Undoubtedly, every scientist and technologist working in the field of plasma etching has heard about the research carried out by Coburn and Winters, two research scientists that worked at the IBM San Jose Research Laboratory for almost their full professional career. In 2016 Harold Winters, at the age of 83, sadly passed away and this was the reason for several initiatives to commemorate the career of Harold. One of these initiatives was a Special Issue Tribute to Harold Winters of the Journal of Vacuum Science and Technology to which we also contributed with an article . Another initiative was an all-invited session, entitled “The Science of Plasmas and Surfaces: Commemorating the Career of Harold Winters”, during the 64th AVS International Symposium and Exhibition (Tampa FL, Oct. 29 – Nov. 3, 2017). I had the honor to speak during this special session and I devoted a post to it on this blog.
Only about 2 years later, we received the very sad news that also John Coburn had passed away at the age of 85. Please read this article by his colleague and friend Eric Kay that was published in Physics Today. In tribute to his important contributions to plasma science, this was a good reason to organize another special issue in the Journal of Vacuum Science and Technology and to have also a special all-invited session during the 66th AVS International Symposium and Exhibition (Columbus OH, Oct. 20-25, 2019): “Commemorating the Career of John Coburn”. I had again the honor to speak during this special session and I will report about my presentation below.
In my earlier blog post, I already briefly described the profound contributions of Harold Winters and John Coburn to the field of plasma science. In the same post, I also described how both Harold and John greatly inspired me with their work. The presentation that I gave during the Special Session was entitled: Plasma ALD – A discussion of mechanisms. This title was chosen in analogy with the title of a paper published by Coburn and Winters in the Journal of Vacuum Science and Technology in 1979: Plasma etching – A discussion of mechanisms . I started my presentation by going back to 1999 when I was the happy winner of Coburn and Winters Student Award of the Plasma Science and Technology Division at the 46th AVS International Symposium in Seattle WA in 1999. I commented on this in my earlier post but now I highlighted a study that I did as graduate student to get insight into the surface reaction probability of plasma radicals during plasma deposition (plasma-enhanced chemical vapor deposition, PECVD) of hydrogenated amorphous silicon (a-Si:H) films. This study involved so-called “aperture-well experiments” [3,4] using a kind of small cavity to trap the radicals and look at the resulting deposition profile. I will come back to these experiments later below.
After winning the award, the names Coburn and Winters obviously meant much more to me than before that time. However things really changed in 2004 when I started doing a sabbatical with Prof. David Graves (Graves lab) at the University of California Berkeley. As I also wrote in my previous post, John and Harold were retired from IBM at that time but they were still interested in being involved in research with PhD students. Therefore they came into Dave’s lab every Tuesday (during the time I was in Berkeley) to join the group meetings and to discuss with the PhD students about their experiments and results (by the way David Fraser, retired from Intel, did the same). This gave me ample opportunity to discuss science but also many other things with both John and Harold. John was most involved in the experiments of students and he was also a co-author of several papers published by Dave’s PhD students. Besides some interesting papers about the detection of plasma radicals by threshold ionization mass spectrometry [5,6], there was one particular other paper that was very relevant for me. This was not a paper related to plasma etching, but it was a publication about radical-enhanced atomic layer deposition (ALD) or basically plasma-enhanced ALD as it probably would be called now. This paper, with Frank Greer as a first author, appeared in the Journal of Vacuum Science Technology A in 2003 . Its title was Fundamental beam studies of radical enhanced atomic layer deposition of TiN. The reason why this article – published before I came to Berkeley – was so relevant was that we had just started research on ALD in Eindhoven. As a matter of fact, we had just started working on plasma-assisted ALD of TiN using TiCl4 as precursor and a H2-N2 plasma as co-reactant, exactly the same system as described in the article by Greer et al. This work was part of my first project on ALD that I submitted to the Dutch Technology Foundation STW in January 2003: Plasma-assisted Atomic Layer Deposition for Processing at the Nano-scale. When the project was granted, we decided to work first on the aforementioned process of TiN which fitted well within the class of processes for nitrides of titanium and tantalum as reported by Steve Rossnagel and his postdoc – at the time – Hyungjun Kim [8,9]. This process was recommended to us by Suvi Haukka from ASM Microchemistry. According to her it was best to start with a material and a process that would not be too difficult to get started with and of course we followed her wise advice. By the way, the two PhD students working on this process – my first students working on ALD – were Stephan Heil and Erik Langereis.
As the title indicates, the article by Greer et al. is not a regular study of an ALD process, it is fundamental study involving beam experiments with which the sticking and reaction probabilities of the most important species involved (TiCl4, H and N) were determined as a function of temperature (see also an overview for plasma ALD in Ref. ). These kind of studies – beam studies and work on sticking and reaction probabilities – are exactly what can be expected when John Coburn is involved in the research! Our own first work in the field of ALD was more classical, we first carefully established that the process was indeed ALD with self-limiting reactions. We also studied the TiN materials properties as well as aspects as conformality and initial growth. The method of in situ spectroscopic ellipsometry had a very prominent position in this work as I think that it can be seen as the starting point for wide scale implementation of in situ spectroscopic ellipsometry during ALD studies, albeit not the first report of in situ (spectroscopic) ellipsometry during ALD. In the blog post In situ spectroscopic ellipsometry and ALD – How ellipsometry became a popular technique to monitor ALD film growth one can read much more about this. But coming back to our first steps into ALD, we were writing our very first paper about ALD – about plasma-enhanced or plasma-assisted ALD – at the time I was spending my sabbatical in Berkeley and interacting frequently with John. The article Plasma-assisted atomic layer deposition of TiN monitored by in situ spectroscopic ellipsometry was published in the Journal of Vacuum Science and Technology A in 2005 . Only after this first paper appeared, we could really claim that we were working on ALD!
The aforementioned aspects were all briefly addressed during my presentation in the special session commemorating the career of John Coburn. However, I also devoted a large part of this presentation to some of our very recent work in the field of plasma ALD. Recently, we have been studying reaction probabilities – as a matter of fact: surface recombination probabilities – of plasma radicals during plasma-enhanced ALD. In this work, which is part of the PhD project of Karsten Arts and which is a collaboration with the Finnish VTT Technical Research Center and Aalto University, we use so-called PillarHall samples as provided by VTT. These PillarHall samples are basically cavities micromachined in silicon which resemble the aperture-well experiments that I described in the beginning of this post. The PillarHall samples are however only much more microscopic and the distance between the bottom of the cavity and the top (membrane) is much smaller. You can find a description of these samples in articles published by VTT [12,13] whereas we have also published some research on thermal ALD of Al2O3 with these samples earlier this year .
During my presentation, I briefly described how information on the sticking probability of the precursors during ALD can be obtained  and I mentioned that for TiCl4 during ALD of TiN a fairly good agreement was achieved between experiments with the PillarHall sample and the beam experiments by Greer and Coburn and co-workers. I also reported on our recent results on the surface recombination probability of plasma radicals during plasma-enhanced ALD. As a matter of fact, I discussed that the surface recombination probability of O atoms varies a lot for different oxide materials prepared by ALD. This was also shown in a dedicated oral presentation by our PhD student Karsten Arts earlier at the same AVS International Symposium. More importantly, the work was very recently published in the Journal of Physical Chemistry C in the article: Film Conformality and Extracted Recombination Probabilities of O Atoms during Plasma-Assisted Atomic Layer Deposition of SiO2, TiO2, Al2O3, and HfO2 by Karsten Arts et al. . Hence, I will not describe the experiments and remarkable results in great detail here but I just refer you to this publication. I do want to emphasize that insight into the surface recombination probability is very important for understanding and predicting the conformality that can be achieved by plasma ALD processes. Furthermore, the quantitative information is also key for modelling the surface reactions and film growth taking place during plasma ALD.
With this work we have shed more light on the surface recombination of plasma radicals and its effect on the conformality of plasma ALD films. One more open question in the field of plasma ALD has therefore been answered and we have gained again more understanding mechanisms underlying plasma ALD. This brings me back to the title of this blog and the title of my presentation during the special session commemorating the career of John. To me, it shows how greatly John has influenced my research, in a direct way but also in an indirect way. I also remember how John praised our work on ALD carried out at the TU/e after giving an invited presentation at the 30th Symposium of Dry Process in Tokyo in 2008 (we were working on ALD only a couple of years then). He was invited there as well and he said something along the lines “you have something good going on”. John was such a warm and encouraging person.
Per initiative by Jane Chang of the University of California Los Angeles – one of the Coburn and Winters Student Award winners – a special poster was prepared that was on display during 66th AVS International Symposium and Exhibition (Columbus OH, Oct. 20-25, 2019). On this poster, past awardees of the Coburn and Winters Student Award wrote briefly about their interaction with John Coburn or what the award meant to them to show his impact and legacy.
 Revisiting the growth mechanism of atomic layer deposition of Al2O3: A vibrational sum-frequency generation study, V. Vandalon and W.M.M. Kessels, J. Vac. Sci. Technol. A 35, 05C313 (2017)
 Plasma etching – A discussion of mechanisms, J.W. Coburn and H.F. Winters, J. Vac. Sci. Technol. 16, 391 (1979)
 Surface-reaction probability of film-producing radicals in silane glow-discharges, D.A. Doughty, J.R. Doyle, G.H. Lin, A. Gallagher, J. Appl. Phys. 67, 6220 (1990).
 Surface reaction probability during fast deposition of hydrogenated amorphous silicon with a remote silane plasma, W.M.M. Kessels, M.C.M. van de Sanden, R.J. Severens, and D.C. Schram, J. Appl. Phys. 87, 3313 (2001).
 Mass spectrometric detection of reactive neutral species: beam-to-background ratio, H. Singh, J.W. Coburn, D. Graves, J. Vac. Sci. Technol. A 17, 2447 (1999)
 Appearance potential mass spectrometry: discrimination of dissociative ionization products, H. Singh, J.W. Coburn, D. Graves, J. Vac. Sci. Technol. A 18, 299 (2000)
 Fundamental beam studies of radical enhanced atomic layer deposition of TiN, F. Greer, D. Fraser, J. W. Coburn, and D.B. Graves, Vac. Sci. Technol. A 21, 96 (2003)
 Plasma-enhanced atomic layer deposition of Ta and Ti for interconnect diffusion barriers, S.M. Rossnagel, A. Sherman, F. Turner, J. Vac. Sci. Technol. B 18, 2016 (2000)
 Atomic layer deposition of metal and nitride thin films: Current research efforts and applications for semiconductor device processing, H. Kim, J. Vac. Sci. Technol. B 21, 2231 (2003)
 Conformality of plasma-assisted ALD: physical processes and modeling, by H.C.M. Knoops, E. Langereis, M.C.M. van de Sanden, and W.M.M. Kessels, J. Electrochem. Soc. 157, G241 (2010).
 Plasma-assisted atomic layer deposition of TiN monitored by in situ spectroscopic ellipsometry, S.B.S. Heil, E. Langereis, A. Kemmeren, F. Roozeboom, M.C.M. van de Sanden, and W.M.M. Kessels, J. Vac. Sci. Technol. A 23, L5 (2005).
 Microscopic silicon-based lateral high-aspect-ratio structures for thin film conformality analysis, F. Gao, S. Arpiainen, R.L. Puurunen, J. Vac. Sci. Technol. A 33, 010601 (2015).
 Modeling growth kinetics of thin films made by atomic layer deposition in lateral high-aspect-ratio structures, M. Ylilammi, O.M.E. Ylivaara, R.L. Puurunen, J. Appl. Phys. 123, 205301 (2018).
 Sticking probabilities of H2O and Al(CH3)3 during atomic layer deposition of Al2O3 extracted from their impact on film conformality, K. Arts, V. Vandalon, R. Puurunen, M. Utriamninen, F. Gao, W.M.M. Kessels, and H.C.M. Knoops, J. Vac. Sci. Technol. A 37, 030908 (2019).
 Film conformality and extracted recombination probabilities of O atoms during plasma-assisted atomic layer deposition of SiO2, TiO2, Al2O3, and HfO2, K. Arts, M. Utriainen, R.L. Puurunen, W.M.M. Kessels, and H.C.M. Knoops, J. Phys. Chem. C 123, 27030 (2019)
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