The process of sputter deposition of thin films relies upon inert gas ions impacting the surface of targets to knock out target species and send them in the direction of the substrate surface to be deposited. This process heavily depends upon the absence of other gas species in the path of the inert gas ions. Therefore sputter deposition is performed under high vacuum condition. The gas inside the chamber is drawn out using powerful rotary and cryogenic pumps. The level of vacuum is indicated by the pressure value present inside the chamber with respect to the atmosphere. This is known as the base pressure. Chambers are typically pumped to base pressure values better than 0.1 mPa but for high end systems these can be as good as 1 µPa. Lower base pressures are better to achieve higher purity thin films as there are less impurities to get embedded and also because incoming species don’t lose their energy colliding with the particles in the path and thereby deposit more crystalline films. Once the required base pressure is achieved, the specified amount of inert gas species, typically Ar is filled into the chamber which is chemically inert and has no charge. Then a high bias is applied between the electrodes which strips the Ar of its electrons creating an Ar+ ion and electron pair. This is known as ionization of the gas and the resulting cloud of ionised gas particles behaves as a single entity and is known as plasma. The ions from the plasma cloud can be directed by the bias process and made to impacts the target surface to start the sputtering process.
The inert gas is applied in measures of Standard cubic centimetres per minutes or Sccm. This raises the chamber pressure to a higher value compared to base pressure, typically around 0.1-0.7 Pa and this pressure is known as the working pressure or deposition pressure. Deposition pressure has profound effect on the morphology and crystallinity of the deposited thin films and thereby starts affecting the final film properties as well. Thornton’s structure zone model corelates the deposition pressure and the substrate temperature at the time of deposition to the surface features expected in the film.
The first effect of deposition pressure is seen on the deposition rate of the thin film. At low Ar pressures, less ions are formed to strike the target surface therefore, resulting in low deposition rate and with increasing Ar pressure this rate increases. However, there is a certain threshold at which this reaches a maximum, because any further increase in gas pressure will result in scattering of the incoming species. Another effect of increasing gas pressure is that the travelling target species will undergo larger collisions on their way to the substrate and will lose energy, thereby forming a porous structure and conversely form denser films at lower gas pressures. This will also be reflected in the surface roughness as films deposited at lower gas pressure will be smoother compared to those at higher gas pressure. The energy of arriving adatoms on the substrate surface affects the crystallization of the films, at lower gas pressure the species would have higher energy to align in their preferred direction giving them better crystallinity when observed through XRD whereas films deposited at higher pressure will have low energy and will therefore show poorer crystallinity compared to those deposited at higher temperature. The crystallinity, surface properties, and morphology will have their combined effect on the desired optical, mechanical or electrical properties. Therefore, it is very important that thin films depositions are performed at optimized deposition gas pressure to achieve the best properties.
Follow our blog to understand the effect that addition of a ternary element like Ag will have on the mechanical and biomedical properties of our thin films.
Comments