Pd-Ge-Au Ohmic Contacts to GaAs: Reliability and Failure Analysis
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Pd-Ge-Au OHMIC CONTACTS TO GaAs: RELIABILITY AND FAILURE ANALYSIS. T. E. KAZIOR, H. HIESLMAIR, and R. C. BROOKS* Raytheon Company, Research Division, 131 Spring St, Lexington MA., 02173 *Present address: Westinghouse Electric Corporation, Electronic Systems Group, P.O.Box 1521, Baltimore, MD 21203 ABSTRACT We report on experiments that were performed to evaluate the temperature stability and long term reliability of non-alloyed Pd-Ge-Au ohmic contacts on N-type GaAs. Low resistance contacts 6 (=1=xl0- .cm 2) were obtained for samples that were sintered in a conventional furnace or flash sintered in a graphite susceptor. Elevated temperature storage (-4000 hours at 280 0C) showed improved contact stability when compared to Ni-AuGe-Ni-Au control samples. Gateless MESFETs subjected to bias temperature stress measurements (ids =300-350mA/mm, 2000-4000 hours at 2000C) showed no significant change in device current. This result is in contrast to devices with NiAuGe-Ni-Au ohmic contacts which exhibited a 6-27% decrease in current under the same test conditions. Failure analysis reveals significant electromigration and Au diffusion in the drain fingers of devices with Ni-AuGe-Ni-Au contacts. In contrast, devices with Pd-Ge-Au contacts show no elect romigration or Au diffusion in the GaAs. INTRODUCTION At present, alloyed Ni-AuGe metallurgy is routinely used to form low resistance ohmic contacts to N-type GaAs. However, chemical analysis (for example, Auger and Transmission Electron Microscopy (TEM)) has revealed a complex and inhomogeneous grain structure consisting of numerous intermediate compounds. This structure is not thermodynamically stable [1] and, as a result, changes in compound structure and continuing interdiffusion under normal device operation will lead to device failure (e.g., increases in contact and source resistance). To address contact stability problems with the Ni-AuGe system numerous novel contact structures are under development [2-6]. These new approaches rely on solid phase reactions for contact formation. Recently much work has focused on Pd-based contacts. In particular, low resistance (=10- 6 fcm2 ) sintered Pd-Ge-based contacts to N+ (=1018 carriers/cm 3 ) GaAs have been demonstrated using two different methods [2,3]. These metal systems have the beneficial feature of being readily inserted into existing FET fabrication sequences. While these approaches show promise, very limited reliability or temperature stability data has been presented [2,3,7,81. In a previous work, we presented data on process optimization and reliability evaluation of PdGe-Au ohmic contacts to N-type GaAs [9]. The objective of this work has been to further evaluate the stability of the sintered Pd-Ge-Au contact metallurgy and begin to determine the failure mechanisms of these contacts. To accelerate chemical or physical reactions that would lead to changes in the contact structure and electrical resistance, samples were subjected to elevated temperature storage as well as conventional bias-temperature stress reliability m
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