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Chapter 3 Basics Semiconductor Devices and Processing

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Title: Chapter 3 Basics Semiconductor Devices and Processing


1
Chapter 3 Basics Semiconductor Devices and
Processing
  • Hong Xiao, Ph. D.
  • www2.austin.cc.tx.us/HongXiao/Book.htm

2
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

3
Topics
  • What is semiconductor
  • Basic semiconductor devices
  • Basics of IC processing

4
What is Semiconductor
  • Conductivity between conductor and insulator
  • Conductivity can be controlled by dopant
  • Silicon and germanium
  • Compound semiconductors
  • SiGe, SiC
  • GaAs, InP, etc.

5
Periodic Table of the Elements
6
Semiconductor Substrate and Dopants
Substrate
P-type Dopant
N-type Dopants
7
Orbital and Energy Band Structure of an Atom
Valence shells
Conducting band, Ec
Nuclei
Band gap, Eg
Valence band, Ev
8
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 gt 1020 mW?cm
Conductors
Semiconductor
Insulator
9
Crystal Structure of Single Crystal Silicon
10
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

11
N-type (Arsenic) Doped Silicon and Its Donor
Energy Band
Conducting band, Ec
Ed 0.05 eV
Electron
Eg 1.1 eV
Valence band, Ev
12
P-type (Boron) Doped Silicon and Its Donor
Energy Band
13
Illustration of Hole Movement
14
Dopant Concentration and Resistivity
Resistivity
P-type, Boron
N-type,
Phosphorus
Dopant concentration
15
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

16
Basic Devices
  • Resistor
  • Capacitor
  • Diode
  • Bipolar Transistor
  • MOS Transistor

17
Resistor
r
h
l
w
r Resistivity
18
Resistor
  • Resistors are made by doped silicon or
    polysilicon on an IC chip
  • Resistance is determined by length, line width,
    height, and dopant concentration

19
Capacitors
k
l
h
d
k Dielectric Constant
20
Capacitors
  • Charge storage device
  • Memory Devices, esp. DRAM
  • Challenge reduce capacitor size while keeping
    the capacitance
  • High-k dielectric materials

21
Capacitors
Dielectric Layer
Dielectric Layer
Poly 2
Poly Si
Si
Poly Si
Oxide
Si
Poly 1
Heavily Doped Si
Parallel plate
Stacked
Deep Trench
22
Metal Interconnection and RC Delay
k
Dielectric,
r
Metal,
I
l
d
w
23
Diode
  • P-N Junction
  • Allows electric current go through only when it
    is positively biased.

24
Diode
25
Figure 3.14
Transition region

-
-

-
-
P
N

-
-

-
-

-
-
V
n
V
0
V
p
26
Intrinsic Potential
  • For silicon V0 0.7 V

27
I-V Curve of Diode
28
Bipolar Transistor
  • PNP or NPN
  • Switch
  • Amplifier
  • Analog circuit
  • Fast, high power device

29
NPN and PNP Transistors
30
NPN Bipolar Transistor
Base
Collector
Emitter
AlCuSi
SiO2
n
n


p
p
p
n-epi
Electron flow
n buried layer
P-substrate
31
Sidewall Base Contact NPN Bipolar Transistor
Metal
CVD oxide
CVD oxide
CVD oxide
Base
Emitter
Collector
p
p
Poly
n
Field oxide
Field oxide
Field oxide
n Epi
n
n Buried Layer
P-substrate
32
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

33
NMOS Device Basic Structure
V
V
G
D
VG
Metal Gate
VD
Ground


n
n
p-Si
Drain
Source
34
NMOS Device
Positive charges
V
gt V
gt 0
V
gt 0
V
0
Electron flow
G
T
V
D
G
D
Metal Gate

SiO
SiO
- - - - - - -
2
2

n



n
n
n
p-Si
p-Si
Source
Drain
Drain
Source
No current
Negative charges
35
PMOS Device
Negative charges
V
lt V
lt 0
Hole flow
V
gt 0
V
0
G
T
V
D
G
D
Metal Gate
- - - - - - -
SiO
SiO
2

2

p



p
p
p
n-Si
n-Si
Source
Drain
Drain
Source
No current
Positive charges
36
MOSFET
37
MOSFET and Drinking Fountain
  • MOSFET
  • 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

38
Basic Circuits
  • Bipolar
  • PMOS
  • NMOS
  • CMOS
  • BiCMOS

39
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
40
Market of Semiconductor Products
41
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.

42
PMOS
  • First MOS field effect transistor, 1960
  • Used for digital logic devices in the 1960s
  • Replaced by NMOS after the mid-1970s

43
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

44
CMOS
  • Most commonly used circuit in IC chip since 1980s
  • Low power consumption
  • High temperature stability
  • High noise immunity
  • Symmetric design

45
CMOS Inverter
Vdd
PMOS
V in
Vout
NMOS
Vss
46
CMOS IC
n Source/Drain
p Source/Drain
Gate Oxide
Polysilicon
STI
USG
p-Si
n-Si
Balk Si
47
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

48
IC Chips
  • Memory
  • Microprocessor
  • Application specific IC (ASIC)

49
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

50
DRAM
  • Major component of computer and other electronic
    instruments for data storage
  • Main driving force of IC processing development
  • One transistor, one capacitor

51
Basic DRAM Memory Cell
NMOS
Capacitor
52
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

53
EPROM
  • Non-volatile memory
  • Keeping data ever without power supply
  • Computer bios memory which keeps boot up
    instructions
  • Floating gate
  • UV light memory erase

54
EPROM
Passivation Dielectric
V
V
G
D
Inter-poly Dielectric
Poly 2
Control Gate
Poly 1
Floating Gate
Gate Oxide


n
n
p-Si
Drain
Source
55
EPROM Programming
Passivation Dielectric
VGgtVTgt0
VD gt 0
Inter-poly Dielectric
Poly 2
Control Gate
e- e- e- e- e- e-
Floating Gate
Gate Oxide
e-


n
n
p-Si
Electron Tunneling
Drain
Source
56
EPROM Programming
Passivation Dielectric
UV light
VGgtVTgt0
VD gt 0
Inter-poly Dielectric
Poly 2
Control Gate
Floating Gate
e- e-
Gate Oxide


n
n
p-Si
Electron Tunneling
Drain
Source
57
IC Fabrication Processes
58
Basic Bipolar Process Steps
  • Buried layer doping
  • Epitaxial silicon growth
  • Isolation and transistor doping
  • Interconnection
  • Passivation

59
Buried Layer Implantation
SiO
2
n

P-silicon
60
Epitaxy Grow
n-epi
n
buried layer

P-silicon
61
Isolation Implantation
p
p


n-epi
n

buried layer
P-silicon
62
Emitter/Collector and Base Implantation
p
n

n

p

p

n-epi
n
buried layer

P-silicon
63
Metal Etch
SiO
2
Emitter
Base
Collector
AlCuSi
p

n

n
p

p

n-epi
n

buried layer
P-silicon
64
Passivation Oxide Deposition
SiO
AlCuSi
Emitter
Base
Collector
2
CVD
oxide
p

n

n
p

p

n-epi
n

buried layer
P-silicon
65
MOSFET
  • Good for digital electronics
  • Major driving forces
  • Watches
  • Calculators
  • PC
  • Internet
  • Telecommunication

66
1960s PMOS Process
  • Bipolar dominated
  • First MOSFET made in Bell Labs
  • Silicon substrate
  • Diffusion for doping
  • Boron diffuses faster in silicon
  • PMOS

67
PMOS Process Sequence (1960s)
68
Wafer clean, field oxidation, and photoresist
coating
69
Photolithography and etch
70
Source/drain doping and gate oxidation
71
Contact, Metallization, and Passivation
72
Illustration of a PMOS
Gate Oxide
CVD Cap Oxide
p
p
N-Silicon
73
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

74
Self-aligned S/D Implantation
Phosphorus Ions, P
Polysilicon
Field oxide
Gate
n
n
p-silicon
Source/Drain
Gate oxide
75
NMOS Process Sequence (1970s)
76
NMOS Process Sequence
Field Oxidation
Clean
p-Si
p-Si
Oxide Etch
Gate Oxidation
p-Si
p-Si
poly
poly
Poly Dep.
Poly Etch
p-Si
p-Si
P Ion Implant
poly
poly
Annealing
n
n
p-Si
p-Si
77
NMOS Process Sequence
PSG
PSG Reflow
PSG
PSG Dep.
poly
poly
p-Si
p-Si
AlSi
Metal Dep.
PSG
PSG
PSG Etch
poly
poly
p-Si
p-Si
AlSi
AlSi
SiN
Nitride Dep.
Metal Etch
PSG
PSG
poly
poly
n
n
p-Si
p-Si
78
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

79
Advantages of CMOS
  • Low power consumption
  • High temperature stability
  • High noise immunity

80
CMOS Inverter, Its Logic Symbol and Logic Table
V
dd
V
V
in
out
PMOS

V

V

in
out
NMOS

In Out 0 1 1 0
V
ss
81
CMOS Chip with 2 Metal Layers
Nitride
PD2
Oxide
PD1
Metal 2, AlCuSi
USG dep/etch/dep
IMD
AlCuSi
BPSG
PMD
SiO2
LOCOS
p
p
n
n
p
p
N-well
Poly Si Gate
P-type substrate
82
CMOS Chip with 4 Metal Layers
Lead-tin alloy bump
Passivation 2, nitride
Passivation 1, USG
Metal 4
Copper
Tantalum barrier layer
FSG
FSG
Copper
Metal 3
Nitride etch stop layer
FSG
Nitride seal layer
Copper
Metal 2
FSG
Tungsten plug
Tantalum barrier layer
FSG
M 1
Cu
Cu
FSG
T/TiN barrier adhesion layer
Tungsten local Interconnection
Tungsten
PSG
n

p

p

n

STI
USG
PMD nitride barrier layer
P-well
N-well
P-epi
P-wafer
83
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

84
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

85
Summary
  • Rr l/A
  • Ck 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

86
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

87
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.
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