Nanofabrication

The following sections are included:

• CONTENTS

• PREFACE

Over the last decade, scanning probe microscopy (SPM), including scanning tunneling microscopy (STM) and atomic force microscopy (AFM), has become a powerful manipulation technique by virtue of its ability to interact with individual adsorbed nanoparticles with nanoscale precision on the surface. In this article, the principles, procedures and applications of both STM and AFM-based technologies for manipulation of atoms, molecules, and nanoclusters are reviewed with an emphasis on their ability to create a wide variety of nanostructures. In the manipulation of single atoms and molecules, the interaction among the atoms/molecules, surface, and tip are specifically discussed first. The approach for positioning the atom/molecule from and to the desired locations and precisely controlling its movement is also elaborated for each specific manipulation technique. The applications of these techniques for fabricating different nanostructures and nanosystems are then presented. In the manipulation of nanoclusters, different nanocluster-substrate pairs in different environments with their potential applications in electronics, biology, and medicine are specifically evaluated. Finally, concluding remarks are provided, where the scopes for technological improvement and future research are recommended.

Atomic force microscopy (AFM) was originally developed for atomic resolution surface topography observations. Nowadays, it is also widely used for nanolithography. AFM-based lithography is an effective method compared to conventional photolithographic processes due to its simplicity, high resolution, and low cost. It can provide nanoscale stage control and the probing tip can be used as a lithographic tool. Therefore, various AFM-based nanoscale fabrication methods have been proposed using electrochemical oxidation, material transfer, mechanical lithography, and thermally induced modifications. This chapter will introduce the detailed processes and applications of AFM-based lithographic techniques.

Nowadays tools based on Scanning Probe Methods (SPM) have become indispensable in a wide range of applications such as cell imaging and spectroscopy, profilometry, or surface patterning on a nanometric scale. Common to all SPM techniques is a typically slow working speed which is one of their main drawbacks. The SPM speed barrier can be improved by operating a number of probes in parallel mode. A key element when developing probe array devices is a convenient read-out system for measurements of the probe deflection. Such a read-out should be sufficiently sensitive, resistant to the working environment, and compatible with the operation of large number of probes working in parallel. In terms of fabrication, the geometrical uniformity i.e. the realisation of large numbers of identical probes, is a major concern but also the material choice compatible with high sensitivity, the detection scheme and the working environment is a challenging issue. Examples of promising applications using parallel SPM are dip-pen-nanolithography, data storage, and parallel imaging.

Nature offers an astonishing array of complex structures and functional devices. The most sophisticated examples of functional systems with multiple interconnected nano-scale components can be found in biology. Biology uses a limited number of building blocks to create complexity and to extend the size and the functional range of basic nano-scale structures to new domains. Three main groups of molecular tools used by biology include oligonucleotides (linear chains of nucleotides), proteins (folded chains of amino acids), and polysaccharides (chains of sugar molecules). Nature uses these tools to store information, to create structures, and to build nano-scale machines.

Recent advances in understanding the structure and function of these building blocks has enabled a number of novel uses for them outside the biological domain. Of particular interest to us is the use of these building blocks to self-assemble nano-scale electronic, photonics, or nanomechanical systems. In this chapter we will look at two groups of building blocks (oligonucleotides and proteins) and review how they have been used to self-assemble engineered structures and build functional devices in the nano-scale.

We will begin by a review of the basic structure and properties (both physical and chemical) of oligonucleotides and proteins. This section is meant to be used as a self-contained reference for the readers from the engineering community that may be less familiar with the symbols and jargon of biochemistry. The most salient properties of the biomolecules are emphasized and listed here to facilitate future research in the area. We continue by a review of recent advances in designing artificial nano-scale DNA structures that can be constructed entirely via engineered self-assembly. Rapid advances in the design and construction of self-assembled DNA structures has resulted in an impressive level of understanding and control over this type of nano-scale manufacturing. Polypeptides and proteins are decidedly less understood and their use in engineered self-assembly has been relatively limited. Nevertheless, as we discuss in the concluding sections of the chapter, both genetically engineered polypeptides and proteins can be used to guide self-assembly processes in nano-scale and help in interfacing nano-scale objects with micron-scale components and templates.

A newly emerging route for non-lithographic nano-fabrication based on a powerful combination of self-assembly, top-down and bottom-up nano-fabrication paradigms is presented and applied for a controlled engineering of arrayed nanostructures. The approach relies on the use of self-assembled anodized alumina templates as masks for direct and inverse pattern transfer facilitated by metal deposition and dry-etching processing. The low material specificity inherent in this approach uniquely allows engineering of a vast variety of nano-materials ranging from metal dot arrays to compositionally complex three-dimensional arrays of semiconductor quantum dots. The ability to achieve dense packing, excellent uniformity, and ordering across large length scales provided with the electrochemical processing of aluminum, combined with recent advances in dry-etching techniques, enables a new platform to synthesize advanced nano-materials whose primary electronic, magnetic and optical properties can be tailored precisely as a function of their size, morphology and spatial arrangement of individual nanostructures. This new platform enabled by the technique holds particular promise for a broad range of advanced nano-electronic device applications and future nano-materials processing.

Semiconducting, metallic and insulating nanowires are attractive building blocks in nanotechnology due to their small size and anisotropy. Moreover, it is possible to fabricate homogeneous or heterogeneous nanowires with high purity and crystallinity in a parallel and cost effective manner. Strategies have also emerged to position nanowires precisely on substrates to allow integration of nanoelectronic devices. In this chapter, we describe the fabrication and assembly of nanowires to form functional devices. Several fabrication strategies including vapor-liquid-solid (VLS) and electrodeposition in nanoporous templates are discussed. We detail advances made in the bottom-up integration of nanowires using patterned growth and directed assembly. Finally, some functional devices fabricated using nanowires are reviewed, and strategies to reduce errors and improve defect tolerance are discussed.

A number of techniques have been developed for the fabrication of glass nanowires (e.g., photo- or electron beam lithography, chemical growth, and taper drawing). Of these techniques, the taper drawing technique yields nanowires with the highest uniformity. Using sapphire fibers, flame or laser-heated glass (fibers) can be drawn directly into nanowires with diameters down to tens of nanometers. Nanowires obtained with this technique show extraordinary diameter uniformity, atomic-level surface smoothness, large length, high mechanical strength and pliability for assembling and patterning, making them promising building blocks for the future micro- and nanoscale photonic devices.

This chapter describes an EUVL technology that is expected to be introduced into the manufacturing of the 32 nm-node device from 2009–2011. EUVL consists of a light source of 13.5 nm, a reflective mask, objective optics and wafer. A reflective mask and objective optics with multilayer coating are employed. The reflectivity of multilayer at the wavelength of 13.5 nm is 68%.

The challenging items are high power source, defect-free mask and resist with low LER and high sensitivity. EUV scanner of α-type have delivered and process studies are performed in several institutions. In this chapter, the principle of EUV lithography, concepts of optics design, aspherical mirror fabrication and measurement, mask fabrication process and inspection, recent activities of resist and source are described.

The basic study of EPL has been done under the name of SCALPEL^® by AT&T (Lucent Technologies, then Agere Systems) from the beginning of 1990s and the name of PREVAIL for electron optical system by the joint work of IBM and Nikon. The features of EPL are larger sub-field size and higher acceleration voltage of electron for obtaining usable higher electrical current on wafer and a wide deflection width for obtaining higher throughput.

EPL system has such features as a large sub-field size, large deflection width, high electrical current, high acceleration voltage and thin Si stencil mask. EPL has been considered as one of the promising technologies for hp65nm node and beyond.

Nikon has developed the first EPL tool and it was delivered to Selete in Japan. The tool has been used for EPL technology evaluation and process development in the pilot line of Selete. This chapter describes the various aspects of EPL technology from basic concept through technology evaluation and future extendibility.

Electron beam (EB) lithography has been mainly used for patterning on masks and reticles in the semiconductor industry and has progressed according to the scaling up of circuit integration and the miniaturization of patterns. Among the various techniques in EB lithography, EB direct writing has played an important role in developing advanced devices because it offers higher resolutions and shorter turn-around time, though its patterning speed has not been high enough for mass-production use. Cutting-edge applications in advanced fields have always been investigated and developed using EB direct writing, and future devices and devices for scientific research, such as various quantum devices, which of course require high resolutions, have also been investigated using the technique.

In this chapter, interactions between electrons and materials, EB direct writing apparatuses, and calculation/measurement of EB diameter are discussed in Section 2. The accuracies of patterning dimensions and positioning in EB lithography are discussed in Section 3. The former includes roughness on pattern edges, and the proximity effect due to electron scatterings and its correction, and the latter is for mainly overlay accuracy. Resist materials, fine patterning, and some applications, including the creation of three-dimensional structures are discussed in Section 4.

The electron beam induced deposition (EBID) originated contamination writing as early as 1934, and many studies have been carried out using scanning electron microscopes for various gases and conditions, and the technique was found to be very successful for a number of applications making variously shaped nano-structures. Therefore, EBID is recognized as a very promising nanofabrication technique and the amount of work devoted to this field is rapidly increasing. In this chapter, fundamentals and an overview of EBID are given in [Section Number of FUNDAMENTALS]. Recent progress is discussed in [Section Number of RECENT RESEARCH ACTIVITIES] which includes consideration of resolution improvement using 200 kV scanning transmission electron microscopy, the Monte Carlo-based calculation of a deposit shape, nanowiring and electron conductivities, three-dimensional structure fabrication, magnetic material deposition, and further trials to improve the range of material choices and the fabrication speed.

This article gives an introduction to the principles and practices of high-resolution electron-beam-induced deposition (EBID). In EBID, a small focused electron beam is used to locally dissociate a precursor onto the surface of a substrate giving rise to a small deposit. Recently it has been discovered that the size of the deposited structure can be as small as one nanometer allowing EBID to be used to fabricate very small nanostructures of arbitrary shape. EBID provides an alternative to more traditional fabrication methods such as electron beam lithography (EBL) and ion beam induced deposition (IBID). EBID is a direct write technique requiring no pre-deposited resist or development and it can be applied to planar and nonplanar surfaces. This article reviews all aspects of the technique including instrumentation, gas-solid reactions, electron-beam specimen interaction, deposition parameters and deposit composition. Special attention is devoted to factors that must be understood and controlled in order to achieve a resolution of 1 nm. Examples of very small nanostructures fabricated by performing EBID with high-energy subnanometer focused electron beams (200 kV) are demonstrated. The chapter compares and contrasts EBID with other fabrication techniques and discusses current and future applications for the technique.

A focused ion beam (FIB) was successfully applied to prepare cross-sectional samples for transmission electron microscopy (TEM), scanning TEM (STEM), scanning electron microscopy (SEM), and scanning ion microscopy (SIM). The FIB milling has been prospectively applied also to nanofabrication. The FIB allows to mill with high accuracy in positioning, being in contrast with a broad ion beam with poor accuracy. Controlling beam conditions of ion pixel-dose (or pixel dwell time) and FIB scanning direction/velocity, and sample tilt/rotation, we can extend the FIB milling from cross-sectioning to three-dimensional (3D) fabrication. We review inherent characteristics of the FIB milling such as positioning accuracy, milling speed, uniformity of cross-section, beam damage, and secondary electron emission. Discussions are mainly held from a viewpoint of interaction of ion beam with solids.

Design of reproducible, simple and efficient nanofabrication routes has become a frontier topic in the emerging field of nanotechnologies. In this chapter we discuss the "state of the art" of ceramics micro- and nanofabrication techniques. We pay special attention to progress in this field made during the last five years.

In this chapter we will discuss about the progress on the use of lithographic tools to create nanoscale ceramic patterns or the potential of soft lithography to create ceramic structures by means of liquid ceramic precursors. The chapter also describes advances on the use of self-assembly and self organization to achieve nanostructured ceramic surfaces. In the final part, we discuss about the possibility of combining physical vapour deposition with micromolding techniques to obtain nanostructured ceramic subtrates.

In the race to downsize the features of components in integrated electronics, nanostructure fabrication is a primary challenge. Semiconductor technology has always relied on the top-down approach such as conventional CMOS technology for surface structuring and patterning. The most remarkable example is the amazing miniaturization of transistors and data storage components by ever more sophisticated lithographic techniques. Nevertheless, the so far unbeaten nanofabrication techniques such as deep-UV (DUV), extreme-UV (EUV) or electron-beam lithography (EBL) are particularly dedicated for patterning motifs in photo or electron-beam resists, spin-coated on planar, ultra-flat semiconductor surfaces. Alternative fabrication processes for growth of integrated novel nanostructured functional materials are already foreseen in related areas such as micro/nano-electro mechanical systems (MEMS/NEMS), sensors and actuators, optoelectronics, bio-chips, plastic or molecular electronics, etc. In other words, appropriate patterning methods are explored for creating and positioning structures with nanometer dimensions (<100 nm) on non-planar (e.g. curved or rough) surfaces or other functionalized and often fragile surfaces (membranes, cantilevers, organic layers). In this context, a variety of other forms of parallel lithography such as molding, stamping, imprinting or stenciling are revisited for their potential as nanofabrication alternatives that alleviate conventional lithography limitations. Thus, fabrication of functional structures with controlled size and shape, precisely positioned on a substrate of choice, using a minimal number of processing steps, becomes a central issue in nanotechnology. Furthermore, growth of novel nanostructured complex materials with functionality represents a big challenge for materials development. Probing new routes to prepare these materials and understand the relationship between their size/structure and their properties is also essential to the development of related technologies.

Significant advances in defining nano-patterns have been made by nanoimprint lithography (NIL). NIL is able to deliver features well below 100 nm, rapidly and with high accuracy, at least compared to advanced optical lithography methods. Either by hot embossing technique (HET) or its related variant, step and flash imprint lithography (S-FIL) nanoimprint shows great potential for the semiconductor industry and has been already placed on the International Technology Roadmap for Semiconductors (ITRS) for the next years. Another promising approach, although less investigated to date, is nanostenciling, known also as controlled growth of nanostructures through a shadow-mask. This process has been proposed both in static or dynamic mode and projected as a suitable method to locally grow patterned nanoscale structures, in a single, resist-less, deposition step. While offering a high degree of freedom in choosing the physical vapor deposition method, nanostenciling is in principle applicable to the deposition of arbitrary materials on almost any substrate. It drastically reduces the number of processing operations with respect to resist–based lithography and therefore represents a promising "universal tool" for local deposition of high-resolution and high-purity 3D nanostructures under high or ultra high vacuum (UHV) conditions.

We provide fundamental and extensive descriptions of stenciling and imprint processes and outline the main concepts used for the fabrication of both stencilmasks and molds. Then, through a couple of detailed examples we will emphasize the importance of several particular parameters involved in these processes (e.g. geometry and methods of deposition for stenciling, molds, resists, tools in the case of imprinting). Finally we will present a couple of examples where either nanostenciling or nanoimprint have been successfully used for device applications and in conclusion offer our perspective on future potential applications (prototyping) in areas where other forms of lithography are much less suitable.

Nanoelectromechanical systems (NEMS) are commonly realized in the form of simple movable suspended nanostructures, such as doubly-clamped beams, cantilevered beams or torsion pedals. NEMS come with extremely high fundamental resonance frequencies, diminished effective masses, low spring constants and high in vacuo quality (Q) factors. As such, these structures have received much recent attention for their potential technological applications as well as for realizing a mesoscopic quantum harmonic oscillator.

This chapter presents a review of the variety of approaches for fabricating NEMS devices. The mainstay approach for patterning freely suspended nanostructures is nanomachining based upon electron beam lithography (EBL). This approach has been applied to fabricate silicon, gallium arsenide, silicon carbide, aluminum nitride, diamond and silicon nitride NEMS; variations of EBL based nanomachining have also been used to fabricate nanotube and nanowire NEMS. Among other emerging approaches reviewed herein are approaches based upon nanoimprint lithography, focused ion beams and stencil masks. Important remaining research issues in the field, such as large scale integration of NEMS, are discussed along with concluding remarks.

The following sections are included:

• INDEX