Processing and Properties of PdGe Ohmic contacts to GaAs
Andrew N. MacInnes, Rick Morton, Fabian Radulescu, Jack McCarthy
TriQuint Semiconductor, 2300 N.E. Brookwood Parkway, Hillsboro, OR 97124
The palladium-germanium PdGe metal system has received considerable attention in recent years as a potential replacement for the almost universally employed Au-Ge-Ni ohmic contact metallurgy on GaAs devices. The perceived advantages of this system are in part derived from the absence of eutectic phase formation during alloying; thereby producing a 'non-spiking' ohmic contact with a high degree of morphological stability. However, these reports typically deal with simple test structures (usually TLM patterns) and rarely test the ohmic contact in the more familiar environment whereupon it is further processed and contacted by a local interconnect metal. Literature reports of such structures, whilst much scarcer, do tend to suggest that there may well be a propensity of the PdGe contact to be degraded by the action of some commonly employed processing chemistries. This paper will discuss possible solutions to the potential problem areas for ohmic contact degradation; most notably prolonged exposure to aqueous media. Experimental test data will also supplemented by a microstructural characterization to illustrate both the formation and degradation processes in this ohmic contact metallurgy. Finally a proposed methodology for the attainment of a low and stable contact resistance utilizing PdGe Ohmic contacts will be presented.