電気化学リソグラフィーによる樹脂表面への金属パターン形成

Demand for the development of deposition strategies of well-defined metal thin films with good chemical and physical characteristics has been rapidly growing in the field of various electronics applications. For example, copper electro- or electroless deposition has been widely utilized and investigated in microelectronics industry due to its low electrical resistivity and high electromigration resistance, as compared with aluminum. The conventional plating bath for electro- or electrolless copper deposition involves various chemical reagents such as deposition promoter (or inhibiter), brightening and smoothing agent, etc. These additives, typically organic molecules, sometimes cause mechanical problems due to inclusion of such additives during deposition process, and the discharge from the deposition bath can be a serious environmental issue. Therefore, the development of a facile process that enables additive-free deposition of metallic thin films as well as low environmental toxicity is an important challenge for the fabrication of microelectronic elements. In order to sustain the demand for generating multichip packaging systems for future electronic devices, it would be exceedingly useful to develop novel electrochemical deposition strategy with high-throughput capability that would allow stability of electrochemical reaction at higher deposition rate from plating bath without organic additives. In this context, direct metal deposition processes though ion conducting films have been investigated, such as the fabrication of metallic patterns on polymer substrates using ion-doped precursor films. A variety of direct deposition techniques have been introduced by several groups, including ours,1,2) based on photoinduced chemistry, thermal treatment, and selective chemical reduction for polyimide and other functional polymers. We have also reported a chemical metallization strategy that utilizes electrochemical constructive lithography with post lift-off process, which enables the fabrication of metal patterns on the polyimide substrate.3) The use of ion-doped precursors has triggered the development of new concept for electrochemical deposition. Herein we report for the first time that metallic thin films can be deposited through polyelectrolyte thin layers placed on the cathode electrode by the diffusion of metallic ions from the interior of polyelectrolyte, which is essentially different to conventional solution-phase electrodeposition processes (Fig. 1). By taking advantage of the cation-exchangeable nature of polyelectrolyte layers used for creation of solid interface, we demonstrate in this study how it is possible to merge the electrochemical deposition at cathode-polyelectrolyte interface and diffusion of cations through polyelectrolytes for development of novel solid electrodeposition (SED) process. As a proof of concept study, we herein present the successful electrochemical deposition of copper and nickel thin films at the interface between an electrode and ion-doped polyelectrolyte layers from the baths containing only metal salts (without any additives). The effect of cation concentration, temperature, and electrochemical deposition conditions on the deposition rate, growth process and morphology of the films has been investigated. Several electrochemical, microscopic and quantitative analysis revealed that the metallic films have been successfully deposited at the cathode-polyelectrolyte interface through ion exchange reaction in the polyelectrolyte layers, the deposition rate of which are determined by ion exchange rate of cations. This strategy offers an opportunity to translate solution-phase electrochemical deposition into a high-throughput, cost-effective, environmentally-friendly process for the fabrication of metal thin films on substrates. 1) Matsumura, Y. Enomoto, T. Tsuruoka, K. Akamatsu, H. Nawafune, Langmuir, 26, 12448-12454 (2010). 2) S. Ikeda, H. Yanagimoto, K. Akamatsu, H. Nawafune, Adv. Funct. Mater., 17, 889-897 (2007). 3) K. Akamatsu, Y. Fukumoto, T. Taniyama, T. Tsuruoka, H. Yanagimoto, H. Nawafune, Langmuir, 27, 11761-11766 (2011). Figure 1

This paper proposes a novel design approach for a thin lens with the aim of overcoming fineness limits in the commercial millimeter wave printed circuit board (PCB) manufacturing process. The PCB manufacturing process typically does not allow the fabrication of metallic patterns with a gap and width of less than 100 μm. This hampers expanding thin lens technology to 5G commercial applications, especially when such technology is considered for 60 GHz or higher frequency, which requires a finer gap and width of metallic traces. This paper proposes that problematic process conditions can be mitigated when a lens is designed by establishing single-polarized lumped element models where larger capacitance and inductance values can be obtained for the same patch and grid unit cells. While the proposed design technique is more advantageous at higher target frequencies, a 60 GHz application and a wireless backhaul system is selected because of a limited range of frequencies that can be measured by an available vector network analyzer. The required gap or width of metallic traces can be widened significantly by using the proposed single-polarized unit cells to acquire the same in-plane capacitance or inductance. This enables the lens operating at higher-frequency under the process limits in fabricable fine traces. Finally, the effectiveness of the simulated design procedure is demonstrated by fabricating a 60 GHz thin lens that can achieve a gain enhancement of 16 dB for a 4 × 4 patch antenna array with a gain of 16.5 dBi.

Flexible electronics have been of great interest in the past few decades for their wide-ranging applications in health monitoring, human-machine interaction, artificial intelligence, and biomedical engineering. Currently, transfer printing is a popular technology for flexible electronics manufacturing. However, typical sacrificial intermediate layer-based transfer printing through chemical reactions results in a series of challenges, such as time consumption and interface incompatibility. In this paper, we have developed a time-saving, wafer-recyclable, eco-friendly, and multiscale transfer printing method by using a stable transferable photoresist. Demonstration of photoresist with various, high-resolution, and multiscale patterns from the donor substrate of silicon wafer to different flexible polymer substrates without any damage is conducted using the as-developed dry transfer printing process. Notably, by utilizing the photoresist patterns as conformal masks and combining them with physical vapor deposition and dry lift-off processes, we have achieved in situ fabrication of metal patterns on flexible substrates. Furthermore, a mechanical experiment has been conducted to demonstrate the mechanism of photoresist transfer printing and dry lift-off processes. Finally, we demonstrated the application of in situ fabricated electrode devices for collecting electromyography and electrocardiogram signals. Compared to commercially available hydrogel electrodes, our electrodes exhibited higher sensitivity, greater stability, and the ability to achieve long-term health monitoring.

Fabrication of high aspect ratio metal patterns without rabbit ear shaped defects on rigid Si and flexible polyethylene terephthalate (PET) substrates was demonstrated. This process is composed of UV nanoimprint lithography (UV-NIL), resist pattern transfer step, and lift-off process. The imprinted resist pattern with a positive pattern profile on a water soluble polyvinyl alcohol (PVA) coated transparent substrate was transferred to Si and PET substrates in order to create an undercut profile for the high fidelity lift-off process using an UV curable adhesive. After the pattern transfer step, the PVA coated substrate was released by water soaking. The adhesive residue on the substrate was removed by short O2 reactive ion etching (RIE) without significant change of the resist pattern profile. Subsequently, the metal film was deposited by e-beam evaporation on the sample and the resist pattern was removed by acetone solution. As a result, the metal patterns with 250 nm of linewidth and 80 nm of thickness were formed by this process on Si and flexible PET substrates without rabbit ear shaped defects.

Fabrication of three-dimensional electrical patterns by swollen-off process: An evolution of the lift-off process

Directed self-assembly (DSA) of block copolymers has gained significant attention in recent years as a possible alternative for large area fabrication of future sub-30 nm lithographic patterns. To achieve this patterning, at least three critical pieces are needed: (1) a block copolymer with sufficient immiscibility of the two blocks to drive phase separation at the low molecular weights required to achieve such small phase domains, (2) a method for selectively removing one of the blocks after phase separation to achieve formation of a relief pattern, and (3) a method for producing the templated surfaces used to guide and register the phase separated patterns on the substrate of interest. Current methods for achieving the patterned substrate template, whether they are of chemoepitaxial or graphoepitaxial nature, are generally complex involving a large number of steps that are not easily applied to a variety of different substrate surfaces. For example, numerous substrates have been studied to provide neutral wettability to the styrene-methacrylate (PS-b- PMMA) block copolymers, such as random styrene-methacrylate copolymer films (PS-r-PMMA) or self-assembled monolayer (SAM) modified surfaces, which induce perpendicularly oriented morphologies for PS-b-PMMA self-assembly. In the case of chemical epitaxy processes, a layer of photoresist is generally then coated on such neutral substrate films and patterned to render commensurability to the periodicity of the PS-b-PMMA being used. The open (i.e. space) regions in the resist are then exposed to alter their chemistry, e.g. soft X-ray or oxygen plasma exposures have been used, to achieve hydrophilicity which should preferentially wet PMMA. Finally, the resist is stripped and the block copolymer is coated and assembled on the template surface. Obviously such multi-step processes would not be preferred if alternatives existed. As a step toward that goal of making DSA processes simpler, a photodefinable substrate film that can be used for PS-b-PMMA self-assembly has been developed in this work that is coated, exposed, and baked in a manner analogous to current optical photoresists. The polymer resin for this material is a form of protected poly(hydroxystyrene) that is cross-linkable. The new material is suitable for DSA processes using both solvent and thermal annealing methods. Results of PS-b-PMMA DSA on this new material will be discussed.

Plasma etching has become a crucial process step for the microelectronics industry, as computer logic and memory circuits require increasingly precise fabrication of fine-scale patterns in semiconductors, metals, and insulators. Plasma etching generally involves the dissociation and ionization of a neutral precursor gas to form reactive neutral fragments and charged ions and electrons. The substrate to be etched is placed in contact with this mixture, and etching proceeds through the synergistic interaction of the substrate, corrosive neutral species, and directionally impinging plasma ions.

Room-temperature metal stamping by microfluidics

A wide variety of metallic and metal oxide nanoflowers and other exotic patterns have been fabricated using different techniques. We have created copper and cupric oxide nanoflowers using two different techniques: electro-deposition of copper in polymer and anodic alumina templates, and cytyltrimethal ammonium bromide (CTAB)-assisted hydrothermal method, respectively. Zinc oxide and manganese oxide nanoflowers have been synthesized by thermal treatment. Characterization of nanoflowers is done in the same way as for nanowires using XRD, SEM, TEM and FESEM. Scanning Electron Microscope (SEM) images record some interesting morphologies of metallic copper nanoflowers. Field Emission Scanning Electron Microscope (FESEM) has been used to determine morphology and composition of copper oxide nanoflowers. X-ray diffraction (XRD) pattern reveals the monoclinic phase of CuO in the crystallographic structure of copper oxide nanoflowers. Nanoflowers find interesting applications in industry. There is an element of random artistic design of nature, rather than science, in exotic patterns of nanoflowers fabricated in our laboratory.

Due to the increasing variety and capacity of information and communication media, systems of microwave band communication have branched out extensively. Surface acoustic wave (SAW) devices are commonly used in microwave applications, primarily because these devices can be fabricated by using photolithographic processes, and easily integrated into other passive and active devices. SAW devices have been successfully employed for various commercial applications, such as intermediate-frequency (IF) filters for televisions and radio-frequency (RF) filters for mobile telecommunications, using small size and mass-production fabrication processes. We apply the principles of surface acoustic wave and microwave theory, by properly designed circuit with cross-coupled combination, the conventional photolithography and lift-off technique were applied for metal pattern fabrication on the 128/spl deg/-rotated YX-cut LiNbO/sub 3/ piezoelectric material substrate.

We report a fully additive-based electrochemical approach to the site-selective deposition of silver on a polyimide substrate. Using a cathode coated with ion-doped precursor polyimide layers, patterns of metal masks used as anodes were successfully reproduced at the cathode-precursor interface through electrochemical and ion-exchange reactions, which resulted in the generation of silver patterns on the polyimide films after subsequent annealing and removal from the substrate. Excellent interfacial adhesion was achieved through metal nanostructures consisting of interconnecting silver nanoparticles at the metal-polymer interface, which are electrochemically grown "in" the precursor layer. This approach is a resist- and etch-free process and thus provides an effective methodology toward lower-cost and high-throughput microfabrication.

Dual-side Cu patterns with well-defined boundaries were plated onto fabric by combining wax-dot printing with electroless copper plating.

Cellulose electro-active paper (EAPap) has attracted much attention as a new smart electronic material to be utilized as mechanical sensors, bio compatible applications and wireless communications. The thin EAPap film has many advantages such as lightweight, flexible, dryness, biodegradable, easy to chemically modify, cheap and abundance. Also EAPap film has a good reversibility for mechanical performance, such as bending movement, under electric field. The main actuation mechanism governed by piezoelectric property can be modulated by material direction and stretching ratio during process. In this paper we present the overview as well as fabrication process of cellulose EAPap as a novel smart material. Also we propose the method to enhance the piezoelectricity, its mechanical and electromechanical properties. In addition, the fabrication of high quality metal patterns with Schottky diode on the cellulose surface is an initiating stage for the integration of the EAPap actuator and electronic components. The integration of flexible actuator and electronic elements has huge potential application including flying magic carpets, microwave driven flying insets and micro-robots and smart wall papers.

Reliable fabrication of multiscale metallic patterns with precise geometry and size at both the nanoscale and macroscale is of importance for various applications in electronic and optical devices. The existing fabrication processes, which usually involve film deposition in combination with electron-beam patterning, are either time-consuming or offer limited precision. Inspired by the kirigami, an ancient handicraft art of paper cutting, this work demonstrates an electron-beam patterning process for multiscale metallic structures with significantly enhanced efficiency and precision. Similar to the kirigami, in which the final pattern is defined by cutting its contour in a paper and then removing the unwanted parts, we define the target multiscale structures by first creating nanotrench contours in a metallic film via an electron-beam-based process and then selectively peeling the separated film outside the contours. Compared with the conventional approach, which requires the exposure of the whole pattern, much less exposure area is needed for nanotrench contours, thus enabling reduced exposure time and enhanced geometric precision due to the mitigated proximity effect. A theoretical model based on interface mechanics allows a clear understanding of the nanotrench-assisted selective debonding behaviour in the peeling process. By using this fabrication process, multiscale metallic structures with sub-10-nm up to submillimetre features can be reliably achieved, having potential applications for anti-counterfeiting and gap-plasmon-enhanced spectroscopy.

Selective metal-vapor deposition on organic surfaces is an interesting phenomenon and is applicable to prepare fine metal patterns for various electronic and photonic devices based on vacuum evaporation without a shadow mask. However, selective metal-vapor deposition with photochromic diarylethenes has not been successful for noble metals such as Au, Ag and Cu, which are generally used in the electronics field. In this paper, we report deposition modulation of noble-metal vapor on a photocurable polydimethylsiloxane (PDMS) film. When Au, Ag or Cu was vacuum-deposited on the UV-curable PDMS, Au and Cu formed a film on the surface even on the uncured film. Ag, however, was desorbed from the uncured film, and a small amount of evaporated Ag atoms was absorbed into the film. This metal species dependence on metal-deposition/desorption tendency was correlated with the intrinsic vapor pressure of metal species; metals with a high vapor pressure tend to desorb. The uncured film with a small amount of Ag deposition was easily removed with a hexane rinse, leaving only Ag on the cured film. Using this principle, various Ag-film patterns were prepared by irradiating upon UV light with a photomask to form a cured pattern and using maskless Ag deposition. Furthermore, we succeeded in fabrication of fine Ag patterns with a width of several micron by UV-laser scanning and maskless Ag-vapor deposition. This method would be applied to the electrode/wiring for various electronic devices.