1. © 1997, Angus Rockett
Section I
Evaporation

2. Conventional Evaporators
© 1997, Angus Rockett
Simple medium vacuum evaporators are used for thin coatings for applications such as metallization of electronic circuit elements.
Laboratory Scale System
Flux monitor
Substrate
Bell jar
e-beam
Effusion cell
Open boat
Turbomolecular or diffusion pump

3. High Performance Evaporation System
Molecular beam epitaxy (MBE) systems are ultrahigh vacuum evaporators optimized for high quality film deposition.
Basic MBE System
Chamber wall
Sample load-lock
Cryoshroud
Electron gun for RHEED
RHEED
screen
shutters
Source flange
Pumping:
cryo, ion or
Effusion cells in separate,
turbomolecular
shuttered, water-cooled tubes
(not shown)
© 1997, Angus Rockett

4. Open Source Evaporators
© 1997, Angus Rockett
W filament
Filament Evaporators
Boat Evaporators
Boats of many materials and many shapes are used.

5. Effusion Cells Evaporated flux Crucible Evaporant charge
Heater windings
Thermocouple
Ta foil
heat shielding
Power conductors
Conflat flange
© 1997, Angus Rockett

6. Evaporant Flux 0°
10°
10°
20°
Material leaving a full effusion cell crucible is distributed according to a cosine: L 
= 0 d 1 2
F = F cos  5 o
As the source material is evaporated and the level sinks into the cell the flux is more directional
Total flux, F, from a Knudsen effusion cell: L
molecules per second
p: pressure, A: area, M: mass, T: temperature all in SI units d
© 1997, Angus Rockett

7. Evaporant Flux Typical evaporators have tapered sidewalls.
Tapering cell sidewalls:
Reduces the effect of the amount of material in the cell.
Changes the pattern of evaporant.
© 1997, Angus Rockett

8. Evaporant Flux Vapor Pressure (Torr) p = p0 e -/kT Temperature (K)
only selected elements are shown
1000
100 10 1
0.1
0.01 -9
-10 -4 -5 -6 -7 -8 Al S As Mg In B W
0.001
Vapor Pressure (Torr)
p = p0 e -/kT
Typical functional form
200
400
600
800
2000
3000
5000
7000
Temperature (K)
p: vapor pressure
p0: “constant”
: latent heat of vaporization
Circles indicate melting points.
Note: melting points are not strongly correlated with vapor pressure.
© 1997, Angus Rockett

9. Effusion source section
Effusion Source With Cracker
A solid source material is evaporated from the effusion cell section into the cracker. In this section, the source material condenses on a catalytically active material and is dissociated.
Effusion source section
Cracking section
© 1997, Angus Rockett

10. Magnetic field into the drawing
Electron Beam Evaporation Sources
Operate at high temperatures.
The source material is its own crucible.
Electrons are
thermionically emitted from a filament,
accelerated, electromagnetically focused, and
generally rastered to spread heating.
Cyclotron orbit
Flux
Electrons
Local heating of evaporant
Water cooling
Magnetic field into the drawing
Filament
Cu crucible
© 1997, Angus Rockett

11. Metalorganic Gas Source
Metalorganic gasses are fed into the source through tubes. The gasses are decomposed in the heated cracking section and effuse from the front of the source.
Heat shield
Heater wires
Thermocouple
Boron nitride cracker
© 1997, Angus Rockett

12. Flux Monitors
Quartz crystals are used in an oscillator circuit. The resonant frequency changes as material is deposited on the quartz.
Electron-induced emission spectroscopy analyzes the wavelength and intensity of light emitted when an ion and an electron recombine.
Resonant oscillator
Wavelength filter -
Electron
Electron
Detector -
Ionizing collision
Deposited film
Incident flux +
Emitted photon
Ion +
Electron -
Quartz crystal -
Filament
© 1997, Angus Rockett

13. Flux Monitors
Ionization gauges can be moved into the growth position to directly measure the beam pressure.
A mass spectrometer samples the flux during growth and can monitor several masses individually. Spectrometer output can be used to control source temperatures.
Filament
Collector
Grid

14. Reflection High Energy Electron Diffraction (RHEED)
Reciprocal
Lattice
Rods
Reciprocal
Lattice
Rods
Ewald sphere
Ewald sphere
Allowed
Reciprocal
Lattice Vectors
Reciprocal
lattice
points
Used to measure growth rate, film thickness, and lattice parameter.
First
Order
Second
First
Order
Second
First
Order
Second
Perfectly flat surface Reciprocal rods have no width
Surface with monolayer roughness. Broadened rods.
Surface with large roughness. Transmission features.
© 1997, Angus Rockett

15. The image of the incident electron beam
RHEED
Ideal RHEED Pattern
The spacing gives the surface lattice constant
Fractional-order spots in some azimuths indicate surface reconstruction
Splitting of the beams indicates a tilted surface. The separation gives the surface tilt.
This distance gives the angle of incidence
The image of the incident electron beam
© 1997, Angus Rockett

16. RHEED Oscillations
The intensity of the spots oscillates as layers are deposited if the surface becomes alternately rough and smooth.
The period of the oscillation gives the growth rate and can be used to determine composition
in some cases.
Intensity
Envelope shows the surface roughness evolution.
Surface smoothness recovers if growth is stopped.
time
one monolayer
© 1997, Angus Rockett

17. Ellipsometry Measurements
Ellipsometry measures change in polarization of light when reflected from the sample surface. From this, film thickness and dielectric constant can be deduced.
Light source
Elliptically
polarized incident beam
Detector
Polarizer
Analyzer
Compensator
Linearly polarized exit beam
Sample
Spectroscopic ellipsometry in which the detector includes wavelength sensitivity adds additional accuracy and allows determination of values for several stacked layers.
© 1997, Angus Rockett