Chapter 3 Basics Semiconductor Devices and Processing 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 2
Topics What is semiconductor Basic semiconductor devices Basics of IC processing 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. 4
Periodic Table of the Elements 5 Semiconductor Substrate and Dopants P-type Dopant Substrate N-type Dopants 6
Orbital and Energy Band Structure of an Atom Valence shells Conducting band, E c Nuclei Band gap, E g Valence band, E v 7 Band Gap and Resistivity E g = 1.1 ev E g = 8 ev Aluminum 2.7 µω cm Sodium 4.7 µω cm Silicon ~ 10 10 µω cm Silicon dioxide > 10 20 µω cm Conductors Semiconductor Insulator 8
Crystal Structure of Single Crystal Silicon Si Shared electrons Si Si Si Si Si Si Si Si Si Si Si - Si 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 10
N-type (Arsenic) Doped Silicon and Its Donor Energy Band Si Si Si Si As Si Conducting band, E c Extra Electron E g = 1.1 ev E d ~ 0.05 ev Si Si - Si Valence band, E v 11 P-type (Boron) Doped Silicon and Its Donor Energy Band Si Si Si Si B Si Si - Si Si Hole Electron Conducting band, E c E g = 1.1 ev Valence band, E v E a ~ 0.05 ev 12
Illustration of Hole Movement Conducting band, E c Conducting band, E c Conducting band, E c Electron E g = 1.1 ev E a ~ 0.05 ev Electron E g = 1.1 ev Electron E g = 1.1 ev Hole Valence band, E v Hole Valence band, E v Valence band, E v Hole 13 Dopant Concentration and Resistivity Resistivity P-type, Boron N-type, Phosphorus Dopant concentration 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 p- type silicon at the same dopant concentration 15 Basic Devices Resistor Capacitor Diode Bipolar Transistor MOS Transistor 16
Resistor ρ l l R = ρ wh ρ: Resistivity h w 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 18
Capacitors κ l d h hl C = κ d κ: Dielectric Constant 19 Capacitors Charge storage device Memory Devices, esp. DRAM Challenge: reduce capacitor size while keeping the capacitance High-κdielectric materials 20
Capacitors Poly Si Oxide Si Dielectric Layer Poly 1 Poly 2 Heavily Doped Si Dielectric Layer Poly Si Si Parallel plate Stacked Deep Trench 21 Metal Interconnection and RC Delay Dielectric, κ Metal, ρ l I d w 22
Diode P-N Junction Allows electric current go through only when it is positively biased. 23 Diode V1 V2 P1 P2 V1 > V2, current P1 > P2, current V1 < V2, no current P1 < P2, no current 24
Figure 3.14 Transition region P + + + + + + + + + + N Vp V0 Vn 25 Intrinsic Potential kt V 0 = ln q N N a 2 ni d For silicon V 0 ~ 0.7 V 26
I-V Curve of Diode I V -I 0 27 Bipolar Transistor PNP or NPN Switch Amplifier Analog circuit Fast, high power device 28
NPN and PNP Transistors E B E C B N P N C C E B C B P N P E 29 NPN Bipolar Transistor Emitter Base Collector Al Cu Si SiO 2 n p n p + + + n-epi Electron flow n + buried layer P-substrate p + 30
Sidewall Base Contact NPN Bipolar Transistor Base CVD oxide Emitter Metal CVD oxide Collector CVD oxide Poly Field oxide p n Epi Field oxide Buried Layer n + n + p n + Field oxide P-substrate 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 32
NMOS Device Basic Structure V G V D Metal Gate V G n + Source p-si n + Drain Ground V D 33 NMOS Device Positive charges V G = 0 V D Electron flow V G > V T > 0 V D > 0 Metal Gate SiO 2 n + Source p-si n + Drain SiO 2 n + Source + + + + + + + p-si Drain + n No current Negative charges 34
PMOS Device Negative charges V G = 0 V D Hole flow V G < V T < 0 V D > 0 Metal Gate SiO 2 p + Source n-si p + Drain SiO 2 p + Source + + + + + + + n-si Drain p + No current Positive charges 35 MOSFET 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 37 Basic Circuits Bipolar PMOS NMOS CMOS BiCMOS 38
Devices with Different Substrates Silicon Bipolar MOSFET BiCMOS Dominate IC industry Germanium Compound Bipolar: high speed devices GaAs: up to 20 GHz device Light emission diode (LED) 39 Market of Semiconductor Products 100% Bipolar Compound } 8% 4% 50% MOSFET 88% 1980 1990 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. 41 PMOS First MOS field effect transistor, 1960 Used for digital logic devices in the 1960s Replaced by NMOS after the mid-1970s 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 43 CMOS Most commonly used circuit in IC chip since 1980s Low power consumption High temperature stability High noise immunity Symmetric design 44
CMOS Inverter V dd PMOS V in V out NMOS V ss 45 CMOS IC n + Source/Drain Gate Oxide p + Source/Drain Polysilicon p-si STI Balk Si n-si 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 47 IC Chips Memory Microprocessor Application specific IC (ASIC) 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 49 DRAM Major component of computer and other electronic instruments for data storage Main driving force of IC processing development One transistor, one capacitor 50
Basic DRAM Memory Cell NMOS Word line Capacitor Bit line V dd 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 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 53 EPROM Passivation Dielectric V G V D Inter-poly Dielectric Gate Oxide n + Source Poly 2 Poly 1 p-si n + Drain Control Gate Floating Gate 54
EPROM Programming Passivation Dielectric V G >V T >0 V D > 0 Inter-poly Dielectric Gate Oxide n + Source Poly 2 e - e - e - e - e - e - e - p-si n + Drain Control Gate Floating Gate Electron Tunneling 55 EPROM Programming Passivation Dielectric V G >V T >0 UV light V D > 0 Inter-poly Dielectric Gate Oxide n + Source e - e - Poly 2 p-si n + Drain Control Gate Floating Gate Electron Tunneling 56
IC Fabrication Processes Adding Ion implantation, Diffusion Grown thin film, SiO 2 Deposited thin film CV PVD Epi, Poly Dielectri Meta Electrical IC Fab. Removing Heating Patterning Wafer Clean Etch CMP Annealing Reflow Alloying Photolithography Patterned etch Blanket Strip Meta Oxid Implantati Dielectri Meta Exposure (heating) PR coating (adding) Baking (heating, Developing 57 Basic Bipolar Process Steps Buried layer doping Epitaxial silicon growth Isolation and transistor doping Interconnection Passivation 58
Buried Layer Implantation SiO 2 P-silicon n + 59 Epitaxy Grow n + buried layer n-epi P-silicon 60
Isolation Implantation p + n + buried layer n-epi p + P-silicon 61 Emitter/Collector and Base Implantation p + n + p n + n + buried layer n-epi p + P-silicon 62
SiO 2 Metal Etch Emitter Base Collector Al Cu Si p + n + p n+ p + n-epi n + P-silicon buried layer 63 SiO 2 Passivation Oxide Deposition Emitter Base Collector Al Cu Si p + n + p n+ p + n-epi n + P-silicon buried layer CVD oxide 64
MOSFET Good for digital electronics Major driving forces: Watches Calculators PC Internet Telecommunication 65 1960s: PMOS Process Bipolar dominated First MOSFET made in Bell Labs Silicon substrate Diffusion for doping Boron diffuses faster in silicon PMOS 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) (P) Test and packaging 67 Wafer clean, field oxidation, and photoresist coating Native Oxide N-Silicon N-Silicon Field Oxide Primer Field Oxide Photoresist N-Silicon N-Silicon 68
Photolithography and etch Source/Drain Mask Field Oxide Source/Drain Mask UV Light Photoresist PR N-Silicon N-Silicon Field Oxide Field Oxide PR PR N-Silicon N-Silicon 69 Source/drain doping and gate oxidation Field Oxide Field Oxide N-Silicon p + p + N-Silicon Field Oxide Gate Oxide Field Oxide p + p + N-Silicon p + p + N-Silicon 70
Contact, Metallization, and Passivation Gate Oxide Field Oxide Gate Oxide Al Si Field Oxide p + p + N-Silicon p + p + N-Silicon Gate Oxide Field Oxide Gate Oxide CVD Cap Oxide p + N-Silicon p + p + N-Silicon p + 71 Illustration of a PMOS Gate Oxide CVD Cap Oxide p + N-Silicon p + 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 73 Self-aligned S/D Implantation Phosphorus Ions, P + Gate Polysilicon n + n + p-silicon Field oxide Source/Drain Gate oxide 74
NMOS Process Sequence (1970s) Wafer clean Grow field oxide Mask 1. Active Area Etch oxide Strip photo resist/clean Grow gate oxide Deposit polysilicon Mask 2. Gate Etch polysilicon Strip photo resist/clean S/D and poly dope implant Anneal and poly reoxidation CVD USG/PSG PSG reflow Mask 3. Contact Etch PSG/USG Strip photo resist/clean Al deposition Mask 4. Metal Etch Aluminum Strip photo resist Metal anneal CVD oxide Mask 5. Bonding pad Etch oxide Test and packaging 75 NMOS Process Sequence Clean p-si p-si Field Oxidation Oxide Etch p-si p-si Gate Oxidation Poly Dep. p-si poly p-si poly Poly Etch P + Ion Implant p-si poly poly n + p-si n + Annealing 76
NMOS Process Sequence PSG Dep. PSG poly p-si PSG poly p-si PSG Reflow PSG Etch PSG poly p-si Al Si PSG poly p-si Metal Dep. Al Si Al Si SiN Metal Etch PSG poly p-si PSG poly n + p-si n + Nitride Dep. 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 78
V dd V in V out Advantages of CMOS Low power consumption High temperature stability High noise immunity 79 CMOS Inverter, Its Logic Symbol and Logic Table PMOS V in V out NMOS In Out V ss 0 1 1 0 80
CMOS Chip with 2 Metal Layers PD2 Nitride PD1 Oxide Metal 2, Al Cu Si p + n + IMD PMD Poly Si Gate n + Al Cu Si BPSG LOCOS SiO 2 p + P-type substrate USG dep/etch/dep p + p + N-well 81 CMOS Chip with 4 Metal Layers Tantalum barrier layer Passivation 2, nitride Passivation 1, USG Metal 4 Lead-tin alloy bump Copper FSG Tungsten plug Tungsten local Interconnection Metal 3 Copper FSG FSG Metal 2 Copper FSG M 1 Cu Cu FSG FSG PSG Tungsten STI n + n + USG p + p + P-well N-well P-epi P-wafer Nitride etch stop layer Nitride seal layer Tantalum barrier layer T/TiN barrier & adhesion layer PMD nitride barrier 82 layer
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 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 84
Summary R=ρ l/a C=κ 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 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 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. 87