film deposition part v: pvd
TRANSCRIPT
Xing Sheng, EE@Tsinghua
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Xing Sheng盛兴
Department of Electronic EngineeringTsinghua University
Film DepositionPart V: PVD
Principles of Micro- and Nanofabrication for Electronic and Photonic Devices
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Film Deposition
PVD: Physical Vapor DepositionCVD: Chemical Vapor Deposition
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PVD vs. CVD
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Chemical reactionsPhysical process
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PVD
4Evaporation (蒸发) Sputter (溅射)
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Units 1 Pa = 1 N/m2
1 atm = 760 torr = 760 mm Hg = 1.013*105 Pa
1 bar = 105 Pa = 750 torr
1 torr = 133.3 Pa
Vacuum Basics
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Evaporation< 10-7 Pa
Sputter~ 10-1 Pa
Pressure cooker~ 1.5 atm
outer space< 10-10 Pa
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Vacuum Systems
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vacuum ~ 10-10 Pa
MBE: Molecular Beam Epitaxy
分子束外延
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Vacuum Pumpdown
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Vacuum Pumpdown
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1. air2. water3. hydrogen
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Evaporation
9Q: Why do we need the vacuum?
Thermal Electron Beam(Ebeam)
Pulsed Laser Deposition(PLD)
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Evaporation
10Q: Required pressure?
Reduce the impurities (N2, O2, H2O, ...)
Prevent oxidation e.g. Cu + O2 -> CuO
Ballistic transport
molecular mean free path
pr
kT22
(Pa)
5.0(cm)
pressure
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Mass Transport
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(Pa)
5.0(cm)
pressure
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Mass Transport
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absorption - movement - desorption
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Evaporation Rate
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esevap PT
mAR 21083.5
Langmuir-Knudsen Theory
Revap: Evaporation rate (g/s)
As: area of sources (cm2)
m: molecular weight (g/mol)
T: temperature (K)
Pe: vapor pressure of sources (Torr) (not chamber pressure)
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Evaporation Sources
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Deposition rate
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Question:1. RA : RB = ?2. RA : RC = ?
A
B
C
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Effects of Substrate Temperature
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Higher Temperature-> Larger Atom Mobility-> Larger Grain Size
Zone Model
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MBE: Molecular Beam Epitaxy
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Ultrahigh VacuumHigh Substrate TemperatureLattice Matched Substrate
High Quality, Single Crystal Films
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Step Coverage
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Substrate rotation and heating improve step coverage
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Challenges of Evaporation
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Materials with high melting points / low vapor pressure W, Mo, SiO2, ...
Compounds and alloys (non-stoichiometry) FeCoB alloy
TiO2 -> TiOx
Radiation damage generated by Ebeam electron beam and X-ray radiation
Poor step coverage via filling
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Plasma (e.g. Ar) assisted transport high energy
high deposition rate
Sputter (溅射)
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Copper in Ar
Plasma
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Deposit more than one material composition control
Co-Sputter
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Advantages Higher pressure than evaporation
Higher deposition rate
Better uniformity and step coverage
Better stoichiometry control
Work for most materials
Disadvantages Plasma induced damages (etching)
More impurities and defects
Not good for single crystal epitaxy
Mostly polycrystalline and amorphous films
Sputter: Pros. & Cons.
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Sputter
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Process Parameters Type: DC, RF/AC, Magnetron, ...
Substrate temperature
Gas type (Ar, O2, N2, ...)
Chamber pressure
Sputter power
...
Control Parameters Deposition rate
Crystallinity
Film quality (defects, ...)
...
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Sputter
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Thornton's Zone Model
25J. A. Thornton, Ann. Rev. Mater. Sci. 7, 239 (1977)
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Refined Zone Model
26A. Anders, Thin Solid Films 518, 4087 (2010)
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Evaporation vs. Sputter
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Evaporation:
- higher temperature- radiation (Ebeam)- lower pressure- poor step coverage
Sputter:
- lower temperature- plasma damage- higher pressure- better step coverage
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Step Coverage
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surface reaction ballistic transport
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Al has good adhesion on Si and SiO2
Al has high solubility in Si
Al + SiO2 = Al2O3 + Si
Film Adhesion
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Q: How about Cu and Au?
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Film Adhesion
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solubility of metals in Si
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Film Adhesion
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Ellingham diagram
Formation of metal oxides
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Metals like Ag and Au tend to have poor adhesion on Si and SiO2
Substrate clean
Deposit a thin (~5 nm) Ti or Cr layer for adhesion
Plasma treatment
Monolayer bonding
...
Film Adhesion
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Typical Ohmic Contacts for III-V
GaAs p-GaAs Be/Au, Ti/Pt/Au, ...
n-GaAs Ge/Ni/Au, Pd/Ge, ...
GaN p-GaN Ni/Au
n-GaN Ti/Al/Au
Schottky contact Ohmic contact
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Thin Film Patterning
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Avoid Step Coverage
PVD instead of CVD avoid high temperature
Photoresist (PR) process negative PR preferred (Why?)
increase PR thickness
multilayer PR
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Requirements for Liftoff
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Focused Ion Beam (FIB) Deposition
world's smallest toilet
Etch: GaDeposition: Pt
in SEM machine