Fast Fabrication of InP-based HBT using a Novel Coplanar Design

48 Annual Report 1998 - Solid-State Electronics Department4.3.2 Fast Fabrication of InP-based HBT using a Novel CoplanarDesignScientistS-O. Kim, U. Auer, M. AgethenIntroductionThe rf-performance of heterojunction bipolar transistors (HBTs) is limited in the case ofconventional emitter-up triple-mesa design by the base access resistance (R b ) and the base-collectorfeedback capacitance (C fb ). In order to reduce the values of these parasitics, a time consuming andsophisticated technology (three metalization and three mesa steps, air-bridge technology) is aprerequisite [1-2].The most advanced approach of simultaneously reducing R b and C fb is presented by Rodwell’sgroup [2]. Due to the transferred substrate technique both the base and the collector mesa arereduced to submicron dimensions. This technique results in an outstanding frequency performanceabove 400 GHz and proves the above mentioned assumption, however at the expense of anextremely sophisticated technology which is hardly applicable for todays industrial needs. Onealternative is the use of a single base electrode placed between two emitter electrodes (E-B-Econfiguration) design instead of a conventional two base electrodes with one emitter electrode (B-E-B configuration) design which was simulated by Madihian et. al. [3] showing a reduced C fb . In adirect comparison a better performance of the E-B-E configuration was experimentally shown [4].But in this work the conventional HBT technology (three metalization steps, air-bridge crossing)with a fairly large parasitic base mesa was used. In this report a novel coplanar design in B-E-Ccontact configuration with a submicron base width is presented (fig. 1 left).A’(3)(3)EL BCEA(1’)(2’)(1) (1)E-CapE-CapEmitter(3)BaseCollectorEmitter(3)B-C MetalA’A(4)SubcollectorBEmitter-MetalS.I. SubstrateB-C-MetalFig. 1: left: the Novel Emitter/Base/Collector designright: schematics of the fully self-aligned HBT-process in two metalization steps

Annual Report 1998 - Solid-State Electronics Department 49This approach combines low base-collector feedback capacitance, low access resistance due to thefull self-alignment and a very fast fabrication process avoiding air-bridge crossings. Base width (L B )smaller than 0.5 µm and emitter to base mesa area ratio near to 1 are possible using conventionalphotolithography. Additionally, during base and collector mesa etching the T-shaped base activearea, which is nearly completely surrounded by the emitter area, is efficiently protected.Fabrication of InGaAs/InP HBT and ResultsThe key technology step is the simultaneous and fully self-aligned evaporation of base and collectormetalization which is enabled by the new design. The metalic contacts in the active as well as in thepad area are simultaneously evaporated and the separation of active and pad areas is performed bywet chemical underetching realizing a quasi air-bridge configuration.Table 1 shows the used epitaxial layers grown on a semi-insulating InP substrate by low-pressuremetal-organic chemical vapor deposition (LP-MOCVD). Si and C are used as the n-type and p-typedopants, respectively. The p-type dopant source is carbon-tetrachlroride (CCl 4 ). Between the InPemitter and p + -InGaAs base an undoped InGaAs spacer layer (10 nm) is inserted in order to improvethe electron injection into the base. The detailed epitaxy growth process is described in reference[5]. The HBT is fabricated by optical lithography, wet chemical etching and with non-alloyedcontacts.Table 1: Epitaxial layerEmitter CapEmitterSpacerBaseCollectorSub-CollectorBufferSubstraten + -InGaAs (150 nm)n-InP (100 nm)un-InGaAs (10 nm)p + -InGaAs (150 nm)un-InGaAs (280 nm)n + -InGaAs (400 nm)un- InP (50 nm)s.i-InPThe first step of the fabrication is the Ti/Pt/Au emitter metalization on the n + -InGaAs cap layer (step1 in fig. 1 right). The emitter metal is used as a mask for emitter mesa etching. H 3 PO 4 : H 2 O 2 : H 2 Oand H 3 PO 4 : HCl are used for etching of the InGaAs emitter cap layer and the InP emitter layer,respectively. The second step is base definition followed by etching to the n + -InGaAs sub-collectorlayer (step 2). The base/collector metal of Pt/Ti/Pt/Au is then evaporated in one step and lifted off(step 3). The final step is the collector mesa isolation (step 4).Fig. 2 shows the SEM-picture of the fabricated InP/InGaAs HBT with emitter area 4 x 10 µm 2 . Thelayer stack underneath the connecting electrodes is completely removed by underetching. A verylow base finger width (L B ≤ 0.5 µm) and an almost ideal emitter/base mesa area ratio (A E /A Bmesa ≈0.9) is realized. Current gain (β) of 20 and very small offset voltage (< 30 mV) are achieved. Theon-wafer S-parameter measurements were performed using an HP8510C network analyzer in the

50 Annual Report 1998 - Solid-State Electronics Departmentfrequency range from 45 MHz to 45 GHz. The cutoff frequency (f T ) of 100 GHz was extrapolated(slope of 20 dB/decade) at V CE of 2V and J C =8x10 4 A/cm 2 .Fig. 2:SEM-picture of fabricated InP/InGaAs HBT.In summary, we have presented a novel coplanar HBT design with inserted small area T-shapedbase contact which can realize an almost ideal emitter to base mesa area ratio of nearly 1 and smallfeed back capacitance (C fb ). The key fabrication technology step is the simultaneous and fully selfalignedevaporation of base and collector metalization. The design of two emitter contact pads resultin a direct coplanar device topology without any air-bridge crossing. A InP/InGaAs HBT with thisdesign and technology have shown promising rf-performance (f T >100 GHz). This technology showsthat high performance InP/InGaAs HBT can be produced in a cost-effective manner.References[1] Y. Matsuoka, S.Yamahata, K. Kurishima and H. Ito: "Ultrahigh-Speed InP/InGaAs Double-Heterostructure Bipolar Transistors and Analyses of Their Operation", Jpn. J. Appl. Phys.,1996, 35 pp. 5646-5654[2] Q. Lee, B. Agarwal, D. Mensa, R. Pullela, J. Guthrie, L. Samoska and M. J. W. Rodwell:"A > 400 GHz fmax Transferred-Substrate Heterojunction Bipolar Transistor IC Technology"IEEE Electron Device Lett., 1998, 19 pp. 77-79[3] M. Madihian, K. Honjo, H. Toyoshima, and S. Kumashiro: "The Design, Fabrication, andCharacterization of a Novel Electrode Structure Self-Aligned HBT with a Cutoff Frequency of45 GHz", IEEE Tran. Electron Devices, 1987, ED-34, pp. 1419-1428[4] W. U. Liu, D. Costa, and J. Harris: "Novel Doubly Self-Aligned AlGaAs/GaAs HBT",Electronics Letters, 1990, 26, pp. 1361-1362[5] A. Lindner, P. Velling, W. Prost, A. Wiersch, E. Kuphal, A. Burchard, R. Margerle,M. Deicher, F.J. Tegude: "The Role of Hydrogen in Low-Temperature MOVPE Growth andCarbon Doping of In 0.53 Ga 0.47 As for InP-based HBT“, J. Crystal Growth, 1997, 170 pp. 287-291.