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

Abstract

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 <10N, the machine must be extremely rigid in order to achieve the necessary surface quality with a reasonable grinding feedrate. Assuming a feed-rate of 5m/min and a total allowable error motion of 5nm, a stiffness of >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.