If a surface is bombarded with an energetic particle it is possible to cause ejection of surface atoms. This process is called sputtering. These ejected atoms could be condensed on to a substrate to form a thin film; this process can be realized by forming positive ions of heavy neutral gas such as Argon gas and bombarding the surface of target material by making the surface of cathode in an electrical circuit.
Basic Principle
When a charged particle strikes a surface a variety of interactions are possible. The most important reactions are shown in figure.
(a) Ejection of neutral atoms of surface material.
(b) Ejection of small no of charger atoms of surface material.
(c) Ejection of free electrons, the no. of free electrons usually greaker than io for each arriving incident ion.
Observed points
1) The first major effect in this method is that the neutral ejected atoms can be collected on a suitably placed substrate to form a film.
2) The second most important effect is that the electrons ejected can be accelerated from target cathode to a suitably anode.
3) On the way to anode they can cause for their ionization which helps to enhance sputtering head.
a) Cathode to Anode spacing : 15 cm
b) Pressure: 10-2 Torr.
Summary of observed salient features
i) The sputtering yield defined as the average no. of atoms ejected from target per incident ion, increases with increasing energy of incident ion.
ii) Also depends on atomic weight of target.
iii) Sputtering threshold exist within 5eV and 25ev for most of materials. Yield increased rapidly beyond the threshold shows some linearity in beginning and then approaches saturation.
iv) The yield depends on angle of incidence. It increases approximately as cosent angle between normal to the target surfaces and beam direction.
v) The yield as a function of atomic number of target displays an unduality behavior.
vi) The yield is rather in sensitive to target temperature.
vii) The ejected atoms have considerable energy. The distribution is Maxwellian.
viii) With increasing bombarding energy, the peak of curve shifts slightly towards high energy.
ix) The peak energy of ejected atoms increases with angle of ejection. However a decrease occurs for large angles greater than angle of ejection.
x) The ejected particles are considerably single atoms however in case of copper or silver at high energies some percent of ejected particles, clusters consists of two or three atoms.
xi) The ejected atoms will be in excited state and show characteristic recombination light emission.
Advantages
sputtering offers many advantages over other deposition techniques such as
1) You can get film of multi component material such as Alloys.
2) Irrespective of melting point, refractive materials films can also be prepared.
3) Insulating materials.
4) Good adhesion
5) Low temperature epitaxy
6) Thickness uniformity over large plane of areas.
Limitations
i) The source material must be available in sheet form.
ii) Deposition rates are usually less than 2000
per minute.
iii) Substrate material should be cooled.
THEORY OF SPUTTERING
1) Transfer of energy
2) Transfer of momentum
3) Radiation
The hot spot of evaporation theory yield at experimental evidence. Later developments showed that sputtering is not an energy transfer but rather a momentum transfer process. It considers sputtering as resulting of double or triple collisions of ion with in lattice atoms followed by its back reflection by lattices.
More sophisticated theories consider the sputtering as essentially a radiation damage phenomenon. Accordingly the incident ion displaces a no. of atoms (knocks on) during its passage thru material and thus loses its energy. Some fraction of knocked – on atoms will diffuse to surface and emerge as sputtering atoms.
The Knocked on atoms may also have sufficient energy to produce additional atoms which contribute to total sputtering yield.
The exact details of interaction of an ion with target atom depends on
a) Target atom
b) Momentum Transfer
c) Collision means free path.
Different Kinds of Sputtering techniques:
1) Glow discharge sputtering
2) Reactive sputtering
3) Bias sputtering
4) Triode sputtering
5) Ion beam sputtering
6) RF sputtering
CATHODIC SPUTTERING
The deposition of metal film by sputtering form a cathode by glow discharge method was first observed by groove.
The ejection of atoms from cathode surface by impinging of energetic positive ions of noble gases such as helium, argon, neon, krypton at a reduced pressure under high dc voltage gives rise to sputtering phenomenon.
It is now possible to make various resistive, semiconducting, superconducting and magnetic films by this technique in better way.
If the process doesn’t involve any chemical reaction between bombarding gas ions and the cathode is known as Physical sputtering.
On the other hand, some reactions are involved then it is termed as Reactive sputtering.
Both the types of sputtering are carried out in poor vacuum and are known as high pressure sputtering.
The mechanism of process involves a momentum-transfer between the impinging energetic ions and cathode surface atoms as a result of which physical removal of atoms takes place.
Sputtering yield increases with the energy and mass of bombarding ions and also with decrease of angle of incidence to the target. A minimum energy is required to start the sputtering process. Sputtered atoms have much higher energies than those of the thermally evaporated ones. Sputtering yield also decreases with large increases of ion energy because of deeper penetration of ions or neutrals inside the lattice.
Sputtering is also accompanied by the emission of secondary electrons from cathode surface. Auger transitions (radiation less) also take place along with emission of secondary electrons.
