Monday, 4 February 2019

On how I came up with the "periodic table of ALD processes"


IUPAC, https://www.iypt2019.org/: "1869 is considered as the year of discovery of the Periodic System by Dmitri Mendeleev. 2019 will be the 150th anniversary of the Periodic Table of Chemical Elements and has therefore been proclaimed the "International Year of the Periodic Table of Chemical Elements (IYPT2019)" by the United Nations General Assembly and UNESCO."

In atomic layer deposition, for which Tuomo Suntola in 2018 received the Millennium Technology Prize, the periodic table is a central tool. This was recently highlighted in the Atomic Limits blog, who created a colourful update of the periodic table of ALD processes which I originally created for a review article in 2005 and which was updated for another review article in 2013. In this post, I tell a bit more on how that "periodic table of ALD" was born and updated. But first: a joint picture with Dmitry Mendeleev in St. Petersburg State University, taken - while visiting Prof. Victor Drozd in November 2015 - to honor this great chemist from St. Petersburg.

Then-Dr. Riikka Puurunen (now Prof.) and statue of Prof. Dmitry Mendeleev in the hall of St. Petersburg State University. Photo taken upon request by Prof. Victor Drozd. November 11, 2015. Originally published in http://aldhistory.blogspot.com/2015/11/travel-notes-st-petersburg-puurunen.html.

To the story.


For my review article on the surface chemistry of ALD, published in 2005 in J. Appl. Phys. (Applied Physics Reviews) http://dx.doi.org/10.1063/1.1940727, among other things I went through all two-reactant processes developed by ALD. I tabulated them, according to the atomic number. The table became huge, it runs over several pages. How could I condense the message of the tabulated material, in a shorter way that one can grasp by looking at once? The only solution that I could think of was to turn to the core tool of the chemists: the periodic table of the elements. But how would I summarize the processes this way? It would be impossible to fit everything - what could I fit in and how, and what would I leave out?

I ended up creating a way, where for each element's entry is divided in smaller cells, which tell which ALD-made compounds have been grown, and whether the element itself has been grown. Not all compound materials tabulated could be fitted in the this division, hence the addition of the class "other materials". The key is copied in the image below, screen capture from http://dx.doi.org/10.1063/1.1940727 (it has also an open access companion: https://www.vtt.fi/inf/julkaisut/muut/2010/Puurunen.pdf).

Original, Puurunen 2005, J. Appl. Phys. 2005, http://dx.doi.org/10.1063/1.1940727 

The review from 2005 http://dx.doi.org/10.1063/1.1940727, which I wrote as sole author as a postdoctoral researcher at IMEC and finished whiled at VTT, has been now cited over 1300 times (WoS, Feb 1 2019, capture here). In hindsight, this review has been transformational for the field, not just for (i) introducing this way of summarizing of ALD process literature in the tabulated form later nicknamed "mammoth" (here) and in the form of a periodic table, but also for example because (ii) it initiated the ALD history activities still continuing in the Virtual Project in the History of ALD (http://vph-ald.com) and (iii) it overviewed all surface chemistry research results on the trimethylaluminium-water ALD process, which since then has become some kind of "model" or "archetype" ALD process (discussion on the details still continues, see e.g. here, here and here).

Years later, as senior research scientist at VTT, I initiated a joint project between VTT (research Prof. Kattelus) and University of Helsinki (Prof. Leskelä) together with several international partners (ALEBOND, financed by Tekes). We were investigating the evolution of crystallinity of ALD films and controlling that to our benefit; a few articles were published, e.g. on reducing stiction in MEMS (https://doi.org/10.1016/j.sna.2012.01.040) and on wafer bonding (https://doi.org/10.1016/j.sna.2012.05.006). By necessity, we overviewed the crystallinity of ALD films, and writing a review article on this topic was included in the original project plan. Once we went through the ALD literature anyway to collect the crystallinity info, it was rather easy to update the ALD process table, so that was done too. Another highly cited review article followed J. Appl. Phys. (Applied Physics Reviews) in 2013, http://dx.doi.org/10.1063/1.4757907. Once more thanks to Ville Miikkulainen, who as a postdoc went through the literature for this review - that was a massive work to do. Below is copied the explanation key of the periodic table - identical to the one from 2005.   
From the first update, Miikkulainen, Leskelä, Ritala, Puurunen, J. Appl. Phys, http://dx.doi.org/10.1063/1.4757907

On Jan 28, 2019, the Atomic Limits blog from the Prof. Kessels group announced that they have updated the summary of atomic layer deposition processes, "often used to show which materials have been prepared by ALD". Kessels continues: "As far as I know, Riikka Puurunen started with this in her highly-cited review paper that appeared in Journal of Applied Physics [1] in 2005." I now confirm this statement to be true. Kessels further discusses: "Although extremely valuable for all of us that are interested in ALD, both papers only contained a black-and-white version of the figure which is not well suited for fancy presentations about ALD. Furthermore, having arrived in 2019, it is very urgent to have an update the Periodic Table with ALD-prepared materials available." A colourful update was then published, and the authors promise to update missing materials on a yearly basis. Thank you to Prof Kessels and collaborators for this update - obviously needed and useful for the field.

A closer look at the update shows that the core logics behind organizing the material have remained the same as in the system I created in 2005. There have been superficial changes which improve the presentation: a fourth row has been added. Now, the atomic number can be accommodated in the table and the chemical symbols presented in large letters. The addition of a fourth row has necessitated the shuffling around of the individual material indicators. Had I (as the original creator of the table) been asked, I would probably have proposed a slightly different order reflecting the relevance of the different classes of compounds, but this one is of course fine and useful, too.  

From Twitter, https://twitter.com/AtomicLimits/status/1090147015462436864
As a last note, I would like to point out - as I did already in Twitter and in the comments section of the blog post - that there is one reference and material missing from the periodic table of the J. Appl. Phys. 2013 review, which the Atomic Limits update of the periodic table summary (accessed 4.2.2019) misses as well. We missed it because it did not contain the keywords we used in the literature search. It is the SnSe process by Drozd and coworkers, https://doi.org/10.1088/0022-3727/42/12/125306. Same person, who took the photo above of me and Mendeleev.

February 4, Espoo, Finland
Riikka Puurunen

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Virtual Project on the History of ALD (VPHA) - in atmosphere of Openness, Respect, and Trust

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