1. 10. Pattern Transfer: Additive techniques-Physical Vapor
Deposition, Chemical Vapor Deposition and
Electrochemical Deposition
Dr. Marc Madou, Fall 2013
Prof. Marc Madou
MSTB 120
3. 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.
4. Physical vapor deposition (PVD):
thermal evaporation
6
The number of molecules
leaving a unit area of evaporant
per second
5. Physical vapor deposition (PVD): thermal
evaporation
The cosine law
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)
6. 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:
7. Physical vapor deposition (PVD): sputtering
-V working voltage
- i discharge current
- d, anode-cathode distance
- PT, gas pressure
- k proportionality constant
Momentum transfer
10. 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
11. 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)
Physical vapor deposition (PVD):Ion
cluster plating and ion plating
12. Chemical vapor deposition (CVD): reaction
mechanisms
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
CVD: Diffusive-convective transport of
depositing species to a substrate
with many intermolecular
collisions-driven by a concentration
gradient
SiH4
SiH4
Si
13. Chemical vapor deposition (CVD):
reaction mechanisms
Energy sources for deposition:
– Thermal
– Plasma
– Laser
– Photons
Deposition rate or film growth rate
(Fick’s first law)
(gas viscosity h, gas density r, gas stream velocity U)
(Dimensionless Reynolds number)
Laminar flow
L
d(x)
dx
(U)
(Boundary layer thickness)
(by substitution in Fick’s first law and Dx=d)
14. 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
Chemical vapor deposition (CVD)
: reaction mechanisms
15. 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 =
a
w
z
=1800
=2700
=900
is angle of arrival
17. The L-CVD 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, fibers
were grown from other substrates including
silicon, carbon, boron, oxides, nitrides,
carbides, borides, and metals such as
aluminium. The L-CVD process can operate at
low and high chamber pressures. The growth
rate is normally less than 100 µm/s at low
chamber pressure (<<1 bar). At high chamber
pressure (>1 bar), high growth rate (>1.1
mm/s) has been achieved for small-diameter (<
20 µm) amorphous boron fibers.
Chemical vapor deposition (CVD) : L-CVD
18. 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
20. Electrochemical deposition: electroless
Electroless metal displacement
Electroless sustainable oxidation of a
reductant
– Metal salt (e.g.NiCl2)
– Reductant (e.g.hypophosphite)
– Stabilizer:bath 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) ------> Zn 2+(aq) + Cu(s)
2. Reduction (cathode reaction) :
Ni+2 + 2e- —> Ni
Oxidation (anode reaction):
H2PO 2- + H2O—> H2PO3
- +2H+ +2e- -------------
-----------------------------
Ni+2 + H2PO2
- + H2O —> Ni + H2PO3
- + 2H+
e.g. electroless Cu: 40 µmhr-1
Cu
21. Electrochemical deposition: electroless
Evan’s 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 I=A 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
Evan’s diagram
F= 96,500 coulombs=1, 6 10 -19 (electron charge) x 6. 02 10 23 (Avogadro’s number)
+
-
22. Electrochemical deposition :electrodeposition-
thermodynamics
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)
23. Electrochemical deposition
:electrodeposition-thermodynamics (E)
E2 > E1 : - battery
E2 < E1 : + E ext > E cell to afford deposition
(Nernst equation)
1. Free energy change for ion in the solution to atom in the metal (cathodic reaction):
or also
2. The electrical work, w, performed in electrodeposition
at constant pressure and constant temperature: and since DV =0
3. Substituting Equation (2) in (1) one gets
(1)
(2)
4. Repeat (1) and (2) for anodic reaction:
or
25. Electrochemical deposition :electrodeposition-
kinetics-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)
26. Electrochemical deposition :electrodeposition-
kinetics-activation control
(Butler-Volmer)
(Tafel law)
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:
27. Electrochemical deposition :electrodeposition-
kinetics-diffusion control
From activation control to diffusion
control:
Concentration difference leads to
another overpotential i.e. concentration
polarization:
Using Faraday’s law we may write
also:
At a certain potential C x=0=0 and then:
we get :
28. 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 :
29. 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:
30. 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.
