AVS2018 Session TF+PS-ThM: Deposition Processes for 3D and Extreme Geometries

Deposition of conformal and uniform coatings onto high aspect ratio high surface area substrates and materials is a unique domain of ALD impacting many important applications, including energy generation and storage, semiconductor device manufacturing, chemical and biosensing and many others. However, ALD process development on such substrates is hindered by complex material geometry leading to poorly understood transport conditions and high cost of real substrates, such as large Si wafers at later staged of processing. In addition, modern ALD reactors, designed and optimized for high speed processing of flat substrates, require validation of uniformity and conformality of ALD coatings both inside the pores and laterally across the high surface area substrates with complex multiscale topology.

Nanoporous Anodic Aluminum Oxide (AAO) – a self-organized material resembling nanoscale honeycomb – is a controllable, well-defined, inexpensive and convenient nanomaterial platform to support the development and validation of high surface area ALD. The combination of ALD with AAO enables control of both the nanoscale geometry and chemistry with unprecedented precision to provide new avenues for design and engineering of functional materials and devices.

Some of the benefits of AAO as a reference ALD substrate are:

• Highly uniform and parallel cylindrical pores, complex pore structures (modulated, branched, etc) available to emulate different pore size distributions of other materials.
• A broad spectrum of transport conditions (pore diameters from 2 nm to 500 nm and pore lengths from <100 nm to >300 µm for aspect ratio (L/D) from 1 to as high as 50,000) that could be tested to aid better understanding and rapid development of new ALD processes.
• Reproducible and well-understood surface chemistry of alumina makes it nearly universally suitable for many binary reactions involved in ALD.
• High pore densities (10¹² to 10⁷ cm^-2) allow straightforward characterization of elemental depth profiles by EDS even for very thin (5-10 nm) coatings.
• Available in different form factors and on different substrates (flexible foils and tapes, rigid plates, Si wafers, glass blanks, etc) to accommodate different types of ALD reactors, such reel-to-reel reactors for coating battery electrodes or reactors designed to process stacks of Si wafers or PV substrates.

• Scaleable to large sizes, low cost.

Several case studies will be presented where AAO was used as a reference substrate to aid the development of ALD for high surface area materials.

An important building block for detector devices are amplifiers, such as microchannel plates (MCPs). Due to the geometry of several periodically arranged microchannels, incident electrons or irradiation can be amplified by several orders of magnitude, making MCPs highly efficient in several applications such as neutron detectors and night-goggles. The efficiency of state-of-the-art MCPs is strongly related to functional coatings, acting as resistive coating and secondary electron emission (SEE) layer. While the SEE material (e.g. Al₂O₃ or MgO) should exhibit a high SEE coefficient, the resistive coating must facilitate a uniform and stable electrostatic field along the pores and during operation. It was shown previously that atomic layer deposition (ALD) is capable to produce highly efficient SEE coatings (MgO) as well as a fine-tuned resistive coating directly on the MCP substrate, thereby improving the overall performance of the detector devices.^[1,2] By producing highly conformal thin films over large areas and on large aspect ratios, ALD is the method of choice to produce functionalized MCPs.

One challenge in manufacturing reliable MCPs for advanced applications are external conditions like ambient temperature. With temperature gradients of 100 K or higher, the thermal coefficient of resistance (TCR) of the resistive coating plays a major role with respect to electrical transport phenomena. Positive TCRs can cause increased resistance at elevated temperatures and in due turn a collapse of the MCP’s performance. Similar determining factors are on hand for strongly decreased temperatures. A second challenge is the fluorine-based chemistry, which is currently often employed to produce ALD-functionalized MCP coatings.

We present here the development of ALD-fabricated materials, being free of fluorine chemistry and exhibiting tailored electric resistance over a broad temperature range as well as promising performance as resistive coating in MCPs. Apart from thorough process development, those transition metal-based materials were analyzed regarding their composition, structure and electrical behavior employing XPS, SEM, XRD and extended IV-measurements. Furthermore, the coatings were applied to MCPs, which were tested for their performance.

[1] A. U. Mane and J. W. Elam, Chem. Vap. Deposition, 2013 (19), 186-193

[2] M. J. Minot, B. W. Adams, M. Aviles, J. L. Bond, C. A. Craven, T. Cremer, M. R. Foley, A. Lyashenko, M. A. Popecki, M. E. Stochaj, W. A. Worstell, A. U. Mane, J. W. Elam, O. H. W. Siegmund, C. Ertley, H. Frisch and A. Elagin, Proceedings Volume 9968, Hard X-Ray, Gamma-Ray, and Neutron Detector Physics XVIII, 2016, DOI: 10.1117/12.2237331

Reproducibly and homogeneously synthesizing single-site transition metal catalysts on exceedingly high surface area supports with stability under catalytic conditions remains a grand challenge. To address this challenge, we utilize atomic layer deposition (ALD) in metal-organic frameworks (MOFs), a process we call AIM. Here, ALD provides a straightforward gas phase route to access a wide variety of small precision clusters with spatial and dimensional homogeneity. The well-defined structure of MOF NU-1000 framework and nodes allows for detailed characterization of their size and uniforming as well as insight into their inherent stability. This talk will discuss several classes of ALD precursors and processing conditions that have been identified to be compatible with few-atom cluster deposition in Zr-based MOF NU-1000.

Atomic layer deposition (ALD) involves layer-by-layer deposition due to sequential exposure of two precursors to as substrate . ALD was developed in the late 1970s by Suntola and co-workers as a modification of chemical vapor deposition (CVD). In ALD, the dosing and purging steps are separated to ensure that each precursor saturates the surface before the other precursor enters the chemical reactor. ALD is widely used in the semiconductor and microelectronics industry to deposit thin films of various oxides on different substrates. ALD can be operated in two modes: plasma assisted and thermally assisted. Thermally assisted ALD has been widely used due to its high uniformity and conformality in depositing thin films on a variety of substrates.

Here we report alumina deposition by ALD using trimethylaluminum and water as precursors on silicon substrates. We optimized the process on flat surfaces like silicon shards, and extrapolate the process to high aspect ratio devices and powders, ultimately confirming the deposition on these high aspect ratio materials. Achieving high degrees of consistency and uniformity for deposition on powders through a frit using ALD has been difficult. We tried multiple recipes for alumina deposition, optimizing the dose time, purge time and effective exposure of the precursor on the substrate to achieve uniform results. Our thin films are characterized by spectroscopic ellipsometry (SE), water contact angle goniometry, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and/or transmission electron microscopy (TEM). The thin films we have obtained on planar substrates using ALD vary in thickness from 0.5 – 41.0 nm. These thin films were used to conduct a multiple sample analysis (MSA) in SE to obtain the optical constants of alumina. Since alumina is one of the most commonly used material in microfabrication, its optical constants will be useful in the field.

CeO₂ has attracted lots of attention due to its superior oxygen storage and donating ability as a catalyst support. CeO₂ nanorods has the best donating ability than all the other types of CeO₂ materials. However, the nanorods change its morphology and lose its activity at 400°C or above. Therefore, improving the thermal stability of CeO₂ nanorods can unlock the potential applications of CeO₂ nanorods in the high temperature applications. In this work, we introduce a diffusion-limited Al₂O₃ atomic layer deposition to selectively passivate the surface site of CeO₂ nanorods, largely enhancing its thermal stability and its oxygen storage capacity simultaneously.