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Chapter 3 Basics Semiconductor Devices and Processing Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 1 Objectives • Identify at least two semiconductor materials from the periodic table of elements • List n-type and p-type dopants • Describe a diode and a MOS transistor • List three kinds of chips made in the semiconductor industry • List at least four basic processes required for a chip manufacturing Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 2 Topics • What is semiconductor • Basic semiconductor devices • Basics of IC processing Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 3 What is Semiconductor • • • • Conductivity between conductor and insulator Conductivity can be controlled by dopant Silicon and germanium Compound semiconductors – SiGe, SiC – GaAs, InP, etc. Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 4 Periodic Table of the Elements Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 5 Semiconductor Substrate and Dopants Substrate P-type Dopant N-type Dopants Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 6 Orbital and Energy Band Structure of an Atom Valence shells Conducting band, Ec Nuclei Band gap, Eg Valence band, Ev Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 7 Band Gap and Resistivity Eg = 1.1 eV Eg = 8 eV Aluminum Sodium Silicon Silicon dioxide 2.7 mWcm 4.7 mWcm ~ 1010 mWcm > 1020 mWcm Conductors Hong Xiao, Ph. D. Semiconductor www2.austin.cc.tx.us/HongXiao/Boo k.htm Insulator 8 Crystal Structure of Single Crystal Silicon Shared electrons Hong Xiao, Ph. D. Si Si Si Si Si Si Si Si www2.austin.cc.tx.us/HongXiao/Boo k.htm - Si 9 Why Silicon • Abundant, inexpensive • Thermal stability • Silicon dioxide is a strong dielectric and relatively easy to form • Silicon dioxide can be used as diffusion doping mask Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 10 N-type (Arsenic) Doped Silicon and Its Donor Energy Band Si Si Si As Si Si Conducting band, Ec Si Extra Electron Si - Ed ~ 0.05 eV Eg = 1.1 eV Si Valence band, Ev Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 11 P-type (Boron) Doped Silicon and Its Donor Energy Band Si Si Conducting band, Ec Si Hole Si B Eg = 1.1 eV Si Ea ~ 0.05 eV Si Si - Si Electron Hong Xiao, Ph. D. Valence band, Ev www2.austin.cc.tx.us/HongXiao/Boo k.htm 12 Illustration of Hole Movement Conducting band, Ec Electron Eg = 1.1 eV Ea ~ 0.05 eV Hole Valence band, Ev Hong Xiao, Ph. D. Conducting band, Ec Electron Hole Eg = 1.1 eV Valence band, Ev www2.austin.cc.tx.us/HongXiao/Boo k.htm Conducting band, Ec Electron Eg = 1.1 eV Valence band, Ev Hole 13 Dopant Concentration and Resistivity Resistivity P-type, Boron N-type, Phosphorus Dopant concentration Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 14 Dopant Concentration and Resistivity • Higher dopant concentration, more carriers (electrons or holes) • Higher conductivity, lower resistivity • Electrons move faster than holes • N-type silicon has lower resistivity than ptype silicon at the same dopant concentration Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 15 Basic Devices • • • • • Hong Xiao, Ph. D. Resistor Capacitor Diode Bipolar Transistor MOS Transistor www2.austin.cc.tx.us/HongXiao/Boo k.htm 16 Resistor r h l w l Rr wh r: Resistivity Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 17 Resistor • Resistors are made by doped silicon or polysilicon on an IC chip • Resistance is determined by length, line width, height, and dopant concentration Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 18 Capacitors l h d hl C d : Dielectric Constant Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 19 Capacitors • Charge storage device • Memory Devices, esp. DRAM • Challenge: reduce capacitor size while keeping the capacitance • High- dielectric materials Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 20 Capacitors Dielectric Layer Dielectric Layer Poly Si Oxide Si Poly 2 Poly Si Si Poly 1 Heavily Doped Si Parallel plate Hong Xiao, Ph. D. Stacked www2.austin.cc.tx.us/HongXiao/Boo k.htm Deep Trench 21 Metal Interconnection and RC Delay Dielectric, Metal, r I l d w Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 22 Diode • P-N Junction • Allows electric current go through only when it is positively biased. Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 23 Diode V1 V2 P1 • V1 > V2 , current • V1 < V2 , no current Hong Xiao, Ph. D. • P1 > P2 , P2 current • P1 < P2, no current www2.austin.cc.tx.us/HongXiao/Boo k.htm 24 Figure 3.14 Transition region P ------ ++ ++ ++ ++ ++ N Vn Vp Hong Xiao, Ph. D. V0 www2.austin.cc.tx.us/HongXiao/Boo k.htm 25 Intrinsic Potential kT N a N d V0 ln 2 q ni • For silicon V0 ~ 0.7 V Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 26 I-V Curve of Diode I V -I 0 Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 27 Bipolar Transistor • • • • • Hong Xiao, Ph. D. PNP or NPN Switch Amplifier Analog circuit Fast, high power device www2.austin.cc.tx.us/HongXiao/Boo k.htm 28 NPN and PNP Transistors E B E C N B P N C C B E B P N C P E Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 29 NPN Bipolar Transistor Emitter p+ n+ Base p Collector n+ Al•Cu•Si SiO2 n-epi p+ Electron flow n+ buried layer P-substrate Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 30 Sidewall Base Contact NPN Bipolar Transistor Metal CVD oxide Base CVD oxide Emitter CVD oxide Collector Poly Field oxide p p n+ n Field oxide Buried Layer Epi n+ n+ Field oxide P-substrate Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 31 MOS Transistor • Metal-oxide-semiconductor • Also called MOSFET (MOS Field Effect Transistor) • Simple, symmetric structure • Switch, good for digital, logic circuit • Most commonly used devices in the semiconductor industry Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 32 NMOS Device Basic Structure VG VD VG “Metal” Gate Ground n+ Source Hong Xiao, Ph. D. p-Si VD n+ Drain www2.austin.cc.tx.us/HongXiao/Boo k.htm 33 NMOS Device Positive charges VG = 0 VD Electron flow VG > V T > 0 VD > 0 “Metal” Gate SiO 2 n+ Source n+ p-Si Drain No current Hong Xiao, Ph. D. SiO 2 n+ Source +++++++ ------p-Si n+ Drain Negative charges www2.austin.cc.tx.us/HongXiao/Boo k.htm 34 PMOS Device Negative charges VG = 0 VD VG < V T < 0 Hole flow VD > 0 “Metal” Gate SiO 2 p+ Source p+ n-Si Drain No current Hong Xiao, Ph. D. SiO 2 p+ Source ------+++++++ n-Si p+ Drain Positive charges www2.austin.cc.tx.us/HongXiao/Boo k.htm 35 MOSFET Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 36 MOSFET and Drinking Fountain MOSFET Drinking Fountain • Source, drain, gate • Source/drain biased • Voltage on gate to turn-on • Current flow between source and drain • Source, drain, gate valve • Pressurized source • Pressure on gate (button) to turn-on • Current flow between source and drain Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 37 Basic Circuits • • • • • Hong Xiao, Ph. D. Bipolar PMOS NMOS CMOS BiCMOS www2.austin.cc.tx.us/HongXiao/Boo k.htm 38 Devices with Different Substrates Silicon • Bipolar • MOSFET • BiCMOS Dominate IC industry Germanium • Bipolar: high speed devices Compound • GaAs: up to 20 GHz device • Light emission diode (LED) Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 39 Market of Semiconductor Products Compound 100% Bipolar 50% MOSFET 1980 Hong Xiao, Ph. D. 1990 www2.austin.cc.tx.us/HongXiao/Boo k.htm 4% 8% 88% 2000 40 Bipolar IC • • • • Earliest IC chip 1961, four bipolar transistors, $150.00 Market share reducing rapidly Still used for analog systems and power devices • TV, VCR, Cellar phone, etc. Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 41 PMOS • First MOS field effect transistor, 1960 • Used for digital logic devices in the 1960s • Replaced by NMOS after the mid-1970s Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 42 NMOS • Faster than PMOS • Used for digital logic devices in 1970s and 1980s • Electronic watches and hand-hold calculators • Replaced by CMOS after the 1980s Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 43 CMOS • Most commonly used circuit in IC chip since 1980s • Low power consumption • High temperature stability • High noise immunity • Symmetric design Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 44 CMOS Inverter Vdd PMOS V in Vout NMOS Vss Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 45 CMOS IC n+ Source/Drain p+ Source/Drain Gate Oxide Polysilicon p-Si Hong Xiao, Ph. D. STI Balk Si n-Si www2.austin.cc.tx.us/HongXiao/Boo k.htm USG 46 BiCMOS • • • • • • • Combination of CMOS and bipolar circuits Mainly in 1990s CMOS as logic circuit Bipolar for input/output Faster than CMOS Higher power consumption Likely will have problem when power supply voltage dropping below one volt Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 47 IC Chips • Memory • Microprocessor • Application specific IC (ASIC) Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 48 Memory Chips • Devices store data in the form of electric charge • Volatile memory – Dynamic random access memory (DRAM) – S random access memory (SRAM) • Non-volatile memory – Erasable programmable read only memory (EPROM) – FLASH Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 49 DRAM • Major component of computer and other electronic instruments for data storage • Main driving force of IC processing development • One transistor, one capacitor Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 50 Basic DRAM Memory Cell Word line NMOS Capacitor Vdd Bit line Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 51 SRAM • Fast memory application such as computer cache memory to store commonly used instructions • Unit memory cell consists of six transistors • Much faster than DRAM • More complicated processing, more expensive Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 52 EPROM • Non-volatile memory • Keeping data ever without power supply • Computer bios memory which keeps boot up instructions • Floating gate • UV light memory erase Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 53 EPROM Passivation Dielectric Inter-poly Dielectric Gate Oxide VD Poly 2 Control Gate Poly 1 Floating Gate n+ n+ Source Hong Xiao, Ph. D. VG p-Si Drain www2.austin.cc.tx.us/HongXiao/Boo k.htm 54 EPROM Programming Passivation Dielectric Inter-poly Dielectric Gate Oxide VD > 0 Poly 2 Control Gate e- e- e- e- e- e- Floating Gate e- n+ Source Hong Xiao, Ph. D. VG>VT>0 p-Si n+ Drain www2.austin.cc.tx.us/HongXiao/Boo k.htm Electron Tunneling 55 EPROM Programming Passivation Dielectric Inter-poly Dielectric Gate Oxide VG>VT>0 VD > 0 Poly 2 Control Gate e- e- Floating Gate n+ n+ Source Hong Xiao, Ph. D. UV light p-Si Drain www2.austin.cc.tx.us/HongXiao/Boo k.htm Electron Tunneling 56 IC Fabrication Processes Ion implantation, Diffusion Adding IC Fab. Removing Heating Epi, Poly silicon Grown thin film, SiO 2 Dielectri CV cMeta D Deposited thin film PVD Electrical l plating Wafer Clean Patterned etch (RIE) Etch Blanket Dielectri etch Strip c CMP Meta Meta lOxid Annealing l eImplantati Reflow on Alloying Exposure (heating) Patterning Hong Xiao, Ph. D. Photolithography www2.austin.cc.tx.us/HongXiao/Boo k.htm PR coating (adding) Baking (heating, removing) Developing 57 Basic Bipolar Process Steps • • • • • Hong Xiao, Ph. D. Buried layer doping Epitaxial silicon growth Isolation and transistor doping Interconnection Passivation www2.austin.cc.tx.us/HongXiao/Boo k.htm 58 Buried Layer Implantation SiO2 P-silicon Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm n+ 59 Epitaxy Grow n-epi n+ buried layer P-silicon Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 60 Isolation Implantation p+ n-epi p+ n+ buried layer P-silicon Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 61 Emitter/Collector and Base Implantation p+ n+ p n+ n-epi p+ n+ buried layer P-silicon Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 62 Metal Etch SiO2 Emitter p+ n+ Base Collector p Al•Cu•Si n+ n-epi p+ n+ buried layer P-silicon Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 63 Passivation Oxide Deposition SiO2 Emitter Base Collector Al•Cu•Si CVD oxide p+ n+ p n+ n-epi p+ n+ buried layer P-silicon Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 64 MOSFET • Good for digital electronics • Major driving forces: – – – – – Watches Calculators PC Internet Telecommunication Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 65 1960s: PMOS Process • • • • Bipolar dominated First MOSFET made in Bell Labs Silicon substrate Diffusion for doping – Boron diffuses faster in silicon – PMOS Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 66 PMOS Process Sequence (1960s) Wafer clean (R) Etch oxide (R) Field oxidation (A) Strip photo resist (R) Mask 1. (Source/Drain) (P) Al deposition (A) Etch oxide (R) Mask 4. (Metal) (P) Strip photo resist/Clean (R) Etch Aluminum (R) S/D diffusion (B)/Oxidation (A) Strip photo resist (R) Mask 2. (Gate) (P) Metal Anneal (H) Etch oxide (R) CVD oxide (A) Strip photo resist/Clean (R) Mask 5. (Bonding pad) (P) Gate oxidation (A) Etch oxide (R) Mask 3. (Contact) Hong Xiao, Ph. D. (P) Test and packaging www2.austin.cc.tx.us/HongXiao/Boo k.htm 67 Wafer clean, field oxidation, and photoresist coating Native Oxide N-Silicon N-Silicon Primer Field Oxide Field Oxide Photoresist N-Silicon Hong Xiao, Ph. D. N-Silicon www2.austin.cc.tx.us/HongXiao/Boo k.htm 68 Photolithography and etch Source/Drain Mask Source/Drain Mask Field Oxide PR Photoresist N-Silicon N-Silicon Field Oxide Field Oxide PR PR N-Silicon Hong Xiao, Ph. D. UV Light N-Silicon www2.austin.cc.tx.us/HongXiao/Boo k.htm 69 Source/drain doping and gate oxidation Field Oxide Field Oxide p+ N-Silicon N-Silicon Field Oxide p+ p+ N-Silicon Hong Xiao, Ph. D. p+ Gate Oxide p+ Field Oxide p+ N-Silicon www2.austin.cc.tx.us/HongXiao/Boo k.htm 70 Contact, Metallization, and Passivation Gate Oxide Field Oxide p+ p+ p+ N-Silicon Gate Oxide Gate Oxide Al∙Si N-Silicon Field Oxide p+ Field Oxide p+ p+ N-Silicon Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 71 Illustration of a PMOS Gate Oxide CVD Cap Oxide p+ p+ N-Silicon Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 72 NMOS Process after mid-1970s • Doping: ion implantation replaced diffusion • NMOS replaced PMOS – NMOS is faster than PMOS • Self-aligned source/drain • Main driving force: watches and calculators Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 73 Self-aligned S/D Implantation Phosphorus Ions, P+ Gate n+ p-silicon Source/Drain Hong Xiao, Ph. D. n+ Gate oxide www2.austin.cc.tx.us/HongXiao/Boo k.htm 74 NMOS Process Sequence (1970s) Wafer clean PSG reflow Grow field oxide Mask 3. Contact Mask 1. Active Area Etch PSG/USG Etch oxide Strip photo resist/Clean Strip photo resist/Clean Al deposition Grow gate oxide Mask 4. Metal Deposit polysilicon Etch Aluminum Mask 2. Gate Strip photo resist Etch polysilicon Metal anneal Strip photo resist/Clean CVD oxide S/D and poly dope implant Mask 5. Bonding pad Anneal and poly reoxidation Etch oxide CVD USG/PSG Test and packaging Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 75 NMOS Process Sequence Clean Oxide Etch p-Si p-Si poly p-Si P+ Ion Implant poly Hong Xiao, Ph. D. Poly Etch p-Si poly p-Si Gate Oxidation p-Si p-Si Poly Dep. Field Oxidation poly n+ www2.austin.cc.tx.us/HongXiao/Boo k.htm p-Si n+ Annealing 76 NMOS Process Sequence PSG Dep. PSG Etch PSG PSG poly poly p-Si p-Si PSG Al·Si PSG poly poly p-Si p-Si PSG Reflow Metal Dep. Al·Si Al·Si Metal Etch PSG PSG poly poly p-Si Hong Xiao, Ph. D. SiN n+ www2.austin.cc.tx.us/HongXiao/Boo k.htm p-Si Nitride Dep. n+ 77 CMOS • • • • • In the 1980s MOSFET IC surpassed bipolar LCD replaced LED Power consumption of circuit CMOS replaced NMOS Still dominates the IC market • Backbone of information revolution Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 78 Advantages of CMOS • Low power consumption • High temperature stability • High noise immunity Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 79 CMOS Inverter, Its Logic Symbol and Logic Table Vdd Vin Vout PMOS Vin Vout NMOS Vss Hong Xiao, Ph. D. In Out 0 1 1 0 www2.austin.cc.tx.us/HongXiao/Boo k.htm 80 CMOS Chip with 2 Metal Layers Nitride PD2 PD1 Oxide Metal 2, Al·Cu·Si IMD USG dep/etch/dep PMD p+ n+ n+ Al·Cu·Si BPSG LOCOS SiO2 p+ p+ p+ N-well Poly Si Gate P-type substrate Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 81 CMOS Chip with 4 Metal Layers Passivation 2, nitride Passivation 1, USG Metal 4 Lead-tin alloy bump Copper Tantalum barrier layer FSG Metal 3 Copper FSG Nitride etch stop layer FSG Metal 2 Nitride seal layer Copper FSG Tungsten plug M1 Cu Cu Tantalum barrier layer FSG FSG Tungsten local Interconnection Hong Xiao, Ph. D. PSG n+ USG P-well www2.austin.cc.tx.us/HongXiao/Boo P-epi k.htm P-wafer STI T/TiN barrier & adhesion layer Tungsten n+ p+ N-well p+ PMD nitride 82 layer barrier Summary • Semiconductors are the materials with conductivity between conductor and insulator • Its conductivity can be controlled by dopant concentration and applied voltage • Silicon, germanium, and gallium arsenate • Silicon most popular: abundant and stable oxide Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 83 Summary • Boron doped semiconductor is p-type, majority carriers are holes • P, As, or Sb doped semiconductor is p-type, the majority carriers are electrons • Higher dopant concentration, lower resistivity • At the same dopant concentration, n-type has lower resistivity than p-type Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 84 Summary R=r l/A C= A/d Capacitors are mainly used in DRAM Bipolar transistors can amplify electric signal, mainly used for analog systems • MOSFET electric controlled switch, mainly used for digital systems • • • • Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 85 Summary • MOSFETs dominated IC industry since 1980s • Three kinds IC chips microprocessor, memory, and ASIC • Advantages of CMOS: low power, high temperature stability, high noise immunity, and clocking simplicity Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 86 Summary • The basic CMOS process steps are transistor making (front-end) and interconnection/passivation (back-end) • The most basic semiconductor processes are adding, removing, heating, and patterning processes. Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Boo k.htm 87
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