Anisotropic Etching of GaAs Using CCl2 F 2 / CCl4 Gases to Fabricate 200 μm Deep Via Holes for Grounding MMICs

In this study we have investigated the reactive ion etching of 60 μm diam, 200 μm deep holes in 3 in. diam semi-insulating GaAs wafer using a combination of CCl 2 F 2 and CCl 4 gases for fabrication of through substrate via holes for grounding in monolithic microwave integrated circuits (MMICs). The effect of process parameters viz. pressure, CCl 4 /CCl 2 F 2 ratio, and power on GaAs etch rate and resultant etch profile was investigated. Two kind of masks, photoresist and Ni, were used to etch GaAs and their performance was compared by investigating effect on etch rate, etch depth, etch profile, and surface morphology. The etch profile, etch depth, and surface morphology of as-etched samples were characterized by scanning electron microscopy. The desired 200 μm deep strawberry profile, with a top diam = 60 ′ 10 μm and bottom diam = 180 ′ 10 μm, was obtained at 40 mTorr process pressure with an average etch rate ∼1.3 μm/min using Ni mask. The vias were then metallized by depositing a thin seed layer of Ti/Au (1000 A) using radio frequency sputtering and Au (5 μm) electroplated to connect the front side pad and back side ground plane. The parasitic inductance offered by these vias was ∼76 pH. The developed process was then integrated into the MMIC process line and a 16-18 GHz amplifier was fabricated using grounding vias with yield >90%.