Sputtering methods and nucleation Flashcards
Explain DC sputtering.
Two electrodes with high voltage in between - 5kV. The target is connected to the neg. electrode and the substrate that one wants to cover to the pos. electrode. The target is bombarded by Ar+ ions from the plasma to kick loose Au atoms and electrons. The Au atoms then hit the substrate on the other side. The electrons interact with Ar atoms to make more ions.
When the Ar-ion hits the target can three possible scenarios happen. Either:
- Ar-ion is scattered from the surface along with a scattered electron.
- the ion goes into the material as an implanted ion
- the ion does not go so deep into the material and kicks out an Au atom from the surface, called sputtered atom.
Describe the trade-off between having high or too high gas pressure when doing sputtering.
High pressure is good because have larger probability of sputtering because there are more Ar-ions. But if the pressure is too high have too many Ar-ions close to the target and they will screen the field and other ions are not attracted to the electrode.
What is the advantage of AC sputtering?
By switching the pos. and neg. electrode an accumulation of Ar-ions cannot be built up.
Explain what DC magnetron sputtering is.
It works like DC sputtering but a magnet is put behind the target which makes electrons stay close to the target, because don’t want them to go to the target and heat it up and also to ionize more Ar close to the target. Which means at same amount of plasma by the target can be sustained but at lower pressure.
Give an example of an experiment using sputtering.
By DC sputtering decorate PS domain with Au, Au prefers PS and will go to these domains on the substrate. If the substrate is analysed by GISAXS during the sputtering one can see two peaks appearing due to the interparticle distance between the Au atoms.
Explain what nucleation means. What are the effects that decide if a nucleation process takes place or not?
Atoms forming a larger particle. When there are a lot of atoms on the surface, ie. the surface/bulk ratio is high, it means that a lot of atoms have high energy which is unfavorable, therefore is there a barrier due to the surface effect. In other words the surface gives a pos. contribution to the energy difference.
The volume on the other hand gives a neg. contribution to the energy, because it is most favorable to stay inside the bulk.
–> The energy gain of creating a particle need to be larger than the cost of creating the surface, due to surface tension.
The free energy difference:
delta G = 4pir^3/3 * delta G_vol + 4pir^2 * surface tension
Explain the three different types of growth. Ostwald ripening, coalescence due to mobility and coalescence due to growth.
Ostwald ripening means that big particles eat smaller one, atoms on the surface of the smaller ones migrate to the larger particles, because they are more energetically stable, until the small are gone.
Coalescence by mobility means that particles move closer to each other and merge. The main driving force is attractive van der Waals interaction.
Coalescence due to growth means that one particle grows so it comes close to other particles and they merge, liquid-like behavior.
Explain the method ion beam co-sputtering.
In this sputtering method are atoms shot onto a target eg. of Ag and a polymer eg. PET, atoms fall down on the substrate as usual sputtering. But in this case have both Ag and polymer molecules since the target contain both. On the substrate they arrange into domains of irregular shapes, approx. 30nm.
