Understanding plasma ALD – Why ions matter… and should be considered!

 [Note: a recording of my presentation for the 2020 Coburn and Winters Student Award competition can be viewed at the bottom of this page]

This year I had the honor of being a finalist in the AVS Coburn and Winters Student Award competition, for my research on fundamental aspects of plasma-assisted atomic layer deposition (plasma ALD). My nomination was largely based on the work that was recently published in Applied Physics Letters: “Evidence for low-energy ions influencing plasma-assisted atomic layer deposition of SiO₂: Impact on the growth per cycle and wet etch rate”.¹

Although the AVS 67^th International Symposium & Exhibition in Denver was cancelled due to Covid-19, the Plasma Science and Technology Division made sure that the student award competition could continue online (thanks a lot!). It was a very nice opportunity to share my work and I would like to congratulate the award winner Ryan Gasvoda for his excellent talk on strategies for enhancing etch selectivity during ALE.

The cancellation of the ‘real conference’ did mean that only a very limited number of people could see my presentation. Therefore, I would like to highlight the work and share my presentation in this blog post. To already give away the main message: always consider the (often significant and beneficial) influence of ions during plasma ALD!

The historical context of how we came to this conclusion is described in the figure below, which gives a somewhat simplified overview of how research on plasma ALD has developed over the past two decades. Regarding the research performed on the role of ions, a more detailed overview will be provided in a later blog post.

In the early 2000s the interest in using plasmas during ALD rose particularly due to the fact that plasmas provide a flux of radicals to the surface.^2,3 The high reactivity provided by the radicals allows for the preparation of a wider range of materials and can also enable film growth at low temperatures. While this high reactivity is thus an important advantage, it also means that the radicals are prone to loss through recombination without contributing to film growth. For example, when an O radical hits the growth surface, it can recombine to form a stable, non-reactive O₂ molecule. Because of this loss mechanism,⁴ it was commonly assumed that the film conformality provided by plasma ALD would be limited compared to thermally-driven ALD.^2–4

Around 2010 research within our group at the TU/e pointed out that also ions could have a significant influence on plasma ALD processes. It was measured that for most remote plasma ALD systems, such as inductively coupled plasma systems, a considerable flux of ions is present: 10¹² up to 10¹⁴ ions·cm^-2·s^-1, typically with a relatively low energy of less than 30 eV.⁵ Subsequently, it was demonstrated for instance by Profijt et al.⁶ and more recently by Faraz et al.⁷ that ions can indeed be used to tune material properties during plasma ALD (see also this previous post). This was done by increasing the energy of the ions up to ~200 eV through the application of RF substrate biasing. The work therefore proved that ions can play an important role during plasma ALD. Yet, the significant influence of ions under ‘normal operating conditions’, so without substrate biasing, remained largely unrecognized.

In my work as a PhD student on the NWO-funded project “Taking plasma ALD to the next level”, I have provided important insights into the two aforementioned topics of debate:

1. Plasma ALD can provide very high film conformalities. Specifically, I have shown that for certain plasma ALD processes the loss of reactive radicals through surface recombination is very low, such that deposition can be achieved up to extremely high aspect ratio values of ~900:1.⁸
2. Also ions with energies as low as ~10 eV can have a strong impact during plasma ALD. Therefore, the influence of ions should also be considered when using a grounded substrate and a remote plasma source.¹

Here, both points are demonstrated specifically for plasma ALD of SiO₂, which has become one of the most important ALD processes used in the semiconductor industry, particularly for nanopatterning but also for gap-filling and applying dielectric liners.^9,10

The main method used for investigating the role of ions is based on all-silicon PillarHall^™ lateral-high-aspect-ratio cavity structures. As illustrated in the figure below, these horizontal cavities are formed by a membrane that is supported by pillars and suspended above part of the substrate. Since the flux of ions is anisotropic, only the surface in the opening is exposed to ions, while the surface underneath the membrane is shielded from ions. In contrast, both the surface in the opening and underneath the membrane receive a flux of radicals, since these neutrals can diffuse into the cavity. After deposition, the membrane can be removed using adhesive tape to study the thickness and material properties (e.g., the wet etch rate) of the SiO₂ film grown in the ion-exposed region and ion-shielded region.

There are two reasons why the experimental results shown below are probably the most surprising and relevant results I encountered. First of all, the dark areas visible in the top view microscope images, corresponding to surface area where SiO₂ has been deposited, indicates that film growth penetrated very deep (i.e., roughly halfway) into the cavity. Since the cavity has an extremely high aspect ratio of 2000:1, deposition was achieved up to an aspect ratio of ~900:1 (!). This was much deeper than we ever expected for any plasma ALD process. As also described in my previous post, I derived that this high penetration depth can be directly related to the low surface recombination probability of O atoms on SiO₂, providing a more profound insight into the film conformality achievable by plasma ALD.⁸

Secondly, the figure above reveals that plasma ALD of SiO₂ is significantly influenced by ions, even when these ions have a low energy (here ~10 eV average). In the optical microscope images for example, it can be seen that the surface area in the ion-exposed region is less dark than the surface area in the ion-shielded region. Moreover, this difference increases when using longer plasma exposures (here varied from 3.8 s to 120 s per ALD cycle). To illustrate what is happening, the local growth per cycle (GPC) is plotted at the right for the deposition done using plasma steps of 12 s. Interestingly, the graph shows that a significantly lower GPC is obtained with exposure to (low-energy) ions. We later confirmed this effect using an ion-selective quartz crystal microbalance sensor from the company Impedans.^1,11

In addition, we observed that the wet etch rate of the deposited SiO₂ in buffered HF strongly decreases with exposure to (low-energy) ions and actually approaches the low value measured for a thermal oxide film. This indicates that excellent material quality can be obtained using the beneficial influence of ions.

While a significant influence of ions is already observed under mild plasma conditions with low ion energies, it should be noted that the influence will be even stronger when supplying more energetic ions and/or a higher flux of ions. Therefore, the influence will depend on the used plasma system and conditions, which makes it difficult to predict the growth behavior in general. This brings me to the part of the work that I am most proud of, which is given in the following figure.

The figure above provides values of the GPC obtained with exposure to ions, for a wide range of plasma conditions. The graph at the left illustrates that this wide range of plasma conditions results in a large spread in GPC values. Still, when we plot the values as a function of the ion energy dose, i.e., the plasma time × ion flux × mean ion energy,¹² we find that the GPC follows a clear trend with this parameter. The ion energy dose can thus be used to predict and tailor the influence of ions during plasma ALD in a universal way, which should hold for any plasma ALD setup.

To conclude, we have demonstrated that also low-energy ions can play an important and beneficial role during plasma ALD, in terms of the growth per cycle and material quality. The influence of ions should therefore also be considered when using a grounded substrate. Moreover, the magnitude of this influence can be universally predicted using the supplied ion energy dose. We believe that these insights are valuable for scientific understanding of the plasma-surface interaction, with important implications for the usage of plasma ALD in industry. This is expected to not only hold for plasma ALD of SiO₂, but also for other processes such as plasma ALD of TiO₂ where similar effects can play a role.^13,14

References

(1)         Arts, K.; Deijkers, J. H.; Faraz, T.; Puurunen, R. L.; Kessels, W. M. M.; Knoops, H. C. M. Evidence for Low-Energy Ions Influencing Plasma-Assisted Atomic Layer Deposition of SiO₂: Impact on the Growth per Cycle and Wet Etch Rate. Appl. Phys. Lett. 2020, 117, 031602. https://doi.org/10.1063/5.0015379.

(2)         George, S. M. Atomic Layer Deposition: An Overview. Chem. Rev. 2010, 110 (1), 111–131. https://doi.org/10.1021/cr900056b.

(3)         Profijt, H. B.; Potts, S. E.; van de Sanden, M. C. M.; Kessels, W. M. M. Plasma-Assisted Atomic Layer Deposition: Basics, Opportunities, and Challenges. J. Vac. Sci. Technol. A 2011, 29 (5), 050801. https://doi.org/10.1116/1.3609974.

(4)         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.

(5)         Profijt, H. B.; Kudlacek, P.; van de Sanden, M. C. M.; Kessels, W. M. M. Ion and Photon Surface Interaction during Remote Plasma ALD of Metal Oxides. J. Electrochem. Soc. 2011, 158 (4), G88–G91. https://doi.org/10.1149/1.3552663.

(6)         Profijt, H. B.; van de Sanden, M. C. M.; Kessels, W. M. M. Substrate-Biasing during Plasma-Assisted Atomic Layer Deposition to Tailor Metal-Oxide Thin Film Growth. J. Vac. Sci. Technol. A 2013, 31 (1), 01A106. https://doi.org/10.1116/1.4756906.

(7)         Faraz, T.; Knoops, H. C. M.; Verheijen, M. A.; van Helvoirt, C. A. A.; Karwal, S.; Sharma, A.; Beladiya, V.; Szeghalmi, A.; Hausmann, D. M.; Henri, J.; et al. Tuning Material Properties of Oxides and Nitrides by Substrate Biasing during Plasma-Enhanced Atomic Layer Deposition on Planar and 3D Substrate Topographies. ACS Appl. Mater. Interfaces 2018, 10, 13158–13180. https://doi.org/10.1021/acsami.8b00183.

(8)         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.

(9)         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.

(10)      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.

(11)      Arts, K.; Deijkers, J. H.; Faraz, T.; Puurunen, R. L.; Kessels, W. M. M.; Knoops, H. C. M. Evidence for low-energy ions influencing plasma-assisted atomic layer deposition of SiO₂ as measured with Impedans’ Quantum System [application note QC03]. https://impedans.com/sites/default/files/pdf_downloads/2020_august_quantum_qc03.pdf.

(12)      Faraz, T.; Arts, K.; Karwal, S.; Knoops, H. C. M.; Kessels, W. M. M. Energetic Ions during Plasma-Enhanced Atomic Layer Deposition and Their Role in Tailoring Material Properties. Plasma Sources Sci. Technol. 2019, 28 (2), 024002. https://doi.org/10.1088/1361-6595/aaf2c7.

(13)      Arts, K.; Deijkers, J. H.; Utriainen, M.; Puurunen, R. L.; Kessels, W. M. M.; Knoops, H. C. M. Role of Ions in Film Conformality and Quality during Plasma-Assisted ALD of SiO₂ and TiO₂, Presented at the AVS 20th International Conference on Atomic Layer Deposition, June 28 – July 1, 2020 (Virtual Meeting).

(14)      Arts, K.; Kessels, W. M. M.; Knoops, H. C. M. Impact of Low-Energy Ions on the Growth of Conformal TiO₂ Thin Films by Plasma-Assisted Atomic Layer Deposition (Manuscript in Preparation).