Question

What is the definition of sputter yield? What are the two main parameters that influence the sputter yield?

Answer 1

When a surface is bombarded with high velocity positive ions, it is possible to cause ejection of the surface atoms. This process of ejecting atoms from the surface by bombardment of positive ions (usually inert gas ions), by momentum transfer process between the sputter gas and target atoms is commonly known as sputtering (cathode sputtering) Argon is commonly used as the sputtering gas.

The ejected atoms can be made to condense on a substrate at an optimal distance from the target to form a film. Apart from the neutral atoms, charged atoms and electrons are also emitted from the surface. The sputtering yield (denoted by ‘S’) is defined as the number of atoms ejected from the target surface per incident ion).

It depends on the target material composition, binding energy, characteristics of the incident ion and the experimental geometry. It also depends on the voltage and current (sputter power) at which sputtering takes place.

Parameters affecting sputter yield:

1) Base Vacuum

The chemical purity of evaporated films depends on the nature and level of impurities that are initially present in the sputtering chamber, in the source (target) or contaminates from the support materials during deposition. Also, it may originate from the residual gases present in the vacuum system. Hence it is necessary to obtain a good base vacuum prior to thin film deposition. Sometimes sputtering system is also thoroughly baked for improving base vacuum.

2) Target substrate geometry

The target-substrate geometry, is an important factor that influences the ultimate film uniformity. This includes the orientation of the source (target) and substrate and their distance of separation. In sputtering, apart from target atoms there are also electrons emitted from the target (cathode) by ion bombardment, and these are accelerated towards the substrate where the electrons collide with the gas atoms. These electrons help in sustaining the plasma by causing ionization as the ionized gas atoms in turn may bombard the target producing the secondary electrons.