As many people involved in Atomic Deposition (ALD) might know, area-selective ALD is a hot topic at the moment.
Papers, workshops, and blog posts are numerous (e.g., the recent atomic limits blog post or posts on BALD engineering) and they explain well what exciting new options there are to go towards area-selective growth. In a sense area-selective ALD utilizes effects that historically have annoyed ALD users, namely that due to the chemical of ALD, nucleation can depend on the initial . In general, work in the ALD was undertaken to minimize this , for instance plasma ALD generally has negligible delay, but for area-selective ALD, nucleation delay effects have been cleverly enlarged instead.
Interestingly, even though plasma ALD generally has no nucleation delay, it can still be used for area-selective ALD, as my university colleagues in Eindhoven have recently shown (http://pubs.acs.org/doi/10.1021/acsnano.7b04701). Key aspect is to re-apply any inhibitor used, after the plasma exposure and this can be done in a three step ABC ALD process as seen below.
One question that came to my mind is what kind of ALD system would people want to study area-selective ALD, and I believe our systems have quite some features that could be beneficial. For instance:
Our tools in general also provide excellent control, for instance they have:
The ideas of clustering and combining with etching which have been proposed for area-selective ALD lead me again to the idea of atomic scale processing, which I discussed in a blog post before (The toolbox for Atomic Scale Processing). I can imagine that by combining area-selective ALD and other processes you could grow novel metamaterials and unique structures. For instance: by growing on selectively exposed Cu, or only effectively grow on sidewalls of structures by periodically etching planar deposition away (i.e., topographically selective ALD). To this end combinations with direction ion exposure could be a benefit, either in a separate plasma etcher or in a FlexAL with the substrate biasing feature. Overall I am expecting a lot of new exciting discoveries and new structures and materials to be able to be made in a controlled fashion.
Author: Dr Harm Knoops