1 InGaAs/InP DHBTs in a planarized, etch-back technology for base contacts Vibhor Jain, Evan Lobisser, Ashish Baraskar, Brian J Thibeault, Mark Rodwell ECE Department, University of California, Santa Barbara, CA D Loubychev, A Snyder, Y Wu, J M Fastenau, W K Liu IQE Inc., 119 Technology Drive, Bethlehem, PA International Symposium on Compound Semiconductors 2011

2 Outline HBT Scaling Laws Refractory base ohmics Fabrication DHBT – Epitaxial Design and Results Summary

3 Ohmic contacts Lateral scaling Epitaxial scaling Bipolar transistor scaling laws To double cutoff frequencies of a mesa HBT, must: (emitter length L e ) WeWe TbTb TcTc W bc Keep constant all resistances and currents Reduce all capacitances and transit delays by 2

4 InP bipolar transistor scaling roadmap Emitter Width (nm) 8421 Access ρ (Ω·µm 2 ) Base Contact width (nm) Contact ρ (Ω·µm 2 ) Collector Thickness (nm) Current density mA/µm 2 Breakdown voltage V fτfτ GHz f max GHz Performance Design Rodwell, Le, Brar, Proceedings of IEEE, 2008

5 Pd contacts diffuse in base (p-InGaAs) Contact resistance ↑ for thin base Limits base thickness  Scaling Limitation 100 nm InGaAs grown in MBE 15 nm Pd diffusion Need for non-diffusive, refractory base metal Contact diffusion TEM by E Lobisser Ashish Baraskar

6 Refractory base ohmics DopingMetalType  c (  -  m 2 ) 1.5E20MoAs deposited E20Ru/MoAs deposited E20W/MoAs deposited E20Ir/MoAs deposited E20Ir/MoAs deposited E20Ir/MoAnnealed0.8 Ashish Baraskar et al., EMC 2010 Refractory metal base contacts Require a blanket deposition and etch-back process Ashish Baraskar et al., Int. MBE 2010

7 Emitter process flow Mo contact to n- InGaAs for emitter W/TiW/SiO 2 /Cr dep SF 6 /Ar etch SiN x Sidewall SiO 2 /Cr removal InGaAs Wet Etch Second SiN x Sidewall InP Wet Etch W/TiW interface acts as shadow mask for base lift off Collector formed via lift off and wet etch BCB used to passivate and planarize devices

8 W Mo InGaAs p+ InGaAs Base Ti 0.1 W 0.9 InP W Mo InGaAs p+ InGaAs Base Ti 0.1 W 0.9 InP Base process flow – I PR Blanket refractory metal PR Planarization Isotropic Dry etch of metal Removes any Emitter-Base short

9 W Mo InGaAs p+ InGaAs Base Ti 0.1 W 0.9 InP W Mo InGaAs p+ InGaAs Base Ti 0.1 W 0.9 InP SiN x Base process flow – II Lift-off Ti/Au Low base metal resistance Blanket SiN x mask Etch base contact metal in the field

10 Base Planarization Planarization: Emitter projecting from PR for W dry etch Etch Back Planarization Boundary

11 Epitaxial Design T(nm)MaterialDoping (cm -3 )Description 10In 0.53 Ga 0.47 As 8  : Si Emitter Cap 15InP 5  : Si Emitter 15InP 2  : Si Emitter 30InGaAs 9-5  : C Base 4.5In 0.53 Ga 0.47 As 9  : Si Setback 10.8InGaAs / InAlAs 9  : Si B-C Grade 3InP 6  : Si Pulse doping 81.7InP 9  : Si Collector 7.5InP 1  : Si Sub Collector 7.5In 0.53 Ga 0.47 As 2  : Si Sub Collector 300InP 2  : Si Sub Collector SubstrateSI : InP V be = 1 V, V cb = 0.7 V, J e = 25 mA/  m 2 Low Base doping  Good refractory ohmics not possible  Pd/W contacts used

12 Results - DC Measurements BV ceo = 2.4 J e = 1 kA/cm 2 β = 26 J KIRK = 21 mA/  m f ,f max J e = 17.9 mA/  m 2 P = 30 mW/  m 2 Gummel plot Common emitter I-V

GHz RF Data I c = 22.4 mA V ce = 1.67 V J e = 17.9 mA/  m 2 V cb = 0.7 V Single-pole fit to obtain cut-off frequencies

14 Equivalent Circuit Hybrid-  equivalent circuit from measured RF data R ex = 6  m 2

15 TEM Large undercut in base mesa Pd/W adhesion issue  High R bb  Low f max 0.1  m Pd/W adhesion issue Large mesa undercut

16 Summary Demonstrated a planarized, etch back process for refractory base contacts Demonstrated DHBTs with peak f  / f max = 410/690 GHz Higher base doping, thinner base and refractory base ohmics needed to enable higher bandwidth devices

17 Questions? Thank You This work was supported by the DARPA THETA program under HR C-006. A portion of this work was done in the UCSB nanofabrication facility, part of NSF funded NNIN network and MRL Central Facilities supported by the MRSEC Program of the NSF under award No. MR