Vacuum Evaporation Recap
? Use high temperatures at high vacuum to evaporate (eject) atoms or molecules off a material surface.
? Use ballistic flow to transport them to a substrate and deposit.
? Film uniformity can be an issue.
? Alloy evaporation is very complicated and in most cases, not possible.

An Alternative Method
? Instead of using heat to eject material from a source, we can bombard them with high speed particles.
? The momentum transfer from the particles to the surface atoms can impart enough energy to allow the surface atoms to escape.
? Once ejected, these atoms (or molecules) can travel to a substrate and deposit as a film.
? There are several considerations here: – Creating, controlling and directing a high speed particle stream.
   – Interaction of these particles with the source surface and emission yields.
   – Deposition of the emitted atoms on the substrate and film quality.

Some Terminology
? Atomic particles can best be easily controlled by electromagnetic methods if they are charged. A weakly charged gas of particles that exhibit collective behavior is called a plasma.
? The source material is called the target and the emitted atoms or molecules are said to be sputtered off.

Sputtering
? So in sputtering, the target material and the substrate is placed in a vacuum chamber.
? A voltage is applied between them so that the target is the cathode and the substrate is attached to the anode.
? A plasma is created by ionizing a sputtering gas (generally a chemically inert, heavy gas like Argon).
? The sputtering gas bombards the target and sputters off the material we’d like to deposit.

Generating and Controlling the Plasma
? Ions can be generated by the collision of neutral atoms with high energy electrons.
? The interaction of the ions and the target are determined by the velocity and energy of the ions.
? Since ions are charged particles, electric and magnetic fields can control these parameters.
? The process begins with a stray electron near the cathode is accelerated towards the anode and collides with a neutral gas atom converting it to a positively charged ion.
? The process results in two electrons which can then collide with other gas atoms and ionize them creating a cascading process until the gas breaks down.
? The breakdown voltage depends on the pressure in the chamber and the distance between the anode and the cathode.
? At too low pressures, there aren’t enough collisions between atoms and electrons to sustain a plasma.
? At too high pressures, there so many collisions that electrons do not have enough time to gather energy between collisions to be able to ionize the atoms.

Glow Discharge Formation
? Initially the current (charge flow) is small. As charges multiply the current increases rapidly but the voltage, limited by supply, remains constant.
? Eventually, there are enough ions and charges for the plasma to be self-sustaining.
? Some of the electron-atom collisions will produce light instead of electrons and ions and the plasma will also glow accompanied by a voltage drop (normal glow)
? If the input power is increased further, the current density becomes uniform across the cathode and we’ll be in the abnormal discharge regime. This is where sputtering operates.

Plasma Pressures
? Unless there are enough collisions, the plasma will quickly die.
? In order to have a self-sustaining plasma, each electron has to generate enough secondary emission.
? Since we want collisions to occur, the pressure can not be too low. – The mean free path should be a tenth or less than the typical size of the chamber.
? Also, since we want the electrons to gain enough energy between collisions, the pressure can not be too high.
? This means discharge tube pressures around 10-1000 mTorr and plasma densities around 1010 – 1012 cm-3.

Ion-Surface Interactions
? When ions bombard a surface, several things can happen:
   – Reflection
   – Sticking (adsorption)
   – Sputtering
   – Ion implantation
   – Chemical reactions
   – Electron and photon emission
? The ion beam energy is the critical parameter.
   – < 5 eV : Adsorption or reflection
   – 5 - 10 eV : Surface damage and migration
   – 10 - 3 keV : Sputtering
   – > 10 keV : Ion implantation

How Ions Sputter Atoms
? When ions collide with surface atoms on the target, the energy transfer can knock some of these atoms off the surface.
? The key principle is energy and momentum conservation.
? In any collision, momentum is conserved.
? If the collision is elastic, kinetic energy is also conserved.
? The energies required for sputtering are much higher than lattice bonding or vibrational energies (which are the causes of inelastic interactions), therefore sputtering collisions can be considered elastic.