May 2010 Compound Semiconductors: Process Flow, Process Integration, Devices and Testing Ravi Ramanathan Skyworks Solutions, Inc. 1 4/28/2010
Prelude • Compound Semiconductor Market Value • An Example: Process Technologies in a Radio • Industry Trend in Power Amplifier/Front-end Modules • Multi-Mode, Multi-Band Radio • Process Technology Evolution • Widely Used Compound Semiconductor Devices • Heterostructures 2 4/28/2010
Compound Semiconductor Market Value Smart Grid Mobile Internet HB-LED/SSL http://www.bccresearch.com/report/SMC032C.html 3G, 4G Data Cards Laser Diodes CAGR of 10%-18% Embedded Wireless Platforms GaN LED: http://www.lbl.gov/Science-Articles/Archive/assets/images/2002/Dec-17-2002/indium_LED.jpg Laser Diode: http://www.aeropause.com/archives/Blu-ray-cover_plat.jpg 3 4/28/2010
Process Technologies in Radio Module Mainstream Tech. CMOS RF-CMOS, BiCMOS GaAs HBT, BiCMOS, LDMOS E/D pHEMT D-pHEMT, SOI, SOS Optimum choice of technologies and MCM packaging approach dominate the cellular hardware architecture! 4 4/28/2010
Industry Trend in PA/Front End Modules -05’ 2006 2007-08 >2009 Standard TX / MHz TX / MHz WCDMA I WCDMA I 1920 -1980 1920 -1980 PCS / WCDMA II PCS / WCDMA II 1850 -1910 1850 -1910 DCS / WCDMA III DCS / WCDMA III 1710 -1785 1710 -1785 WCDMA IV WCDMA IV 1710 -1755 1710 -1755 GSM 850 / WCDMA V GSM 850 / WCDMA V 824 -849 824 -849 WCDMA VI WCDMA VI 830 -840 830 -840 WCDMA VII WCDMA VII 2500 -2570 2500 -2570 E-GSM / WCDMA VIII E-GSM / WCDMA VIII 880(890) 880(890) -915 -915 WCDMA IX WCDMA IX 1749.9 -1784.9 1749.9 -1784.9 WCDMA X WCDMA X 1710 -1770 1710 -1770 Seamless Mobility & Wi-Fi + RFID + DTV + GPS +FM Rich Applications Single Band FEMs FEMs with Converged PA/FEM U-FEM PAs Cellular, WiFi Pass-thru Ports Architectures Discrete PA to multiple band “converged” PA architectures to multimode FEM – drives complex integration, on-chip as well as MCM 5 4/28/2010
Multi Mode and Multi Band Radio • Radios are becoming complex with the introduction of multiple modes and multiple bands in a single package (SiP). • Compound Semiconductors dominate the power amplifier, front-end modules and switch applications. – Power amplifiers and front-end modules are fabricated using heterojunction bipolar transistors • N. Sokal, RF Power Amplifiers - Classes A Through F (Overview), IEEE, 1996. • S.C. Cripps, RF Power Amplifiers for Wireless Communications, Artech House, Book publication. – Antenna switch modules (ASMs) and diversity switches are fabricated using depletion mode pHEMT and J-pHEMT devices, low noise amplifiers using E/D pHEMT devices. • H. Tosaka et al., An Antenna Switch MMIC Using E/D Mode p-HEMT for GSM/DCS/PCS/WCDMA Bands Application, 2003 IEEE Radio Frequency Integrated Circuits Symposium, page 519. • S. Tamari et al., Recessed JPHEMT Technology for Low Distortion and Low Insertion Loss Switch, CS MANTECH Conference, May 2009, Tampa, Florida, USA 6 4/28/2010
Process Technology Evolution • MOSFET, HEMT and MESFETs are lateral conduction devices while BJT, HBT devices are vertical conduction devices MOSFET: Racing down the μ m nm curve Evolution in RF Transistors Source: INTEL End of traditional Vcc, GoX, Lg scaling era! Figures of Merit Rule of Thumb • CMOS : Gate Length, Threshold, Gate Leakage f T ~ 10 × f op (LNA) • BiCMOS : Unit Current gain frequency (Ft) Unit Power gain frequency (Fmax) f T ~ 3-5 × f op (PA) & DC/RF Gain (Beta, Hfe) • HBT : DC Gain, BVceo, Ft, Fmax f max ~ f T • pHEMT : Gate Length, Ron, g m , Idss, Coff BV ~ 2-4 x V supply (PA) Lower cost • SoI/SoS : Gate Length, Csub, Ron, Threshold J.J. Liou, F. Schwierz, Journal of Telecommunications & Information Technology, page 99, 2004 7 4/28/2010
Compound Semiconductor Devices • Heterojunction Bipolar Transistors – HBT used for power amplification • pHEMT (Pseudomorphic High Electron Mobility Transistors) – Used as diversity switches, antenna switch module, low noise amplifier, mmWave amplifiers • BiFET (Integration of Heterojunction Bipolar and Metal- Semiconductor Field Effect Transistors) – HBT used for power amplification – FET used in the bias circuit and as a “switch” • BiHEMT (Integration of Heterojunction Bipolar and High Electron Mobility Transistors) – HBT used for power amplification – HEMT used in the bias circuit as well as a “switch” to turn some HBT arrays “ON” and “OFF” 8 4/28/2010
Heterostructures • Heterostructure junction based devices dominate the mainstream compound semiconductor applications. • In heterostructure devices, the band offset between conduction bands and/or the valence bands of the adjoining materials is exploited to get the desired device performance. • InGaP HBT as well as pHEMT are heterostructure based devices. • Some of the commonly used heterostructures are: Al 0.3 Ga 0.7 As GaAs In 0.52 Al 0.48 As In 0.48 Ga 0.52 P GaAs Al 0.3 Ga 0.7 N GaN In 0.53 Ga 0.47 As 1.422 1.422 1.8 1.451 4.04 1.9 3.44 0.737 Δ Ev = 0.22 eV Δ Ev = 0.194 eV Δ Ev = 0.18 eV Δ Ev = 0.33 eV Δ Ec = 0.15 eV Δ Ec = 0.52 eV Δ Ec = 0.42 eV Δ Ec = 0.14 eV** ** Depends on Orderliness D. Arnold et al., APL, 45, 1237 (1984) of the InGaP 9 4/28/2010
Organization of Process Flows • Front-end of Line Device Processing – HBT – pHEMT – BiFET vs. BiHEMT – Diodes (Schottky, Junction Diodes) • Back-end of Line Interconnect Processing – General Process Flow – Passives • Resistors • Capacitors • Inductors • Bondpads • Backside Process Flow (through Wafer Via) 10 4/28/2010
HBT Zhores Alferov & Herbert Kroemer (Nobel Prize 2000) DC Gain − ⎛ Δ ⎞ I I I E N V β = = ⎜ ⎟ β = c n r exp g E nB ⎜ ⎟ + + max I n ⎝ ⎠ I I I I N V kT I r B p r s B pE * I c I s RF 1 1 I b For = = f Speed: πτ π(τ + τ + τ + τ t 2 2 ) Low R b ec eb bc b c Low C bc f = t f Low C e I p π MAX 8 R C B BC Emitter Current: I e = I n + I p + I s Influenced by Collector Current: I c = I n – I r BV = Breakdown CBO BV collector Base Current: Ib = I p + I r + I s β CEO n doping & thickness • Thinner the base, higher the beta • High-base doping is needed to reduce base resistance → high F max . • High-base doping reduces emitter injection efficiency (hole storage in emitter) • In HBT, barrier for hole is higher than that for electrons → high-base doping can be used 11 4/28/2010
InGaP/GaAs HBT Key Features: (i) Very low output conductance (high base doping), (ii) Fairly constant gain as function of I b , (iii) Low turn on voltage, (iv) High breakdown voltage • HBT devices must be protected with proper ballasting scheme to ensure InGaP (Emitter) thermal stability GaAs (Base) Base Ballasting R bb Emitter Ballasting R eb Emitter contact: Ti/Pt (n-type ohmic contact) R bb = β R eb Base contact: Pt/Ti (p-type ohmic contact) Collector contact: AuGeNi (n-type alloyed contact) Careful design of epitaxial layer structures as well as optimized device layout geometry are critical to achieve consistent device and circuit performance. 12 4/28/2010
HBT Fabrication – Carving the device Layout Typical Process Flow Device Nitride Nitride MOCVD HBT MOCVD HBT MOCVD HBT MOCVD HBT Nitride Nitride Passivation Deposition Wafers Wafers Wafers Wafers Deposition Deposition Emitter Contact Ti/ Pt Contact Vias E/ B/ C Nitride Emitter Contact Emitter Contact E/ B/ C Nitride E/ B/ C Nitride Emitter Contact Photo & Metal Photo & Etch E/ B/ C Photo & Metal Photo & Metal Photo & Etch Photo & Etch TEM Epitaxial Material Emitter Mesa Emitter Mesa Collector Photo Collector Emitter Mesa Emitter Mesa Collector Photo Collector Photo Dry or Dry/ Wet etch Photo & Etch Definition (Wet Etch) Etch & Metal Photo & Etch Photo & Etch Etch & Metal Etch & Metal Base Pedestal Base Pedestal Au-Ge-Ni Base Pedestal Base Pedestal RTP Alloy Photo & Etch Dry or Wet etch RTP Alloy RTP Alloy Alloy Process Photo & Etch Photo & Etch I solation I mplant I solation I solation I solation (He + + ) TEM Photo & I mplant Contact Metal Photo & I mplant Photo & I mplant of I nGaAs Cap Emitter Base Photo Base Contact of Base Photo Base Photo Emitter Mesa Etch & Metal Definition Etch & Metal Etch & Metal HBT I nGaP structure Establishing “Statistical Process Control” at device fabrication steps is vital to achieve high yield and consistent performance. 13 4/28/2010
pHEMT • pHEMT is an acronym for pseudomorphic high electron mobility transistor. First AlGaAs-GaAs HEMT device was reported by Fujitsu Corporation, Japan in 1982 and the AlGaAs/InGaAs pHEMT in 1986. • pHEMT is an heterojunction field effect transistor. • Pseudomorphic refers to the fact that there is strain in and between layers due to the lack of crystal lattice matching (AlGaAs – InGaAs: below t critical ). • Primarily one tries to minimize the channel sheet resistance, R ch by increasing electron density and mobility. • Higher bandgap discontinuity ( Δ E c ) between AlGaAs & InGaAs → higher carrier sheet density and higher current; helps noise and linearity due to better carrier confinement. • Mobility can be thought of as the relative ease at which carriers can move through a material; higher mobility translates into higher speed transistors. 14 4/28/2010
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