Sputtering deposition is one of the main physical vapor deposition techniques to deposit thin films of metals or other materials over a substrate.
The technique is useful for microelectronics applications and has been widely popular in the industry for several reasons as it can be seen as the natural evolution of thermal evaporation, one of the earliest techniques for thin film physical vapor deposition.
While thermal evaporation requires only a relatively simple vacuum chamber where the material is heated in a controlled way and evaporated, sputtering needs much larger and complicated machinery that allows the substrate to be literally “bombarded” by ions in order to eject particles that get deposited over a substrate placed in front of the target itself
Sputtering is now the deposition method of choice for industrial applications while thermal evaporation is little more than a lab experiment as sputtering allows deposit a large variety of materials in a much tightly controlled way.
While with thermal evaporation the deposition rate can only roughly be controlled by setting the material source at a predetermined temperature, sputtering allows to control more factors such as: the gas pressure inside the chamber, the potential difference between anode and cathode, the target and substrate temperatures, etc.
The main differences between thermal evaporation and sputtering are:
- With thermal evaporation only a limited number of materials can be deposited, with sputtering the variety is much larger;
- With sputtering it is possible to control the deposition speed down to almost one atomic layer per second while with thermal deposition hundreds if not thousands of atomic layers are deposited in a second. In industrial applications, where the deposition rate must be strictly controlled, thermal evaporation is then not an option
- It is possible to deposit larger areas with sputtering than with thermal evaporation
- The decomposition of the target material and its erosion during sputtering are uniform and therefore the process makes better use of the target material, which in many cases can be expensive (gold, ruthenium, etc.)
However, sputtering has also some disadvantages if compared with thermal evaporation as:
- It needs by far more expensive machinery and devices than thermal evaporation, which basically needs only a simple vacuum chamber coupled with a precise thermometer
- The substrate can be damaged due to particle hit during sputtering
From the above considerations, it is clear that the advantages of sputtering over thermal evaporation far outweigh the disadvantages for industrial applications.
Nowadays, sputtering is widely considered the physical vapor deposition method of choice for compact disks, large area displays, and even for applications not related with microelectronics, such as deposition of TiN, CrN, TiC or CrAlN over saw blades and bearing gears.
The sputtering process mechanism works as follows: during the deposition, the disc of materials which needs to be deposited (called as the “target”) has a negative potential and it is bombarded by positive ions of inert gases such as argon or xenon. By purely kinetic energy transfer between the ions colliding on the target and the atoms on the target surface, the latter get ejected from the target surface and collide with the substrate, creating a thin film of material over it.
Keeping all other variables fixed, sputtering deposition rate is proportional to the plasma energy within a certain range and therefore it can be easily controlled.
This is the main reason why sputtering deposition is superior to other thin film physical vapor deposition methods such as thermal evaporation
However, as hinted above, the whole process of sputtering requires a much larger system as it takes at least two or three order of magnitude more energy to liberate one atom from the target during sputtering than during thermal evaporation.
Moreover, the target and the substrate require considerable cooling and therefore a proper refrigerating system needs to be put in place.
The whole sputtering station needs to be into vacuum, and to be kept free from contamination from atmospheric element.
The formation of sustained plasma is also crucial to the reliability of the whole process.
The reason why the conditions of the plasma need to be carefully controlled are several, probably one of the most important parameters is plasma pressure, a factor that is pivotal to the reliability of the whole process.
If the pressure of the plasma is too high, a large number of the sputtered atoms cannot pass through the gas and get reflected back to the cathode, while if the pressure is too low, the number ionizing collisions between secondary electrons released from the cathode and the inert gas is not enough to keep the plasma sustained at the needed levels.
Another point that needs to be carefully considered is the power supply for the whole system as the type of power supply is heavily dependent on the type of material that needs to be sputtered.
For conductors, a typical DC power supply can be used, for insulating materials an RF power supply is needed. Insulating materials are usually harder to be sputtered and therefore sometimes sputtering may not be the best physical vapor deposition method, plasma lased deposition is a another possible option
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