Ultra-fast rotary ultrasonic micro-hole machining of silicon wafers: a comparative study between twist drill and diamond tools.

Silicon plays a crucial role in today's digital world, particularly in the semiconductor industry. Micro-hole machining is essential in manufacturing different silicon-based semiconductor devices including sensors, actuators, and microsystems. However, silicon's high brittleness poses significant machining challenges such as reduced machinability, rapid tool wear, and the risk of cracking. Although various thermal and chemical-based non-traditional machining (NTM) processes are generally used for micro-hole fabrication in silicon, they face limitations like forming heat-affected zones, recast layers, residual stresses, and chemical usage. Rotary ultrasonic micro-hole machining (RUµM), a mechanical NTM process, has already shown the potential to overcome these issues in previously reported studies. However, those studies primarily used relatively low tool rotation speeds, which caused poor drilling efficiency, low tool stability, and large edge damage. To address these issues and fill the knowledge gap, this research explores the impact of ultra-fast tool rotation (tens of thousands of rpm) on RUµM and compares the performance of micro-twist drill and diamond tools. This study experimentally investigates ultra-fast RUμM, with and without ultrasonic vibration to evaluate tool performance, material removal mechanisms, and overall machining quality. Results indicate that ultra-fast tool rotation significantly reduces cutting forces while improving surface integrity. Drilling-based RUµM with twist drills produces more uniform micro-holes but suffers from higher tool wear. In contrast, grinding-based RUµM with diamond tools enhances tool life and lowers cutting forces. Additionally, while machining with ultrasonic vibrations improves material removal efficiency, it increases chipping and micro-cracks at the hole entrance but minimizes damage at the hole exit.