Formation of high aspect ratio through-glass vias by the combination of Ultrasonic micromachining and copper electroplating

Abstract

Fabrication of high aspect ratio (>5) multiple copper-filled through-holes in non-conductive materials like glass and single-crystal‑silicon is reported by using a faster (>8 μm/s) ultrasonic micromachining (USM). A customized stainless tool having multiple tips arranged in a 6 × 6 area-array manner was used with ~1 μm-size Silicon carbide (SiC) abrasive particles. Numerical simulations were performed to find a suitable horn to reduce the losses during the transmission of ultrasonic wave propagation. A tapered horn profile was chosen to transmit the vibrations uniformly from the ultrasonic head to the multiple tips. A novel technique was proposed to measure the vibration amplitude at different ultrasonic power ratings. >250 experiments were carried out to investigate the effect of ultrasonic power ratings and tool feed rate on the geometric characteristic of microholes. Average etch rates as high as 8 μm/s and 6 μm/s were obtained for glass and silicon, respectively. These average etch rates are higher than those obtained with plasma etching and electrochemical discharge machining. Through-holes were created in a 1.1 mm thick glass substrate in <3 min. The importance of post-USM cleaning by ultrasonic agitation was highlighted, which helped in removing the abrasive particles stuck inside the through-holes. Copper was deposited inside these through-holes using a bottom-up electroplating approach. These copper-filled through-holes can be used as 3-dimensional (3D) interconnects required in various microsystems packaging-related applications.