PROCESS DEVELOPMENT FOR SMALL-AREA GaN/AlGaN HBT s K.P.Lee (1), A.P.Zhang (1), G.Dang (1), F.Ren (1), J.Han (2), W.S.Hobson (3), J.Lopata (3), C.R.Abernathy (1), S.J.Pearton (1), J.W.Lee (4) (1) University of Florida Gainesville Florida (2) Sandia National Laboratories, Albuquerque, NM 87185 (3) AT&T Bell Labs, Lucent Technologies Murray Hill, NJ 07974 (4) Unaxis, St. Petersburg FL 33716
Introduction What are the Advantages of HBT s over HFETs High power density capability Better linearity Uniform threshold voltage High tranconductance What are potential applications of GaN/AlGaN HBTs High frequency switching and communications application To fulfill high frequency ---(small emitter area, small τ B, graded base, etc) Microwave power amplifiers 1-5 GHz range, Operating temperature > 400 C
Introduction Currently limiting factor of GaN/AlGaN npn HBTs Low p-type doping high ionization energy of Mg : 170meV Low electron lifetime in neutral base electron mobility is relatively low in p-base High leakage currents in collector-emitter junction material quality is bad. High recombination rates.
Overview of Process Scheme Self-aligned process to get high cut-off frequency characteristics self-aligned emitter pattern self-aligned base metal Using dielectric side wall spacers reduce junction leakage current protect metal bridge base to emitter enforce masking for self aligned dry etching Selective growth of GaAs on p-base GaN reduce p-base contact resistance Lift-off metallization
Layer Structure emitter GaN (2000Å, 1.8E19) AlGaN to GaN ( 200Å, 1.8E19) AlGaN ( 800 Å, 5.0e17) base AlGaN (50Å, UID) GaN (1500Å, ( Mg, 3e17) GaN (5000Å, 3.0E16) collector GaN (1.44 um, 1.8E19)
Process Review Photo-lithography resist : AZ1818, soft bake : 90 C, 20 min Etch : ICP etcher (Plasma Therm 790) low source power : 300W (2MHz) ICP / 40W (13.56MHz) rf Cl 2 /Ar chemistries for GaN/AlGaN Dielectric films : Plasma Therm CVD (300mT, 300 C) Selective growth of GaAs on p-base GaN---MOCVD carbon doped (~10 20 /cm 3 ), at 650 C. Remove Dry etching damage or cleaning : dip in dilute HCl (1:20)
Schematic of Completed Device Emitter metal (Ti/Al/Pt/Au) Final Metal (Ti/Pt/Au) Collector metal (Ti/Al/Pt/Au) Emitter(AlGaN) Base (p-gan) Base metal (Ti/Pt/Au) p+ GaAs n-gan substrate
SEM Micrographs at different stages emitter spacer base mesa collector mesa
Problems for HBT s Process Need tight realignment for lithography Need high dry etch selectivity specially emitter over thin-base Need improved epi growth dry etch rate depends on material quality surface morphology is rough device performance is not good
SEM Micrographs of Small Area HBT Base Base metal Emitter metal Base metal collector metal Emitter Emitter metal
Contact resistance on p-gaas base Materials p-gaas(c) P-GaN(Mg) ρ C (Ω-cm 2 ) as depo. 9~14 x 10-6 8 x 10-2 ρ C (Ω-cm 2 ) 800 C anneal 3~6 x 10-6 4 x 10-3 P-GaAs/p-GaN 7 x 10-3 2 x 10-3 (Measured at 250 C)
I-V Characteristics (large area device) Power density : 20.4 kw/cm 2 off-set voltage : 2~3 V Common-base Gain β : ~10 Common-emitter
I-V Characteristics (small area device) Common-base Current density is same order as for the large devices
Conclusion and Future work Develop self-aligned small area GaN/AlGaN npn HBT Future work Process optimization Need precise mask alignment Need high dry etch selectivity finely defined lift-off metallization High quality epi materials for device yield
Acknowledgments This work is partially supported by NSF DMR DARPA/EPRI NSF CTS ONR
sapphire P.R AlGaN p+gan n-gan AlGaN Ti/Al/Pt/Au SiN E-metal PR 2nd spacer(sin) SiO 2 2000 Å AlGaN 1st spacer (SiO 2 ) p-gan
SiO 2 GaAs etching 3rd spacer (SiO 2 ) Base metal PR p+gaas Base-metal Metal lift-off Emitter re-open PR Base mesa PR
Base mesa etch Collector mesa Base-metal PR n-gan substrate Collector mesa etch PR substrate Ti/Al/Pt/Au Collector metal SiN Via hole substrate substrate
Emitter metal (Ti/Al/Pt/Au) Final Metal (Ti/Pt/Au) Collector metal (Ti/Al/Pt/Au) Emitter(AlGaN) Base (p-gan) n-gan Base metal (Ti/Pt/Au) p+ GaAs substrate
Base Base metal collector metal Emitter Emitter metal Emitter metal Base metal