Wet bulk micromachining of Borofloat glass towards the fabrication of through-holes in different concentrations of hydrofluoric acid

Abstract

Glass through-holes are essential for wafer-level packaging of microelectromechanical systems (MEMS) devices and are often fabricated through wet bulk micromachining. For efficient through-hole fabrication, there is a need for the development of cost-effective masking layers and faster etching processes. This work presents an economical method for fabricating through-holes of various dimensions in 500 µm-thick Borofloat glass wafers with a relatively high etch rate using wet bulk micromachining. The process employs wet isotropic etching in 25% and 30% hydrofluoric acid (HF), utilizing a masking layer of sputter-deposited Cr thin film and spin-coated positive photoresist. The masking layer revealed strong adhesion to the wafers during the entire etching process, enabling the fabrication of through-holes with sharp edges. Additionally, the masking layer delivered excellent resistance to both HF concentrations, establishing effective protection, and subsequently resulting in minimal defects on the wafer surface. Through-holes are fabricated in 190 min using 25% HF and in 150 min using 30% HF, with the latter facilitating comparatively faster fabrication due to its higher HF concentration. The present work demonstrates the best output in terms of faster etching time for through-holes fabrication in glass wafers using a Cr thin film combined with a photoresist as a masking layer.