MSE 550

1
MSE 550

• Presentation 2

2

• What is a thin film?
• Science background
• overview of film growth
• crystal structure and defects (dislocations,
  grain boundaries)
• diffusion
• properties of vacuum
• Film formation
• Thermal accommodation
• Sticking and surface diffusion
• nucleation of film
• growth modes (island, layer by layer, mixed)
• coalescence of film
• continued growth (zone models)
• other factors
• energetic deposition
• amorphous films
• epitaxial growth
• Deposition parameters and techniques
• (relate the knobs on equipment to what happens on
  the film)

3
(No Transcript)
4

• How are thin films made?
• What are the basic parts of deposition systems?
• source
• transport region
• substrate - deposition region
• What parameters can we control?
• temperature
• deposition rate
• deposition energy
• Within the framework established above, examine
  each of the following deposition methods in
  detail
• evaporation
• cathodic arc vaporization
• sputter deposition
• DC
• RF
• Molecular Beam Epitaxy
• Chemical Vapor Deposition
• Ion Beam assisted deposition / ion implantation
• plasma enhanced deposition (PECVD, ECR . . . )

5

• What is a "thin film" ?
• thin less than about one micron ( 10,000
  Angstroms, 1000 nm)
• film layer of material on a substrate
• (if no substrate, it is a "foil")
• Applications
• microelectronics - electrical conductors,
  electrical barriers, diffusion barriers . . .
• magnetic sensors - sense I, B or changes in them
• gas sensors, SAW devices
• tailored materials - layer very thin films to
  develop materials with new properties
• optics - anti-reflection coatings
• corrosion protection
• wear resistance

6

• Typical steps in making thin films
• emission of particles from source ( heat, high
  voltage . . .)
• transport of particles to substrate (free vs.
  directed)
• condensation of particles on substrate (how do
  they condense ?)
•  

Simple model
7

• Mechanisms?
• thermodynamics and kinetics
• phase transition - gas condenses to solid
• nucleation
• growth kinetics
• activated processes
• desorption
• diffusion
• allowed processes and allowed phases

8

• Kinetics and Diffusion
• Kinetics how fast it will happen
• we will concentrate on mass transport
• atoms diffusing through a solid
• Diffusion in one dimension - Fick's 1st and
• Fick's 1st Law "stuff moves from where you have
  lots to where you have little"

9
2nd Ficks law
10

• There is always a potential energy barrier to
  diffusion (activation energy).
• What do we expect mathematically for the flux to
  the right (from position1 to 2)
• Similarly we can find the flux to the left
• (note if we had used the gas constant, R,
  instead of Boltzmann constant, k, then the energy
  would be the diffusion energy/mole)

11
Examine what happens when we apply a field
12
How fast do atoms diffuse?
13
Nucleation and Growth

• Connection to Phase Diagrams
• Can phase diagrams help us in understanding rates
  ?
• Consider cooling a liquid into a solid through a
  eutectic point

at point A solid is not stable so will not
form at point B solid and liquid are both stable
so no driving force to solid at point C liquid
is unstable - will form solid at point D liquid
is unstable - will form solid further from
equilibrium gt greater driving force to form
solid
14
Nucleation

• depends on
• liquid phase instability
• driving force toward equilibrium (as above)
• increases as we move to lower temperatures
• diffusion of atoms into clusters
• increases at higher temperatures
• combine these two terms (multiplication) to
  determine the total nucleation rate
• The maximum rate of nucleation is at some T lt Te

15
Growth

• growth of the phase is diffusion controlled gt
  increases with temperature
• Transformation rate
• total rate of forming solid is product of
  nucleation rate and growth rate

16
Nucleation details

• When moving into a 2 phase region on phase
  diagram - how does the new phase form ?
• Two issues
• Thermodynamics Is nucleation possible ? (energy
  minimization)
• Kinetics How fast does it happen ? (nucleation
  rate)
• Homogeneous Nucleation
• vapor --gt liquid (solid) for a pure material with
  NO substrate

17

• Energy minimization involves two terms
• volume transition
• surface formation
• volume transition

where W is the atomic volume, PS is the pressure
above the liquid (solid), and PV is the pressure
in the vapor. We want PV gt PS so that ÆG is
negative gt supersaturation provides the driving
force.
18
surface formation
Change in surface energy is always positive when
forming surfaces. Total energy change
19

• note
• initial formation of nuclei has increase in G gt
  metastable
• if r lt r then nuclei shrink to lower G
• if r gt r then nuclei grow to lower G
• r is a critical radius for nuclei

20

• Nucleation rate
• How fast will the critical nucleus continue to
  grow ?
• Consider the rate at which atoms will join the
  critical nuclei

expect nucleation rate to be given by
N concentration of critical nuclei
(nuclei/cm3) A critical surface area of
nuclei w flux of atom impingement (atoms /
cm2sec) Consider each of these three terms
21
(No Transcript)
22

• Film Formation I
• Competing Processes
• adding to film
• impingement (deposition) on surface
• removing from film
• reflection of impinging atoms
• desorption (evaporation) from surface
• We can characterize the process of getting atoms
  onto a surface with
• sticking coefficient mass deposited / mass
  impinging
• Steps in Film Formation
• thermal accommodation
• binding
• surface diffusion
• nucleation
• island growth
• coalescence
• continued growth
• We will examine each of these steps in turn.

23

• 1. Thermal accommodation
• impinging atoms must lose enough energy thermally
  to stay on surface
• assume that E kT so we can talk about energy or
  temperature equivalently

thermal accommodation coefficient (aT)
24
if rebound is strong enough - atom escapes if not
- atom is trapped - oscillates and loses energy
to lattice RESULTS atom is trapped if Ev lt 25
Edesorb Edesorb is typically 1-4 eV trapped if
Ev lt 25 - 100 eV equivalently Tv lt 2500 - 10,000
K most deposition processes have Ev lt 10 eV MOST
ATOMS ARE TRAPPED thermal accommodation is very
fast around 10-14 seconds
25

• 2. Binding
• two broad types of surface bonds
• physisorption (physical adsorption)
• Van der Waals type
• weak bonds
• 0.01 eV
• chemisorption (chemical adsorption)
• chemical bonds
• strong bonds
• 1 - 10 eV
• Can we keep the atoms on the surface ?
• competition between impinging atoms (deposition)
  and desorption of atoms
• deposition determined by deposition rate
  (atoms/cm2sec) desorption determined by DGdes
  free energy of desorption
• TS temperature of substrate
• no frequency of adsorbed atom attempting to
  desorb lattice vibration frequency

26
Consequences heat up substrate gt lower
coverage stop depositing gt lower coverage until
not film films are not stable !!! What is wrong
with this model ? missing surface diffusion
27

• 3. Surface diffusion
• allows clusters of adsorbed atoms to form
• clusters are stable gt film forms
• How far do they diffuse ?
• from random walk analysis see F. Reif
  "Fundamentals of Statistical and Thermal Physics"
  p. 486
• diffusion distance (X) is given by Consider two
  cases

28

• 4. Nucleation
• How do clusters form ? gt nucleation
• Two competing processes in cluster formation
• clusters have a condensation energy per unit
  volume (DGV) which lowers the desorption rate
  (higher barrier)
• clusters have a higher surface energy than
  individual atoms
• clusters want to break up to minimize energy
• Capillarity Model ( heterogeneous nucleation)
• nucleation on a substrate
• assume nuclei are spherical caps

29
as with homogeneous nucleation, we can plot ÆG
against r and determine a critical nucleus size
30
(No Transcript)
31
How do nuclei grow initially ?
Substrates are NOT flat steps, kinks, etc. have
higher Edes barrier gt longer residence time on
surface gt preferred sites for nucleation
32
Nucleation Rate How quickly do nuclei form ?
33
Nucleation Rate
34

• Nucleation Rate
• What can we learn from the capillarity model
  about effects of deposition rate and substrate
  temperature on nucleation ? from before

35
To see how the lab variable (deposition rate,
substrate temperature) change the basic physics
examine the derivatives (and plug in some typical
values)
36

• Summary
• high T and/or low deposition rate gt large
  crystal grains
• low T and/or high depostion rate gt small
  polycrystalline structure
• Problem Can we apply macroscopic thermodynamics
  to nuclei of 2-100 atoms ?

37

• Atomistic (Statistical) Nucleation Model
• Walton - Rhodin Theory
• treat clusters of atoms like molecules rather
  than solid caps
• consider the bonds between atoms
• similar to capillarity model, but now include Ei
  energy to break apart a critical cluster of i
  atoms into individual atoms.
• other terms
• Ni concentration of critical clusters per unit
  area
• N1 concentration of single atoms per unit area
• no total density of adsorption sites on surface

38
advantages of this model depends on microscopic
parameters includes crystallographic information
since bonds between atoms are included critical
size (i) depends on substrate temperature model
shows transitions in growth modes preferred i
increases with T
39

• Film Formation II
• 5. Island Growth
• observe 3 growth modes experimentally
• 1. Island growth (Volmer - Weber)
• form three dimensional islands
• source
• film atoms more strongly bound to each other than
  to substrate
• and/or slow diffusion

40

• 2. Layer by layer growth (Frank - van der Merwe)
• generally highest crystalline quality
• source
• film atoms more strongly bound to substrate than
  to each other
• and/or fast diffusion

41

• 3. Mixed growth (Stranski - Krastanov)
• initially layer by layer
• then forms three dimensional islands
• gt change in energetics

42

• When would we expect to see each of these ?

The layer growth condition with cosine greater
than 1 looks odd. This is the case where the
angle theta is undefined because for layer growth
there really is no point where the substrate,
vapor and film come together and therefore, no
way to define the angle.
43

• 6. Island Coalescence
• three common mechanisms
• 1. Ostwald ripening
• atoms leave small islands more readily than large
  islands
• more convex curvature gt higher activity gt more
  atoms escape

2. Sintering reduction of surface energy
44

• 3. Cluster migration
• small clusters (lt100 Å across) move randomly
• some absorbed by larger clusters (increasing
  radius and height)

45

• 7. Thick films - zone models
• Further growth depends on
• bulk diffusion
• surface diffusion
• desorption
• geometry
• shadowing (line of sight impingement)

• Relative importance of these processes depends on
• substrate temperature (T)
• deposition rate

these variables to find regions with similar film
structure (similar properties)
46
(No Transcript)
47

• Columnar structures
• very common
• from limited atomic mobility
• often oriented slightly toward source

Films are typically lower density than bulk more
porosity at macro, micro and nano scales. Grain
size dependence on deposition rate and substrate
temperature grain size typically increases with
increasing film thickness, increasing substrate
temperature, increasing annealing temperature,
and decreasing deposition rate.
48
(No Transcript)
49

• Other factors affecting film growth
• 1. Substrate
• not really a featureless plane
• atomic structure gt epitaxy
• relationship of film crystal structure to
  substrate crystal structure
• defects
• nucleation sites
• 2. Contamination
• from
• poor background pressure
• impure deposition source
• dirty substrate
• changes the energies (surface energies,
  desorption energy, surface diffusion energy)

50

• 3. Impinging particle energy
• 0.5 eV -------------------gt 10 - 20 eV --------gt
  100-1000 eV
• thermal evaporation ----- sputtering ---------
  accelerated (bias)
• interactions of incident particles with
  film/substrate produce
• sputter removal of surface atoms
• insertion of particles into film or substrate
• increased local temperature
• defects
• shock (pressure) waves