The ability of atomic layer deposition (ALD) to grow highly conformal thin films is one of the main reasons why ALD is so popular in research and heavily employed in the semiconductor industry.1,2 This not only holds for thermally-driven ALD. As explained in a previous post, also plasma-assisted ALD can penetrate very deep into high-aspect-ratio structures, since the loss of reactive radicals at the surface can be very low.3,4 This is, for example, the case for plasma ALD of SiO2.3,4 Accordingly, plasma ALD of SiO2 has several applications where a high film conformality is essential. This includes the growth of sidewall spacers in self-aligned multiple patterning,1,5 dielectric lining of high-aspect-ratio channels in 3D NAND,6 and gap-filling of regions around electrical contacts.6,7 Still, conformal film growth by plasma ALD has the reputation of being challenging.
In this blog post, we want to share a collection of transmission electron microscope (TEM) images that clearly visualize that plasma ALD can, indeed, provide excellent film conformality and seamless gap-filling. Part of these images have been used in our recent publication in Chemistry of Materials.9 There, we studied to what extent ions − as they hit the surface anisotropically − can influence film conformality during plasma ALD of TiO2 and SiO2. While a significant and tunable influence of ions was observed, the TEM images also beautifully demonstrate the capabilities of plasma ALD. As a result, a customized version of one of the images made it to the front cover of Chemistry of Materials (issue 13, July 13, 2021), where the paper is highlighted (see below). Moreover, the published TEM images received quite some attention on social media, which made us realize that the images could serve the community very well. Therefore, we decided to provide all our (unpublished) images in this blog, such that anyone can freely re-use them. With the exception of the Chemistry of Materials cover, all images are licensed under a Creative Commons Attribution 4.0 International License and can be used freely when providing an appropriate credit line. We suggest: Courtesy of K. Arts, M.A. Verheijen, W.M.M. Kessels and H.C.M Knoops (CC BY 4.0 license), image library at www.AtomicLimits.com, 2021.
A few examples of the original images are given below. These cross-sectional images were obtained using high-angle-annular-dark-field scanning transmission electron microscopy (HAADF-STEM). In addition, elemental maps measured by energy-dispersive X-ray spectroscopy (EDS) are provided at the right. The images display a stack of alternated layers of TiO2 (green) and SiO2 (blue), and a single layer of Al2O3 (red). All layers were prepared by plasma ALD, using an Oxford Instruments FlexAL tool in our NanoLab@TU/e cleanroom. The substrate with vertical trench nanostructures was provided by LAM Research. Further experimental details can be found here.
Cross-sectional HAADF-STEM images and corresponding EDS maps, showing a stack of alternated TiO2 and SiO2 layers and a single layer of Al2O3, all grown by plasma ALD on nanoscale trench structures.
As shown below, the trench structures have different initial aspect ratios in the range of approximately 0.6 to 6. The conformality of the individual layers can therefore be visualized for several geometric conditions. Moreover, as the trenches become more narrow during deposition, in principle also ALD in higher aspect ratios is investigated.
Combined figure of the individual HAADF-STEM images, demonstrating the conformality of the deposited stack for a variety of initial aspect ratios in the range of approximately 0.6 to 6.
Finally, we would like to very much thank Marcel Verheijen (Eindhoven University of Technology and EAG Eindhoven – Eurofins Materials Science Netherlands) for making these beautiful images. Moreover, we would like to thank Beatriz Barcones Campo (NanoLab@TU/e) for the FIB preparation. An overview of the rest of the TEM images is given below. Enjoy!
(1) Knoops, H. C. M.; Faraz, T.; Arts, K.; Kessels, W. M. M. Status and Prospects of Plasma-Assisted Atomic Layer Deposition. J. Vac. Sci. Technol. A 2019, 37 (3), 030902. https://doi.org/10.1116/1.5088582.
(2) Cremers, V.; Puurunen, R. L.; Dendooven, J. Conformality in Atomic Layer Deposition: Current Status Overview of Analysis and Modelling. Appl. Phys. Rev. 2019, 6, 021302. https://doi.org/10.1063/1.5060967.
(3) Arts, K.; Utriainen, M.; Puurunen, R. L.; Kessels, W. M. M.; Knoops, H. C. M. Film Conformality and Extracted Recombination Probabilities of O Atoms during Plasma-Assisted Atomic Layer Deposition of SiO2, TiO2, Al2O3, and HfO2. J. Phys. Chem. C 2019, 123 (44), 27030–27035. https://doi.org/10.1021/acs.jpcc.9b08176.
(4) Arts, K.; Deijkers, S.; Puurunen, R. L.; Kessels, W. M. M.; Knoops, H. C. M. Oxygen Recombination Probability Data for Plasma-Assisted Atomic Layer Deposition of SiO2 and TiO2. J. Phys. Chem. C 2021, 125 (15), 8244–8252. https://doi.org/10.1021/acs.jpcc.1c01505.
(5) Raley, A.; Thibaut, S.; Mohanty, N.; Subhadeep, K.; Nakamura, S.; Ko, A.; O’Meara, D.; Tapily, K.; Consiglio, S.; Biolsi, P. Self-Aligned Quadruple Patterning Integration Using Spacer on Spacer Sub-32nm Pitch Applications. Proc. SPIE 2016, 9782, 97820F. https://doi.org/10.1117/12.2219321.
(6) ASM International Analyst and Investor Technology Seminar, presented at Semicon West, July 9, 2019. https://www.asm.com/Downloads/ASMI Analyst and Investor Technology Seminar July 9 2019 revREL.PDF.
(7) Tang, W.; Park, J. D.; Schravendijk, B. Van; Ashtiani, K. US 9.406,544 B1 – Systems and Methods for Eliminating Seams in Atomic Layer Deposition of Silicon Dioxide Film in Gap Fill Applications. United States Pat. 2016.
(8) Knoops, H. C. M.; Langereis, E.; van de Sanden, M. C. M.; Kessels, W. M. M. Conformality of Plasma-Assisted ALD: Physical Processes and Modeling. J. Electrochem. Soc. 2010, 157 (12), G241–G249. https://doi.org/10.1149/1.3491381.
(9) Arts, K.; Thepass, H.; Puurunen, R. L.; Kessels, W. M. M. (Erwin); Knoops, H. C. M. Impact of Ions on Film Conformality and Crystallinity during Plasma-Assisted Atomic Layer Deposition of TiO2. Chem. Mater. 2021, 33 (13), 5002–5009. https://doi.org/10.1021/acs.chemmater.1c00781.