The Basics of Vacuum Evaporation
Traditional vacuum evaporation methods work, essentially, by heating materials to high temperatures.
This is required for common thin-film evaporants so that they are vapourised from a source. The evaporants then move up through a process chamber to condense on a substrate. Higher source temperatures allow for the deposition of materials with higher evaporation/sublimation temperatures.
Low Temperature Evaporation
Recently, there has been a surge of interest in the use of Physical Vapour Deposition (PVD) methods for forming thin films of materials that have relatively low vapourisation temperatures (i.e., high volatility). Examples include small-molecule compounds with properties useful for organic electronics applications such as OFETs, OLEDs and OPVs.
Vaporisation of these materials usually takes place at <600 °C. This is much lower than, for example, metals used for contact electrodes. Furthermore, for organic materials, the onset of vaporisation under vacuum conditions is extremely sensitive to temperature, i.e., a small temperature increase can make the difference between a zero deposition rate and ejection from the source of the entire load!
The secret source – LTE Cells
Given the above, vacuum evaporation of organic materials requires sources with a design that is very different to those seen in standard thermal evaporation. These sources, often referred to as low-temperature evaporation (LTE) cells, separate the heating element and material support to allow for fine control of material heating power. This ensures that evaporant temperatures can be varied gradually – enabling precise variation of the deposition rate.
It is common to run LTE sources as part of a feedback loop based on in-situ rate and thickness measurements from a quartz crystal sensor head, as this provides for the best control performance.
Like all evaporation techniques, LTE is line-of-sight, with evaporant vapour moving in straight lines between the source and the substrate. As such, it provides highly anisotropic coating useful for, among others, lift-off applications. However, in some cases, for evaporants with very high vapour pressures, some isotropic nature may be seen due to high local pressures in the deposition plume.