© 1997, Angus Rockett Section I Evaporation

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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