During my postdoc with Chris Summers's group at Georgia Tech, I learned about atomic layer deposition (ALD) and built several ALD systems. We used ALD to create inverse opal photonic crystals and to manipulate the properties of 2D slab photonic crystals.
Atomic layer deposition (ALD) is a process for depositing thin films in a slow but very precise way. It’s better to think of growing films a few atoms at a time rather than depositing them. Chemical vapor deposition (CVD) is also a film “growth” technique, but in CVD, films are grown at somewhat higher temperatures using chemicals that don’t spontaneously react with each other. In CVD, the chemicals react with a hot surface to form the film.
In ALD, more reactive chemicals are used so that the surface temperature can be lower (e.g., maybe the surface is plastic and would melt at higher temps). Since the chemicals are more reactive, the surface can be cooler, which is good, but the chemicals also can react with each other in as vapors, which is typically bad. To prevent this, the chemicals are added to the surface sequentially until they can’t react with the surface any further, and this process is repeated over and over. In this way, films are grown in a cyclic, layer-by-layer manner where the amount of material that is deposited in each cycle is controlled by self-limiting surface chemistry (cool!).
There are lots of good cartoons and references for learning more about ALD.
In my group, we use homebuilt ALD systems as well as a commercial system that we modified.
- Tube furance ALD system built by Laurie Stephey and Chris Gorman at Rollins College in 2007. Lots of modifications since.
Four inlet ports; capable of full stop flow (12500:1 aspect ratio record)