Rapid Coating of Through-Porous Substrates by Atomic Layer Deposition

Abstract

Uniform coating of macroscopic porous substrates is a great challenge with a huge potential impact. In principle, atomic layer deposition (ALD) is an ideal method of coating porous substrates but it has, so far, suffered significantly from the long processing times needed with high aspect ratio (depth to diameter ratio) structures. This paper is a report of a new ALD reactor design that makes it possible to uniformly coat macroscopic through-porous substrates nearly as quickly as planar surfaces. The improvement gained by this new reactor essentially brings a third macroscopic dimension into ALD processing, and is expected to open up entirely new application areas for coated porous substrates.Coating of substrates that are through-porous is needed in a wide variety of applications, such as 3D integrated circuits, printed circuit boards, electrodes for electrochemical applications like fuel cells and gas generators, 3D capacitors, X-ray optics based on microchannel plates, scaffolds for hard-and soft-tissue engineering and replacement, and heterogeneous catalysts and filters of various kinds. However, in only some of these applications can the coating be performed satisfactorily, and even in these cases there are certain minimum dimensions the pores must have. As the aspect ratio increases most of the coating methods will face problems in terms of film uniformity inside the pores. Methods such as thermal oxidation and anodization do not suffer from this limitation but are very limited in material selection, for example, with anodization only an oxide of the substrate material can be formed on the surface.Among the various thin-film deposition techniques, ALD [1][2][3] offers the greatest potential in coating high aspect ratio structures, including macroscopically porous substrates. The superior conformality of ALD is a consequence of its unique self-limiting growth process where alternately supplied precursor vapors react with each other on the surface in a saturating manner. The self-limiting growth mechanism ensures not only excellent conformality, but also large area uniformity, as well as simple and accurate control of film thickness and composition down to an atomic layer level. A wide variety of thin film materials can be produced using ALD, [1] giving it a considerable advantage over the few other methods, such as oxidation and anodization, that can be used in coating very high aspect ratio structures. When porous substrates are to be coated by ALD, the precursors need to diffuse into and out of the pores during each precursor pulse, i.e., twice during one ALD cycle and hundreds, or even thousands, of times when a film with a thickness in the range of 100 nm is needed. In typical ALD reactors the precursors flow over the surface of the substrate, carried by an inert carrier gas ( Fig. 1 top and middle), and the transportation of the precursor molecules

Pt–Al2O3dual layer atomic layer deposition coating in high aspect ratio nanopores

“…This requires that the exposures (pulse 1 and 3 in the ALD cycle) are sufficiently long to allow gas diffusion into the nanopores. An arbitrary duration would results in an only partially covered pore wall, if too short, or in a waste of precursor/reactant gas, if too long [88]. Models for determination of the required exposure for coating the pores have been presented by Gordon et al [85] and Adomaitis et al [89].…”

Atomic Layer Deposition of Al2O3, TiO2 and ZnO Films into High Aspect Ratio Pores

“…Our total cycles times of 5-10 seconds are similar to ones reported in [9] but in that work a specially designed through-flow ALD reactor was used. This is considerably less than the 5 minutes pulses used in [10] for coating nanoporous silica aerogel with ZnO.…”

“…More surprisingly, closed pores and through-wafer open pores are coated similarly. In through-wafer pores the conductance is in fact fairly mildly scaling as a function of pore size: according to [9] conductance is linearly proportional to pore diameter.…”

Coating of Highly Porous Fiber Matrices by Atomic Layer Deposition

“…First, the dimensions of the metal catalyst and the underlying oxide film can be adjusted accurately by the number of ALD cycles applied. Second, it is likely that a homogeneous distribution of the noble metal on the catalyst support surface can be achieved relatively easily by ALD, especially when the flow-through arrangement [25] is used. Third, post-deposition treatments such as drying, calcination, or reduction are not necessary.…”