ALD－ナノ構造作製のための多機能ツール

Mato Knez

Max-Planck-Institute of Microstructure Physics, Weinberg 2, D-06120 Halle, Germany

ald@mpi-halle.eu

はじめに

ALDは、原子層堆積法（atomic layer deposition）の略語です。ALDプロセスは1970年代に開発されましたが、当初この手法の用途はほとんどがエレクトロニクスに限定されており、ニッチなプロセスに過ぎない状態に留まっていました。近年、ALDは、ナノ構造やマイクロ構造などの非常に微小な構造でも制御しながらコーティングできる能力があるため、大きな関心が持たれるようになってきました。世界中の多数のグループが、さまざまな戦略や改質法を用いて、新規構造や官能化材料を作り出してきました。最も革新的で有望な方法には、鋳型の指示に従った新規構造体合成、材料の選択領域での堆積、温度に敏感な基板への低温ALD堆積、ALDの多機能性を拡大する新プロセスの開発などがあります。

これらのALDの応用はすべて、ALD堆積方法の能力と、先端材料研究への影響を示すものです。特に、単純で効果的なプロセスと、市販されているALD反応装置の数の増加とが相まって、「ナノALD」分野の科学論文の増加にも表されるように、ALDは世界中の研究者の関心を集めています。

この論文では、前述の分野も含めた最近の研究例をいくつか示します。しかし、これは現在の開発状況の一端に過ぎません。さらに包括的なレビューは、他で見ることができます¹。

ALDプロセス

ALDプロセスとは、気相による薄膜堆積方法で、化学的気相成長（CVD：Chemical Vapor Deposition）と化学的には大変よく似ています。この類似性は、ALD前駆体材料をCVDに使用可能な事実からわかりますが、その逆は必ずしも可能ではありません。物理的には大きな違いがあります。例として、Al₂O₃の堆積を、トリメチルアルミニウム（TMA）と水を用いて行う場合のCVDとALDの主な差異を説明します。

CVDプロセスでは、TMAとH₂Oの2つの前駆体材料が共に反応室内に導かれ、Al₂O₃が生成され基板に堆積しますが、ALDプロセスでは、化学反応は2つの半反応に分かれます。最初に基板をTMAに曝すと、化学吸着した（サブ）単分子膜が形成されます（図1a）。吸着の後、気相中の余分なTMAをパージにより除去します。次に基板をH₂Oに曝すと、TMAの（サブ）単分子膜と反応してAl₂O₃の層が形成されます（図1b）。反応生成物（この場合メタン）と余分なH₂Oを除去すると、1サイクルの成長が完了しますが、これを所望する厚さの層が得られるまで繰り返すことができます。「ALD窓」と呼ばれる前駆体物質固有の温度範囲内でプロセスを実行すれば、膜の成長が直線的になり厚みをÅ単位で制御することが可能です。

図1 Schematic of one cycle of an ALD process. The schematic illustrates a simplified model for the deposition of Al2O3 using TMA and water as precursors.

The big advantage of ALD is that the process is driven by the chemical saturation of surfaces with the precursor (e.g., TMA) rather than a directed deposition, as is the case with CVD.

鋳型を用いたナノ構造の合成

Template-directed synthesis of nanostructures is the fastest growing area in ALD. Various templates can be used for conformal coating and replication or functionalization of nanostructures. These include nano- and microporous substrates, arrangements of nanospheres, nanowires, nanotubes or even single nanoparticles.

Probably, the easiest method for templated nanostructure synthesis involves the use of porous materials.

図2 Electron micrographs (scanning, SEM; transmission, TEM) of iron oxide tubes. スケールバー： 100 nm. (a) SEM of an array of narrow tubes (11±4 nm Fe2O3, green circles) embedded in the alumina template; contrast enhanced by colorization. (b) TEM of a single thick and short tube (42±4 nm Fe3O4) isolated by dissolution of the template; the inset zooms in on the very smooth wall. (c) SEM of an array of thick ZrO2/Fe2O3/ZrO2 tubes (12±2/26±4/12±2 nm) embedded in the template: edge view at a crack, with tubes broken in their length and emerging on the top side of the membrane. Image reprinted with permission from J. Am. Chem. Soc. 2007, 129, 9554–9555, Copyright 2007 American Chemical Society.

Similar to nanoporous materials, nanowires can also be used as templates for ALD deposition on and successive removal of the initial nanowire.

A trend towards the synthesis of functional nanostructures has been observed.

図3 TEM images of Cu nanoparticle chains prepared by reduction of CuO nanowires with 20 nm Al2O3 shells with H2 for 1 h at different temperatures: (a,b) sample prepared at 600 °C; (c,d) sample prepared at 750 °C. Panels (b) and (d) are TEM images corresponding to panels (a) and (c) at higher magnification. Image reprinted with permission from Nano Lett. 2008, 8, 114–118. （Copyright 2008 American Chemical Society）。

Advanced optical nanostructures produced by ALD already exist. Due to the benefits of this deposition method the production of inverse opal structures from a variety of materials by replicating highly ordered arrangements of nanospheres became possible. Research groups at Georgia Institute of Technology and Harvard University have been very active in this field. A number of various inverse opals were synthesized by ALD and characterized. Thanks to this simple synthesis method such structures can and surely will be further developed in the coming years.

A comparatively difficult area is the use of carbon nanotubes (CNTs) as templates. As the surface of carbon nanotubes is rather inert, it becomes very difficult to coat CNTs by ALD. Nevertheless, due to their shape and stability CNTs remain very interesting as templates. Therefore strategies were developed in order to achieve uniform coatings. However, technological use will still require further research and development.

Probably the most difficult nanoscale template for ALD is a single nanoparticle. Although it might be easily coated, difficulties in handling such materials appear frequently. If one wants to obtain a continuous conformal coating, one has to prevent any contact of the nanoparticles with each other and/or with the walls of the reactor. Nevertheless, here too some progress has been achieved.

領域を選択しての材料堆積

A highly interesting field of development in ALD is area-selective deposition. Although the most powerful application of ALD is the conformal coating of all accessible surfaces, there is the possibility to direct the deposition to discrete areas by chemically tuning them. The principle itself is simple as well as effective. Most likely this strategy will be broadly applied to deposit a variety of materials in a structured manner in order to obtain electronically or optically active materials.

低温ALD堆積

Low-temperature ALD (LT-ALD) is playing an increasingly important role. In thin film deposition the ability to coat materials that are temperature-sensitive and cannot be coated with other methods (e.g. CVD); substrates include polymers or biological templates.

If one considers the possibility to deposit metallic electrodes on polymer structures in order to obtain flexible electrodes, the significance of LT-ALD becomes obvious. Development in this area is at the very beginning and one can expect that more materials will be deposited by LT-ALD technique in the near future.

A particularly interesting application of LT-ALD is the possibility to coat or functionalize biological nanostructures. Nature has already applied nanotechnology for millions of years. If one considers the hydrophobicity of lotus leaves, which is related to micro- and nanostructures, it becomes obvious that mankind can still learn from nature. In some cases nature provides perfect nanostructures, which could easily be replicated thus avoiding more complex growth and fabrication with nanometer precision. However, one of the limiting factors here is the technology involved; for example, in the case of ALD the vacuum process and the deposition temperature. As the vacuum process cannot be avoided, for structures that are vacuum resistant, the deposition temperature is of significance. A few attempts have already been reported to coat biological nanoand microstructures by ALD.

図4 TEM (200 kV) images of ferritin molecules treated with Al2O3(a) and TiO2(b) by ALD. The images show ferritin molecules embedded in amorphous freestanding Al2O3 and TiO2 films. The darker gray areas originate from the holes in the carbon film on the TEM grid. The black spots in image (b) show the iron oxide core of ferritin. The films have cracks, and on image (b) the free-standing film is rolled up from the side, which was caused by the electron beam from the TEM. Image reprinted with permission from Nano Lett. 2006, 6, 1172–1177. Copyright 2006 American Chemical Society。

その後の研究で、ナノ構造のチョウの羽根をAl₂O₃でコーティングする可能性と、これによりコーティングの厚さに応じ色が変化する構造化されたサンプルを得られる可能性が示されました²²。この分野は緩やかに発展していますが、近い将来活動が活発になり進歩が見られると予想されます。

新規プロセス

Since the development of ALD, much of the work was dedicated to the development of new processes. A large number of materials, mainly metal-oxides, but also nitrides, carbides, sulfides, phosphides and metals have been successfully deposited. The known instances cover a major part of the Periodic Table either as pure elements or as binary or ternary compounds.

While most of the developed processes concentrated on materials, which in one way or another are interesting for electronic or optical applications, there are also other important areas of application.

Another highly interesting novel group of materials deposited by ALD are polymers or their derivatives. Since then the ALD process has become a really versatile tool for thin film deposition, as not only inorganic materials can be deposited, but also organic molecules. Moreover, studies on the synthesis of hybrid materials of organic and inorganic molecules stacked in a layer-by-layer manner were performed. More insight into this particular field of ALD (MLD) is given in the article on page 34 by S.M. George et al. in this issue of Material Matters.

有機－無機ハイブリッド材料は、特異な特性を示し、さまざまな生物医学や環境用途でに役立つことが証明される可能性があるため、この方向でのさらなる開発に大いに興味が持たれます。

結論

ALD has emerged as the method of choice for the controlled and conformal deposition of thin layers involving micro- or nanostructures. A large number of materials can be deposited by ALD. The possibility of obtaining a reactor commercially as well as the steadily increasing number of precursors make the ALD process especially convenient and attractive for newcomers to this field. Even though there are still limitations in particular processes or precursors for very special purposes, in general, the only limitation to further applications of ALD appears to be the imagination and creativity of the researchers involved.

Acknowledgment

Dr. Mato Knez gratefully acknowledges financial support by the German Ministry of Education and Research (BMBF) under the contract number 03X5507.

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