Sputter deposition

1. Sputterdeposition

2. An accelerated ion incident on a material can transfer momentum, and thereby eject atoms or molecules from it. • Utilization: • Dry etch • Depth profiling (SIMS, AES) • Deposition of thin films Sputtering

3. Low substrate temperature • High melting point materials can be deposited • Good adhesion • Good step coverage compared to evaporation • Less radiation damage than e-beam evaporation • Well suited for alloys and compounds Advantages of sputter deposition

4. Steps of the sputtering process • Plasma provides ions • Ions accelerated in electricfieldbetweentarget (cathode) and substrate (anode) • Sputteringof target • Transport ofsputtered material • Adsorption to substrate • Surfacediffusion • Nucleation and film formation Sputter deposition setup

5. A gas with ionized atoms and electrons • Choices for sputtering plasma • Chemically inert gas to avoid reactions • Efficient momentum transfer when the mass of the sputtering ion is close to the atomic mass of the target atom • Argon (Neon for light target elements, and Krypton or Xenon for heavy) Plasma

6. Plasma • Ar ions are accelerated towards the target for sputtering • Releaseofsecondaryelectrons • Sufficiently low pressure • So electrons achieve necessary energy before collisions • Too low pressure gives too few collisions to sustain the plasma • The glow comes from de-excitation of atoms after collision with electrons that has too • low energy for complete ionization

7. Increasing ion energy • E<10eVAdsorption, bouncing off surface, or surface damage • 10eV-5keV Sputtering • E > 5keV Ion implantation • At sputteringenergies • Nuclear stopping is effective • Interaction with top layers • Sputtered atoms typically have 10-50eV of kinetic energy • Two orders of magnitude larger than for evaporation • This leads to better surface mobility when the atoms reach the substrate • Elastic collision: Conservation of momentum and kinetic energy • A qualitative view of sputtering can be achieved by considering an elastic model • But for a thorough analysis one need to consider the coupled effect of bond breaking and physical displacement Ion interaction with target

8. Depend on • Ion and target atomic mass • Ion energy • Target crystallinity • Angle of incidence Sputter yield

9. Deposition on substrate

10. Surface diffusion happens until nuclei of critical size are formed. • Capture of further ad-atoms by the nuclei forms islands. • If diffusivity is high the islands will merge at small sizes and yield a smooth continuous film • Three-zone modelgives the morphology as function of substrate temperature and incident ion energy • 1: Amorphous, low density • T: Specular, small grains • 2: Columnar grains with facets • 3: Larger grains, equiaxed Morphology

11. Depositedstoichiometrydependondifferences in thermalisation in the plasma • Multiple targets • Different areas on target • Use target composition to yield the wanted film composition • Base pressure is also important for the film quality, as contamination by N and O can affect the reflectivity of the film. • Step coverage improvement by: • Heating • Diffusion • Biasing • Resputtering Stoichiometry and step coverage

12. Choice of ions • Plasma pressure • Voltage for acceleration • Angle • Substrate bias and temperature Key parameters

13. So far: DC sputtering • Possiblechanges • RF sputtering • Avoid charge build-up when material is isolating • Magnetron sputtering • Increased ionization of Ar • Reactivesputtering • adding a reactive gas that reacts with the sputtered atoms to form the compound • Enables sputtering of compounds consisting of materials with very different sputter yields as TiW DC/RF/Magnetron/Reactive