Chapter 3 Basics Semiconductor Devices and Processing

Name: Chapter 3 Basics Semiconductor Devices and Processing
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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/Book.htm

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

Topics What is semiconductor Basic semiconductor devices
Basics of IC processing Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

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

Periodic Table of the Elements
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

Semiconductor Substrate and Dopants
P-type Dopant N-type Dopants Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

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/Book.htm

Band Gap and Resistivity
Eg = 1.1 eV Eg = 8 eV Aluminum 2.7 mWcm Sodium 4.7 mWcm Silicon ~ 1010 mWcm Silicon dioxide > 1020 mWcm Conductors Semiconductor Insulator Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

Crystal Structure of Single Crystal Silicon

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

N-type (Arsenic) Doped Silicon and Its Donor Energy Band
As Extra Conducting band, Ec Ed ~ 0.05 eV Electron Eg = 1.1 eV Valence band, Ev Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

P-type (Boron) Doped Silicon and Its Donor Energy Band
Valence band, Ev Eg = 1.1 eV Conducting band, Ec Ea ~ 0.05 eV Electron - Si B Hole Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

Illustration of Hole Movement
Valence band, Ev Eg = 1.1 eV Conducting band, Ec Ea ~ 0.05 eV Electron Hole Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

Dopant Concentration and Resistivity
P-type, Boron N-type, Phosphorus Dopant concentration Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

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

Basic Devices Resistor Capacitor Diode Bipolar Transistor
MOS Transistor Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

Resistor r: Resistivity r h l w Hong Xiao, Ph. D.
www2.austin.cc.tx.us/HongXiao/Book.htm

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

Capacitors k: Dielectric Constant l h d k Hong Xiao, Ph. D.

Capacitors Charge storage device Memory Devices, esp. DRAM
Challenge: reduce capacitor size while keeping the capacitance High-k dielectric materials Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

Capacitors Parallel plate Stacked Deep Trench Dielectric Layer
Poly 2 Poly Si Si Poly Si Oxide Si Poly 1 Heavily Doped Si Parallel plate Stacked Deep Trench Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

Metal Interconnection and RC Delay
Dielectric, k Metal, r I l d w Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

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

Diode Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

Figure 3.14 P N V V V Transition region n p - - + + - - + + - - + + -
V p Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

Intrinsic Potential For silicon V0 ~ 0.7 V Hong Xiao, Ph. D.

I-V Curve of Diode Hong Xiao, Ph. D.

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

NPN and PNP Transistors
E B C P N E B C Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

NPN Bipolar Transistor
Emitter Base Collector Al•Cu•Si SiO2 n + p n + p+ p+ n-epi Electron flow n+ buried layer P-substrate Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

Sidewall Base Contact NPN Bipolar Transistor
Metal CVD oxide CVD oxide CVD oxide Base Emitter Collector Poly p p n+ Field oxide Field oxide Field oxide n Epi n+ n+ Buried Layer P-substrate Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

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

NMOS Device Basic Structure
G D VG “Metal” Gate VD Ground n + n + p-Si Source Drain Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

NMOS Device No current Positive charges V > V > 0 V = 0 V > 0
Electron flow G T G D D “Metal” Gate SiO SiO 2 2 n + n + n + n + p-Si p-Si Source Drain Source Drain No current Negative charges Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

PMOS Device Negative charges Hole flow V < V < 0 V = 0 V > 0
“Metal” Gate SiO SiO 2 2 p + p + p + p + n-Si n-Si Source Drain Source Drain No current Positive charges Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

MOSFET Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

MOSFET and Drinking Fountain
Source, drain, gate Source/drain biased Voltage on gate to turn-on Current flow between source and drain Drinking Fountain 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

Basic Circuits Bipolar PMOS NMOS CMOS BiCMOS Hong Xiao, Ph. D.

Devices with Different Substrates
Bipolar MOSFET BiCMOS Dominate IC industry Silicon Germanium Bipolar: high speed devices GaAs: up to 20 GHz device Light emission diode (LED) Compound Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

Market of Semiconductor Products
MOSFET 100% 50% 1980 1990 2000 Compound Bipolar 88% 8% 4% Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

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

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

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

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

CMOS Inverter Vdd PMOS V in Vout NMOS Vss Hong Xiao, Ph. D.

CMOS IC n+ Source/Drain p+ Source/Drain Gate Oxide Polysilicon STI
p-Si n-Si USG Balk Si Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

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

IC Chips Memory Microprocessor Application specific IC (ASIC)

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

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

Basic DRAM Memory Cell Word line NMOS Capacitor Vdd Bit line

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

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

EPROM Gate Oxide n n p-Si Source Drain V V Passivation Dielectric
Inter-poly Dielectric Poly 2 Control Gate Poly 1 Floating Gate Gate Oxide n + n + p-Si Source Drain Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

EPROM Programming Gate Oxide n n p-Si Source Drain VG>VT>0
Passivation Dielectric VG>VT>0 VD > 0 Inter-poly Dielectric Poly 2 Control Gate e- e- e- e- e- e- Floating Gate Gate Oxide e- n + n + p-Si Source Drain Electron Tunneling Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

EPROM Programming Gate Oxide n n p-Si Source Drain VG>VT>0
Passivation Dielectric UV light VG>VT>0 VD > 0 Inter-poly Dielectric Poly 2 Control Gate e- e- Floating Gate Gate Oxide n + n + p-Si Source Drain Electron Tunneling Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

IC Fabrication Processes

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

Buried Layer Implantation
SiO 2 n + P-silicon Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

Epitaxy Grow n-epi n buried layer P-silicon + Hong Xiao, Ph. D.

Isolation Implantation
+ p + n-epi n + buried layer P-silicon Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

Emitter/Collector and Base Implantation
+ n + p + p + n-epi n + buried layer P-silicon Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

Metal Etch SiO Emitter Base Collector Al•Cu•Si p n n p p n-epi n
2 Emitter Base Collector Al•Cu•Si p n + n + p + p + n-epi n + buried layer P-silicon Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

Passivation Oxide Deposition
SiO Al•Cu•Si Emitter Base Collector 2 CVD oxide p n + n + p + p + n-epi n + buried layer P-silicon Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

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

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

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) 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

Wafer clean, field oxidation, and photoresist coating

Photolithography and etch

Source/drain doping and gate oxidation

Contact, Metallization, and Passivation
N-Silicon p + Gate Oxide Field Oxide Al∙Si Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

Illustration of a PMOS Gate Oxide CVD Cap Oxide N-Silicon p+ p+

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

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

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 Al deposition Grow gate oxide Mask 4. Metal Deposit polysilicon Etch Aluminum Mask 2. Gate Strip photo resist Etch polysilicon Metal anneal CVD oxide S/D and poly dope implant Mask 5. Bonding pad Anneal and poly reoxidation CVD USG/PSG Test and packaging Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

NMOS Process Sequence Field Oxidation Clean Oxide Etch Gate Oxidation
p-Si p-Si Oxide Etch Gate Oxidation p-Si p-Si Poly Dep. poly poly Poly Etch p-Si p-Si P+ Ion Implant poly poly Annealing n+ n+ p-Si p-Si Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

NMOS Process Sequence PSG PSG PSG Reflow PSG Dep. Al·Si PSG Etch PSG
poly poly p-Si p-Si Al·Si PSG Etch PSG PSG Metal Dep. poly poly p-Si p-Si Al·Si Al·Si SiN Nitride Dep. Metal Etch PSG PSG poly poly n+ n+ p-Si p-Si Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

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

Advantages of CMOS Low power consumption High temperature stability
High noise immunity Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

CMOS Inverter, Its Logic Symbol and Logic Table
dd V V in out PMOS V V in out NMOS In Out 0 1 1 0 V ss Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

CMOS Chip with 2 Metal Layers
Nitride PD2 PD1 Oxide Metal 2, Al·Cu·Si IMD USG dep/etch/dep Al·Cu·Si PMD BPSG LOCOS SiO2 n+ n+ p+ p+ p+ p+ N-well Poly Si Gate P-type substrate Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm

CMOS Chip with 4 Metal Layers
Lead-tin alloy bump Passivation 2, nitride Passivation 1, USG Metal 4 Copper Tantalum barrier layer FSG Metal 3 Copper FSG Nitride etch stop layer FSG Nitride seal layer Metal 2 Copper FSG Tungsten plug Tantalum barrier layer M 1 Cu Cu FSG FSG T/TiN barrier & adhesion layer Tungsten local Interconnection PSG Tungsten STI n + n + USG p + p + PMD nitride barrier layer P-well Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm N-well P-epi 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

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

Summary R=r l/A C=k 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

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

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

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