Ch3

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Chapter 3
Device Fabrication Technology
About 1020 transistors (or 10 billion for every person in the
world) are manufactured every year.
VLSI (Very Large Scale Integration)
ULSI (Ultra Large Scale Integration)
GSI (Giga-Scale Integration)
Variations of this versatile technology are used for flat-panel
displays, micro-electro-mechanical systems (MEMS), and
chips for DNA screening...
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-1
3.1 Introduction to Device Fabrication
Oxidation
Lithography &
Etching
Ion Implantation
Annealing &
Diffusion
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-2
3.2 Oxidation of Silicon
Quartz tube
Si Wafers
Flow
controller
O2 N2
H 2O or TCE(trichloroethylene)
Resistance-heated furnace
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-3
3.2 Oxidation of Silicon
Dry Oxidation :
Si + O2  SiO2
Wet Oxidation :
Si +2H2O  SiO2 + 2H2
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-4
3.2 Oxidation of Silicon
EXAMPLE : Two-step Oxidation
(a) How long does it take to grow 0.1m of dry oxide at 1000 oC ?
(b) After step (a), how long will it take to grow an additional
0.2m of oxide at 900 oC in a wet ambient ?
Solution:
(a) From the “1000oC dry” curve in Slide 3-3, it takes 2.5 hr to
grow 0.1m of oxide.
(b) Use the “900oC wet” curve only. It would have taken 0.7hr to
grow the 0.1 m oxide and 2.4hr to grow 0.3 m oxide from
bare silicon. The answer is 2.4hr–0.7hr = 1.7hr.
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-5
3.3 Lithography
(c) Development
(a) Resist Coating
Positive resist
Negative resist
Photoresist
Si
Oxide
(b) Exposure
Si
Deep Ultraviolet Light
Optical
Lens system
Si
(d) Etching and Resist Strip
Photomask with
opaque and
clear patterns
Si
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Si
Slide 3-6
3.3 Lithography
Photolithography Resolution Limit, R
• R kl due to optical diffraction
• Wavelength l needs to be minimized. (248 nm, 193 nm,
157 nm?)
• k (<1) can be reduced will
• Large aperture, high quality lens
• Small exposure field, step-and-repeat using “stepper”
• Optical proximity correction
• Phase-shift mask, etc.
• Lithography is difficult and expensive. There can be 40
lithography steps in an IC process.
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-7
3.3 Lithography
Wafers are being loaded into a stepper in a clean room.
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-8
3.3.1 Wet Lithography
Photo Mask
Water
Photoresist
Wafer
(a)
(b)
conventional dry lithography
wet or immersion lithography
l
l
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-9
Extreme UV Lithography (13nm wavelength)
Reflective “photomask”
Laser produced
plasma emitting
EUV
No suitable lens material at this
wavelength. Optics is based on mirrors
with nm flatness.
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-10
Beyond Optical Lithography
• Electron Beam Writing : Electron beam(s) scans and exposed
electron resist on wafer. Ready technology with relatively low
throughput.
• Electron Projection Lithography : Exposes a complex
pattern using mask and electron lens similar to
optical lithography.
• Nano-imprint : Patterns are etched into a durable material to
make a “stamp.” This stamp is pressed into a liquid film over
the wafer surface. Liquid is hardened with UV to create an
imprint of the fine patterns.
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-11
3.4 Pattern Transfer–Etching
Isotropic etching
Anisotropic etching
photoresist
SiO 2
photoresist
SiO 2
(1)
(1)
photoresist
SiO 2
photoresist
SiO 2
(2)
SiO 2
(2)
SiO 2
(3)
(3)
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
(a) Isotropic wet etching
(b) Anisotropic dry etching.
Slide 3-12
3.4 Pattern Transfer–Etching
Reactive-Ion Etching Systems
Gas Baffle
Wafers
Gas Inlet
RF
Vacuum
RF
Cross-section View
Top View
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-13
3.4 Pattern Transfer–Etching
Dry Etching (also known as Plasma Etching, or
Reactive-Ion Etching) is anisotropic.
• Silicon and its compounds can be etched by plasmas
containing F.
• Aluminum can be etched by Cl.
• Some concerns :
- Selectivity and End-Point Detection
- Plasma Process-Induced Damage or Wafer Charging
Damage and Antenna Effect
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-14
Scanning electron microscope view of a plasma-etched
0.16 m pattern in polycrystalline silicon film.
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-15
3.5 Doping
3.5.1 Ion Implantation
Dopant ions
•
•
•
•
The dominant doping method
Excellent control of dose (cm-2)
Good control of implant depth with energy (KeV to MeV)
Repairing crystal damage and dopant activation requires
annealing, which can cause dopant diffusion and loss of
depth control.
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-16
3.5.1 Ion Implantation
Schematic of an Ion Implanter
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-17
3.5.1 Ion implantation
Phosphorous density
profile after
implantation
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-18
3.5.1 Ion Implantation
Model of Implantation Doping Profile (Gaussian)
Ni
( x  R ) 2 / 2 R 2
N ( x) 
e
2  (R)
Ni : dose (cm-2)
R : range or depth
R : spread or sigma
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-19
Other Doping Methods
• Gas-Source Doping : For example, dope Si with P
using POCl3.
• Solid-Source Doping : Dopant diffuses from a doped
solid film (SiGe or oxide) into Si.
• In-Situ Doping : Dopant is introduced while a Si
film is being deposited.
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-20
3.6 Dopant Diffusion
Junction depth
SiO 2
n-type
diffusion layer
p-type Si
No
 x 2 / 4 Dt
N ( x, t ) 
e
  Dt
N : Nd or Na (cm-3)
No : dopant atoms per cm2
t : diffusion time
D : diffusivity, Dt is the approximate distance of
dopant diffusion
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-21
3.6 Dopant Diffusion
• D increases with
increasing temperature.
• Some applications need
very deep junctions (high
T, long t). Others need
very shallow junctions
(low T, short t).
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-22
3.6 Dopant Diffusion
Shallow Junction and Rapid Thermal Annealing
• After ion implantation, thermal annealing is required. Furnace
annealing takes minutes and causes too much diffusion of dopants
for some applications.
• In rapid thermal annealing (RTA), the wafer is heated to high
temperature in seconds by a bank of heat lamps.
•In flash annealing (100mS) and laser annealing (<1uS), dopant
ddiffusion is practically eliminated.
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-23
3.7 Thin-Film Deposition
Three Kinds of Solid
Crystalline
Example:
Silicon wafer
Polycrystalline
Thin film of Si or metal.
Amorphous
Thin film of
SiO2 or Si3N4.
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-24
3.7 Thin-Film Deposition
Examples of thin films in integrated circuits
• Advanced MOSFET gate dielectric
• Poly-Si film for transistor gates
• Metal layers for interconnects
• Dielectric between metal layers
• Encapsulation of IC
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-25
3.7.1 Sputtering
Schematic Illustration of Sputtering Process
Sputtering target
Ion (Ar +)
Atoms sputtered out of the target
YY
Y
Target material
deposited on wafer
YY
YYYYY
YY YYY
YYYYYYYY
YYYYYYYY YYYYYY
YYYY
YYYYYY
YYYYYYYY
YYYYYY YYYYYYYYYYYY
YYYYYYYY YYYYYY YYYYYYYYYYYY YY
Y
YYYY
YYYYY YYYYYYY YYYYYYY
YYYY
YY
YYYYYY
YYYYYYY
YY
YYYY YYYYYYYY
YYYYYYYYYYYY
YYY
YYYYYY
Y
YY
Si Wafer
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-26
3.7.2 Chemical Vapor Deposition (CVD)
Thin film is formed from gas phase components.
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-27
Some Chemical Reactions of CVD
Poly-Si : SiH4 (g)
Si (s) + 2H2 (g)
Si3N4 : 3SiH2Cl2 (g)+4NH3 (g)
Si3N4 (s)+6HCl(g)+6H2 (g)
SiO2 : SiH4 (g) + O2 (g)
SiO2 (s) + 2H2 (g)
or
SiH2Cl2 (g)+2N2O (g)
SiO2 (s)+2HCl (g)+2N2 (g)
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-28
3.7.2 Chemical Vapor Deposition (CVD)
Two types of CVD equipment:
• LPCVD (Low Pressure CVD) : Good uniformity.
Used for poly-Si, oxide, nitride.
• PECVD (Plasma Enhanced CVD) : Low temperature
process and high deposition rate. Used for oxide,
nitride, etc.
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-29
3.7.2 Chemical Vapor Deposition (CVD)
Pressure sensor
Resistance-heated furnace
Quartz tube
Trap
To exhaust
Si Wafers
Pump
Gas control
system
Source
gases
LPCVD Systems
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-30
3.7.2 Chemical Vapor Deposition (CVD)
Cold Wall Parallel Plate
Gas Injection
Ring
Wafers
Pump
Heater Coil
Hot Wall Parallel Plate
Wafers
Pump
Gas
Inlet
Power leads
Plasma Electrodes
PECVD Systems
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-31
3.7.3 Epitaxy (Deposition of Single-Crystalline Film)
Epitaxy
Selective Epitaxy
SiO 2
Si Substrate
Epi film
Si Substrate
(a)
SiO2
Si Substrate
SiO 2
Epi film
SiO2
Si Substrate
(b)
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-32
3.8 Interconnect – The Back-end Process
AlAl-Cu
or Cu
SiO2
Dopant diffusion region
Si
(a)
Encapsulation
Metal 3
Dielectric
Metal 2
via or plug
Dielectric
Metal 1
Dielectric
silicide
CoSi2
diffusion region
Si
(b)
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-33
3.8 Interconnect – The Back-end Process
SEM: Multi-Level Interconnect (after removing the dielectric)
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-34
3.8 Interconnect – The Back-end Process
Copper Interconnect
• Al interconnect is prone to voids formation by
electromigration.
• Cu has excellent electromigration reliability
and 40% lower resistance than Al.
• Because dry etching of copper is difficult (copper
etching products tend to be non-volatile), copper
patterns are defined by a damascene process.
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-35
3.8 Interconnect – The Back-end Process
Copper Damascene Process
dielectric
dielectric
(a)
•Chemical-Mechanical
Polishing (CMP)
removes unwanted
materials.
(b)
Cu
Cu
liner
liner
dielectric
dielectric
(c)
(d)
•Barrier liner prevents
Cu diffusion.
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-36
3.8 Interconnect – The Back-end Process
Planarization
• A flat surface is highly desirable for subsequent
lithography and etching.
• CMP (Chemical-Mechanical Polishing) is used
to planarize each layer of dielectric in the
interconnect system. Also used in the front-end
process.
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-37
3.9 Testing, Assembly, and Qualification
•
•
•
•
•
•
•
•
•
Wafer acceptance test
Die sorting
Wafer sawing or laser cutting
Packaging
Flip-chip solder bump technology
Multi-chip modules
Burn-in
Final test
Qualification
Modern Semiconductor Devices for Integrated Circuits (C. Hu)
Slide 3-38
P-Si
(3)
SiO2
(11)
SiO2
3.10 Chapter Summary–A Device Fabrication
Example
P-Si
Arsenic implantation
Wafer
(0)
(4)
P-Si
(8)SiO
2
SiO
Ion
Implantation
Al
SiO2 S iO
2
S iO2
P-Si N
+
P
SiO2
(1)
P-Si
Oxidation
(5)
SiO2
UV
+
NSi
(9)
P
(6)
Annealing &
Al
SiO Diffusion
P2
SiO
PSi
3 N4
Al
UV
SiO2 UV
SiO2
M as k
SiO2
P-Si
(7)
(11)Res is t
+ 3 N4
NSi
SiO2
P
SiO2
Al
P
SiO 2
N+
(4)
SiO2
P
SiO2
P-Si
Modern Semiconductor
Devices for Integrated Circuits (C. Hu)
(12)
SiO2
SiO2
Al
SiO2
Si3 N4
Si
(13)
Photoresist
Al
Arsenic implantation
SiO 2
Al
Sputtering
N+
P-Si
SiO2
SiO
2
N+
Positive resist
SiO2
Etching
N4
Al
SiO2
(10)
(3)
3
N+
M ask
Lithography
(12)
SiO2
SiO2
UV
(2)
SiO 2
Lithography
Slide 3-39
Al
Photoresist
SiO2
SiO2
(3)
P-Si
Si3 N4
Al
Arsenic implantation
(4)
3.10 Chapter Summary–A Device Fabrication Example
N+
Metal (8)
etching SiO
(5)
P
SiO2
SiO2
Al
P-Si
S iO2
S iO2
Si3 N4
(12)
N+
SiO2
Al
P
2
N+
SiO 2
SiO2
P
(9)
N
P
Al
Al SiO2
SiO2
SiO2
Au
N SiO2
+
N+
P
P
(10)
Lithography
and etching
Back side
metallization
+
Si3 N4
CVD
nitride
(6)
deposition
wire
Si3 N4
UV
Al
SiO2
SiO2
+
M as kN
Si3N 4
(13)
Al
SiO2
SiO2
+
N
P
(7)
SiO 2
SiO2
P
Res is t
(11) Al
Photoresist
SiO2
SiO2
Al
Au
Plastic package
SiN3 N4
+
Back Side
milling
P
Al
SiO2
SiO 2
metal leads
N+
Dicing, wire bonding,
and packaging
P
(12)
Si3 N4
Modern Semiconductor
Al Devices for Integrated Circuits (C. Hu)
SiO2
SiO 2
N+
Slide 3-40
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