VIII' Pattern Transfer: Additive techniquesPhysical Vapor Deposition and Chemical Vapor Deposition

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VIII. Pattern Transfer Additive
techniques-Physical Vapor Deposition and Chemical
Vapor Deposition Winter 2009
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Content

• Physical vapor deposition (PVD)
• Thermal evaporation
• Sputtering
• Evaporation and sputtering compared
• MBE
• Laser sputtering
• Ion Plating
• Cluster-Beam

• Chemical vapor deposition (CVD)
• Reaction mechanisms
• Step coverage
• CVD overview
• Epitaxy
• Electrochemical Deposition

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Physical vapor deposition (PVD)

• The physical vapor deposition technique is based
  on the formation of vapor of the material to be
  deposited as a thin film. The material in solid
  form is either heated until evaporation (thermal
  evaporation) or sputtered by ions (sputtering).
  In the last case, ions are generated by a plasma
  discharge usually within an inert gas (argon). It
  is also possible to bombard the sample with an
  ion beam from an external ion source. This allows
  to vary the energy and intensity of ions reaching
  the target surface.

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Physical vapor deposition (PVD) thermal
evaporation
The number of molecules leaving a unit area of
evaporant per second
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Physical vapor deposition (PVD) thermal
evaporation
This is the relation between vapor pressure
of the evaporant and the evaporation rate. If a
high vacuum is established, most molecules/atoms
will reach the substrate without intervening
collisions. Atoms and molecules flow through the
orifice in a single straight track,or we have
free molecular flow
The fraction of particles scattered by collisions
with atoms of residual gas is proportional to
The source-to-wafer distance must be smaler than
the mean free path (e.g, 25 to 70 cm)
The cosine law
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Physical vapor deposition (PVD) thermal
evaporation
From kinetic theory the mean free path relates
to the total pressure as
Since the thickness of the deposited film, t, is
proportional To the cos b, the ratio of the film
thickness shown in the Figure on the right with
? 0 is given as
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Physical vapor deposition (PVD) sputtering
Momentum transfer
-V working voltage - i discharge current - d,
anode-cathode distance - PT, gas pressure - k
proportionality constant
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Evaporation and sputteringcomparison
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Physical vapor deposition (PVD) MBE, Laser
Ablation

• MBE
• Epitaxy homo-epitaxy hetero-epitaxy
• Very slow 1µm/hr
• Very low pressure 10-11 Torr
• Laser sputter deposition
• Complex compounds (e.g. HTSC, biocompatible
  ceramics)

-
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Physical vapor deposition (PVD) Ion cluster
plating

• Ionized cluster it is possible to ionize atom
  clusters that are being evaporated leading to a
  higher energy and a film with better properties
  (adherence, density, etc.).
• From 100 mbar (heater cell) to 10-5 to 10-7 mbar
  (vacuum)--sudden cooling
• Deposits nanoparticles
• Combines evaporation with a plasma
• faster than sputtering
• complex compositions
• good adhesion

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Physical vapor deposition (PVD)Ion cluster
plating and ion plating

• Gas cluster ions consist of many atoms or
  molecules weakly bound to each other and sharing
  a common electrical charge. As in the case of
  monomer ions, beams of cluster ions can propagate
  under vacuum and the energies of the ions can be
  controlled using acceleration voltages. A cluster
  ion has much larger mass and momentum with lower
  energy per atom than a monomer ion carrying the
  same total energy. Upon impact on solid surfaces,
  cluster ions depart all their energy to an
  extremely shallow region of the surface. Cluster
  plating material is forced sideways and produces
  highly smooth surfaces.
• Also individual atoms can be ionized and lead to
  ion plating (see figure on the right, example
  coating very hard TiN)

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Chemical vapor deposition (CVD) reaction
mechanisms

• CVD Diffusive-convective transport of depositing
  species to a substrate with many intermolecular
  collisions-driven by a concentration gradient

• Mass transport of the reactant in the bulk
• Gas-phase reactions (homogeneous)
• Mass transport to the surface
• Adsorption on the surface
• Surface reactions (heterogeneous)
• Surface migration
• Incorporation of film constituents, island
  formation
• Desorption of by-products
• Mass transport of by-produccts in bulk

SiH4
SiH4
Si
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Chemical vapor deposition (CVD) reaction
mechanisms

• Energy sources for deposition
• Thermal
• Plasma
• Laser
• Photons
• Deposition rate or film growth rate

(Ficks first law)
(Boundary layer thickness)
(gas viscosity h, gas density r, gas stream
velocity U)
(Dimensionless Reynolds number)
(by substitution in Ficks first law and Dxd)
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Chemical vapor deposition (CVD) reaction
mechanisms

• Mass flow controlled regime (square root of gas
  velocity)(e.g. AP CVD 100-10 kPa) FASTER
• Thermally activated regime rate limiting step is
  surface reaction (e.g. LP CVD 100 Pa----D is
  very large) SLOWER

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Chemical vapor deposition (CVD) step coverage

• Step coverage, two factors are important
• Mean free path and surface migration i.e. P and T
• Mean free path l

0
q180
q is angle of arrival
z
0
q90
q270
0
a
w
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Chemical vapor deposition (CVD) overview

• CVD (thermal)
• APCVD (atmospheric)
• LPCVD (lt10 Pa)
• VLPCVD (lt1.3 Pa)
• PE CVD (plasma enhanced)
• Photon-assisted CVD
• Laser-assisted CVD
• MOCVD

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Chemical vapor deposition (CVD) L-CVD

• The LCVD method is able to fabricate continuous
  thin rods and fibres by pulling the substrate
  away from the stationary laser focus at the
  linear growth speed of the material while keeping
  the laser focus on the rod tip, as shown in the
  Figure . LCVD was first demonstrated for carbon
  and silicon rods. However, fibres were grown from
  hundreds of substrates including silicon, carbon,
  boron, oxides, nitrides, carbides, borides, and
  metals such as aluminium. The LCVD process can
  operate at low and high chamber pressures. The
  growth rate is normally less than 100 µm/s at low
  chamber pressure (ltlt1 bar). At high chamber
  pressure (gt1 bar), high growth rate (gt1.1 mm/s)
  has been achieved for small-diameter (lt 20 µm)
  amorphous boron fibres.

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Epitaxy

• VPE
• MBE (PVD) (see above)
• MOCVD (CVD) i.e.organo-metallic CVD(e.g.
  trimethyl aluminum to deposit Al) (see above)
• Liquid phase epitaxy
• Solid epitaxy recrystallization of amorphous
  material (e.g. poly-Si)

Liquid phase epitaxy
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Epitaxy

• Selective epitaxy
• Epi-layer thickness
• IR
• Capacitance,Voltage
• Profilometry
• Tapered groove
• Angle-lap and stain
• Weighing

Selective epitaxy
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Electrochemical deposition electroless

• Electroless metal displacement
• Electroless sustainable oxidation of a reductant
• Metal salt (e.g.NiCl2)
• Reductant (e.g.hypophosphite)
• Stabilizerbath is thermodynamically unstable
  needs catalytic poison (e.g. thiourea)
• Complexing agent prevent too much free metal
• Buffer keep the pH range narrow
• Accelerators increase deposition rate without
  causing bath instability (e.g. pyridine)

• Deposition on insulators (e.g. plastics) seed
  surface with SnCl2/HCl
• 1. Zn(s) Cu 2(aq) ------gt Zn 2(aq) Cu(s)
• 2. Reduction (cathode reaction)
• Ni2 2e- gt Ni
• Oxidation (anode reaction)
• H2PO 2- H2Ogt H2PO3- 2H 2e-
  ------------------------------------------
• Ni2 H2PO2- H2O gt Ni H2PO3- 2H
• e.g. electroless Cu 40 µmhr-1

Cu
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Electrochemical deposition electroless

Evans diagram

• Evans diagram electroless deposition is the
  combined result of two independent electrode
  reactions (anodic and cathodic partial reactions)
• Mixed potential (EM) reactions belong to
  different systems
• ideposition ia ic and IA x i deposition
• Total amount deposited m max I t M/Fz (t is
  deposition time, Molecular weight, F is the
  Faraday constant, z is the charge on the ion)
• CMOS compatible no leads required

-
F 96,500 coulombs1, 6 10 -19 (electron charge)
x 6. 02 10 23 (Avogadros number)
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Electrochemical deposition electrodeposition-ther
modynamics

• Electrolytic cell
• Au cathode (inert surface for Ni deposition)
• Graphite anode (not attacked by Cl2)
• Two electrode cells (anode, cathode, working and
  reference or counter electrode) e.g. for
  potentiometric measurements (voltage
  measurements)
• Three electrode cells (working, reference and
  counter electrode) e.g. for amperometric
  measurements (current measurements)

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Electrochemical deposition electrodeposition-ther
modynamics (E)
1. Free energy change for ion in the solution to
atom in the metal (cathodic reaction)
or also
(1)
2. The electrical work, w, performed in
electrodeposition at constant pressure and
constant temperature
and since DV 0
(2)
3. Substituting Equation (2) in (1) one gets
(Nernst equation)
4. Repeat (1) and (2) for anodic reaction
or
E2 gt E1 - battery E2 lt E1 E ext gt E cell
to afford deposition
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Electrochemical deposition electrodeposition-ther
modynamics (h)

• A thermodynamic possible reaction may not occur
  if the kinetics are not favorable
• Kinetics express themselves through all types of
  overpotentials
• E -E o h ( anodic and - is cathodic)

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Electrochemical deposition electrodeposition-kine
tics-activation control

• Understanding of polarization curves consider a
  positive ion transported from solution to the
  electrode
• Successful ion jump frequency is given by the
  Boltzmann distribution theory (h is Planck
  constant)

(without field)
(with field)
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Electrochemical deposition electrodeposition-kine
tics-activation control

• At equilibrium the exchange current density is
  given by
• The reaction polarization is then given by
• The measurable current density is then given by
• For large enough overpotential

(Butler-Volmer)
(Tafel law)
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Electrochemical deposition electrodeposition-kine
tics-diffusion control
we get

• From activation control to diffusion control
• Concentration difference leads to another
  overpotential i.e. concentration polarization
• Using Faradays law we may write also
• At a certain potential C x00 and then

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Electrochemical deposition electrodeposition-non-
linear diffusion effects

• Nonlinear diffusion and the advantages of using
  micro-electrodes
• An electrode with a size comparable to the
  thickness of the diffusion layer
• The Cottrell equation is the current-vs.-time on
  an electrode after a potential step
• For micro-electrodes it needs correction

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Electrochemical deposition electrodeposition-non-
linear diffusion effects

• The diffusion limited currents for some different
  electrode shapes are given as (at longer times
  after bias application and for small electrodes)
• If the electrodes are recessed another correction
  term must be introduced

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Homework

• Homework demonstrate equality of l
  (pRT/2M)1/2 h/PT and l kT/2 1/2 a 2 p PT (where
  a is the molecular diameter)
• What is the mean free path (MFP)?  How can you
  increase the MFP in a vacuum chamber?  For metal
  deposition in an evaporation system, compare the
  distance between target and evaporation source
  with working MFP.  Which one has the smaller
  dimension?  1 atmosphere pressure ____ mm Hg
  ___ torr.  What are the physical dimensions of
  impingement rate?
• Why is sputter deposition so much slower than
  evaporation deposition?  Make a detailed
  comparison of the two deposition methods.
• Develop the principal equation for the material
  flux to a substrate in a CVD process, and
  indicate how one moves from a mass transport
  limited to reaction-rate limited regime.  Explain
  why in one case wafers can be stacked close and
  vertically while in the other a horizontal
  stacking is preferred.  
• Describe step coverage with CVD processes.
   Explain how gas pressure and surface temperature
  may influence these different profiles.