Preface

Applications of low-temperature plasmas for nanofabrication is a very new and quickly emerging area at the frontier of physics and chemistry of plasmas and gas discharges, nanoscience and nanotechnology, solidstate physics, and materials science. Such plasma systems contain a wide range of neutral and charged, reactive and non-reactive species with the chemical structure and other properties that make them indispensable for nanoscale fabrication of exotic architectures of different dimensionality and functional thin films and places uniquely among other existing nanofabrication tools. By nanoscales, it is typically implied that the spatial scales concerned are above 1 nm (= 10^-9 m) and below few hundred nm.

In the last decade, there has been a strong trend towards an increasing use of various plasma-based tools for numerous processes at nanoscales, including plasma-aided nanoassembly of individual nanostructures and their intricate nanopatterns, deposition of nanostructured functional materials (including biomaterials), nanopatterns and interlayers, synthesis of quantum confinement structures of different dimensionality (e.g., zero-dimensional quantum dots, one-dimensional nanowires, twodimensional nanowalls and nanowells, and intricate three-dimensional nanostructures), surface profiling and structuring with nanoscale features, functionalization of nanostructured surfaces and nanoarrays, ultra-high precision plasma-assisted reactive chemical etching of sub100 nm-wide and high-aspect-ratio trenches and several others.

In many applications (such as in commonly used plasma-assisted reactive chemical etching of semiconductor wafers in microelectronics), plasma-based nanotools have shown superior performance compared to techniques primarily based on neutral gas chemistry such as chemical vapor deposition (CVD). However, compared to neutral gas routes, in low-temperature plasmas there appears another level of complexity related to the necessity of creating and sustaining a suitable degree of ionization and a much larger number of species generated in the gas phase, which is no longer neutral. Furthermore, in many cases uncontrollable generation, delivery and deposition of a very large number of radical and ionic species, further complicated by intense physical (physisorption, sputtering, etc.) and chemical (chemisorption, bond passivation, reactive ion/radical etching) plasma-surface interactions substantially compromise the quality and yield of plasma-based processes.

This overwhelming complexity leads to a number of practical difficulties in operating and controlling plasma-based processes. In many cases, instead of nicely ordered arrays of nanoscale objects one obtains poor quality and very disordered films nowhere near having any nanoscale features. Moreover, improper use of plasmas may lead to severe and irrepairable damage to nanoscale objects already synthesized. On the other hand, plasma-based processes can be used to create really beautiful nanostructures and nanofeatures such as single and multiwalled carbon nanotubes and high-aspect ratio straight trenches in silicon wafers. These common facts give us a lead to think that certain knowledge and skills are required to operate and use plasma discharges to synthesize and process so delicate objects as nanoscale assemblies.

In our daily life we always use a broad range of appliances and tools. Some of them are so simple to operate so that no one even reads a user's guide. However, the more complex the tool or appliance becomes, the more options it offers, to everyone's benefit. On the other hand, as the complexity increases, it becomes increasingly difficult to operate them. Some of the new and uncommon features are very difficult to enable merely relying on the already existing knowledge and experience. It is of course possible to enable some of these features via trial and error but a chance of damaging the (presumably expensive!) tool or appliance becomes higher after each unsuccessful attempt. The more complex the object of our experimentation becomes the larger number of trials we need to undertake. Above a certain level of complexity, trial and error simply become futile and way too risky and the best way in this case would be simply to read the user's manual. Fortunately, it is a norm nowadays that manufacturers of household appliances and related tools and devices provide handy user's instructions and manuals.

The situation changes when one tries to experiment and create something uncommon and unusual, by suitably modifying the commonly used tools. This is a typical situation in nanotechnology, which aims to create exotic ultra-small objects with highly-unusual properties compared with their bulk material counterparts. Apparently, creation of such small objects would most likely require different tools, approaches and techniques. Since the nanoscale objects are usually more complex than their corresponding bulk materials, they also require more complex fabrication tools and processes. Moreover, the costs involved in nanoscale processing are usually substantially higher compared to treatment of similar bulk materials. For example, multi-step nanostructuring of silicon semiconductor wafers (which may involve pre-patterning, surface conditioning, etching, deposition, etc. stages) is far more expensive than its coating by a plain dielectric film. The complexity of processing and therefore, the associated cost continuously increase as the feature sizes become smaller and smaller. Taken that even a single faulty interconnect or a short-circuited gate of a field effect transistor (which is more and more difficult to fabricate as they reduce in size) may disable proper functioning of the whole microchip.

Hence, the price of even simple errors in nanoscale processing may be way too high to simply afford them! For example, a 45nm-sized nanoparticle attached to the surface of a 5 μm-thick film will most likely make no difference in terms of the film properties and performance. However, the same particle can reconnect (and hence, short-circuit) the two gate electrodes of a field effect transistor (FET) fabricated using a 45nm node technology. This particle can be mistakenly grown in the gate area (e.g., when a nucleus was formed in an uncontrollable fashion) or grown in the gas phase and then dropped onto the transistor's gate. In either cases the associated damage to the integrated circuit may become irrecoverable and the whole effort spent on fabricating a huge number of transistors, vias, interconnects, interlayer dielectrics, etc. may go to waste simply because of a single nanoparticle-damaged transistor!

Therefore, it becomes clear that as the complexity of nanoscale processing increasses, the cost of a single error becomes higher and eventually any "trial and error" approach in adjusting the nanofabrication tool and/or process may become inappropriate. First of all, the more complex the tools and processes become, the more reliant the researchers, students, process engineers and technicians become on user's manuals and detailed process specifications. For precise materials synthesis and processinig these guides should be as precise as possible. But who is supposed to write these detailed instructions? Engineers should write such guides for technicians, researchers for engineers, but who is supposed to write these for researchers? In the sister monograph "Plasma-Aided Nanofabrication: From Plasma Sources to Nanoassembly" [1] published by Wiley-VCH in July 2007 we tried to give some most important practical advices to researchers how to develop plasma-based nanoassembly processes, select the right plasma type, design appropriate plasma tools and reactors, and provided specific process parameters that led us and our colleagues to the synthesis of a wide range of nanoscale objects. However, the number of recipes given in that book is limited to certain types of low-temperature plasmas and specific nanoscale objects. So, where to find advice what to do when, for example, a 45 nm-sized nanoparticle was found in the gate area of an FET?

A typical advocate of a "trial and error" approach would suggest to change some process parameters and see what happens. But what if this trial will not work or continue causing more problems? On the other hand, a typical advocate of strictly following the prescribed guidelines would suggest to check a troubleshooting guide. But what if there is nothing about which knob to turn to eliminate the above particles? Moreover, taken the huge number of nanoscale processes that involve highercomplexity environments such as low-temperature plasmas, how could one possibly develop suggestions to troubleshoot every possible problem? The more complex the system becomes, the more opportunities for better, faster, more precise synthesis and processing it offers; on the other hand, a chance that something will go wrong will increase substantially. No wonder, the system is complex and may cause even more complex problems!

There are no exhaustive recipes to eliminate and troubleshoot all possible problems in a myriad of plasma-based/assisted processes that either already exist or being developed. In fact, if the nanofabrication system is very complex, then it would be physically impossible to foresee everything that can go wrong... So, what to do in this case? There is only one clear advice in this regard: do research, find a cause of the problem and then eliminate it. Therefore, the more complex systems we use in nanofabrication (as well as in any other area of technology and everyday's life), the more important is to understand how they work, how to make them operate smoothly and how to prevent and eliminate any potential problems at a minimum cost and effort. The importance of this rather simple commonsense statement becomes crucial when dealing with nanoscale materials synthesis and processing and I hope that anyone involved in related research will agree with me without any major arguments.

We are almost near the point where it becomes very clear what is the main point of this book. It should already become perfectly clear that it is about plasma-based nanotechnology. This nanotechnology is based on tow-temperature plasmas, which represent a significantly more complex nanofabrication environment as compared with neutral feed gases where such plasmas are generated. So, how to properly handle plasmabased nanoassembly, avoid costly errors and troubleshoot any potential problems? To do this, we have to understand which plasmas to use, which plasma reactors and processes to design, how exactly to operate the plasma and control the most important surface processes.

These are among the most important issues the Plasma Nanoscience deals with and this monograph primarily aims to introduce the main aims and approaches of the Plasma Nanoscience to a reasonably broad audience which includes not only experts in the areas of plasma processing, materials science, gas discharge physics, nanoscience and nanotechnology and other related areas but also other researchers, academics, engineers, technicians, school teachers, graduate, undergraduate and even high school students.

As we will see from this monograph, the "microscopic" key to overcome the above problems and ultimately improve the overall perfomance of plasma-aided nanofabrication tools is to control generation, delivery, deposition, and structural incorporation of the required building units (BUs) complemented by appropriately manipulating other functional species [hereinafter termed "working units" (WUs)] that are responsible, e.g., for preparing the surface for deposition of the BUs. This task is impossible without properly identifying the purpose of each species (that is, as a BU, WU, functionless, or even a deleterious species) and numerical modelling of number densities of such species in plasma nanofabrication facilities and their fluxes onto nanostructured solid surfaces being processed.

Thus, the fundamental key to the ability to properly operate and troubleshoot highly-complex plasma nanotools is in comprehensive understanding of underlying elementary physical and chemical processes both in the ionized gas phase and on the solid surfaces exposed to the plasma. This is one of the main objectives of this monograph.

In my decision to write this book I was motivated by the fact that even though basic properties and applications of low-temperature plasma systems and even a range of useful recipes how to use such plasmas have been widely discussed in the literature, there have been no attempt to systematically clarify and critically examine what actually makes lowtemperature plasmas a versatile nanofabrication tool of the "nano-age". One of the aims of this work is to discuss, from different perspectives and viewpoints, from commonsense intuition to expert knowledge, numerous specific features of the plasma that make them particularly suitable for synthesizing a wide range of nanoscale assemblies, epitaxial films, functionalities and devices with nano-features.

Richard Feynman's visionary speech "There is plenty of room at the bottom" [2] and a recent rapid progress in nanotechnology gave me a source of additional inspiration and provoked a couple of simple questions:

• Is there a room, in the global nanoscience context, for atomic manipulation in the plasma?

• Since the plasma is an unique, the fourth (ionized) state of the matter associated in our minds to a collection of interacting charged particles, what is the difference between nanoscale objects assembled in ionized and non-ionized gas environments?

Moreover, as we will stress in Chapter 1 of this monograph, since more than 99% of the visible matter in the Universe finds itself in an ionized (plasma) state (and contains charged atoms and electrons), the formation of the remaining ~ 1 % of the matter should have inevitably passed through the nano-scale synthesis process (termed nanoassembly hereinafter) step. The nanoassembly is basically a rearrangement of gasphase borne subnanometer-sized atomic building units into more ordered macroscopic liquid and solid-like structures. Thus, one can intuitively suspect (even without any specialist knowledge apart from the sizes of the atoms and macroscopic ordered structures) that the process of formation of solid matter in the Universe did include the nano-assembly step in the plasma environment. Meanwhile, our commonsence tells us that the Nature always chooses the best option for arranging the things! So, could the plasma environment was chosen by the Nature for a specific purpose? As we will see from this monograph, the plasma environment could serve as an accelerator of nanoparticle creation in stellar outflows. Amazingly, without the plasma, there might have been insufficient dust particles, which are essential to maintain chemical balance in the Universe!

Another interesting area where in-depth investigation of the elementary plasma-based processes may shed some light on many existing mysteries is possible creation of building blocks of life such as DNA, RNA, proteins and living cells. There is a number of theories suggesting that these building blocks might have formed from simple organic molecules through a chain of elementary chemical reactions in methane, hydrogen, and water vapor-rich atmosphere of primordial Earth. The most amazing related fact is that at that time electrical discharges in the Earth's atmosphere (e.g., lightnings, coronas and sparks) were so frequent so that they may have played a significant role in chemical synthesis of macromolecules that eventually led to the formation of DNA, RNA and more complex building blocks of life. Despite more than 50 years of intense research and related debates about the creation and the origins of life which involve an extremely broad audience, this issue is far from being complete. On a positive note, reactive plasmas have been used to synthesize, in laboratory conditions close to those in primordial Earth, many complex organic macromolecules whithout which the existence of more complex building blocks of life would be impossible.

Even though this particular issue is only briefly mentioned in this monograph, here we stress that creation of building blocks of life is as important for the Plasma Nanoscience as the plasma-assisted synthesis of cosmic dust (building blocks of the Universe) and various building blocks (nanostructures, nanoarrays, etc.) of modern nanotechnology. These seemingly very different and unrelated issues have one most important thing in common: plasma environment which is used for deterministic creation of the above building blocks.

Since the Nature's nanofab uses the plasma in the Universe and quite possibly used quite similar ideas to synthesize building blocks of life in the atmosphere of primordial Earth, it sounds quite logical that so many companies and research institutions presently use cleanroom and laboratory plasma environments to synthesize a variety of nano-sized objects and nanodevices. Indeed, if a so reputed authority as Nature uses lowtemperature plasmas to create many useful nanoscale things, then why should not one use that in terrestrial labs and commercial fabs? However, human mind always aims to create something that the Nature either cannot create or creates way too slow and inefficiently.

It is remarkable that the number of nanofilms, nano-sized structures, architectures, assemblies, and micronanodevices fabricated by using low-temperature plasmas, has been enormous in the last ten years. Amazingly, using catalyzed plasma-assisted growth, it is possible to synthesize carbon nanotubes which are not among the common products of the Nature's astrophysical nanofab, and moreover, at rates which are orders and orders of magnitude higher.

Interestingly, the competition for priority synthesis and improved performance of nano-objects has been very tough in the last decade and gave rise to currently prevailing "trial and error" (followed by a rapid dissemination of the results) practice in the nanofabrication area. Furthermore, there is presently a wide gap between the practical performance of numerous plasma-based nanofabrication facilities and in-depth understanding of fundamental properties and operation principles of such devices and tools and elementary processes involved at every nanofabrication step. Indeed, if a particular plasma tool works well and allows one to fabricate nanostructured wafers and integrated circuits with a huge number of nano-sized transistors and synthesize a myriad of different nanostructures and materials, what is the point to research why it does so? Does one really need to?

Yes, one has to do that, and for a number of reasons. The most important reason for in-depth study of elementary physical and chemical processes involved is the need to keep the pace with miniaturization and ever-increasing demands for better quality nanomaterials and high performance functionalities and nanodevices. At some stage the existing pool of tools will fail to meet the requirements, and what shall one do next? Do the trial and error as we discussed earlier?

After several years of active and productive research in the area, my colleagues and myself realized that the capabilities of "trial and error" approaches will soon be exhausted and deterministic "cause and effect" approaches to nanofabrication will need to be widely used to achieve any significant improvement in the properties and performance of the targeted nano-assemblies, nanomaterials, and nanodevices, which was quite easy to achieve several years ago by "trial and error". Indeed, in early and mid-90s, after a pioneering discovery of carbon nanotubes by Iijima [3], almost every carbon nanostructure synthesized under different process conditions, might have had quite different properties. But it is very difficult to impress anyone by synthesizing a carbon nanotube in 2008, when such a work has become a routine exercise in the third year chemistry or nanotechnology undergraduate programs.

Therefore, there is a vital demand for the development and wider practical use of sophisticated, and yet simple, deterministic "cause and effect" approaches. It is important to mention that such approaches would be impossible without a comprehensive understanding and generic recipes on the appropriate use and control, at the microscopic level (and more importantly, both in the ionized gas phase and on the solid surfaces), of the "causes" to achieve the envisaged and pre-determined goals ("effects").

Evidently, in the nanofabrication context, one can use the building blocks (e.g., specific atoms or radicals) of the nanoassemblies as the "cause" and the nanoassemblies themselves as the "effect". Indeed, the "building block" has been among the most commonly used and popular terms of the nanoscience and nanotechnology in the last decade. This term usually encompasses both elementary building units of atomic and molecular assemblies and some nanostructures and nanoparticles that are in turn used to build more complex nanoscale functionalities and nanodevices. However, merely praising the building units of the plasma-aided nanoassembly would be unfair, since many other particles also serve for other, merely than as building material, purposes.

For this reason, in this monograph we introduced the expanded notion of "working units" that encompasses all the relevant plasma species that contribute to any particular nanofabrication step. For instance, without appropriate surface preparation by suitable plasma species, the deposition and stacking of the building units into a nanostructure would be impossible. Thus, one should be fair in acknowledging contributions from all working units and realize that every one of them has to do their specific job properly to achieve the overall success. This is how the "nanoteam effort" work!

It should also be emphasized that despite an enormous number of research monographs and textbooks related to nano-science and nanotechnology, only a few of them report on and analyze superior performance of plasma enhanced chemical vapor deposition (PECVD) and other plasma-based systems in nanofabrication of a wide variety of common nanostructures, such as carbon nanotubes, quantum dots, nanowalls, nanowires, etc. Therefore, there is a significant gap between the knowledge and information related to basic properties and applications of low-temperature plasmas and numerous nanoassembly processes that merely use such plasmas as a tool. Thus, the question about the actual role of the plasma in a large number of relevant processes remains essentially open. This book is intended to fill this obvious gap in the literature.

This monograph introduces the Plasma Nanoscience as a distinct research area and shows the way from Nature's mastery in assembling nano-sized dust grains in the Universe to deterministic plasma-aided nanoassembly of a variety of nanoscale structures and their arrays, a base of the future nanomanufacturing industry. We also introduce a concept of deterministic nanoassembly together with a multidisciplinary approach to bridge the spatial gap of nine orders of magnitude between the sizes of plasma reactors and atomic building units that selfassemble, in a controlled fashion, on plasma-exposed surfaces. By discussing the results of ongoing numerical simulation and experimental efforts on highly-controlled synthesis of various nanostructures and nanoarrays we show potential benefits of using ionized gas environments in nanofabrication.

In this monograph, we systematically discuss numerous advantages of using low-temperature plasmas to synthesize various nano-scale objects, and also introduce basic concepts of Plasma Nanoscience as a distinctive research area. For consistency of illustrating the benefits of using the advocated "cause and effect" approach, the majority of the examples come from own research experience of the author and his colleagues. Nevertheless, we will also attempt to provide a reasonable coverage of relevant ongoing reserach efforts that ultimately aim at achieving the goal of plasma-based deterministic synthesis of various nanostructures and elements of nanodevices.

In a systematic and easy-to-follow way, this monograph highlights the fundamental physics and relevant nanoscale applications of lowtemperature plasmas and attempts to give detailed comments on what exactly makes the plasma a versatile nanofabrication tool of the "nanoage". An initial attempt to answer this very intriguing and timely puzzle of modern interdisciplinary science was made in a Colloquium article of Reviews of Modern Physics published in 2005 [4]. This original effort was further supported by a Special Cluster Issue of the Journal of Physics D on plasma-aided fabrication of nanostructures and nanoassemblies. For more details about this Special Issue please refer to the editorial review [5] and a cluster of 19 articles in the same issue. This monograph continues this series of efforts and aims to consolidate, in a single publication, some of the most important bits of knowledge about the unique properties and outstanding performance of the plasma-based systems in nanofabrication, as well as about possible ways of controlling the plasma-based nanoassembly.

Main attention is paid to the conditions relevant to the laboratory gas discharges and industrial plasma reactors. A specialized and comprehensive description of the most recent experimental, theoretical and computational efforts to understand unique properties of low-temperature plasma-aided nanofabrication systems involving a large number of associated phenomena is provided. Special emphasis is made on fundamental physics behind the most recent developments in major applications of relevant plasma systems in nanoscale materials synthesis and processing.

This monograph covers a specific area of the cutting-edge interdisciplinary research at the cross-roads where the physics and chemistry of plasmas and gas discharges meet nanoscience and materials physics and engineering. It certainly does not aim at the entire coverage of the existing reports on the variety of nanostructures, nanomaterials, and nanodevices on one hand and on the plasma tools and techniques for materials synthesis and processing at nanoscales and plasma-aided nanofabrication on the other one. Neither does it aim to introduce the physics of low-temperature plasmas for materials processing. We refer the interested reader to some of the many existing books that cover some of the relevant areas of knowledge [6-20]. From the perspective of fundamental studies, one of the purposes of this book is to pose a number of open questions to foresee the future development of this research area and also urge the researchers to look into fundamental, elementary bits (and not merely limited to the building units!) that make their nano-tools work.

The author extends his very special thanks to S. Xu, his principal collaborator in the last 8 years and a co-author of the sister monograh [1] and I. Levchenko, a co-author of Chapter 6, who also made substantial original contributions to a large number of original publications used in this monograph and created many exciting visualizations of original computational results and excellent illustrations for this book.

I am particularly grateful to my co-authors (alphabetic order) Q.J. Cheng, U. Cvelbar, I. Denysenko, J. C. Ho, S. Y. Huang, M. Keidar, J. D. Long, A. B. Murphy, A. E. Rider, P. P. Rutkevych, E. Tarn, Z. L. Tsakadze, H.-J. Yoon, L. Yuan, X. X. Zhong, and W. Zhou, who made major contributions to the original publications used in this monograph.

I greatly acknowledge contributions and collaborations of other present and past members and associates of the Plasma Nanoscience (The University of Sydney, Australia) and Plasma Sources and Applications Center (NTU, Singapore) teams Y. Akimov, K. Chan, J. W. Chai, M. Chan, H. L. Chua, Y. C. Ee, S. Fisenko, N. Jiang, Y A. Li, V. Ligatchev, W. Luo, E. L. Tsakadze, C. Mirpuri, V. Ng, L. Sim, Y.P. Ren, M. Xu, and all other co-authors of my research papers and conference presentations.

I also greatly appreciate all participants of the international research network and Plasma Nanoscience enthusiasts around the globe, as well as fruitful collaborations, mind-puzzling discussions, and critical comments of (alphabetic order) A. Anders, M. Bilek, I. H. Cairns, L. Chan, P. K. Chu, K. De Bleecker, C. Drummond, C. H. Diong, T. Desai, C. Foley, F. J. Gordillo-Vazquez, M. Hori, N. M. Hwang, A. Green, B. James, H. Kersten, S. Komatsu, U. Kortshagen, S. Kumar, O. Louchev, X. P. Lu, D. Mariotti, D.R. McKenzie, M. Mozetic, A. Okita, X.Q. Pan, F. Rosei, P. A. Robinson, P. Roca i Cabarrocas, F. Rossi, Y. Setsuhara, M. Shiratani, M. P. Srivastava, L. Stenflo, R. G. Storer, H. Sugai, H. Toyoda, S. V. Vladimirov, M. Y. Yu, and many other colleagues, collaborators and industry partners. I also thank all the authors of original figures for their kind permission to reproduce them. I sincerely appreciate the interest of a large number of undergraduate and postgraduate students at the University of Sydney in our special and summer vacation projects.

Last but not the least, I thank my family for their support and encouragement and extend very special thanks to my beloved wife Tina for her love, inspiration, motivation, patience, emotional support, and sacrifice of family time over weekends, evenings and public holidays that enabled me to work on this book and a large number of associated original publications, review papers, and project applications. My special thanks to my beloved pet Grace The Golden Retriever, who inspired me on a number of occasions during long evening walks around the suburb where we live.

This work was partially supported by the Australian Research Council, the University of Sydney, CSIRO, Institute of Advanced Studies (NTU, Singapore), and the International Reserach Network for Deterministic Plasma-Aided Nanofabrication.

Sydney, June 2008

Kostya (Ken) Ostrikov