A customer recently asked us about the index of refraction of their Silicon Nitride Thin Films:
Stoichiometric Silicon Nitride (Si3N4) is a white, high melting point, thermally stable, insulating compound that is the most thermodynamically stable of all the various silicon nitrides. Si3N4 is a difficult material to consolidate into a bulk sputtering target form. It disassociates at temperatures above 1850 C, below the melting point of 1900 C. This makes it difficult to sinter under normal Hot Pressing environments or even under Hot Isostatic Pressing conditions. Therefore, a proprietary Spark Plasma Sintering Technique is used to produce a dense, near net shape, solid suitable for sputtering target applications. This material is slightly silicon rich, making it grey, or translucent black, in color; but it is still highly insulating in nature.
So when one of our customers asked why their resultant films were not providing the desired Index of Refraction (n=2.1) it was necessary to look at the deposition parameters that they were using to resolve what the resolving issues might be. As an insulating material, Si3N4 has a resistivity greater than 10 – 12 Ohm cm and cannot be deposited with a conventional magnetically enhanced cathode assembly utilizing either an AC or DC power supply. It is necessary to revert to using either an rf generator, with an associated matching network, or utilizing a mid frequency range pulsed DC supply such as the Advanced Energy Pinnacle plus unit.
In either case, there will be some increased disassociation of the host Si3N4 target material composition in the plasma, freeing up the elemental Si and N constituents. These will likely be pumped away in the vacuum chamber. So, to produce stoichiometric Si3N4 resultant films with the desired physical and electrical properties, it is necessary to utilize a reactive deposition process. The argon working gas needs to be augmented with an active partial pressure of some other gaseous constituent, typically either pure nitrogen or a combined ammonia/silane mixture in the ratio of 0.7-1. In any case, deposition must be done at a relatively slow rate to allow enough collisions in the mean-free-path to allow for an equilibrium Si3N4 resultant film. It may take some Trial and Error manipulations to dial in the correct partial pressure of the reactive gas constituent to produce the desired films, based on the configuration of the deposition system and the other operating parameters, but once configured, reproducibility should not be an issue.