Copper-Based Ohmic Contacts For The SiGe/Si Heterojunction Bipolar Transistor (HBT) Structure
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ABSTRACT A Cu-based metallization scheme has been studied for establishing low resistance contacts for a Si/SiGe/Si heterojunction bipolar transistor (HBT) structure. As-grown doped layers were further implanted with BF 2 and As ions for the p-type base and n-type emitter layers, respectively, in order to produce a low sheet resistance surface layer. Contacts were metallized using an e-beam deposited multilayer structure of Ti/Cu/Ti/Al. Specific contact resistances of the order of 10-7 0? cm2 or lower were obtained. INTRODUCTION Silicon based HBTs with Si/Ge base are beginning to be important in high-speed and highfrequency microelectronics (1). Currently the fabrication of these devices uses standard Si process lines with minimal modifications. Electrical contacts employed for these devices are also standard metallization schemes developed for Si integrated circuit devices. However, since the HBT structure is not identical to the standard Si bipolar structure, a common approach may not yield optimal low resistance ohmic contacts. Therefore, a study was conducted with the purpose of developing optimal low resistance contacts to films with doping concentrations corresponding to the highly doped p-type base and n-type emitter regions of SiGe base HBTs. Following the recent trend in VLSI interconnect metallization (2-4), a Cu based system, namely Ti/Cu/Ti/Al was employed for ohmic contacts in this preliminary study. The authors are not aware of any reports pertaining to the application of Cu-based metallization for Si or SiGe devices as ohmic contacts. EXPERIMENTAL Two different 100 mm diameter wafers were used in this study. One of these wafers had a complete HBT structure, for the study of emitter contacts while a second wafer was terminated after the deposition of the base layer. Doping concentrations of individual layers and their thicknesses are shown in Fig. 1. The deposition procedure was similar to that described in reference (5). These wafers were sawed into lcm x 1cm samples. Samples were RCA cleaned before processing of contacts. For both p and n layers, a high-dose ion implantation 5step (doses of x 1015, 3 x 1015 and 5 x 1015 cm"2 for As and lxl015, 3 x 1015 and 7 x 10,1 , x 1016 cmn2 7x10, for BF 2 all at 30 keV, nominally at room temperature) was included in order to introduce a surface layer with a high dopant concentration. A post-implantation anneal was conducted in forming gas at a temperature of approximately 550 0C for 60 min followed by 30 s at 850'C in order to recrystallize the amorphized surface layer and to activate the implanted dopant. A linear transfer length method (TLM) (6) pattern (rectangular 400 ýtm x 100 ýLtm contact pads separated
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Mat. Res. Soc. Symp. Proc. Vol. 564 ©1999 Materials Research Society
n+ Si
n Si i Sio.8 Ge0 .2 p+ Si . Ge 0 8 0 .2 i Si 0 .8Ge 0 .2 n-
Si Si p- Si n+
emitter contact emitter spacer base spacer collector sub-collector substrate
4x10' 9 cm-3 2x10"8 cm-3 UID 3 1x 1020 cmUID lx l016 cm-3 lxl019 cm-3 -lxl013 cm-3
2000A• 1000A
50A 400A0
50A 2500A
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