishing lap. In addition, the polishing lap shape changes with time and must be periodically reconditioned to obtain consistent removal efficiency. The traditional method therefore does not offer a flexible, cost-effective option for finishing, particularly for advanced optical shapes such as aspheres or conformal optics. A new precision polishing method called magnetorheological finishing (MRF) has been developed to overcome fundamental limitations of traditional finishing techniques [1]. MRF may produce surface accuracy of the order of 10 nm peak to valley and surface micro-roughness less than 10A on optical glasses, single crystals (calcium fluoride, silicon) and ceramics. High precision spheres and flats, nearly any asphere, square and rectangular aperture optics, prisms and cylindrical optics may be corrected and finished with MRF. Currently manufactured by QED Technologies, MRF equipment is being employed in the production of high precision optics in North America, Europe, and Asia. In this paper, it will be shown that the future utilization of unique properties of magnetorheological fluid offers several advantages over current MRF systems. The next generation of MRF machines practically is not limited in the size of large optics, has the advantage in polishing concave optics and has a more stable and easier to maintain delivery system.