Thin films are deposited onto substrates for various reasons. They can be used as functional components of a device, for instance in the case of insulating or conductive layers. A thin layer can also be applied to serve as a mask for etching processes, in which case it is usually removed after the process is completed. Thin films consist mostly of metals or dielectric materials. Depending on the desired function, the thin film material is chosen. The type of material and exact function of the layer then determines the deposition method.
The deposition of thin layers on silicon or glass substrates is carried out through physical or chemical processes. The most commonly used processes are Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD). The thickness of the layers ranges from a few nanometers to several micrometers.
Physical Vapor Deposition (PVD)
Physical Vapor Deposition is the general term describing a multitude of methods to deposit thin films on various surfaces. They involve ablating material from a ‘target’, bringing it into a gaseous/vapour state, and allowing it to be deposited as a thin coating onto a substrate. These deposition methods consist of pure physical processes, such as e-beam evaporation (vaporization of the material is caused by an electron beam), sputtering (the material is atomized through an ion bombardment) or Pulsed Laser Deposition (PLD): this method uses pulses of laser energy to remove the material.
What can you expect, using PVD?
- Typically metal layers, for instance: Cr, Au, Ni, Al, Pt, Pd, Ti, Ta, Cu, SiO2, ITO, CuNi, etc
- Thickness up to several hundreds of nm (depending on stress and adhesion)
- Annealing can be used to modify properties after deposition (re-crystallization, stress reduction, and diffusion of alloys)
Chemical Vapor Deposition (CVD)
In Chemical Vapor Deposition, a chemical step takes place before the source material is deposited onto the substrate, or the material itself is the result of a chemical reaction. The two most commonly used methods are Plasma Enhanced Chemical Vapor Deposition (PECVD) and Low Pressure Chemical Vapour Deposition (LPCVD). PECVD is a process that uses a plasma to facilitate the reaction of precursor gases, resulting in deposition of the thin layer onto the substrate, whereas in LPCVD the required energy is delivered by heat, in a low pressure oven tube.
What can you expect, using CVD?
- Most common materials:
- silicon dioxide (SiO2) PECVD, LPCVD (TEOS, LTO)
- silicon nitride (SiN) PECVD, LPCVD (stoichiometric Si3N4), low stress nitride
- Polysilicon LPCVD
- ONO (oxide-nitride-oxide) layers
- Silicon-oxynitride (adjustable refractive index / stress) LPCVD
Other Furnace processes
- Wet oxidation of silicon
- Dry oxidation of silicon
- Annealing (e.g. bond annealing)
All of these processes produce pure, high-performance materials. The choice of technique depends on the type of material and the type of layer that is desired.