Elements for the design of precision machine tools and their application to a prototype 450mm Si-wafer grinder

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 155-160).Next generation precision machines will require ever more rigid elements to achieve the required machining tolerances. The presented work focuses on the application of ultra stiff servo-controllable kinematic couplings and hydrostatic bearings to minimize the structural loop of multi-axis precision grinding machines while reducing complexity. The fundamental importance of these ultra stiff, adjustable machine elements is demonstrated in the design of a grinding machine for 450mm diameter silicon wafers. A new generation of silicon wafer grinding machines is needed to back-grind large (450mm diameter) wafers from the production thickness of up to 1 mm down to less than 50pm so as to reduce the cost of Si-wafer based components. The grinding process needs to be done in about 90 sec (fine-grinding, e.g. -200 micron) to 160 sec (coarse grinding, e.g. -600 micron). After completion of the fine grinding process the wafer must be flat to 0.1 pm/o45mm and parallel to 0.6pm/450mm diameter. The surface roughness must be less than Rymax 0.1 pm and Ra 0.01 pm. Even though the required machining forces are 1 N/nm is required, which is many times stiffer than a typical machine tool (0.1 to 0.3 N/nm). In cooperation with industry, this work had the aim of creating a new machine design philosophy, with an example application that focuses on nano-adjustable kinematic coupling and feedback controlled water hydrostatic bearing technology. This new design philosophy is needed to enable the design of a relatively small footprint, compact precision machines. In particular, a ball screw preloaded height adjustable kinematic coupling and a magnetically preloaded hydrostatic thrust bearing were designed and built. The adjustable kinematic coupling allows for up to 8mm of vertical height adjust and 7N/nm stiffness at 26 kN preload. By varying the preload on the coupling by +/- 10%, in-process nm to micron height and tilt adjustment at >95% of the nominal stiffness is possible. Under the assumption of a constant flow supply, the hydrostatic bearing achieves a theoretical stiffness of 1 N/ nm at a 20 micron bearing gap and 7000 N combined gravitational and magnetic preload. In practice, the stiffness is limited by the pressure flow characteristics of the supplying pumps. To increase the bearing stiffness to a required 4N/ nm, various control loops have been developed and tested.by Gerald Rothenhöfer.Ph.D

Design, dynamic modeling, simulation and feedback control of hydrostatic bearing

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2007.Includes bibliographical references (p. 113).A hydrostatic bearing carriage (Hydrocline) that uses an open face linear motor to drive the carriage as well as to preload the hydrostatic bearings has been developed by Professor Alexander Slocum and Gerald Rothenhöfer of the Massachusetts Institute of Technology's Mechanical Engineering Department. The Hydrocline is made to cope with the increasing requirements for accuracy in silicon wafer grinding machines. The prototype was built of aluminum oxide ceramic due to its high stiffness-to-weight ratio, low thermal expansion and corrosion resistance. In order to keep the cost of manufacturing as low as possible, a modular design that uses micron-level precision ceramic beams was chosen. Initial testing of the prototype carriage indicates that it has the following performance specifications: a vertical load capacity exceeding 5000N (theoretical limit at 12000N, max. pressure of pumps); a carriage pitch error of 0.7arc seconds; a yaw error of 0.7arc seconds; a roll error of +/- 0.6arcsec a vertical straightness at the center of the carriage of +/-0.75microns; and a vertical stiffness of the carriage of 900N per micron (load range from 0 to 1000N).(cont.) A dynamic model of the hydrostatic bearing and fluid supply system has been developed and accurately predicts the performance of the Hydrocline. The model has been used to simulate a feedback control loop that adjusts the bearing supply flow such that changes in load can be compensated and theoretically infinite stiffness can be reached. In first experiments on a specially designed test setup the measured static stiffness of the single pocket test bearing could be increased by a factor 8 (load range 45 to 270N). The dynamic stiffness of the bearing could be increased by a factor of 2.5.by Gerald S. Rothenhöfer.S.M