● Sputtering technique:

We have so far seen the deposition techniques based on the evaporation mechanism. The sputtering methods are characterized by a nonthermal mechanism for the transfer of the solid target material into the vapor phase. In the sputtering method [4], the ejection of atoms from the cathode surface by impinging the energetic positive ions of noble gases such as helium, argon, neon, krypton, at a reduced gas pressure under high voltage gives rise to the sputtering (see Figure 39.1). This process involves a momentum-transfer between the impinging positive ions and the cathode surface atoms as a result of which a physical removal of atoms takes place. The sputtering yield is defined as ratio between the mean number of emitted atoms to the number of incident ions on the target surface.

Figure 39.1: Schematic drawing of sputtering process.

1. The sputtering yield increases with a) energy b) mass of bombarding atom c) decrease of angle of incidence to the target.

2. A minimum energy is required to start the sputtering process. Sputter atoms have higher energy than those of the thermally evaporated atoms.

3. The sputter atoms are ejected along the crystallographic directions of the cathode metal lattices.

4. Sputtering yield decreases with larger increase of the ion-energy because of the deeper penetration of ions inside the lattice. Also, it is insensitive to the cathode temperature.

5. Sputtering is also accompanied by the emission of secondary electrons from the cathode surface.

6. If the sputtering process does not involve any of the chemical reaction between bombarding gas ions and the cathode, it is known as physical sputtering. If some reactions takeplace then it is named as reactive sputtering.