Investigations into velocity decay, initial tool-workpiece gap, and material removal behaviour in ultrasonic micromachining

A dynamic impact-based numerical model is presented to estimate the cross-sectional profiles of deep vias having depths up to 1 mm, created in glass by ultrasonic micromachining (USM). Finite element method (FEM) analysis was used to calculate the indentation volume caused by a single abrasive particle in a single impact. Considering the normal distribution of a large number of abrasive particles under the vibrating tool, the cross-sectional profiles were estimated. For the first time, the relationship between the velocity and the distance travelled by the abrasive in a viscous slurry after its impact with the tool is established. An exponential reduction in the abrasive velocity within the tool-glass gap was observed. This model predicted the minimum USM power, the minimum impact velocity and the maximum initial tool-workpiece gap required to cause significant machining. Experiment results confirmed that no machining happened at USM power <inverted exclamation> 60 W, which the simulation model earlier predicted. Using this model, the initial gap required to prevent the glass breakage at different power ratings can also be estimated. In this work, the initial gaps were estimated to be 17 mu m, 30 mu m, 42 mu m, and 51 mu m for the USM power ratings of 60 W, 100 W, 140 W, and 180 W, respectively. The opening sizes and depths of vias, machined at varying USM power ratings, were predicted and later validated by detailed experiments. The percentage error in the simulation and experimental depths was within +/- 5%. High-aspect ratio through-holes were created in a 1.1 mm thick glass using optimized USM parameters, thus resulting in an etch rate of 350 mu m/min. The results conclude that the USM process can be used in various MEMS packaging and advanced interconnect applications thanks to its faster etching and room-temperature processing.