Laser-assisted thermal imprinting of glass guided mode resonant (GMR) optical filter

Laser-assisted thermal imprinting of glass nanostructures is demonstrated. Compare to the existing thermal imprinting, this method significantly reduced the contact imprinting time. The quality of the replicated glass nanostructures revealed by field emission scanning electron microscope ( SEM) and atomic force microscope ( AFM) exhibited a very smooth surface finish that closely matched the profile of the silicon mold. As proof-of-concept, the utility of laser-assisted, imprinted glass nanostructures as guided-mode resonant (GMR ) optical filter was evaluated. The peak spectral values obtained were satisfactory; which yielded an average FWHM and PWV of 4.6 nm and 691.39 nm respectively.

Double-Deck Metal Solenoids 3D Integrated in Silicon Wafer for Kinetic Energy Harvester

A silicon-chip based double-deck three-dimensional (3D) solenoidal electromagnetic (EM) kinetic energy harvester is developed to convert low-frequency (<100 Hz) vibrational energy into electricity with high efficiency. With wafer-level micro electro mechanical systems (MEMS) fabrication to form a metal casting mold and the following casting technique to rapidly (within minutes) fill molten ZnAl alloy into the pre-micromachined silicon mold, the 300-turn solenoid coils (150 turns for either inner solenoid or outer solenoid) are fabricated in silicon wafers for saw dicing into chips. A cylindrical permanent magnet is inserted into a pre-etched channel for sliding upon external vibration, which is surrounded by the solenoids. The size of the harvester chip is as small as 10.58 mm × 2.06 mm × 2.55 mm. The internal resistance of the solenoids is about 17.9 Ω. The maximum peak-to-peak voltage and average power output are measured as 120.4 mV and 43.7 μW. The EM energy harvester shows great improvement in power density, which is 786 μW/cm3 and the normalized power density is 98.3 μW/cm3/g. The EM energy harvester is verified by experiment to be able to generate electricity through various human body movements of walking, running and jumping. The wafer-level fabricated chip-style solenoidal EM harvesters are advantageous in uniform performance, small size and volume applications.

Transparent and Flexible Mayan-Pyramid-based Pressure Sensor using Facile-Transferred Indium tin Oxide for Bimodal Sensor Applications

Transparent and conducting flexible electrodes have been successfully developed over the last few decades due to their potential applications in optoelectronics. However, recent developments in smart electronics, such as a direct human-machine interface, health-monitoring devices, motion-tracking sensors, and artificially electronic skin also require materials with multifunctional properties such as transparency, flexibility and good portability. In such devices, there remains room to develop transparent and flexible devices such as pressure sensors or temperature sensors. Herein, we demonstrate a fully transparent and flexible bimodal sensor using indium tin oxide (ITO), which is embedded in a plastic substrate. For the proposed pressure sensor, the embedded ITO is detached from its Mayan-pyramid-structured silicon mold by an environmentally friendly method which utilizes water-soluble sacrificial layers. The Mayan-pyramid-based pressure sensor is capable of six different pressure sensations with excellent sensitivity in the range of 100 Pa-10 kPa, high endurance of 105 cycles, and good pulse detection and tactile sensing data processing capabilities through machine learning (ML) algorithms for different surface textures. A 5 × 5-pixel pressure-temperature-based bimodal sensor array with a zigzag-shaped ITO temperature sensor on top of it is also demonstrated without a noticeable interface effect. This work demonstrates the potential to develop transparent bimodal sensors that can be employed for electronic skin (E-skin) applications.

Fabrication of Convex PDMS–Parylene Microstructures for Conformal Contact of Planar Micro-Electrode Array

Polymer-based micro-electrode arrays (MEAs) are gaining attention as an essential technology to understand brain connectivity and function in the field of neuroscience. However, polymer based MEAs may have several challenges such as difficulty in performing the etching process, difficulty of micro-pattern generation through the photolithography process, weak metal adhesion due to low surface energy, and air pocket entrapment over the electrode site. In order to compensate for the challenges, this paper proposes a novel MEA fabrication process that is performed sequentially with (1) silicon mold preparation; (2) PDMS replica molding, and (3) metal patterning and parylene insulation. The MEA fabricated through this process possesses four arms with electrode sites on the convex microstructures protruding about 20 μm from the outermost layer surface. The validity of the convex microstructure implementation is demonstrated through theoretical background. The electrochemical impedance magnitude is 204.4 ± 68.1 kΩ at 1 kHz. The feasibility of the MEA with convex microstructures was confirmed by identifying the oscillation in the beta frequency band (13–30 Hz) in the electrocorticography signal of a rat olfactory bulb during respiration. These results suggest that the MEA with convex microstructures is promising for applying to various neural recording and stimulation studies.

Contact Photolithography at Sub-Micrometer Scale Using a Soft Photomask

This paper proposes a method for improving the patterning resolution of conventional contact photolithography from the micrometer, down to the sub-micrometer scale. The key element is a soft polydimethylsiloxane (PDMS) photomask, which is first replicated from a silicon mold and then patterned with a black photoresist (PR) layer to selectively block ultraviolet (UV) light. This soft PDMS photomask can easily form an intimate and conformable contact with a PR-coated substrate and hence can perform contact photolithography with high pattern resolution. The fabrication processes of this black-PR/PDMS soft photomask are experimentally carried out. Using the fabricated soft photomask, UV patterning by contact photolithography with the smallest line-width of 170 nm over a 4” wafer area was successfully achieved. The advantages and potentials of this new type of contact photolithography will be addressed.

Machining of Lenticular Lens Silicon Molds with a Combination of Laser Ablation and Diamond Cutting

Lenticular lenses are widely used in the three-dimensional display industry. Conventional lenticular lens components are made of plastics that have low thermal stability. An alternative is to use glass to replace plastic as the lenticular lens component material. Single crystal silicon is often used as the mold material in the precision glass molding process. It is, however, difficult to fabricate a lenticular lens silicon mold that has a large feature size compared to the critical depth of cut of silicon. In order to solve the problems of machining lenticular lens silicon molds using the conventional diamond cutting method, such as low machining efficiency and severe tool wear, a hybrid machining method that combined laser ablation and diamond cutting was proposed. A feasibility study was performed to investigate the possibility of using this method to fabricate a lenticular lens silicon mold. The influence of the laser parameters and machining parameters on the machining performance was investigated systematically. The experimental results indicated that this hybrid machining method could be a possible method for manufacturing lenticular lens silicon molds or other similar microstructures.

Sustainable Fabrication of Glass Nanostructures Using Infrared Transparent Mold Assisted by CO2 Laser Scanning Irradiation

Direct thermal imprinting of nanostructures on glass substrates is reliable when manufacturing net-shaped glass devices with various surface functions. However, several problems are recognized, including a long thermal cycle, tedious optimization, difficulties in ensuring high level replication fidelity, and unnecessary thermal deformation of the glass substrate. Here, we describe a more sustainable and energy efficient method for direct thermal imprinting of nanostructures onto glass substrates; we use silicon mold transparent to infrared between 2.5 and 25 μm in wavelength combined with CO2 laser scanning irradiation. The glass strongly absorbed the 10.6 μm wavelength irradiation, triggering substantial heating of a thin layer on the glass surface, which significantly enhanced the filling of pressed glass material into nanostructured silicon mold cavities. For comparison, we conducted conventional direct glass thermal imprinting experiments, further emphasizing the advantages of our new method, which outperformed conventional methods. The thermal mass cycle was shorter and the imprint pattern quality and yield, higher. Our method is sustainable, allowing more rapid scalable fabrication of glass nanostructures using less energy without sacrificing the quality and productivity of the fabricated devices.

Perfluoropolyether (PFPE) Intermediate Molds for High-Resolution Thermal Nanoimprint Lithography

Among soft lithography techniques, Thermal Nanoimprint Lithography (NIL) is a high-throughput and low-cost process that can be applied to a broad range of thermoplastic materials. By simply applying the appropriate pressure and temperature combination, it is possible to transfer a pattern from a mold surface to the chosen material. Usually, high-resolution and large-area NIL molds are difficult to fabricate and expensive. Furthermore, they are typically made of silicon or other hard materials such as nickel or quartz for preserving their functionality. Nonetheless, after a large number of imprinting cycles, they undergo degradation and become unusable. In this paper, we introduce and characterize an innovative two-step NIL process based on the use of a perfluoropolyether (PFPE) intermediate mold to replicate sub-100 nm features from a silicon mold to the final thermoplastic material. We compare PFPE elastomeric molds with molds made of the standard polydimethylsiloxane (PDMS) elastomer, which demonstrates better resolution and fidelity of the replica process. By using PFPE intermediate molds, the nanostructured masters are preserved and the throughput of the process is significantly enhanced.

An interlocked flexible piezoresistive sensor with 3D micropyramidal structures for electronic skin applications

An interlocked micropyramidal flexible sensor was fabricated using an anisotropically-etched silicon mold and modelled using a finite-element-assisted constriction resistance model.