Thin Film Deposition

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Thin Film Deposition

• Michael Oye
• NASA-ASL / MACS

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Overview

• Overview of thin film stack
• Fundamentals of sputtering
• Glow discharge, atoms in a gas state
• Techniques of sputtering
• DC and RF sputtering
• DC Magnetron and reactive sputtering
• Glow discharge
• CVD and PVD processes
• MO-CVD, PE-CVD, plasma deposition

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Thin Film Applications

• Window coatings
• Magnetic media
• Semiconductors
• Biomedical devices
• Optics
• Automotive
• Aerospace

http//spectrum.ieee.org/energy/renewables/first-s
olar-quest-for-the-1-watt/0
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Thin Film Stack

• A thin film Bragg reflector consists of a
  multilayer-stack of alternate high- and low-index
  films, all one quarter wavelength thick (see
  figure left). The geometrical thicknesses of the
  high- und low-index films are tHl/(4nH) and
  tLl/(4nL) respectively.nH and nL are the
  indices of refraction of the high- and low-index
  films, respectively and l is the center
  wavelength of the Bragg mirror.  On every
  interface in the stack a part of the incident
  beam is reflected. The reflected parts have a
  phase shift of 180 only if the incident light
  goes from low-index medium in a high-index
  medium. The relative phase difference of all
  reflected beams is zero or a multiple of 360 and
  therefore they interfere constructively. The
  intensity of the incident light beam decreases
  during his travel trough the quarter-wave stack
  and at the same time the reflected light
  increases, if the absorbance A of the stack is
  negligible.

http//www.batop.de/information/r_Bragg.html
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Thin film stack SEM image (FIB cross section

• SEM cross-section image of a thin film stack.
  Each layer is approximately 500 Angstroms thick.
  - http//www.jwdstaging6.com/gallery

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Southwall XIR Film
AES depth profile of Southwall Technologies XIR
Film data from Nanolab Technologies
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Deposition Processes

• Evaporation (thermal)
• Sputter deposition (DC, RF, PE)
• CVD, MO-CVD, PE-CVD
• Plasma polymerized films
• Solution cast (spin coating) films
• Self Assembled Monolayers (SAMs)

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Thin Film Deposition

• Need other types of films besides SiO2
• insulating films for GaAs III-V
• Oxide no good
• Need SiO2 or Si3N4
• Need metal lines for electrical connects
• Need insulation for these metal wires
• Need to deposit other types of semiconductors
• Heterostructures, epitaxy
• 2 categories
• Physical vapor deposition
• Chemical vapor deposition

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Physical Vapor Deposition (PVD)

• Evaporation (8.1.1)
• Simplest case
• Evaporate metal to gaseous state (?T) by heat or
  e- beam
• Evaporated atoms float up to the wafer and
  condense on cool wafer surface
• Equivalent to boiling water and condensation onto
  pot lid
• Solid usually polycrystalline
• Characteristics (film)
• Usually low melting point materials ? metals
• Must be pure (different Tm / vapor pressure)
• Can have multiple sources but difficult to
  control relative fluxes

wafer
evacuated chamber
localized heating - less contamination from
crucible
e- beam
metal source
heat
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Evaporation

• Vacuum Requirements
• Purity of film
• Must have large mean free path (distance before
  collision with some molecule)

T usually room temp. of chamber (?) P
pressure of chamber D diameter of vapor atom
2-5 Å
for P 10-4 Pa ? ? order of meters
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Evaporation

• What is growth rate?
• 3 steps
• Jv flux leaving evaporation source
• J? flux arriving at substrate
• Jinc flux incorporated into the film
• Evaporation rate
• (flux of atoms to the surface)
• Assume ? is greater than distance from source to
  substrate
• J arriving at substrate can be very different
  depending on geometry (amount of source exposed
  to surface)
• for spherical source all flux is the same (like
  coil)
• for crucible source
• perpendicular area of source is pr2
• at an angle, get ellipse
• J?0 Jv
• J? Jvcos?
• If substrate is significantly cooler than the
  source (temp) every atom that hits the surface
  will most likely condense.

Pv vapor pressure of dep. atom (function of
Tv) m mass of atom
Sticking coefficient If S1 ? thickness
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Atoms into Gas State

• At target
• target atoms ejected
• target ions ejected (1 - 2 )
• Electrons emitted
• helps keep plasma going
• Ar ions reflected as Ar neutrals
• Ar buried in target
• Photons emitted

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Vacuum Deposition

• Physical Vapor Deposition

• Chemical Vapor Deposition

• Evaporation HV, UHV, Inert Gas Reactive
• Sputtering HV, UHV, Inert Gas, Reactive
• Pulsed Laser Deposition HV, UHV, Inert Gas,
  Reactive

• Thermal Growth Plasma Assisted
• Polymerization
• Plasma assisted
• Chemical growth
• Plasma enhanced

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Sputter Deposition

• energy of Ar 10eV 10keV
• not as high as ion implantation but still enough
  to mechanically impact target and knock off
  atoms.
• called sputtering ? sputter etch / ion milling
• purely mechanical
• sputtered atoms (generally neutral ) diffuse down
  to anode and deposit there
• anode substrate
• cathode desired film material
• okay for conductive substrates and targets, but
  not for insulators
• charge builds up at cathode
• ? RF (radio frequency) sputtering

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Sputter Deposition

• for low frequencies (lt50 kHz)
• same as D.C. except both target and substrate get
  sputtered (etched)
• use this technique to clean up wafer surface
• for high frequencies (gt50 kHz)
• heavy ions cant keep up with changing field,
  electrons can
• short time in reverse, ions dont make it to
  substrate to sputter but e- can go to cathode to
  neutralize charge
• usually f 13.56 MHz fcc designated frequency

spend longer time at where you sputter the
longest and deposit on substrate
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• Sputter yield atoms / ion
• E ion energy
• U binding energy
• mi mass of ion
• mt target mass
• can increase deposition rate by increasing I
• magnetron sputtering use magnetic field to
  increase path of e-

geometric term ? momentum transfer f(A)
helical path - more probability of colliding with
Ar ? Ar
SA gt SB
compound AB
increase conc. of B, more likely to sputter B
A
B
A
B
A
B
B
B
B
A
B
A
B
A
B
A
B
A
B
A
B
small voltage drop
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Sputter Deposition

• can deposit alloys, compounds, high Tm materials
• requires the use of a plasma
• plasma energized gas consisting of gas atom,
  gas ions, and e-
• relatively low pressure inert gas is flown
  between an anode and a cathode
• small amount of current (e-) may flow between the
  electrodes
• collisions w/ Ar atoms ? ionize Ar
• Ar ions get accelerated towards cathode (target)

d.c.
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Gas Flow Sputtering
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Comparison of Evaporation vs. Sputtering Process
and Films
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PVD Sputtering Tool
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DC Sputtering
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DC Sputter Deposition

• Sputter deposition is a physical vapor
  deposition (PVD) method of depositing thin
  films by sputtering, that is ejecting, material
  from a "target," source, which then deposits onto
  a "substrate," such as a silicon wafer. Sputtered
  atoms ejected from the target have a wide energy
  distribution, typically up to tens of eV (100,000
  K). The sputtered ions (typically only a small
  fraction (1 of the ejected particles are
  ionized) can ballistically fly from the target in
  straight lines and impact energetically on the
  substrates or vacuum chamber (causing
  resputtering).  Energetic ions sputter material
  off the target which diffuse through the plasma
  towards the substrate where it is deposited.
  There is no strong plasma glow around the cathode
  since it takes a certain distance for the plasma
  to be generated by electron avalanches started by
  a few secondary electrons from the sputtering
  process.

http//en.wikibooks.org/wiki/Microtechnology/Addit
ive_Processes
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Sputtering System
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Thin Film Sputtering Line
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Thin Film Sputtering Line
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Molecular Beam Epitaxy

• Generally used for ultra high purity, epitaxial
  (single crystalline) growth
• Same as evaporation
• Usually with other semiconductors
• Why? ? a vs. Eg
• HEMT
• Epitaxy arranged upon
• Just like in boule growth, depositing atoms align
  with the substrate crystal structure
• Growth chamber show figure
• Similar to evaporation
• Effusion cells (flux determined by T)
• Wafer is held at some specific T ? can control
  film characteristics

high mobility channel for electrons
GaAs
InGaAs
GaAs
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Molecular Beam Epitaxy

• Film characteristics
• Burton, Cabrera, Frank (BCF Model)
• Surface of a wafer is never cut exactly along a
  single plane ? miscut
• atoms coming and landing on surface ? adatoms
• show figure
• terrace adatom singly bonded (easy diffusion)
• step adatom double bonded
• kink adatom triply bonded
• morphology of film depends on if adatom can get
  to a step / kink or incorporate w/ crystal
  structure

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Molecular Beam Epitaxy

• Growth Modes
• amorphous and island growth ? defects
• preferred 1) step mediated
• competition between flux and T
• ?T allow surface diffusion to proper sites
• ?F reduce of atoms on surface to incorporate
  before more atoms are introduced
• slow growth rates (layer by layer)? atomically
  abrupt surfaces/interfaces
• grow heterostructures
• composition of film dictated by relative flux (if
  S is equivalent)
• For HEMT
• deposit Ga As

evap.
multiple nucleation sites
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Chemical Vapor Deposition

• consists of precursors reacting at surface to
  form film
• e.g. electronic grade Si
• SiHCl3 ? Si HCl
• e.g. SiO2
• SiH4 (g) O2 (g) ? SiO2 H2
• can get epitaxial growth vapor phase epitaxy
  (5.3)
• same differences apply ?F, ?T

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Chemical Vapor Deposition
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Chemical Vapor Deposition

• Steps
• transport of reactants to wafer
• transfer of reactants from gas stream to
  substrate surface
• reactants adsorb onto surface
• chemical reaction / surface diffusion
• incorporation (for epitaxy)
• desorption of byproducts
• Rate limiting steps (slowest)
• transport of reactant to substrate
• chemical reaction at surface

precursors
wafer
exhaust
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Chemical Vapor Deposition

• flow conditions in reactor
• laminar vs. turbulent
• smooth flow of one layer over another preferred
  for control and uniformity
• stream vs. rapids
• Reynolds number define regime
• Re lt 2300 laminar
• Re gt 2300 turbulent
• under laminar flow ? boundary layer (like boule)
  (stagnant air)

Dr tube diameter n gas velocity r density
of gas m viscosity
reactants must diffuse through stagnant boundary
layer to get to surface.

• velocity
• D diffusivity in gas
• x dist. along substrate

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• Concentration of gaseous reactants
• growth rate similar to oxide
• Diffusion through boundary layer
• Interface reaction (first order)
• at steady state

• growth rate vs. T
• low T regime kltlth
• reaction limited
• dx/dt ? k
• k exponentially dep. on T
• 2. high T regime kgtgtT
• mass transfer limited dx/dt ? h
• less sensitive to T h D/d

endothermic
h ? 1/P
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Chemical Vapor Deposition

• types of CVD
• APCVD atmospheric pressure hltltk
• 760 Torr
• high deposition rate, not very T dep.
• LPCVD low pressure hgtgtk
• (0.25 1 torr)
• low contamination, T controlled
• PECVD plasma enhanced
• low temperature required (dopant dist.)
• use the energy of plasma instead of thermal
  energy for reaction
• e.g. SiN ? 800-900C, Al melts _at_ 660C
• PECVD ? 300C!
• VPE epitaxial films
• ?T, ?pressure
• ?surface diffusion, ?flux
• CVD systems show figure
• horizontal reactor

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Chemical Vapor Deposition

• Examples
• Si
• precursors SiCl4, SiHCl3, SiH2Cl2, SiH4
• in chamber SiH2Cl2 ? SiCl2 H2 gt800C
• at surface 2SiCl2 ? Si SiCl4 1050C -
  1200C
• SiCl4 2H2 ? Si 4HCl
• deposits Si at surface (surface diffusion)
• provides lots of HCl every Si atom ? 2HCl
• reaction can reverse ? etch Si away
• need to remove HCl ? proper exhaust
• can use SiH4 (no chlorine)
• SiH4 ? Si 2H2 (H2 inert)
• Heat of formation
• SiCl4 -153.2 kcal/mol
• SiCl3 -112.1
• SiHCl2 -75
• SiH4 7.8
• SiH4 very volatile ? higher growth rate but can
  precipitate in gas phase
• also SiH4 O2 ? Si H2O in gas precipitates

harder to crack more chlorine
lower T needed
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Chemical Vapor Deposition

• III-Vs
• GaAs
• ease of cracking precursor too early
• byproducts are harmful
• purity
• toxicity
• Halide process
• AsCl3 H2 ? As4 HCl
• Ga As ? GaAs
• GaAs HCl ? GaCl As
• dep. GaAs on substrate
• other precursors

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In Line Deposition Tools
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Roll to Roll Coater
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Spin Coat

• Spin coating is a procedure used to apply
  uniform thin films to flat substrates. In short,
  an excess amount of a solution is placed on the
  substrate, which is then rotated at high speed in
  order to spread the fluid by centrifugal force. A
  machine used for spin coating is called a spin
  coater, or simply spinner. Rotation is continued
  while the fluid spins off the edges of the
  substrate, until the desired thickness of the
  film is achieved. The applied solvent is
  usually volatile, and simultaneously evaporates.
  So, the higher the angular speed of spinning, the
  thinner the film. The thickness of the film also
  depends on the concentration of the solution and
  the solvent. Spin coating is widely used
  in microfabrication, where it can be used to
  create thin films with thicknesses below 10 nm.
  It is used intensively inphotolithography, to
  deposit layers of photoresist about
   micrometer thick. 

http//en.wikipedia.org/wiki/Spin_coating
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Characterization Tools

• SEM-EDX
• Imaging
• X-Ray Analysis
• XPS/AES
• Elemental composition
• Chemical bonding
• Depth composition profiles

• XRF
• Composition
• Film thickness
• XRD
• Crystal structure
• Phase
• TEM
• Imaging
• Atomic structure

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SEM-EDX Analysis
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Al/Pd/GaN Thin Film Annealing
(cross section)
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Al/Pd/GaN Profile Data
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Al/Pd/GaN Atomic Concentration Data
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Summary

• Thin film deposition
• Physical Vapor Deposition
• DC Magnetron, Sputtering, Ion Beam Sputtering
  (IBS)
• Molecular Beam Epitaxy
• Atomic Layer Deposition
• Chemical vapor Deposition
• CVD, PE-CVD, MO-CVD

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References

• AVS American Vacuum Society)
• Wikipedia Materials Science
• Colorado Dept of Physics and Energy
• EAG Labs application notes
• Nanolab Technologies (XPS/AES)
• NASA-ASL / MACS

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Permissions

• Permission to use this PPT presentation
  explicitly granted by Tom Christianson
• Physics of Thin Films
• PES 449 / PHYS 549 Spring 2000
• Department of Physics and Energy Science
  University of Colorado at Colorado Springs P.O.
  Box 7150 Colorado Springs, CO 80933
• emailtchriste_at_uccs.edu
• Michael Oye UCLA 2006
• Please do not repost this PowerPoint lecture