InGaAs/InP DHBTs in a planarized, etch-back technology for base contacts

1. International Symposium on Compound Semiconductors 2011 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 93106-9560 D Loubychev, A Snyder, Y Wu, J M Fastenau, W K Liu IQE Inc., 119 Technology Drive, Bethlehem, PA 18015 vibhor@ece.ucsb.edu, 805-893-3273

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

3. We Wbc Tb Tc (emitter length Le) Bipolar transistor scaling laws To double cutoff frequencies of a mesa HBT, must: Keep constant all resistances and currents Reduce all capacitances and transit delays by 2 Epitaxial scaling Lateral scaling Ohmic contacts

4. InP bipolar transistor scaling roadmap Design Performance Rodwell, Le, Brar, Proceedings of IEEE, 2008

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

6. Ashish Baraskar et al., EMC 2010 Refractory base ohmics Ashish Baraskar et al., Int. MBE 2010 Refractory metal base contacts Require a blanket deposition and etch-back process

7. Emitter process flow Mo contact to n-InGaAs for emitter W/TiW/SiO2/Cr dep SF6/Ar etch SiNx Sidewall SiO2/Cr removal InGaAs Wet Etch Second SiNx 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. Base process flow – I Blanket refractory metal PR Planarization Isotropic Dry etch of metal Removes any Emitter-Base short Ti0.1W0.9 Ti0.1W0.9 W W PR Mo Mo InGaAs InGaAs InP InP p+ InGaAs Base p+ InGaAs Base

9. Ti0.1W0.9 Ti0.1W0.9 SiNx W W Mo Mo InGaAs InGaAs InP InP p+ InGaAs Base p+ InGaAs Base Base process flow – II Lift-off Ti/Au Low base metal resistance Blanket SiNx 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 Vbe = 1 V, Vcb = 0.7 V, Je = 25 mA/mm2 Low Base doping  Good refractory ohmics not possible  Pd/W contacts used

12. Results - DC Measurements Common emitter I-V @Peak ft,fmax Je = 17.9 mA/mm2 P = 30 mW/mm2 BVceo = 2.4 V @ Je = 1 kA/cm2 β = 26 JKIRK = 21 mA/mm2 Gummel plot

13. 1-67 GHz RF Data Ic = 22.4 mA Vce = 1.67 V Je = 17.9 mA/mm2 Vcb = 0.7 V Single-pole fitto obtain cut-off frequencies

14. Equivalent Circuit Rex = 6 Wmm2 Hybrid-p equivalent circuit from measured RF data

15. TEM Large undercut in base mesa Pd/W adhesion issue  High Rbb  Low fmax 0.1 mm Pd/W adhesion issue Large mesa undercut

16. Summary • Demonstrated a planarized, etch back process for refractory base contacts • Demonstrated DHBTs with peakft/ fmax = 410/690 GHz • Higher base doping, thinner base and refractory base ohmics needed to enable higher bandwidth devices

17. Thank You Questions? This work was supported by the DARPA THETA program under HR0011-09-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. MR05-20415