Transcript film_deposition

Deposition and Etching of
Thin Films
Nathaniel J. C. Libatique, Ph.D.
[email protected]
Process Steps
• Start with polished wafers
of chosen r and crystal
orientation
• Films: epitaxial, thermal
oxides, polysilicon,
dielectrics, metals
• Doping: via diffusion or
ion implantation
• Lithography: shadow
masked or projection
• Etching: Wet and Dry
• Sequential Mask Transfer
• Stepper Iteration
Sze, Semiconductor Devices, John Wiley and Sons
Starting Materials
Quartzite + carbon sources (coal,
coke, wood chips)
• SiC(solid) + SiO2(solid) 
Si(solid) + SiO(gas) + CO (gas)
provides metallurgical grade
silicon (98%)
http://csmres.jmu.edu/geollab/Fichter/MetaRx/Rocks/quartzite1.html
• Pulverize silicon and treat with
HCl to produce trichlorosilane
Si(solid) + 3 HCl(gas) 300oC
SiHCl3(gas) + H2(gas)
trichlorosilane liquid at RT.
Fractional distillation.
Purified trichlorosilane in hydrogen reduction
reaction
SiHCl3(gas) + H2(gas)  Si(solid) + 3HCl(gas)
EGS (electronic grade Si) is produced. Ppb
impurities. Poly.
(Elemental Ga and As are the starting materials
for GaAs poly)
Single Crystal
• Furnace
• Pulling Mechanism
• Ambient Control
rotation mechanism, heating
elements and power supply,
seed holder, rotation
mechanism, Ar gas, gas flow,
exhaust, temperature, Si
diameter, pull rate, rotation
speed
Sze, Semiconductor Devices, John Wiley and Sons
Wafer Flats
Sze, Semiconductor Devices, John Wiley and Sons
• Grind to fixed diameter, edge
grind for auto placement algos
• Secondary flats reveal
conductivity and type
• Slice determines orientation,
thickness, taper (t variation), bow
(center to edge)
• Lap with Al2O3 and glycerine,
flatness within 2 mm
• Etch and polish
Deposition Techniques
• Thermal Oxidation
• Evaporation: Thermal & E-Beam
• Sputtering
• Vapor Phase Epitaxy
• Molecular Beam Epitaxy
Thermal Oxidation
Many films deposited
on semiconductors
Native film advantages:
- no deposition reqrd
- relatively pure
- excellent interface
-device passivation
www.cnfusers.cornell.edu/
Silicon Dioxide
• Native Oxide = 15 to 20 Ang.
• Si + O2  SiO2 ; dry
• Si + 2H2O  SiO2 +2 H2 ; wet
• Wet Oxidation: H2 rapid diff ’n
Oxide layer 2.7 times thickness
of consumed silicon
SiO2 on Si under fluorescent lighting
Thermal Evaporation
• Electron Beam Evaporation gun
• A System Controller , Power Supply
• Crucibles for the evaporation material,
Materials for Evaporation
• Material to be coated
PLUS
Substrates < 100 C
High deposition rate
Simple procedure
MINUS
Metals
Poor layer adhesion
Uneven or structured surfaces
100 meV Energies
Sputtering
1 to 20 eV Energies  better adhesion
Oxides, ceramics, alloys, semiconductors, glasses
CVD
AX(gas) + BY(gas)  AB(solid) + XY(gas)
SiH4 + 2 N2O  SiO2 + 2N2 + 2H2
SiH4 + NH3  SixNyHz + H2
LPCVD
Molecular Beam Epitaxy
• Separate effusion
chambers (pyrolitic
boron nitride)
• Ultra-high vacuum
• Arsenic overpressure
• E-Gun for Si
5 to 30 cm
Sze, Semiconductor Devices, John Wiley and Sons
Mean Free Path
S = Collision cross section = p r2
c
n particles in a volume V
d
V
S
c dt
One collision only if
(n/V) pr2 c dt  l = [(n/V) pr2 ]-1
Ultra-High Vacuum Required
l = [(n/V) pr2 20.5 ]-1
l = kT/ (20.5 pr2 P) P = nkT/V
At room temperature
lcm = 5 x 10-3 / P (in Torr)
Typical value, 3 Angstrom for d, so about 0.5 x 103 cm for partial
pressures of 10-5 Torr
Nucleation and Growth
• Incident Flux
• Surface diffusion until attachment
• Desorption, higher probability for crystals adsorbed
on a low binding energy site
• Binding energies also function of surface, <111>
slower growth rate than <100> in silicon
• Surface preparation is key
Binding Energies
1, 2, 3, 4
4, 6, 6, 8
Ghandi, VLSI Fabrication Principles, 2nd Ed., John Wiley and Sons
f1  nearest neighbor, f2  second nearest neighbors
Special Considerations
• Off axis growth, say 2 to 4 degrees, series of steps
and kinks are introduced
• Elevated substrate temperatures increase surface
diffusion (Ea,surface is 25% to 50% Ea,bulk)
• Si (111) planes more easily stacked than (100), plane
to plane distance is 57% of (111), better morphology
• GaAs special.
GaAs
Successive
Layers two
dangling bonds,
alernate Ga and
As deposition
Double layers
separated by a
wide spacing.
One dangling
bond. yy’
signinficantly
different from
xx’. Significant
surface energy
change.
Ghandi, VLSI Fabrication Principles, 2nd Ed., John Wiley and Sons
Surface Morphology
Comparison
[Gerlach and Dotzel]
Etching
Comparison
Etch rates in nm/min for ion beam etch (Ar+, 0.5 keV, ion flow at 1 mA/cm)
• Si  20 .. 40
• SiO2  30 .. 40
• Au  140
• Al  30
• PR AZ 1350  20 .. 30
Wet Etch: Anisotropic
References

G. Gerlach and W. Dotzel, “Introduction to
Microsystem Technology, A Guide for
Students”, Wiley, ISBN 978-0-470-05861-9