In March 2019 the ALD/Database was introduced to the ALD community on www.atomiclimits.com. The ALD/Database is an initiative by Harm Knoops, Bart Macco, Vincent Vandalon and Erwin Kessels and offers a centralized platform for the ALD community to provide an overview of papers reporting on ALD processes and their characteristics. New papers can be added to the database by visitors – and as such the site is a true community-driven effort – which will be reviewed to keep the database consistent and ensure that only papers with an appropriate scope are included.
We would like to seize the end of 2019 as an opportunity to look back and reflect on the first year of the database. In this blog post we will also provide context of its inception, insight into how the ALD/Database has been growing, and explain the review process of the new submissions in more detail.
Why did we start the database?
We identified the need for a central and up-to-date “database” of papers and publications a couple years ago. At the start of 2019, more concrete plans were formulated and the website was built / programmed by Jan Buiter. Practically speaking, we wanted the ALD/Database to provide: (1) A “periodic table” of the materials showing at a glance which materials have been synthesized by ALD. (2) Allow a visitor of the site to quickly track down the relevant papers describing how to perform a specific ALD process. (3) Enable visitors to submit new entries for the database.
The new entries submitted by the community to the ALD/Database are reviewed before they are shown on the website. The main acceptance criteria are the correctness / consistency of the data entered in the database (e.g. naming of the reactants) and whether there are sufficient processing details in the paper. Currently the entries are reviewed by Harm Knoops, Joseph P. Klesko (joining the team mid 2019), Bart Macco, and Vincent Vandalon. See the inset for a more detailed description of the type of publications we want to collect in the database.
A brief comment on our review criteria
The database was started to build up a collection of papers discussing ALD processes and the resulting material characteristics. So we are looking for papers that provide sufficient insight into the ALD process itself and should be helpful to reproduce the process. To do so, a paper should mention practical details such as dose times, growth-per-cycle, and temperature although exceptions do occur. Questionable are, for example, papers reporting on the device performance of a FET where the high-k dielectric was made by ALD without going into further details about the ALD process or the deposited material. On the other hand, a paper reporting on the coating of nanostructures (nanowires, fibers) by an already established ALD process might omit the GPC and saturation curves and still get accepted: The GPC on planar substrates is already known and it is not straightforward to determine the GPC reliably on nanostructures. If the paper gives sufficient insight into the characteristics of the ALD process for coating of nanostructures, it is still valuable to the (intended) audience of the ALD/Database.
How does the ALD/Database work?
An overview of all the processes in the ALD/Database are shown graphically using an annotated periodic table. This user interface was inspired by a figure in the review paper by Puurunen [J. Appl. Phys. 97, 121301 (2005)]. By clicking on an element in the ALD/Database, a list is shown of papers reporting on the synthesis of films containing this element. This allows a visitor to quickly determine if a material has already been grown by ALD and to easily find the appropriate references describing the process (if any).
We encourage the use of the ALD/Database in your presentations and papers. You can download an image of the periodic table (as shown above) for use under the creative commons agreement from the website itself. See the so called “hamburger” menu on the website for the download button; you can cite the ALD/Database using its DOI:10.6100/ALDDatabase. Also let us know if you are using the ALD/Database in your presentation! This is of course fully optional but it allows us to quantify its usage better and it is of course very good for our motivation.
The ALD/Database: setting-up the database and some statistics
To make the ALD/Database a success, we recognized that the website should offer value for the visitors from day one of its introduction. This meant that we wanted to start with a good collection of references. Apart from writing most of the code, Jan Buiter also did some data mining to fill the database. For this purpose, several excellent review papers were used:
- Puurunen [J. Appl. Phys. 97, 121301 (2005)]
- Miikkulainen et al. [J. Appl. Phys. 113, 021301 (2013)]
- Knoops et al. [J. Vac. Sci. Technol. A 37, 030902 (2019)]
- Mackus at al. [Chem. Mater. 31, 1142 (2019)].
Combined these review papers contained 2970 references. On top of that, during the year 1293 entries were contributed to the database by the ALD community (see also inset). The figure below shows the distribution of the number of ALD entries in the database per year of publication (data taken on 27-11-2019). Because of the large number of entries from the review papers, a jump in the data is seen at the cut-off date of the Miikkulainen paper which is a superset of the references in the paper by Puurunen. Over time, we expect that this aberration will fade as more-and-more papers will be added.
In Q1 of 2020 we will release a more comprehensive analysis of publications from 2019 reporting on ALD processes and their resulting material properties mainly based on the entries in the ALD/Database. In this report, we will take a closer look at the materials synthesized by ALD in 2019 reported for the first time as well as trends in new precursor and much more.
Key contributors to the ALD/Database
The ALD/Database is a community driven effort and the current content of the database is the result of a combined effort of the ALD community. Visitors can submit new ALD processes to the database or contribute by adding additional references to an already existing process. We started with 2970 entries from the reviews papers and since this time 1293 entries have been added by the community in less than a year.
The ALD/Database team would like to thank everyone that has contributed to the database. Without these contributions, it would not be possible to keep the database up-to-date and thus relevant. In this context, we would also like to highlight the top 5 contributors for 2019:
- Miika Mattinen [University of Helsinki]: 774 contributions
- Joseph P. Klesko [NIST]: 253 contributions
- Mikko Ritala [University of Helsinki]: 11 contributions
- Saravana Balaji [Eindhoven University of Technology]: 8 contributions
- Tomi Livonen [University of Helsinki]: 6 contributions
Our experiences running the ALD/Database
When starting an initiative such as the ALD/Database you always encounter interesting and unexpected factors. For example, we anticipated a sharp increase in the number of submissions around the AVS, MRS, and ALD conference. When preparing a presentation, you typically go through some literature and if you are presenting a recently published paper you might take the chance to add it to the ALD/Database. On the conference itself, people mentioning the ALD/Database in talks or during the coffee break might trigger people to have a look at it and (hopefully) submit their own works to the database. In practice these peaks in submissions are less prominent than expected and we get a steady stream of new submissions (10’s a week). The review process itself turned out to be more straightforward than expected. With a good set of rules, most papers either clearly meet the requirements or fall well outside the type of papers we are looking for. Keeping the database consistent was expected to be a major challenge and it indeed turns out to be just that. Finding and removing “near” duplicate entries is difficult: Do you submit a nearly stoichiometric materials using integer numbers (e.g. TiO2) or variables (e.g. TiOx)? In practice, people do both and these duplicates can be hard to track down. However, we feel that just forcing either style is too limiting and both have their use. Naming schemes in general are complex – see also the inset – with the main challenges being: keeping the naming consistent and correctly rendering complex chemical equations from plain text chemical equations.
Precursor naming schemes: what’s in a name?
When submitting a new process we ask the contributors to describe the material and the reactants (including precursors). This also means that we are “forced” to use 1 coherent naming scheme for molecules. I never realized how many different notation schemes are being used in the field of ALD: when reading papers, you typically accept the abbreviations introduced in the text even if the “standard” notation is not followed. However, for the ALD/Database this is not a viable solution because we have to group processes together (Al2O3 synthesized using TMA or by AlMe3 are the same process but might result in 2 different “processes” in the ALD/Database). In order to sort this out, we are encouraging users to use the list of abbreviations which can be found in the “hamburger” menu under the “info” button on the website. Furthermore, this a topic we pay special attention to when reviewing.
Looking back on 2019, the response of the community has been very encouraging. We have noticed this in the number of visits to the website (~9000 “visitors” this year) and the amount of new submissions (~1300 in less than a year). Moreover, the ALD/Database was mentioned in a couple of (unaffiliated) conference contributions; a sign that awareness is spreading. For now, we are planning to continue on the same path, working on improving the website (experience) and reviewing new contributions.
If you want to help out, you can of course do so by submitting more references. But we also really appreciate suggestions for improvements and reports of bugs or mistakes. You can let us know using the website itself (see hamburger menu -> submit comment) or contact us directly.
For the related atomic scale processes of atomic-layer etching (ALE) a similar effort is ongoing with the ALE/Database which can also be reached from www.atomiclimits.com. Of course, this field has gained a lot of attention in the last couple of years as witnessed by, for example, the ALE workshops recently introduced on the ALD conference. As a result, far fewer processes have been reported so far for ALE. On the other hand, this means that it is possible to build up a quite accurate and complete collection of works in this database from the inception of the field. So if you are also interested in ALE, also have a look at that resource.
To conclude, the ALD/Database team would like to wish you a happy holidays & lots of ALD in 2020.
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