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/book.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/book.htm 2
Topics What is semiconductor Basic semiconductor devices Basics of IC processing Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.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/book.htm 4
Periodic Table of the Elements Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 5
Semiconductor Substrate and Dopants P-type Dopant Substrate N-type Dopants Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 6
Orbital and Energy Band Structure of an Atom Valence shells Conducting band, E c Nuclei Band gap, E g Valence band, E v Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 7
Band Gap and Resistivity E g = 1.1 ev E g = 8 ev Aluminum Sodium Silicon Silicon dioxide 2.7 µω cm 4.7 µω cm ~ 10 10 µω cm > 10 20 µω cm Conductors Semiconductor Insulator Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 8
Crystal Structure of Single Crystal Silicon Shared electrons Si Si Si Si Si Si Si Si Si Si Si Si Si - Si Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 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/book.htm 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 Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 11
P-type (Boron) Doped Silicon and Its Donor Energy Band Si Si Si Si B Si Hole Conducting band, E c E g = 1.1 ev Si Si - Si E a ~ 0.05 ev Electron Valence band, E v Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 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 Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 13
Dopant Concentration and Resistivity Resistivity P-type, Boron N-type, Phosphorus Dopant concentration Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.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 p- type silicon at the same dopant concentration Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 15
Basic Devices Resistor Capacitor Diode Bipolar Transistor MOS Transistor Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 16
Resistor l ρ h w l R = ρ wh ρ: Resistivity Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.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/book.htm 18
Capacitors κ l d h hl C = κ d κ: Dielectric Constant Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 19
Capacitors Charge storage device Memory Devices, esp. DRAM Challenge: reduce capacitor size while keeping the capacitance High-κ dielectric materials Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 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 Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 21
Metal Interconnection and RC Delay Dielectric, κ Metal, ρ l I d w Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.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/book.htm 23
Diode V1 V2 P1 P2 V1 > current V2, P1 > P2, current V1 < V2, no current P1 < P2, no current Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 24
Figure 3.14 Transition region + + P + + + + + + + + N Vn Vp V0 Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 25
Intrinsic Potential kt V = ln 0 q N N a 2 i n d For silicon V 0 ~ 0.7 V Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 26
I-V Curve of Diode I V -I 0 Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 27
Bipolar Transistor PNP or NPN Switch Amplifier Analog circuit Fast, high power device Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 28
NPN and PNP Transistors E E B C B N P N C C E B C B P N P E Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 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 + Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 30
Sidewall Base Contact NPN Bipolar Transistor Base CVD oxide Emitter Metal CVD oxide Collector CVD oxide Poly Field oxide p n + p n Epi Field oxide n + Buried Layer n + Field oxide P-substrate Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.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/book.htm 32
NMOS Device Basic Structure V G V D V G Metal Gate n + Source p-si n + Drain Ground V D Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 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 SiO2 n + Source + + + + + + + p-si Drain n + No current Negative charges Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 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 SiO2 p + Source + + + + + + + n-si Drain p + No current Positive charges Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 35
MOSFET Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.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/book.htm 37
Basic Circuits Bipolar PMOS NMOS CMOS BiCMOS Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 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) Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 39
Market of Semiconductor Products 100% 50% Bipolar MOSFET Compound } 8% 4% }} 88% 1980 1990 2000 Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 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/book.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/book.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/book.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/book.htm 44
CMOS Inverter V dd PMOS V in V out NMOS V ss Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 45
CMOS IC n + Source/Drain Gate Oxide p + Source/Drain Polysilicon p-si STI Balk Si n-si USG Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 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/book.htm 47
IC Chips Memory Microprocessor Application specific IC (ASIC) Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.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/book.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/book.htm 50
Basic DRAM Memory Cell NMOS Word line Capacitor Bit line V dd Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.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/book.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/book.htm 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 Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 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 Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 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 Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 56
IC Fabrication Processes Adding Ion implantation, Diffusion Grown thin film, SiO 2 Deposited thin film CV PVD Electrical Epi, Poly Dielectri Meta 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, Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 57 Developing
Basic Bipolar Process Steps Buried layer doping Epitaxial silicon growth Isolation and transistor doping Interconnection Passivation Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 58
Buried Layer Implantation SiO 2 P-silicon n + Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 59
Epitaxy Grow n + buried layer n-epi P-silicon Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 60
Isolation Implantation p + n + buried layer n-epi p + P-silicon Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.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/book.htm 62
SiO 2 Metal Etch Emitter Base Collector Al Cu Si p + n + p n+ n-epi p + n + buried layer P-silicon Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 63
SiO 2 Passivation Oxide Deposition Emitter Base Collector Al Cu Si p + n + p n+ p + n-epi n + buried layer P-silicon CVD oxide Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.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/book.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/book.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) (P) Test and packaging Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 67
Wafer clean, field oxidation, and photoresist coating Native Oxide N-Silicon N-Silicon Field Oxide Primer Field Oxide Photoresist N-Silicon N-Silicon Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 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 Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 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 Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 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 + Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 71
Illustration of a PMOS Gate Oxide CVD Cap Oxide p + N-Silicon p + Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.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/book.htm 73
Self-aligned S/D Implantation Phosphorus Ions, P + Polysilicon Field oxide Gate n + n + p-silicon Source/Drain Gate oxide Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.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/book.htm 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 Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 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 + n + p-si Nitride Dep. Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 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/book.htm 78
Advantages of CMOS Low power consumption High temperature stability High noise immunity Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 79
CMOS Inverter, Its Logic Symbol and Logic Table V dd V in V out PMOS V in V out NMOS In Out V ss 0 1 1 0 Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 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 Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm 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 Metal 3 Metal 2 M 1 Copper Cu Copper Cu FSG FSG FSG FSG FSG Nitride etch stop layer Nitride seal layer Tantalum barrier layer T/TiN barrier & Tungsten local PSG Tungsten adhesion layer Interconnection STI n + n + USG p + p + P-well PMD nitride Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.htm N-well 82 P-epi barrier layer P-wafer
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/book.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/book.htm 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 Hong Xiao, Ph. D. www2.austin.cc.tx.us/hongxiao/book.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/book.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/book.htm 87