ABSTRACT
In this chapter, micro-and nanoscale device concepts are introduced. Various micronanofabrication and manufacturing techniques are reviewed. The organization of this chapter is as follows: The first part deals with device fabrication and manufacturing through chemical approaches, which include the solution method, chemical vapor deposition (CVD), and multiphase (vapor-liquid-solid) growth. In the second part, high-energy-beam-assisted catalytic growth is discussed. Afterward, electrochemical approaches, including anodizing, electrochemical etching, electroplating, and electrocodeposition, are introduced. Device fabrication through templates is presented. The subsequent section is on micronanoscale manipulation via lithographic and scanning probe techniques. Finally, the electrospinning technique for micronanoscale device fabrication and manufacturing is discussed. 2.1 IntroductionDesign and fabrication of microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) have caught increasing interest due to the wide applications. In such systems, various sensors play a critical role. Sensors can be viewed as energy
conversion devices that convert signals from mechanical, chemical, optical, thermal, magnetic, and various other energy domains to the electrical energy domain. Actuators work in the reverse way. They convert energy from the electrical energy domain into the mechanical energy domain. Transducers include both sensors and actuators. The concepts of micro-or nanoscale devices come from the unique properties and functions of nanomaterials or structures associated with the devices or systems. For example, fluids containing carbon nanotubes (CNTs) were made for enhanced heat transfer [1-3]. High-density and large-area vertically aligned porous silicon nanowires (Si NWs) were fabricated on the two sides of the silicon substrate using the metal-assisted chemical etching approach. [4] The porous Si NWs show good thermoelectric (TE) properties with the figure of merit (ZT) value of 0.493. A special electrorheological (ER) property can be obtained by incorporating nanomaterials. Under an applied electric field, the added nanoscale entities show actuation behaviors, which allow them to be used as propelling components in nanomachines. The synthesis and characterization of Pb3O2Cl2 NWs and the ER properties of the NW containing fluids have recently been studied [5]. The ER properties of the nanomaterial suspensions were tested via oscillatory shear experiments. It was found that the viscoelasticity of the nanosystems changes with the intensity of the applied DC electric fields. The actuation behavior for the laden suspensions was observed at low voltages and a very low concentration of the reinforcements (0.0125 wt%). Various new nanoscale devices using nanofibers are designed. The concept of an “electronic nose” has been proposed by building nanofiber sensor arrays so that high sensitivity can be achieved to discriminate between different chemicals. Sysoev et al. [6] prepared a nanoscopic electronic nose using an array of individual metal oxide (SnO2, TiO2, and In2O3) nanofiber sensors. The device is capable of discriminating between CO and H2. Single-walled carbon nanotubes (SWNTs) were used to make a nanoscale device for the label-free detection of DNA hybridization [7]. The effect of SWNT-DNA binding on DNA functionality is revealed by the electrical conductance change between the SWNT and gold contact. Adsorption of trace amounts of chemical vapors at the defect sites in the SWNT produces a large electronic response that dominates the capacitance and conductance sensitivity of the SWNT. It is considered to be because
of the increased adsorbate binding energy and charge transfer at defect sites [8]. It is suggested that oxidation defects enhance the sensitivity of a SWNT network sensor to a variety of chemical vapors. Nanoscale field emission devices have been made with arrays of metal NWs [9], CNTs [10], and TaSi2 nanofibers [11]. Hwang et al. [9] fabricated vacuum tube arrays using anodic aluminum oxide (AAO) nanotemplates. Ni NWs were deposited electrochemically within the pores of the AAO and were used as field emitters. A titanium thin film was evaporated on the templates to seal the pores. The Ti film also served as the anode of the field emission device. Since the distances between the tips of Ni NWs and the anode are much smaller than those of conventional designs, a much lower turn-on voltage is needed to trigger electron emission. In the work performed by Sohn et al. [10], well-aligned CNT field emitter arrays were prepared for potential electron emission applications (such as cold-cathode flat-panel displays) and vacuum microelectronic devices (such as microwave power amplifier tubes). The well-aligned CNT arrays by thermal chemical vapor deposition (CVD) at temperatures below 800°C on Fe nanoparticles were deposited by a pulsed laser on a porous Si substrate. The field emitter arrays are vertically well-aligned CNTs on the Si-wafer substrate. The cathode of the field emitter is an array of CNTs. The cathode and anode were separated by a polyvinyl film with the thickness of about 60 µm. The field emission behavior of the device was tested at room temperature in a vacuum chamber below 10-6 Torr. High field emission current densities can be obtained at relatively low electric fields owing to the good adhesion of the CNTs to the Si substrate through Fe nanoparticles. It was found that the CNT field emitter arrays emitted a current of 1 mA/cm2 at an electric field of 2 V/µm. An emission current density as high as 80 mA cm-2 was obtained at 3 V/µm. Nanoscale field emission devices not only have the advantage of very low turn-on electric fields but also demonstrate the ability to hold at very high current density. For example, TaSi2 NW emitters have a remarkably high failure current density, on the order of 10-8 A/cm2, which promises future applications in nanoelectronics and nano-optoelectronics. Chueh et al. [11] introduced the synthesis of TaSi2 NWs on a Si substrate by annealing NiSi2 films at 950°C in tantalum (Ta) vapor. The obtained NWs are as long as 13 µm. The metallic TaSi2 NWs exhibit excellent electrical properties through
field emission measurements. It is found that the turn-on electric field is as low as 4 V µm-1, while the failure current density is as high as 3 × 10-8 A/cm2. 2.2 Chemical ApproachesChemical approaches have found wide applications in synthesizing semiconductors, metals, oxides, and various micronanoscale devices. A very common example is the solution-based crystal growth. Through chemical approaches, the structures and performances of many nanoscale materials can be tailored. Chemical approaches allow the study of size-dependent phenomena at the nanoscale and thus provide the potential for exploring new types of devices with precise functions. In the following discussion, three chemical fabrication methods-the solution method, CVD, and vapor-liquid-solid (VLS) growth-will be presented. 2.2.1 Solution Method: Liquid-Phase DepositionThe solution method, or liquid-phase deposition, is commonly used for the synthesis of nanoparticles, nanotubes (NTs), and NWs. Zhang et al. [12] synthesized an yttrium-aluminum-garnet (YAG) nanopowder with aluminum and yttrium nitrates as the starting materials and an ethanol-water mixture as the solvent. The powder is single-phased YAG with an average grain size of 80 nm. The grain size distribution is in a relatively narrow range. The solution method has also been used to prepare oxide sheets with nanoholes, for example, a titanium oxide sheet containing a nanohole array [13]. The titanium oxide with the nanohole array was fabricated through equilibrium reactions in fluorocomplex solutions. It is found that the crystal structure of titanium oxide with the nanohole array is amorphous in an as-deposited state. After heat treatment, it becomes anatase. Spatially organized nanodots were fabricated by exploiting the solution method under controlled dewetting, ripening, and crystallization conditions [14]. The size of the nanodot features can be as small as 100 nm. Functional nanodots and nanorods with different sizes and shapes have recently been fabricated through a simple solution route, as
shown by Cao et al. [15]. They synthesized single-crystalline ZnO nanorods with the smallest diameter, of about 5 nm, at ambient temperatures in ethanol without using additional surfactants. Recently, there has been considerable interest in the fabrication of 1D coaxial layered NTs because of the remarkable properties that are different from those of single-layered NTs. For semiconductors comprising a core and a shell, improved performance can be obtained. A core-shell motif has permitted enhanced photoluminescence, improved stability against photochemical oxidation, and engineered band structures. Coaxial layered NTs are expected to have great potential applications for photoelectronic nanoscale devices. Thus far, different forms of multilayered coaxial nanostructures have been produced by various methods, such as self-assembling [16], layer-by-layer deposition [17], and atomic layer deposition [18]. Carny et al. [16] designed trilayered (metal-insulator-metal) coaxial nanocables that may have unique and useful electromagnetic properties for applications in micro-and nanoscale systems. They fabricated these coaxial NTs via site-specific metal reduction. To generate the chemical reaction sites, gold nanoparticles were bounded to the surface of peptide NTs through a common molecular recognition element that was included in various linker peptides. Multilayered NTs can be synthesized by a liquid-phase-deposition approach. Figure 2.1 shows the schematic of the liquid-phase deposition and the scanning electron microscopic (SEM) images of the prepared CdS-TiO2 coaxial NTs. Liquid-phase deposition represents a simple fabrication method. Nanoporous templates may be used as the forming molds for the deposition. For example, this method was used for the preparation of CdS-TiO2 hybrid coaxial nanocables within porous AAO templates [19]. The thickness of the TiO2 NTs could be controlled precisely by adjusting the reaction conditions. A polycrystalline CdS layer was deposited onto the titanium oxide NTs during another chemical deposition process so that coaxial CdS-TiO2nanocables were formed. Core-sheath nanocables have potential applications in solar energy cells, semiconductor photocatalysis, water purification reactors, and electrochromic devices [20]. The fabrication procedures for the preparation of CdS-TiO2 hybrid coaxial nanocables include three steps. The first step is the preparation of the TiO2 NT array through deposition from an aqueous solution
mixture of ammonium hexafluorotitanate and boric acid into the nanopores of AAO membranes. The second step is the synthesis of hybrid CdS-TiO2 NTs. CdS-TiO2 NTs are synthesized with TiO2NTs as the templates via a solution reaction approach by injecting reactants into the pores of TiO2-AAO templates. In the third step, the CdS-TiO2 coaxial NTs are harvested by chemically dissolving the AAO templates.
