Chapter 12 Physical Vapor Deposition (PVD)

Chapter 12
Physical Vapor Deposition (PVD)

PVD methods:
Evaporation – condensation of metal vapor in
high vacuum to deposit a thin film on a wafer; unable to
cover severe topology (poor step coverage) which is
beneficial when doing lift-off patterning; also hard to
deposit alloys due to possible difference in melting
points.
Sputtering - use of plasma and acceleration of
ions towards a target; material “sputtered” from the
target and deposited on the wafer; extensively used in Si
technology; moderate step coverage.

Evaporation Process:
*Chamber under high vacuum (low pressure and long mfp)
*metal pieces placed in crucible (charge) and heated to T m
*substrates placed above the crucible, and
*metal deposits on the substrates.
Not shown in picture is a shutter that would be
opened when the evaporation rate reaches a certain
rate indicating pure metal deposition.
View factor associated with deposition (like RTP);
deposition rate depends on location of wafer in
chamber (above crucible will have ↑ dep rate).
For better uniformity and step coverage, planetary
sample holders are rotated during deposition and
samples are heated.
Deposition rate monitored with quartz crystal
(oscillates at a resonance frequency that shifts when
additional mass is deposited on the crystal).
Figure 12.1 A simple diffusion-pumped
evaporator showing vacuum plumbing and
the location of the charge-containing
crucible and the wafers.

For reasonable deposition
rates, the vapor pressure
must be ∼ 10 mTorr. This
makes it nearly impossible
ibl
to evaporate some
materials (refractory
metals: Ta, W, Mo, Ti)
Figure 12.2 Vapor pressure curves for some commonly
evaporated materials (data adapted from Alcock et al.).

Deposition rate depends on position of wafer
View factor, k depends on R, Θ, Φ
Wafers directly above the crucible will be coated more heavily than
wafers off to the side. Wafers can all be mounted on the surface of a
sphere (planetary) for more uniform deposition.
Figure 12.3 The geometry of deposition for a wafer (A) in an arbitrary
position and (B) on the surface of a sphere.

Step Coverage
If the substrate surface is not
planar, the long mfp in
evaporation means that
evaporated material will follow
a straight line; some areas are
“shadowed”
AR = step height
step diameter
Helps to rotate and heat the
substrates.
Planetary holders rotate
simultaneously around two
axes.
Figure 12.5 (A) Time evolution of the evaporative
coating of a feature with aspect ratio of 1.0, with
little surface atom mobility (i.e., low substrate
temperature) and no rotation. (B) Final profile of
deposition on rotated and heated substrates.

Evaporator systems: crucible heating techniques
Resistively Heated
Figure 12.7 Resistive evaporator sources. (A) Simple sources including heating
the charge itself and using a coil of refractory metal heater coil and a charge rod.
(B) More standard thermal sources including a dimpled boat in resistive media.

Evaporator systems: crucible heating techniques
Inductively Heated
Figure 12.8 Example of an inductively heated crucible used to create moderately
charged temperatures.

Evaporator systems: crucible heating techniques
Electron beam
Heat of vaporization is
supplied by the impact
of an electron beam
focused on the charge
and melts a region of the
material to be
evaporated;
Adv: possible to coevaporate
materials with
dual targets,
Disadv: more expensive
than resistance heated
systems, substrates may
have radiation damage
Figure 12.9 Electron beam evaporative sources. (A) A simple low flux
source using a hot wire electron source and a thin movable rod. (B) A
popular p source using a 2707 source arc in which the beam can be rastered
across the surface of the charge. The magnet must be much larger than
shown to achieve the full 270° of arc.

Evaporation
Advantages: Simple process, relatively
inexpensive, high purity films can be
deposited.
Disadvantages: difficult to evaporate alloys
due to different melting points, “line of
sight” deposition results in poor surface
coverage unless there is rotation ti of fthe
samples; deposition of refractory materials
is a problem due to high temperatures
required.
Figure 12.10 A commercial evaporator. Inset
shows a planetary (photographs courtesy of
CHA Industries).

Evaporation of Alloys
Figure 12.11 Methods for evaporating multicomponent films include (A)
single-source evaporation, (B) multisource simultaneous evaporation, and (C)
multisource sequential evaporation.

Sputtering:
Bombard the target (cathode) with energetic ions, usually Ar + , in a
plasma. Target material, not the wafers, must be placed on the electrode
with maximum ion flux.
*Chamber base pressure - high
vacuum,
*Argon flows into chamber
raising pressure to mTorr range,
*power supplied to electrodes,
*target material deposits on the
wafers.
Allows deposition of refractory
metals, alloys, dielectrics if an RF
system is used.
Figure 12.12 Chamber for a simple parallel-plateplate
sputtering system.

Basic steps in sputter deposition:
1) plasma generation – glow discharge formed when inert gas
becomes ionized by an E-field; an electron accelerated towards
anode, ionizes Ar atoms upon collision
2) ion bombardment – Ar + impacts the target with high energies and
transfer their momentum to the target material; these collisions
disrupt the atomic surface causing target atoms, ions, and electrons to
be ejected
3) sputtered atom transport – sputtered atoms, ions, are influenced
by collisions they undergo during transport to the film (determined
by background pressure); sputtered particles will lose their energy as
number of collisions increases so important to control the pressure
4) film growth - sputtered material leaves the target and deposits on
surrounding surfaces. The rate of diffusion is dependent upon the
substrate material and Temp. Growth proceeds by diffusion and form
nuclei, linuclei ligrow and eventually form islands; il islands il grow
together until a continuous film is formed.

When an energetic ion strikes the surface of a material --
1) Ions with low energies may bounce of the surface
2) Ions may adsorb to the surface, giving up its energy to
phonons (heat)
3) Ion penetrates t into material, depositing energy deep into the
substrate
Figure 12.13 Possible outcomes for an ion
incident on the surface of a wafer.

Sputter Yield (S)
Determines rate of sputter deposition
S = # target atoms ejected/number of
ions incident
Depends on:
•Target material
•Mass of bombarding ions
•Energy of bombarding ions
Each target material has a threshold
energy (below that energy no
sputtering occurs), typically 10-30 eV.
Figure 12.14 Sputter yield as a function of ion
energy for normal incidence id argon ions for a
variety of materials (after Anderson and Bay,
reprinted by permission).

S versus Ion atomic number
Figure 12.15 Sputter yield as a function of the bombarding ion
atomic number for 45-keV ions incident on silver, copper, and
tantalum targets (after Wehner, reprinted by permission, AIP).

Magnetron Sputtering – a magnetic field applied at right angles to
the E-field; causes e- to follow spiral paths, increases probability of
ionizing a gas atom, increases ionization efficiency, confines
plasma resulting in a higher deposition rate; also able to form
plasma at lower chamber pressures
Figure 12.17 Planar and cylindrical magnetron sputtering systems T: target; P:
plasma; SM: solenoid; M: magnet; E: electric field; B: magnetic field (after
Wasa and Hayakawa, reprinted by permission, Noyes Publications).

Fig. 12.18 in text, p. 340 shows cross section of a planar magnetron
target using permanent magnets to supply the field
The region of the target beneath the ringshaped
volume where the plasma density is
highest is sputtered the most rapidly and this
target erosion is called the race-track
Target showing erosion in the race-track
From: Fundamentals of High Power Impulse
Magnetron Sputtering, dissertation by Johan Böhlmark
Figure 12.18 Detailed cross section of a
rectangular planar magnetron target using
permanent magnets to supply the field (after
Wasa and Hayakawa, reprinted by
permission, Noyes Publications).

Film morphology
Zone model (Zones 1, 2, 3, T) indicates the films final
characteristics based on the substrate temperature and ion energy;
T-region is characterized by very small grains
Zone1-low T, low ion energy
yields amorphous, porous
materials; Raise T or lower P
moves to T-zone
Zone2-Increase T and/or
increase ion energy will
increase grain size - tall
columnar grains
Zone3-Increase T, film has
large 3-D grains– surface may
be rough and hazy
Figure 12.21 The three-zone model of film deposition as proposed by
Movchan and Demchishin (after Thornton, reprinted by permission, AIP).

Step Coverage
Application of substrate heat will dramatically improve the step
coverage due to surface diffusion; High AR can be a problem
otherwise.
Figure 12.22 Cross section of the time evolution of the typical
step coverage for unheated sputter deposition in a high aspect
ratio contact.

Can improve step coverage by collimated sputtering or
application of a bias to the wafer.
Figure 12.24 In collimated sputtering a
disposable collimator is placed close to the
wafers to increase directionality.
Figure 12.26 In bias sputtering, the ions
incident on the surface of the wafer
redistribute the deposited film to improve
step coverage.

Ionized Metal Plasma (IMP) sputter deposition – ejected
atoms pass through a second plasma; IMP process produces
near-vertical deposition.
Figure 12.25 The Endura system by Applied Materials uses a number of PVD or CVD chambers
fed by a central robot. For conventional and IMP sputtering, targets are hinged to open upward. Two
open chambers are shown, along with the load lock (from Applied Materials).

Reactive sputtering: use of reactive gases (O 2 , CH 4 , NH 3 , N 2 ) rather
than inert gases to sputter oxides, carbide, nitrides.
Example below is for TiN
Figure 12.28 Resistivity and composition of reactively sputtered TiN as a function of the N 2
flow in the sputtering chamber (after Tsai, Fair, and Hodul, reprinted by permission, The
Electrochemical Society, and Molarius and Orpana, reprinted by permission, Kluwer
Academic Publishing).

Figure 12.29 Cross section electron micrograph of a
moderately high aspect ratio contact that has been sputterdeposited
with TiN (after Kohlhase, Mändl, and Pamler,
reprinted by permission, AIP).

A thin film deposited on a substrate can be either in tensile stress or
compressive stress; if stress is too large, film may peel away from
the surface; implications in reliability.
Figure 12.30 The change in wafer deflection may be used to
measure the stress in a deposited layer. This is typically measured
using a reflected laser beam.

Sputtering
Advantages: Moderately ygood step coverage; ;preferred
technique for deposition of alloys, can sputter a wide variety of
materials
Disadvantages: may have some Argon incorporation in the film;
could have some damage to substrate although not as much as in
e-beam evaporation