Elements for the Design of Next Generation, High Stiffness and High Accuracy Precision Machines

摘要

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 1mm down to less than 50μm so as to reduce the cost of Si-wafer based components. The grinding process needs to be done in about 90sec (fine-grinding, e.g. -200micron) to 160sec (coarse grinding, e.g. -600micron). After completion of the fine grinding process the wafer must be flat to 0.1μm/□45mm and parallel to 0.6μm/450mm diameter. The surface roughness must be less than Rymax 0.1 μm and Ra 0.01μm. Even though the required machining forces are <10N, the machine must be extremely rigid in order to achieve the necessary surface quality with a reasonable grinding feed-rate. Assuming a feed-rate of 5m/min and a total allowable error motion of 5nm, a stiffness of >1Nm is required, which is many times stiffer than a typical machine tool (0.1 to 0.3Nm). 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 couplings 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 work spindle thrust bearing as well as a magnetically levitated wheel spindle with self centering, hydrostatic radial bearings were designed and built. The adjustable kinematic coupling allows for up to 8mm of vertical height adjust and 7Nm stiffness at 26kN 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 [2]. Under the assumption of a constant flow supply, the hydrostatic bearing achieves a theoretical stiffness of 1Nm at a 20micron bearing gap and 7000N 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 [3], although we find that there is much room for more advanced pump/control system development. The wheel spindle with axial magnetic bearing and hydrostatic radial bearings achieves a minimum dynamic stiffness of 340N/μm at 120 Hz and a radial bearing stiffness of nearly 600 N/μm at 1.8 MPa water pressure, which corresponds to 160 N/μm radial stiffness at the front end of the shaft.