Thin Film Evaporation

Difference between PVD and CVD

PVD: inorganic source material, no chemical pyrolysis at surface, lines of sight deposition

CVD: Uses inorganic or organic-containing source materials, molecules land on surface, pyrolyze then incorporate into the film

Thermal Evaporator System Parts

crucible, vacuum chamber, Quartz crystal microbalance

What does the crucible do?

holds and heats the source material, sometimes is the source

What types of geometries does a crucible have

Source rod translated through a heating coil

Charge held in a dimpled boat which is part of a resistive circuit

How does the thermal evaporator system work

1. Pumped to presssures <1 mTorr

2. Creates long mean free paths for evaporated atoms

3. Evaporated atoms deposit in a line-of-site manner creating a spherical wavefront

What does the QCM do

the quartz crystal microbalance measures the thickness in sito with angstrom level detection

What is the underlying assumption behind the kinetic theory of gases

you treat gas molecules as non-interacting spheres

In a thermal evaporator what is the relationship between pressure and impurities

As the pressure in the chamber goes down, less impurities are incorporated during growth, and the evaporated fluxes become easier to accurately estimate

In thermal evaporation the temperature for appreciable depoisition depends on the metal’s ________

vapor pressure

What is the equilibrium pressure?

Pressure where rate of evaporation/sublimation is equal to rate of condensation

Why does all of the material arriving to the wafer in thermal evaporation stick to it

Due to the ultra-high vacuum

The deposition rate in the evaporator depends on the ______ and _____ of the wafer in the chamber

location and orientation

How is a crucible heated?

1. Resistive heating: most simple method but has outgassing from holder

2. inductive heating: can be used for refractory metals but there is contamination from the crucible

How does E-beam evaporation work? What are its pros and cons

an electron beam hits only the charge in the crucible using a magnetic field to bend it onto the crucible. This creates a self-contained hot region.

Pros: wider range of materials, faster deposition rate, high purity

Con: radiation damage

What is an issue with line-of-site step coverage

It only coats the surface directly facing the source

How can you improve step coverage

Rotate the wafers increasing intital coverage uniformity

Heat the wafers to increase energy of adatoms (adsorbed atom), allowing them to diffuse further across the surface