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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 mWcm
4.7 mWcm
~ 1010 mWcm
> 1020 mWcm
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
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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
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16
Resistor
r
h
l
w
l
Rr
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
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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
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k.htm
24
Figure 3.14
Transition region
P
------
++
++
++
++
++
N
Vn
Vp
Hong Xiao, Ph. D.
V0
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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
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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
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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
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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
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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
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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.
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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
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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
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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
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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
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58
Buried Layer Implantation
SiO2
P-silicon
Hong Xiao, Ph. D.
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n+
59
Epitaxy Grow
n-epi
n+ buried layer
P-silicon
Hong Xiao, Ph. D.
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60
Isolation Implantation
p+
n-epi
p+
n+ buried layer
P-silicon
Hong Xiao, Ph. D.
www2.austin.cc.tx.us/HongXiao/Boo
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61
Emitter/Collector and Base
Implantation
p+
n+
p
n+
n-epi
p+
n+ buried layer
P-silicon
Hong Xiao, Ph. D.
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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.
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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.
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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
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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.
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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
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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
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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
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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
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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.
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71
Illustration of a PMOS
Gate Oxide
CVD Cap Oxide
p+
p+
N-Silicon
Hong Xiao, Ph. D.
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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.
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73
Self-aligned S/D Implantation
Phosphorus Ions, P+
Gate
n+
p-silicon
Source/Drain
Hong Xiao, Ph. D.
n+
Gate oxide
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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.
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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+
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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+
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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
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Advantages of CMOS
• Low power consumption
• High temperature stability
• High noise immunity
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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
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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
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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
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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
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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
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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
•
•
•
•
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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
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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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