Fabrication of multiple through-holes in non-conductive materials by Electrochemical Discharge Machining for RF MEMS Packaging

Formation of multiple through-holes arrays in a 400 gm thick silica substrate is reported for the first time by a cost-effective electrochemical discharge machining (ECDM). Customized multi-tip array tools having varying pitches and tip sizes were fabricated in low-cost stainless steel material. The average tool tip size, pitch, and tip length were 150 mu m, 1.5 mm and 15 mm, respectively. Experiments were performed with varying voltages, duty cycles, and pulse frequencies to study their effects upon different geometrical characteristics such as through-hole entrance size, overcut and heat affected zone (HAZ). The effect of tool tip length on hydrogen gas bubble entrapment between the tool tips was investigated. Additionally, the effect of three different electrolytes i.e., NaOH, KOH, and H2SO4 were investigated. Significant improvements in the quality of the through-holes were obtained with KOH electrolyte over NaOH electrolyte. However, the material removal with acidic H2SO4 electrolyte was negligible as compared to NaOH and KOH. The least radial overcut (< 50 mu m) and HAZ width (61 mu m) at the optimized parameters were observed in the KOH electrolyte. The depths of machined holes were in good agreement with the FEM based numerical simulation results. Compared to conventional methods like plasma etching and laser drilling, a large number of through-holes can be machined at customized locations in less than 10 min. Upon metal filling, these through-holes can be used as 'through-glass-via' (TGV) interconnects in 3D radio-frequency (RF) MEMS packaging applications.