Wafer Manufacturing
Reading Assignments:
Plummer, Chap 3.1~3.4
1
Periodic Table
of the Elements
Roman letters
give valence
2
Why Silicon?
First transistor, Shockley, Bardeen, Brattain1947
First IC, T.I. Jack Kilby 1958
Made by Germanium
• Abundant, cheap
– 26% of earth’s crust vs. 1.8ppm of Germanium
• Stable dielectric for gate dielectric and doping mask
– Silicon dioxide is very stable, strong dielectric, and it is easy to
grow in thermal process. Perfect interface with Silicon.
Germanium oxide is not stable at >800oC and water soluble.
• Large band gap
– Wide operation temperature and doping range. Higher breakdown
voltage.
3
Structure of Solids
• Amorphous (no short/long-range order)
• Poly-crystalline (short-range order, typically, 1nm - 50μm
range)
• Crystalline (long-range order, irregularity treated as defects,
faults, stacked faults or line faults)
Grain/domain
Amorphous
Grain
boundary
Polycrystalline
Crystal
4
Crystal Structures (Crystallography)
Example: Simple Cubic (SC)
a1 a
3
a1 a2 a3 : primitive translation vectors
a2
Basis (one atom)
Crystal Structure
Space lattice (SC)
There are total 14 distinct Bravais lattices.
Example: Face-Centered Cubic (FCC)
ClNa+
NaCl
5
Simple Cubic Crystal
6
Diamond Crystal Structure
7
Diamond Crystal Structure
Diamond lattice cell
(C, Si, Ge, etc.)
Zincblende lattice cell
(GaAs, AlAs, GaP, ZnS, etc.)
•Diamond (covalent, Si, Ge, C, etc.) and Zinc-blend (ionic,
GaAs, InP, etc.) consist of 2 interleaved FCC with 1/4 diagonal
offset
8
Crystal Planes/Direction
Direction [001]
Indexing Procedure for Planes
z
1. Record where the plane
intercepts the axes in the unit of
the unit cell length. → 3, 2, 2
2
1
1
2
1
2
Plane (233)
3
x
Direction [100]
y 2. Invert the intercept values
→ 1/3, ½, ½
3. Convert to the smallest possible
set of whole numbers → 2, 3, 3
4. Enclose the whole-number set in
parentheses→ (233)
9
Crystal Planes
face atom in FCC
corner atom in FCC
(100)
(110)
(200)
(100)
1
(111)
10
Use of Crystal Orientation
• Electrical anisotropy: (100) has the
highest mobility, also scattering rates
are slightly different
• Mechanical anisotropy: different
surface has different modulus
components (later)
• Chemical anisotropy: e.x., KOH will
not etch (111), Etch rate
(110)>(100)>(111). 54.74° V-groove
etch on (100) plane.
• Angles: (100) vs. (110): 45°, 90 ° ;
(100) vs. (111): 54.74°; (110) vs.
(111): 35.26°, 90 ° or 144.74°
• Important for strain engineering
<110>
Newer wafers (8” and 12”) now
use a groove to mark the
orientation, instead of a cut to
save some area for test structures
11
Wafer Flat
Wafer flats in 4” and 6” silicon wafers
Newer wafers (8” and 12”) now use a groove to mark the orientation,
instead of a cut to save some area for test structures.
Manufacturers Data of incoming wafers:
orientation, thickness, doping type, resistivity.
12
Defects in Crystals
2D
0D
0D
1D
3D
13
Commercial Silicon Wafer
300mm
200mm
150mm
100mm
150mm
200mm
300mm
450mm 14
Starting Material Cost
300mm bare silicon ~ $100/pcs
200mm bare silicon ~ $40/pcs
200mm ASP ~ $1000/pcs
15
Silicon Purification
Heat (2000 ˚ C)
MGS:
SiO2
Sand
+
→
C
Carbon
Si
MGS
+
CO2
Carbon Dioxide
MGS: Metallurgical Grade Silicon, 98% pure, Major impurity Al and Fe
Hydrochloride
EGS:
Reactor,
300 °C
Si + HCl
Silicon
Powder
→ TCS
Condenser
Filters
TCS: Trichlorosilane SiHCl3
Pure TCS with
99.9999999%
Purifier
Heat (1100 ˚ C)
SiHCl3
TCS
+
H2
Hydrogen
→
Si
EGS
+
3HCl
Hydrochloride
EGS: Electronic Grade Silicon, 99.9999999% pure, impurity 1013-1014/cm3
16
Czochralski (CZ) Crystal Growth
Single Crystal Silicon Seed
Quartz Crucible
Molten Silicon
1415 ˚C
Single Crystal
silicon Ingot
Heating Coils
Graphite Crucible
Most common. Use in the large-diameter wafer manufacturing.
The ingot diameter is determined by the pulling speed.
The dislocation due to the thermal stress is terminated at the neck region.
Impurities comes from crucibles (O; 1017-1018/cm3) and susceptors 17
(C;
1015-1016/cm3)
Czochralski (CZ) Crystal Growth
Source: http://www.fullman.com/semiconductors/_crystalgrowing.html
18
Floating (FZ) Crystal Growth
Heating Coils
Movement
Poly Si
Rod
Molten Silicon
Heating
Coils
Single Crystal
Silicon
Seed Crystal
Local melting. No crucible needed.
Low impurity. High resistive wafer possible.
Hard to scale up.
19
Wafer Finishing
Flat, Notch
Wafer Sawing
Coolant
Orientation
Notch
Crystal Ingot
Ingot
Movement
Saw Blade
Flat, 150 mm and smaller
Diamond Coating
Notch, 200 mm and larger
Wafer Rounding
Wafer
Surface Flatten
Pressure
Wafer movement
Slurry
Wafer
Wafer Holder
Wafer Before Edge Rounding
Polishing Pad
Wafer After Edge Rounding
20
Surface Flatten
After Wafer Sawing
After Edge Rounding
After Lapping
Polish away 50μm (DS)
After Etch
76 μm
914 μm
76 μm
914 μm
12.5 μm
814 μm
<2.5 μm
750 μm
Etch-off 20-25μm (DS)
After CMP
Virtually Defect Free
725 μm
Polish away 25μm (SS)
21
Resistivity and Mobility
t
Four Point Probe
Sheet resistance :
Rs(Ω/□)= ρ(Ω．cm) / t (cm)
Jdrift = (qp μp +qn μn )F =σ
F
1
ρ=
qnμn + qpμ p
22