Sputtering is a process that has its roots as far back as 1852. Physical deposition processes are a class of material deposition methods that do not require a chemical reaction for the deposition process to occur. Physical deposition methods have the capability to deposit thin films of conductors and insulators that are used in MEMS application for optical coatings or electrical conductors.
James J. Allen
Chapter 02: Fabrication Processes
The two physical deposition processes are evaporation and sputtering.
The sputtering process utilizes a plasma formed by a large voltage in a low pressure gas (0.1 torr) across a closely spaced electrode pair. The target material (source material to be deposited) is on the cathode. The ions come from an inert gas within the chamber. Bombardment of the cathode by energetic ions gives rise to the sputtering process.
When ions strike a material surface, several things can happen, depending on the energy of the ions:
- Bouncing off the surface
- Absorption by the surface to produce heat
- Penetration of the surface to deposit the energy within the material
- Ejection of surface atoms from the cathode (sputtered)
BETTER MATERIAL COVERAGE
Sputtered atoms have more energy than evaporated atoms, which increase the surface mobility of the sputtered atoms. Increased surface mobility produces better step coverage than is attainable with the evaporation process. Because ion collisions give rise to the sputtering of the target material, a gas with a high atomic weight is advantageous. Argon is a frequently used inert gas in a sputtering process. Sputtered films can be deposited at ambient temperature. The sputter deposition does not depend on the substrate temperature; however, substrate may be heated to promote adhesion or prevent film cracking.
There are several variations to the sputtering process to achieve special effects.
- Reactive sputtering. A chemical combination between the sputtered material and the ambient gas reacts to form a compound (e.g., sputter silicon with a nitrogen ambient to form silicon nitride films).
- Triode sputtering. An additional filament in the chamber is used to increase the sputter rate by producing additional electrons.
- Magnetron sputtering. A magnetic field is used to increase density of electrons, which will increase the sputter rate.