Highly Reliable WGe Ohmic Contact to GaAs-AlGaAs HBTs
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HIGHLY RELIABLE WGe OHMIC CONTACT TO GaAs-AIGaAs HBTs T. R. FULLOWAN*, F. REN*, B. TSENG*, S. J. PEARTON*, C.R. ABERNATHY*, L. R. HARRIOTT*, AND E. LANE** *AT&T Bell Laboratories, Murray Hill, NJ 07974. **AT&T Bell Laboratories, Breiningville, PA. ABSTRACT We report a highly reliable, sputtered WGe emitter contact for npn heterojunction bipolar transistors. A specific contact resistance of 7.5 x 10-7 Q -cm 2 and a transfer resistance of 4.0 x 10-2 f2mm were obtained after 380°C, 1 min alloy. The contact was patterned by SF 6 dry etching at low bias using a Au mask. This novel contact has comparable resistance to conventional AuGe-based metallization while having superior thermal stability. We have studied the dependence of contact properties on the post-deposition annealing conditions by transfer length method of electrical characterization and Auger analysis, and will also report long-term reliability results in comparison to AuGe-based metallization. INTRODUCTION GaAs-based heterojunction bipolar transistors (HBT's) continue to show promise for high-speed digital and microwave analog applications. The reduction of the ohmic contact contribution to parasitic resistance is critical in obtaining optimum device performance of these products. In order to realize high performance as well as LSI complexity, HBT ohmic contact metalization selection must consider contact resistance, manufacturability, process compatibility, uniformity and reliability. Frequently Ni/AuGe-based alloys or W/WSi refractory metals are used for the n-type emitter ohmic contact. In AuGe-based contacts, Au and Ge are applied in proportions that represent a eutectic alloy (88% Au, 12% Ge by weight) which melts at 360'C [1]. It is believed that the mechanism of AuGe ohmic formation is Ge, enhanced by Au, diffuses into the GaAs during alloy and is incorporated in Ga sites thus heavily doping the n-type surface [2-61. Furthermore during this process Au and Ni are also diffused into the GaAs as both As and Ga move into the metal overlayer [7-9]. These AuGe-based contacts have proven thermally unstable and are therefore a potential degradation mechanism in stressed devices. W and/or WSi, while more thermally stable, impose fabrication and material complexities which may yield contacts whose specific contact resistance (Pc) are undesirable. In this paper we present a WGe metalization scheme as a possible alternative for minimizing a performance-reliability trade-off in the emitter electrode of GaAs-based HBT's. Contact fabrication methodology to MOMBE HBT structure is outlined. Electrical characteristics of the ohmic behavior are obtained by transfer length method (TLM) on thermally aged structures and compared to "standard" AuGe control contacts [10]. Auger electron spectroscopy (AES) analysis of contact compositions are presented as well as SEM examination of morphology and metal diffusion. EXPERIMENTAL The layer structure for materials used in this study is included in table 1 and was grown in an INTEVAC Gas source Gen II (MOMBE) by a procedure detai
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