CH4 H-2/AR/CL-2 ELECTRON-CYCLOTRON-RESONANCE PLASMA-ETCHING OF VIA HOLES FOR INP-BASED MICROWAVE DEVICES/

We report on a dry etch process for backside through-wafer via hole fa brication in InP-based transistors which addresses many manufacturing issues including wafer mounting schemes, degradation of the frontside ohmic metal pads, yield across two-inch wafers, and reliability issues . Low pressure electron cyclotron resonance plasma etching using a CH4 /H-2/Ar/Cl-2 gas chemistry is investigated and compared to other InP v ia etching techniques. A photoresist mask was used to define 54 mu m X 54 mu m backside vias in substrates thinned to similar to 50 mu m. Po st-baking the resist served both to strengthen the mask against the pl asma, and to control the profile of the through-via. The etching was p erformed at a temperature of 130 degrees C to enhance etch product vol atility while allowing a reliable wax-mounting scheme for the thinned wafers. Etch rates of similar to 1 mu m/min were obtained for 950 W mi crowave power and 250 W rf power (-260 V). After similar to 95% of the etched depth was achieved, the rf bias was reduced to minimize sputte ring of the frontside metal pad on which the etch terminates. After th e residual resist mask was removed, the samples were metalized, de-mou nted, and tested for electrical connection, resulting in high yield vi a etching across the wafer.