MSE 550 . Presentation #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
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What is a thin film?
overview of film growth
crystal structure and defects (dislocations, grain boundaries)
properties of vacuum
Sticking and surface diffusion
nucleation of film
growth modes (island, layer by layer, mixed)
coalescence of film
continued growth (zone models)
Deposition parameters and techniques
(relate the knobs on equipment to what happens on the film)
Vacuum Arc Deposition
Chemical Vapor Deposition
Ion Beam assisted deposition
plasma enhanceddeposition (PECVD, ECR . . .
How are thin films made?
What are the basic parts of deposition systems?
substrate - deposition region
What parameters can we control?
Within the framework established above, examine each of the following deposition methods in detail:
cathodic arc vaporization
Molecular Beam Epitaxy
Chemical Vapor Deposition
Ion Beam assisted deposition / ion implantation
plasma enhanced deposition (PECVD, ECR . . . )
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")
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
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 ?)
thermodynamics and kinetics
phase transition - gas condenses to solid
allowed processes and allowed phases
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"
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)
Examine what happens when we apply a field:
How fast do atoms diffuse?
Energy minimization involves two terms:
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 > PS so that ÆG is negative
=> supersaturation provides the driving force.
Change in surface energy is always positive when forming surfaces.
Total energy change:
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:
thermal accommodation coefficient (aT)
as with homogeneous nucleation, we can plot ÆG against r and determine a critical nucleus size:
How do nuclei grow initially ?
Substrates are NOT flat
steps, kinks, etc. have higher Edes barrier => longer residence time on surface
=> preferred sites for nucleation
How quickly do nuclei form ?
To see how the lab variable (deposition rate, substrate temperature) change the basic physics examine the derivatives (and plug in some typical values):
high T and/or low deposition rate => large crystal grains
low T and/or high depostion rate => small polycrystalline structure
Problem: Can we apply macroscopic thermodynamics to nuclei of 2-100 atoms ?
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.
Ni* = concentration of critical clusters per unit area
N1 = concentration of single atoms per unit area
no = total density of adsorption sites on surface
Film Formation II
5. Island Growth
observe 3 growth modes experimentally
1. Island growth (Volmer - Weber)
form three dimensional islands
film atoms more strongly bound to each other than to substrate
and/or slow diffusion
2. Layer by layer growth (Frank - van der Merwe)
generally highest crystalline quality
film atoms more strongly bound to substrate than to each other
and/or fast diffusion
3. Mixed growth (Stranski - Krastanov)
initially layer by layer
then forms three dimensional islands
=> change in energetics
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.
6. Island Coalescence
three common mechanisms:
1. Ostwald ripening
atoms leave small islands more readily than large islands
more convex curvature => higher activity => more atoms escape
reduction of surface energy
3. Cluster migration
small clusters (<100 Å across) move randomly
some absorbed by larger clusters (increasing radius and height)
7. Thick films - zone models
Further growth depends on:
shadowing (line of sight impingement)
these variables to find regions with similar film structure (similar properties)
from limited atomic mobility
often oriented slightly toward source
Other factors affecting film growth
not really a featureless plane
atomic structure => epitaxy
relationship of film crystal structure to substrate crystal structure
poor background pressure
impure deposition source
changes the energies (surface energies, desorption energy, surface diffusion energy)
3. Impinging particle energy
0.5 eV -------------------> 10 - 20 eV --------> 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
shock (pressure) waves