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- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Serie: Microsystem and Nanotechnology Series
- Sprache: Englisch
Nanoscale pattern transfer technology using molds is a rapidly advancing area and one that has seen much recent attention due to its potential for use in nanotechnology industries and applications. However, because of these rapid advances, it can be difficult to keep up with the technological trends and the latest cutting-edge methods. In order to fully understand these pioneering technologies, a comprehensive understanding of the basic science and an overview of the techniques are required. Nanoimprint Technology: Nanotransfer for Thermoplastic and Photocurable Polymers covers the latest nanotransfer science based on polymer behaviour. Polymer fluid dynamics are described in detail, and injection moulding, nanoimprint lithography and micro contact printing are also discussed. Cutting-edge nanotransfer technologies and applications are also considered and future trends in industry are examined. Key features: * Covers the fundamentals of nanoimprint technology * Presents cutting-edge techniques and applications * Provides industrial examples and describes the mold fabrication process * Considers nanotransfer of thermoplastics by simulation * Describes the design and evaluation of UV curable polymer Nanoimprint Technology: Nanotransfer for Thermoplastic and Photocurable Polymers is a comprehensive reference for industry engineers as well as graduate and undergraduate students, and is a useful source of information for anyone looking to improve their understanding of nanotransfer mechanisms and methods.
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Veröffentlichungsjahr: 2013
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Table of Contents
Series Page
Title Page
Copyright
About the Editors
List of Contributors
Series Preface
Preface
Chapter 1: What is a Nanoimprint?
References
Chapter 2: Nanoimprint Lithography: Background and Related Techniques
2.1 History of Material Processing: Polymer Processing
2.2 Products with Microstructure and Nanostructure
2.3 Technology for Making Micro- and Nanostructures
References
Chapter 3: Nanopattern Transfer Technology of Thermoplastic Materials
3.1 Behavior of Thermoplastic Materials
3.2 Applicable Processes Used for Nanopattern Transfer
3.3 Pattern Transfer Mechanism of Thermal Cycle NIL
3.4 Modeling of Nanopattern Transfer
References
Chapter 4: Mold Fabrication Process
4.1 Ultra Precision Cutting Techniques Applied to Metal Molds Fabrication for Nanoimprint Lithography
4.2 Nanoimprint Mold Fabrication Using Photomask Technology
References
Chapter 5: Ultraviolet Nanoimprint Lithography
5.1 Orientation and Background of UV-NIL
5.2 Transfer Mechanism of UV-NIL
5.3 UV-NIL Materials and Equipment
5.4 Evaluation Method
References
Chapter 6: Applications and Leading-Edge Technology
6.1 Advanced Nanoimprinting Technologies
6.2 Applications
6.3 High-Accuracy Nanoimprint Technology, Development of Micropatterning Method, and Automatic Process Control Using Batch Press Type, Step and Repeat Type Nanoimprint Machine
6.4 Micro/Nano Melt Transcription Molding Process
6.5 Future Trends
References
Index
Microsystem and Nanotechnology Series Series Editors: Ron Pethig and Horacio Dante Espinosa
This edition first published 2013
© 2013, John Wiley & Sons Ltd
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Library of Congress Cataloging-in-Publication Data
Nanoimprint technology : nanotransfer for thermoplastic and photocurable polymer / edited by Jun Taniguchi, Hiroshi Ito, Jun Mizuno, Takushi Saito.
pages cm
Includes bibliographical references and index.
ISBN 978-1-118-35983-9 (cloth)
1. Nanoimprint lithography. 2. Nanolithography–Materials. 3. Plastics–Molding. 4. Polymers–Thermal properties. 5. Thermoplastics. 6. Microfluidics. 7. Transfer-printing. I. Taniguchi, Jun.
TK7874.843.N36 2013
621.381–dc23
2013007112
A catalogue record for this book is available from the British Library.
Print ISBN: 978-1-118-35983-9
About the Editors
Jun Taniguchi is an Associate Professor at the Department of Applied Electronics, Tokyo University of Science (Tokyo, Japan). He received BE, ME, and PhD degrees from Tokyo University of Science, in 1994, 1996, and 1999, respectively. From 1999 to 2013, he was with the Department of Applied Electronics, Tokyo University of Science. His research interests include electron beam lithography for nanoimprint molding, nanoimprint lithography, roll-to-roll nanoimprint lithography, and nanotechnology applications such as optical devices and moth-eye structures ([email protected]).
Hiroshi Ito graduated from the Department of Polymeric Materials and Engineering at Yamagata University (Yamagata, Japan). He received his Master's degree in engineering from Yamagata University, in 1990. After graduation, he joined Oki Electric Industry Co., Ltd (Tokyo, Japan). In 1993, he became an Assistant Professor at Tsuruoka National College of Technology (Yamagata, Japan), and received his PhD from Yamagata University, in 1996. In this year, he also became an Assistant Professor at the Tokyo Institute of Technology (Tokyo, Japan). In 2007, he became an Associate Professor at Yamagata University. In 2010, he became a Professor at Yamagata University. He is now Chair of the Department of Organic Device Engineering and the Department of Organic Materials Engineering (PhD program). He is also Director of Research at the Center for Advanced Processing GREEN Materials, Yamagata University ([email protected]).
Jun Mizuno received his PhD in applied physics from Tohoku University (Miyagi, Japan) in 2000. He is currently an Associate Professor at Waseda University (Tokyo, Japan) and works at the Nanotechnology Research Center. His current interests are MEMS/NEMS technology, bonding technology at low temperature using plasma activation or excimer laser irradiation, printed electronics, and composite technology for UV/heat nanoimprint lithography combined with electrodeposition ([email protected]).
Takushi Saito is a member of the Department of Mechanical and Control Engineering at the Tokyo Institute of Technology (Tokyo, Japan). He received his PhD in engineering from the Tokyo Institute of Technology in 1996. He began his academic career as a post-doctoral researcher at the University of Minnesota (MN, USA) in 1997. He then became an Assistant Professor at the Tokyo Institute of Technology in 1998. Since June 2002, he has been an Associate Professor at the Tokyo Institute of Technology. His current research topics include visualization and measurement of polymer processing, laser-assisted manufacturing processes, and the development of heat transfer control techniques in material processing ([email protected]).
List of Contributors
Series Preface
The Microsystem and Nanotechnology book series provides a thorough contextual summary of the current methods used in micro- and nanotechnology research and how these advances are influencing many scientific fields of study and practical application. Readers of these books are guided to learn the fundamental principles necessary for the topic, while finding many examples that are representative of the application of these fundamental principles. This approach ensures that the books are appropriate for readers with varied backgrounds and useful for self-study or as classroom materials.
Micro- and nanoscale materials, fabrication techniques, and metrology methods are the basis for many modern technologies. Several books in this series, including Introduction to Microsystem Technology by Gerlach and Dotzel, Microfluidic Technology and Applications edited by Koch, Evans, and Brunnschweiler, and Fluid Properties at Nano/Meso Scale by Dyson, Ransing, P. Williams, and R. Williams, provide a resource for building a scientific understanding of the field. Multiscale modeling, an important aspect of microsystem design, is extensively reviewed in Multiscale Analysis of Deformation and Failure of Materials by Jinghong Fan. Modern topics in mechanics are covered in Nano and Cell Mechanics: Fundamentals and Frontiers edited by Espinosa and Bao. Specific implementations and applications are presented in AC Electrokinetics: Colloids and Nanoparticles by Morgan and Green, Digital Holography for MEMS and Microsystem Metrology edited by Asundi.
This book, edited by Jun Taniguchi, presents the fundamental methods of nanoimprint technologies and the principles of fabrication and materials selection that are essential for their successful implementation. Included in this work are examples of theoretical modeling of the physical phenomena that govern micro- and nanofabrication and the invaluable insight they provide for informing process design and parameters.
Horacio D. EspinosaRon Pethig
Preface
The technique of nanoscale pattern transfer technology using a mold has attracted attention because this technology makes nanotechnology industries and applications possible. This field of technology has evolved rapidly, year by year. However, because of these rapid advances, it is difficult to keep up with the technological trends and the latest cutting-edge methods. In order to fully understand these pioneering technologies, comprehension of the basic science and an overview of the techniques is required. In this book, the latest nanotransfer science—based on polymer behavior and polymer fluid dynamics—is described in detailed but easy-to-understand language. Based on their physical science, injection molding and nanoimprint lithography are explored. These exemplifications of concrete methods will help the reader to create an accurate picture of nanofabrication. Furthermore, the newest cutting-edge nanotransfer technologies and applications are also described. We hope the reader will benefit from knowledge of these new technologies and be left with a basic comprehension of nanotransfer mechanisms and methods.
Jun Taniguchi
Chapter 1
What is a Nanoimprint?
Jun Taniguchi
Department of Applied Electronics, Tokyo University of Science, Japan
The technical term “nanoimprint” first appeared in “nanoimprint lithography,” as used by Professor S.Y. Chou in 1995 [1]. “Nano” means 10–9, and usually refers to nanometer (nm) scale objects and structures. “Imprint” means to press and make engraved marks, and so has almost the same meaning as pressing, embossing, and printing. However, lithography has a special meaning, and is the main technique for fabricating nanopatterns in the semiconductor process. The lithography process is shown in Figure 1.1.
Figure 1.1 Lithography process
First, a photoresist is coated on a silicon (Si) substrate. A photoresist is a material whose solubility changes when exposed to light (photons). The photomask is made of quartz and chromium (Cr), producing a light contrast—the quartz area is transparent whereas the Cr area does not transmit light. Thus, the photomask defines the area of the photoresist that will be exposed to light. An excimer laser (KrF: wavelength 248 nm, ArF: wavelength 193 nm) is used as the light source. The photomask is placed over the photoresist on Si, then light is exposed through the photomask (Figure 1.1(a)) to produce the exposed areas of the photoresist (Figure 1.1(b)). The exposed areas are changed into two types by liquid immersion. This liquid is called the developer, and the liquid immersion process is called development. After development, the photoresist where the exposed areas were removed is called positive type whereas the photoresist where the exposed areas remain is called negative type. These two types form the resist pattern on the silicon wafer. Using the resist patterns, successive semiconductor processes such as dry etching, ion implantation, and metal wiring are carried out. Dry etching is the process of removing silicon substrate using the developed resist for an etching mask. Dry etching uses an active gas such as CF4, SF6, or CHF3 for the silicon substrate, by creating a plasma at low pressure. The activated species (ions or radicals) also etch the development resist, hence the term “photoresist.” The ion implantation process is the process of doping donors and acceptors to create p- and n-type regions. Metal wiring is performed by the lift-off process, as follows: after development, metal is deposited by sputtering or evaporation, then the resist is removed by the remover, which dissolves the resist polymer. After removal of the resist, metal wiring remains on the silicon substrate and this area acts as an electrode and power supply. Therefore, the resolution of the resist pattern is very important for all processes because lithography determines the design rule of silicon devices such as ultra-large-scale integrated circuits (ULSIs). The design rule is the gate length or half pitch of line and space, and this index measures how small the transistor is. A small design rule enables many transistors to be formed per unit area, enabling a densely integrated electronic circuit which can be used to create high value-added devices such as large memory devices and high-performance central processing units (CPUs).
The following photolithography equation determines the resolution and hence the design rule for lithography [2]:
1.1
where R is the resolution, k1 is a process factor depending on the optical system of the stepper or scanner, λ is the wavelength of the light source, and NA is the numerical aperture of the lens, given by
1.2
where n is the refractive index of the light path and θ is the angle of aperture, thus The photolithography exposure system includes a stepper and scanner, which can reduce the exposed pattern area to 1/4 of the mask pattern by reduced-projection optical lenses. Here, “stepper” means the “step and repeat” motion of the Si wafer stage during light exposure and “scanner” means the continuous motion of the Si wafer stage during light exposure. This system has precise stage and optical elements, and so the cost of the system is extremely high. According to eqs (1.1) and (1.2), a fine pattern can be obtained by a small wavelength (λ) and a large NA. Thus, photolithography has a limit to miniaturization; various techniques are required to exceed this limit, which are usually expensive.
In contrast, nanoimprint lithography (NIL) can exceed this limit because the patterning mechanism is merely physical pressing. The NIL process is shown in Figure 1.2.
Figure 1.2 Nanoimprint lithography process
First, a nanoscale patterned mold is prepared. A silicon wafer with resist layer is also prepared (Figure 1.2(a)). Two types of resist layer are mainly used: thermoplastic polymer and photocurable polymer. The thermoplastic polymer is solid at room temperature, but begins to flow (liquefies) upon applying heat. Thus, the shape of the thermoplastic polymer is deformed by heating and pressing of the mold. Meanwhile, the photocurable polymer is liquid at room temperature and so is easily deformed by mold pressing [3]. However, to solidify this resin, exposure to ultraviolet (UV) light is required, for which a mercury lamp i-line (365 nm) is usually used. UV light does not transmit through the silicon wafer, so the mold must be UV-transparent. Quartz or sapphire is transparent to UV light, and so these materials are used for the mold. After preparing the pattern transfer, the mold presses the resist layer on the silicon wafer (Figure 1.2(b)). After solidification of the resist layer, the mold is released from it (Figure 1.2(c)). At this time, the convex part of the mold engraves the concave part of the resist layer, but the convex part of the mold does not contact the silicon wafer. Usually, a residual layer remains above the silicon wafer. This residual layer is unnecessary and is removed by oxygen plasma ashing and so on (Figure 1.2(d)). After these processes, the silicon wafer has a mask pattern as shown in Figure 1.1(c), therefore NIL can act as a lithography process.
The advantages of NIL are as follows. It is a simple process and thus cost-effective; once a nanoscale mold has been prepared, nanoresolution patterns can be obtained at low cost. Furthermore, sub-10 nm feature patterns by NIL were reported in 1997 [4], which is a major step in the semiconductor field because NIL is a simple, cost-effective, and high-resolution process. The potential of NIL is well known worldwide, and many companies and researchers are currently conducting semiconductor research [5]. In addition, NIL is very useful for other fields such as three-dimensional (3D) pattern transfer. When NIL is used for the semiconductor process, the residual layer must be removed, but in other fields it is not necessary to remove it. When the residual layer is removed, a mask pattern for silicon is obtained, but this is a two-dimensional pattern. That is, the silicon surface is painted with or without the resist mask. In contrast, by using a mold with a 3D pattern, the nanoimprint process creates a 3D replica. This kind of 3D fabrication is difficult to achieve by photolithography. Furthermore, 3D replica patterns are widely used for optical elements and surface-modified uses. For example, a moth-eye structure (which is a kind of anti-reflective structure), diffractive optical elements, gratings, Fresnel lenses, polarizers, sub-wavelength plates, and wire-grid polarizers are all optical devices. Surface-modified devices include cell culture plates, hydrophobic surfaces (lotus-effect surfaces), and adhesive surfaces such as gecko finger structures. Therefore, NIL is widely used for 3D nanofabrication, and this versatile process is called “nanoimprint technology.” Therefore, NIL now means not only lithography but also 3D fabrication. This book mainly describes nanoimprint technology for 3D fabrication.
Many preparations are required to perform nanoimprint technology, such as the transfer polymer, mold fabrication process, transfer machine, and measurement system. The main transfer polymers are thermoplastic polymer and photocurable polymer, but their transfer processes are different. In addition, different transfer machines are also required for each polymer. Thus, in this book, thermoplastic and photocurable polymers are dealt with in separate chapters.
The pattern transfer of thermoplastic polymer is described in Chapters 2 and 3. First, Chapter 2 describes the history of polymer processing and the principle of the transfer method. Then, Chapter 3 describes the characteristics of thermoplastic polymer and the transfer method, and also simulation results. These simulations are very helpful for identifying thermoplastic behavior and how deformation develops over time. Nanoimprint technology using thermoplastic polymer requires a thermal cycle, so this kind of NIL is called “thermal cycle NIL” or simply “thermal NIL.” The technical terms “thermoplastic polymer,” “thermoplastic resin,” and “thermoplastic” have almost the same meaning.
Mold fabrication processes are described in Chapter 4. The mold is the key component of nanoimprint technology, and so it is very important to be able to make a fine and precise mold. The mechanical cutting process and electron beam lithography and dry etching process required to obtain a nanoscale 3D shape mold are described in detail. Machine tools and accurate machine positioning and control have been developed, enabling sub-micrometer order 3D cutting shapes to be fabricated. The merit of using cutting tools is rapid fabrication. Electron beam lithography (EBL) involves exposure to an electron beam instead of excimer laser light. An electron beam can be focused to less than several nanometers, so a finer pattern (less than 10 nm) can be delineated. Electron beam lithography is usually used for the photomask in the semiconductor process, but by using EBL and successive dry etching technologies, nanoimprint molds can be fabricated. This book also describes various mold materials. The technical terms “mold,” “stamp,” and “template” have almost the same meaning.
Nanoimprint technology using photocurable polymer is described in Chapter 5. In this case, ultraviolet light is used to harden the photocurable polymer, so this kind of NIL is called “ultraviolet NIL,” or UV-NIL. This chapter describes the UV-NIL mechanism, photocurable polymer science, UV-NIL machine, release agents, and measurement methods. Usually, nanoscale patterns are observed with a scanning electron microscope (SEM) and atomic force microscope (AFM), but these methods are for microscale local observation. In this chapter, a macroscale non-uniform measurement system is described. The release agent is the coating material on the mold surface, which prevents the photocurable polymer from sticking. The technical terms “photocurable polymer,” “photocurable resin,” “UV-curable polymer,” and “resin” have almost the same meaning.
Chapter 6 outlines the latest nanoimprint technologies, as well as actual applications and some devices made by nanoimprint technology.
This book outlines nanoimprint technology using thermoplastic and photocurable polymers, and describes in detail nanoscale transfer technology, materials, machines, know-how, and trends.
References
[1] Chou, S.Y., Krauss, P.R., and Renstrom, P.J. 1995. Imprint of sub-25 nm vias and trenches in polymers. Appl. Phys. Lett.67: 3114–3116.
[2] Mack, C. 2007. Fundamental Principles of Optical Lithography. John Wiley, Chichester, UK, pp. 21–22.
[3] Haisma, J., Verheijen, M., van den Heuvel, K., and van den Berg, J. 1996. Mold-assisted nanolithography: A process for reliable pattern replication. J. Vac. Sci. Technol. B14: 4124–4128.
[4] Chou, S.Y., Krauss, P.R., Zhang, W., Guo, L., and Zhuang, L. 1997. Sub-10 nm imprint lithography and applications. J. Vac. Sci. Technol. B15: 2897–2904.
[5] Colburn, M., Johnson, S., Stewart, M., Damle, S., Bailey, T., Choi, B. et al. 1999. Step and flash imprint lithography: A new approach to high-resolution patterning. SPIE 24th International Symposium on Microlithography: Emerging Lithographic Technologies III, Santa Clara, CA, pp. 379–389.
Chapter 2
Nanoimprint Lithography: Background and Related Techniques
Hiroshi Itoa and Takushi Saitob
aDepartment of Polymer Science and Engineering, Yamagata University, Japan
bDepartment of Mechanical and Control Engineering, Tokyo Institute of Technology, Japan
2.1 History of Material Processing: Polymer Processing
Throughout human history, appliances have been produced using various materials. Very early devices were made of wood or stone, but metal came to be used after several thousand years. We have passed through a long period of development, and our current civilization involves the construction of tall, rigid bridges and skyscrapers using metal materials for their frames.
In contrast, the use of polymer materials began with natural rubber, progressing to natural cellulose, with dramatic development over a very short timeframe. The use of synthetic resins has enabled the production of indispensable components and products that are important to modern society. Such wide use is generally attributable to the superior molding properties of processed polymers. The degrees of freedom in product shape and coloration for such materials are high. In addition, the strength ratios (material strength divided by density) of materials classified as engineering plastics approach those of metal materials. Increasingly, the value of these characteristics is recognized for use in containers, housing, household electrical appliances, and interior panels of cars as well as their bumpers. Moreover, mechanical parts and optical components are made less and less with metal materials and optical glass materials; they are instead made with polymer materials for reasons of performance, machined strength, and optical characteristic.
A number of techniques are used to give shape to polymer materials, but the shaping technology called plastic molding is used most often industrially. The basic concept of plastic molding is divisible into three steps, as follows.
In practice there will be some overlap in the processing steps described above, although polymer processing can generally be divided into three steps. For example, we perform nanoimprinting in one device during steps 1–3, and injection molding during steps 2 and 3, which progress simultaneously inside a die. Close attention to the various parts of the process is required, but such an overlap realizes high productivity of the plastic molding process. The characteristics of the molding technique are explained in detail in other chapters.
2.2 Products with Microstructure and Nanostructure
From the Industrial Revolution in the 18th century up to the mid-20th century, manufacturing processes were undertaken on a millimeter scale. From the latter half of the 20th century to the beginning of the 21st century, that operational scale has been reduced to micrometers and nanometers (Figure 2.1).
Figure 2.1 Schematics of structure dimension and molding scale
The equipment supporting the growth of such miniaturization technology includes electron microscopes and atomic force microscopes. These modes of microscopy were invented in the first half of the 20th century. Subsequently, applications of the equipment rapidly became popular. Using such equipment, the microscopic world has been revealed from the sub-micrometer to the nanometer scale to a degree that was impossible using optical microscopy alone. For example, a certain virus might be only a few hundred nanometers long, despite inflicting great damage on people. Researchers would have been entirely unable to observe a virus on this scale using optical microscopy.
Some products with a fine surface structure on the micrometer to nanometer scale have been produced; their use in many fields is greatly anticipated. In the medical field, regenerative medical techniques have been proposed using tissue scaffolds with a fine nanostructure [1] [2]. Physiological sensors using surface plasmons have been produced [3]. Moreover, in the semiconductor and electronic component manufacturing fields, development of a transistor, memory, and sensor using miniaturization and carbon nanotubes as wiring for CPUs is progressing [4]. New patterned media for high-density memory for data storage are also being considered [5]. Furthermore, a nanostructure on the surface of a film with advanced features of an electrolyte membrane has been produced for use in the energy field of polymer electrolyte fuel cells (PEFCs) and direct-methanol fuel cells (DMFCs) [6, 7]. These products not only have microscale and nanoscale surface structures, they also offer additional value and performance.
For example, it is necessary to give organism affinity to tissue scaffold materials. Electronic components and a fuel cell require thermal resistance and stability during a chemical reaction. Consequently, in these fields, coexistence of product shape and functional characteristics is important. Furthermore, mass production can be realized to reduce the costs of manufacture.
Moreover, memory and CPUs with semiconducting integrated circuits are described as the way to advance the miniaturization of structures. In the International Technology Roadmap for Semiconductors (ITRS) [8], dynamic random access memory (DRAM) of 1/2 pitch is set as the line width standard of semiconductor integrated circuits serving at the 68 nm level in 2007 and around 25 nm in 2015. In this field, ArF liquid immersion exposure technology, using an advanced mode of photolithography, is proposed using an ArF excimer laser as a light source with 193 nm oscillation wavelength [9]. The mode improves resolution using a liquid with a high refractive index as a filling between the substrates used as objective and target. Moreover, as a phase shift method, the phase and hardness of the light which passes a mask are changed to form a phase shift mask and a photomask by preparing the portion for which refractive index and permeability differ [10]. Thereby, resolution is improved. For super-ultraviolet radiation with extreme ultraviolet (E-UV) and soft X-rays of wavelength 13.5 nm light source exposure technologies [11, 12], the resolution at the time of exposure itself is also improved. Using these technologies, mass production conversion of the pattern formation at 45–30 nm level has begun. Consequently, products with a microstructure or nanostructure are always in demand for miniaturization and functional improvement of the fine structure. To achieve miniaturization, top-down construction methods are important which produce the desired shape using a processing machine with a mold stamper. Also increasingly important are bottom-up techniques using materials' own self-organization.
2.3 Technology for Making Micro- and Nanostructures
Several techniques are used to fabricate products of micro- and nano-size structure. For example, a photolithography process is used in the manufacture of semiconductor integrated circuits (Figure 2.2). In this technique, exposure light is irradiated on the photoresist material spread on the silicone substrate through the mask, carving circuit patterns. The difference in callousness of the photoresist material caused by the exposure light irradiation forms a base of minute patterns on the substrate. Then, actual minute patterns are formed by removing the unnecessary photoresist material and performing an etching process on the substrate surface. Because this technique can efficiently create a microscale structure of large area, it has contributed to mass production and a reduction in the cost of semiconductor manufacture. However, the facility cost of the process is very high. Moreover, various contrivances to shorten the wavelength of the light source are needed to create smaller circuits, because the spatial resolution of the circuit pattern on the substrate depends on the wavelength of exposure light.
Figure 2.2 Schematics of the photolithography process
The photolithography process is also used as a method of making a minute structure and a movable mechanism in MEMS (micro-electro-mechanical systems). During the fabrication steps of MEMS, a removal process is needed not only in the plane direction of the substrate but also in the depth direction. Note that the ratio between the size in depth and plane directions is called the aspect ratio. Deep RIE (reactive ion etching) using ICP (inductively coupled plasma) is used to make a structure with a high aspect ratio (structural size in the depth direction higher than that in the plane direction).
The “LIGA process” which makes a minute pattern by irradiating synchrotron radiation or X-rays on the resist film thickly painted on the substrate is also well known as a technique for making minute structures with high aspect ratio [13]. In recent years a great reduction in process cost has been achieved, because a new resist film material (for example, SU-8™) that can use ultraviolet light as exposure light source has appeared [14]. In contrast, there are other techniques which directly fabricate a minute structure on the substrate. For example, EBs (electron beams) and FIBs (focused ion beams) are often used to directly carve minute structures (Figure 2.3). In these techniques, if the optical system for beam focusing is appropriate, the spatial resolution of the process can reach one-digit nano size. However, it usually takes a significant amount of time to fabricate the structure over a large area because the process basically progresses a line at a time.
Figure 2.3 Structure of the electron beam scanning system
Microfabrication by EB and FIB is categorized as the removal process, because electrically accelerated electrons and ions collide at high speed and remove the material. A similar removal process can be realized using an ultra-short pulse laser with high energy. However, it is known that light energy has superior advantages in microfabrication. For instance, the fabrication of a minute statue was reported using UV curing material [15]. A key technique was use of the multiphoton absorption process of a femtosecond laser. In recent years miniaturization of machining tools has also been advanced, and a micro-end mill tens of micrometers in diameter is being marketed. To achieve highly accurate microfabrication by machining, total system development including the machine tool and its driving device has become important. Owing to the size limitation of tools, the minimum scale of current processing is thought to be around several micrometers. However, because an increase in the number of tooling axes results in a processing degree of freedom, more complex shapes can be achieved.
Although the techniques described above have merits and demerits, they are mostly technically established. However, the idea of a process that makes an object directly and individually is inefficient as a technique for mass production. Therefore, the process of a mother pattern made by the techniques explained above is indispensable for the mass production of products with micro- and nanostructure. In such circumstances, an injection molding method and nanoprint/imprint methods considered as the replication process are regarded as promising techniques suitable for mass production.
References
[1] Yang, F., Murugan, R., Ramakrishna, S., Wang, X., Ma, Y.-X., and Wang, S. 2004. Fabrication of nano-structured porous PLLA scaffold intended for nerve tissue engineering. Biomater.25: 1891–1900.
[2] Smith, L.A., Liu, X., and Ma, P.X. 2008. Tissue engineering with nano-fibrous scaffolds. Soft Matter4(11): 2144–2149.
[3] Weilbaecher, C.R., Hossain, M., Gangopadhyay, S., and Grant, S. 2007. Development of a novel nanomaterial-based optical platform for a protease biosensor. Proc. SPIE, Vol. 6759.
[4] Naeemi, A. and Meindl, J.D. 2009. Carbon nanotube interconnects. Ann. Rev. Mater. Res.39: 255–275.
[5] Nanostructured materials in information storage. MRS Bull. 33: 831–834.
[6] Zhang, Y., Lu, J., Shimano, S., Zhou, H., and Maeda, R. 2007. Nanoimprint of proton exchange membrane for MEMS-based fuel cell application. Proc. 6th Int. IEEE Conf. on Polymers and Adhesives in Microelectronics and Photonics, pp. 91–95.
[7] Zhang, Y., Lu, J., Wang, Q., Takahashi, M., Itoh, T., and Maeda, R. 2009. Nanoimprint of polymer electrolyte membrane for micro direct methanol fuel cell application. ECS Trans. Micro Power Sources16: 11–17.
[8] http://www.itrs.net/
[9] Honda, T., Kishikawa, Y., Tokita, T., Ohsawa, H., Kawashima, M., Ohkubo, A. et al. 2004. ArF immersion lithography: Critical optical issues
