engineering materials

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

2. 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

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. This allows to vary the energy and
intensity of ions reaching the target surface.

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

8. Evaporation
and
sputtering:
comparison

9. 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)

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

16. Chemical vapor deposition (CVD) :
overview
 CVD (thermal)
– APCVD (atmospheric)
– LPCVD (<10 Pa)
– VLPCVD (<1.3 Pa)
 PE CVD (plasma enhanced)
 Photon-assisted CVD
 Laser-assisted CVD
 MOCVD

17.  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 (<<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 fibres.
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

19. Epitaxy
 Selective epitaxy
 Epi-layer thickness:
– IR
– Capacitance,Voltage
– Profilometry
– Tapered groove
– Angle-lap and stain
– Weighing
Selective 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

24. Electrochemical deposition
:electrodeposition-thermodynamics (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)

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.