Harutaka Mekaru

Development of Three Dimensional LIGA Process to Fabricate Spiral Microcoil

The LIGA process has been developed as a 2.5-dimensional processing method on Si wafers to date. However, we have succeeded in extending the LIGA process to 3D for the first time. 3D-LIGA was achieved by the technical development of 3D X-ray lithography and worm injection molding replication technology with unscrewing. These technologies began from the development of equipment and have developed into quite original technologies. By combining this 3D-LIGA process with a metallization technique that consists of flat and smooth electroplating and isotropic chemical etching, a spiral copper microstructure with a linewidth of 10 µm, a pitch of 20 µm and a thickness of 2 µm was formed on a cylindrical surface made from LCP with a length of 1 mm and a diameter of 0.48 mm. Furthermore, we applied the process to fabricate a spiral microcoil and estimated the electrical properties of the microcoil. The numbers of turns were 15, the inductance was 91 nH and the quality factor was 5.8 for a frequency of 1 GHz. Direct-current resistance was measured as 99 Ω.

Frequency and amplitude dependences of molding accuracy in ultrasonic nanoimprint technology

We use neither a heater nor ultraviolet lights, and are researching and developing an ultrasonic nanoimprint as a new nano-patterning technology. In our ultrasonic nanoimprint technology, ultrasonic vibration is not used as a heat generator instead of the heater. A mold is connected with an ultrasonic generator, and mold patterns are pushed down and pulled up at a high speed into a thermoplastic. Frictional heat is generated by ultrasonic vibration between mold patterns and thermoplastic patterns formed by an initial contact force. However, because frictional heat occurs locally, the whole mold is not heated. Therefore, a molding material can be comprehensively processed at room temperature. A magnetostriction actuator was built into our ultrasonic nanoimprint system as an ultrasonic generator, and the frequency and amplitude can be changed between dc–10 kHz and 0–4 µm, respectively. First, the ultrasonic nanoimprint was experimented by using this system on polyethylene terephthalate (PET, Tg = 69 °C), whose the glass transition temperature (Tg) is comparatively low in engineering plastics, and it was ascertained that the most suitable elastic material for this technique was an ethyl urethane rubber. In addition, we used a changeable frequency of the magnetostriction actuator, and nano-patterns in an electroformed-Ni mold were transferred to a 0.5 mm thick sheet of PET, polymethylmethacrylate (PMMA) and polycarbonate (PC), which are typical engineering plastics, under variable molding conditions. The frequency and amplitude dependence of ultrasonic vibration to the molding accuracy were investigated by measuring depth and width of imprinted patterns. As a result, regardless of the molding material, the imprinted depth was changed drastically when the frequency exceeded 5 kHz. On the other hand, when the amplitude of ultrasonic vibration grew, the imprinted depth gradually deepened. Influence of the frequency and amplitude of ultrasonic vibration was not observed on the width of imprinted patterns. Moreover, the imprinted depth deepened as the Tg of the molding material lowered, and a progressive change according to conditions of ultrasonic vibration also became remarkable. Therefore, it seems that impressing ultrasonic vibration with a high frequency and large amplitude promotes thermal deformation and improves the molding accuracy in the ultrasonic nanoimprint technology.

Ultrasonic nanoimprint on engineering plastics

The authors developed a new ultrasonic nanoimprint technology that is superior to the current thermal and UV nanoimprint technologies. In the new technology an ultrasonic vibration is impressed along the direction of the loading force during a molding operation at room temperature. The mold in this case is mounted onto an ultrasonic generator with a UV photoresist, where the mold patterns are pushed and pulled on a thermoplastic material at a high speed by employing ultrasonic vibration. The system employs a magnetostriction actuator that generates ultrasonic vibration with frequencies and amplitudes ranging from dc to 20kHz and from 0to±3μm, respectively. Several optimized imprinting conditions had been investigated by using polyethylene terephthalate (Tg=69°C) with a comparatively low glass transition temperature in engineering plastics. These optimized imprinting conditions were found to be frequency of the ultrasonic vibration=10kHz, amplitude=3μm, contact force=500N, contact time=60s, and buffer mate...

Performance of SU-8 Membrane Suitable for Deep X-Ray Grayscale Lithography

In combination with tapered-trench-etching of Si and SU-8 photoresist, a grayscale mask for deep X-ray lithography was fabricated and passed a 10-times-exposure test. The performance of the X-ray grayscale mask was evaluated using the TERAS synchrotron radiation facility at the National Institute of Advanced Industrial Science and Technology (AIST). Although the SU-8 before photo-curing has been evaluated as a negative-tone photoresist for ultraviolet (UV) and X-ray lithographies, the characteristic of the SU-8 after photo-curing has not been investigated. A polymethyl methacrylate (PMMA) sheet was irradiated by a synchrotron radiation through an X-ray mask, and relationships between the dose energy and exposure depth, and between the dose energy and dimensional transition, were investigated. Using such a technique, the shape of a 26-μm-high Si absorber was transformed into the shape of a PMMA microneedle with a height of 76 μm, and done with a high contrast. Although during the fabrication process of the X-ray mask a 100-μm-pattern-pitch (by design) was enlarged to 120 μm. However, with an increase in an integrated dose energy this number decreased to 99 μm. These results show that the X-ray grayscale mask has many practical applications. In this paper, the author reports on the evaluation results of SU-8 when used as a membrane material for an X-ray mask.

Demonstration of fabricating a needle array by the combination of x-ray grayscale mask with the lithografie, galvanoformung, abformung process

We demonstrated fabricating a needle array of polycarbonate (PC) and polymethyl methacrylate (PMMA) by using a 3-D LIGA (lithografie, galvanoformung, abformung) process. The diameter of the bottom of the needle was about 50 µm, and the height was 135 µm. Although the LIGA process is commonly applied for making structures with vertical sidewalls, the use of an x-ray grayscale mask in the LIGA process has made it possible to fabricate needle-shaped structures. The x-ray grayscale mask was composed of a Si x-ray absorber and an SU-8 membrane. The sidewall of the x-ray absorber was diagonally processed by Si tapered-trench-etching technology such that the transmission intensity of x rays could be changed locally. The x-ray lithography experiment was executed by using this x-ray grayscale mask on a beamline BL-4 in the TERAS synchrotron radiation facility at National Institute of Advanced Industrial Science and Technology (AIST). By using this facility, a PMMA resist master with three-dimensional (3-D) structures was made. A Pt layer was then sputter-deposited as a seed layer on the PMMA resist master, and a Ni mold was fabricated by electroforming technology. In addition, a needle array of PC and PMMA was produced by hot embossing technology. Self-assembled monolayers (SAMs) of a release agent were required on the surface of the mold pattern to achieve a complete molding. Thus, we succeeded in extending the LIGA process to three dimensions by the use of an x-ray grayscale mask.

Fabrication and evaluation of a grayscale mask for x-ray lithography using MEMS technology

We propose a new fabrication method of an x-ray grayscale mask using micro-electro-mechanical-systems MEMS technologies, and also report on successful fabrication of three-dimensional 3D mi- crostructures on a polymethylmethacrylate PMMAsheet by using only a single x-ray exposure. We showed that silicon can be diagonally etched by optimizing the etching condition in a reactive-ion-etching RIE pro- cess. It is well known that the absorbers of an x-ray mask can be made into 3-D shapes. Here, we describe how this process can be extended to fabricate an x-ray grayscale mask by using a tapered-trench-etching technique. With such a mask, we carried out experiments on x-ray lithog- raphy XRL using a beam line BL-4 in the synchrotron radiation facility TERAS of National Institute of Advanced Industrial Science and Technol- ogy AIST. The dose energy used for the exposure was 150 mA·h, and the subsequent resist development was done by a GG developer at room temperature for 16 h. The sidewalls in the upper part of the PMMA resist structure were inclined and rounded. In particular, the shape of the PMMA resist structure of the lines with 20-m width also referred as 20-m lines could be processed to achieve a halberd-like shape. Thus, the effectiveness of the grayscale mask in adjusting to the varying thick- nesses of absorber was confirmed by XRL experiments. Moreover, we showed that the final shape of PMMA resist structures after XRL was predictable by calculations.

Glass nanoimprint using amorphous Ni–P mold etched by focused-ion beam

The authors succeeded in glass-nanoimprint lithography of micropatterns and nanopatterns using an amorphous Ni–P alloy mold. Glasslike carbon has been used as a mold material to mold not only Pyrex glass but also quartz, because it is still stable at a temperature of 1650°C. However, it is difficult to process glasslike carbon substrates into arbitrary shapes by machining. They thought that amorphous Ni–P alloy could be used as a mold material for industrial glass molding. If Ni is electroless plated when mixed with suitable amount of P on a Si wafer, the Ni–P alloy layer becomes amorphous. An appropriate ratio of Ni and P was determined by the results of x-ray-diffraction measurements. The optimized composition ratio of Ni–P was Ni:P=92:8wt%. Moreover, line and space patterns and dot arrays with linewidths of as little as 500nm were etched on the mold using focused-ion beam (FIB) and the processing accuracy for the amorphous Ni–P layer was compared with that for the pure Ni layer. The result was that pat...

Optimization of Au mask fabrication processes for LIGA applications

This paper presents approaches to fabricate Au masks for LIGA process. The proposed fabrication process starts with deposition of a thick layer of Au film, then followed by electron cyclotron resonance (ECR) Ar+ etching and coating of a polymer material that is used as a membrane. Finally the ICP DRIE etching is applied from backside to figure out the membrane. The profiles of the Au microstructures on the mask have been improved thanks to the optimization of etching process and photoresist material. The fabricated masks have been used in the X-ray lithography and demonstrated a well acceptable performance. A nickel mold has been successfully realized by electroplating and used in a hot embossing process for forming optical components. Au masks made by using conventional Au-electroplating technique have also been demonstrated for a comparison.

Three-dimensional X-ray lithography using a silicon mask with inclined absorbers

We proposed a new fabrication method of an X-ray gray mask using MEMS technologies, and we also succeeded in fabricating three-dimensional microstructures on a PMMA sheet by using only a single X-ray exposure. Silicon can be diagonally etched by optimizing the etching condition in a RIE process. We thought X-ray absorbers of an X-ray mask were processed to three-dimensional shape, and a gray mask for the X-ray lithography was fabricated by using a tapered-trench- etching technique. Then, we experimented on the X-ray lithography using the beamline BL-4 in the synchrotron radiation facility TERAS of AIST. The total dose energy was 150 mAxh and the development was performed at the room temperature for 16 h using a GG developer. Sidewalls in the upper part of the PMMA resist structure were inclined and rounded. Especially, the shape of the PMMA resist structure of the line width 20 μm was able to be processed to shape like the target. Thus, the effectiveness of the gray mask that adjusted the thickness of absorber was confirmed by X-ray lithography experiments. Moreover, we experimentally showed that the final shape of PMMA resist structures after the X-ray lithography was predictable by the calculation.

A novel fabrication method of needle array combined X-ray gray mask with LIGA process

We have succeeded in fabricating a needle array of polycarbonate by using a three-dimensional LIGA process. The diameter of the bottom of the needle was about 50 μm, and the height was 135 μm. Although a usual LIGA process has been employed to form structure only with vertical sidewalls, it has now become possible to fabricate needle shape structure by employing a technology that combines X-ray gray mask with the LIGA process. The X-ray gray mask was composed of Si X-ray absorbers and a SU-8 membrane. The sidewall of the X-ray absorber was diagonally processed by Si tapered-trench-etching technology where the transmission intensity of X-rays could be varied locally. An X-ray lithography experiment was executed by using the X-ray gray mask on a beamline BL-4 in TERAS synchrotron radiation facility at AIST. Using this technology a PMMA resist master with three-dimensional structures was made. A Pt layer was sputter deposited as a seed layer on the PMMA resist master, and a Ni mold was fabricated by an electroforming technology. In addition, needle arrays of polycarbonate (PC) and of polymethyl methacrylate (PMMA) were produced by hot embossing technology. Thus, we succeeded in extending the LIGA process to a three-dimensional process capability by employing X-ray gray mask.