Precision and Scalability in Ultrasonic Machining for Microscale Features.

Abstract

Micro ultrasonic machining, µUSM, is a non-thermal, nonchemical and non-electrical process that is especially suitable for hard, brittle, and inert insulators such as ceramics. Typically, the µUSM process is capable of machining rates ≥300nm/sec; the resulting surface roughness is Sa≥250nm. There is a compelling need to extend this micromachining approach in precision and resolution for a variety of MEMS, such as for the high resolution trimming of timing references. However, a number of challenges must be addressed including the development of appropriate equipment, methodology of tool design and fabrication, and optimization of machining parameters. The research described in this thesis addresses the challenges for high resolution micro ultrasonic machining (HR-µUSM), providing high resolution and high surface quality, and precise control of machining rates. Experimental results demonstrate that the HR-µUSM process achieves machining rates as low as 10nm/sec averaged over the first minute of machining of fused silica substrates. This corresponds to a mass removal rate of ≈20ng/min. The average surface roughness, Sa, achieved is as low as 30nm, which is an order of magnitude lower than conventional µUSM. The process is used to demonstrate trimming of hemispherical 3-D shells made of fused silica. Additionally, this thesis addresses a challenge of slurry precipitation or settling during 3-D machining using µUSM, which drastically reduces the machining rates to negligible values. A mode of μUSM is developed in which the workpiece is vibrated and not the tool. Experimental evaluations of this process result in machining rates ranging typically from 5–50 nm/sec for vibration levels ranging from 1–8 μm. The workpiece vibration agitated the abrasive particles, alleviating slurry settling. Finally, this thesis explores the resolution limit of µUSM using lithographically patterned silicon micromachined tools. The use of lithography enables the batch mode transfer of complex patterns, greatly enhancing the throughput of the process. Silicon microstructures with high resolution(≤10 µm) and high aspect ratio(≥20:1) can be readily made using deep reactive ion etching (DRIE). Fine featured Si cutting tools are lithographically patterned and fabricated. Machining evaluations result in the successful transfer of patterns with sub-10 μm feature sizes and ≈3:4 aspect ratios.