Silicon carbide micromachining

Wide band gap materials such as diamond like carbon, III–nitride, and silicon carbide have been widely applied in many electronic devices, where conditions include high power and high temperature. Among numerous materials, silicon carbide (SiC) is a preferable choice owing to its excellent physical properties along with the availability of wafers. The main obstacles hindering the wider applications of SiC include the high wafer costs and low etching rate of SiC compared to Si. Cubic silicon carbide thin films provide a valid alternative as they can be grown on a silicon substrate and have been used as an excellent platform for MEMS. Epitaxial SiC films grow on large scale Si wafers, taking advantage of low cost Si wafers, and simplifying the fabrication process of SiC MEMS. To increase the application feasibility of SiC in harsh environments, electrical insulators such as SiO2 can prevent the leakage current from the functional layer (SiC) to the substrate (Si). In addition, the SiC on the SiO2 platform is also of significant interest for optical applications such as SiC waveguides or photonic crystals since SiC offers a higher refractive index than SiO2. To date, the most common method to form SiC on an insulator is based on wafer bonding. However, this method typically requires smooth surface treatment and the removal of Si. The NTNLab has devised a novel technique to form SiC on an insulator by thermally oxidizing the Si substrate. This technique is based on the chemical inertness of SiC, which allows the material to remain unchanged during the thermal oxidation process. NTNLab’s experimental data show that thermal oxidation of silicon makes the development of SiC nano structures (eg nanowires) on an insulator possible without the requirements of wafer-bonding and/or silicon removal. The proposed method could result in SiC based MEMS devices including high temperature sensors, and nanowire waveguides.1

Other work in NTNLab examines strain engineering, a topic which has attracted much attention, particularly for epitaxial films grown on a different substrate. Residual strains of SiC have been employed to form ultra-high frequency and high Q factor resonators. However, the highest residual strain of SiC was reported to be limited to approximately 0.6%. Large strains induced into SiC could lead to several interesting physical phenomena, as well as significant improvement of resonant frequencies. NTNLab research examines an unprecedented nanostrain-amplifier structure with an ultra-high residual strain up to 8% utilizing the natural residual stress between epitaxial 3C-SiC and Si. In addition, the applied strain can be tuned by changing the dimensions of the amplifier structure. The possibility of introducing such a controllable and ultra-high strain will lead to investigating the physics of SiC in large strain regimes and the development of ultra-sensitive mechanical sensors.2

Work from NTNLab has explored the thermoresistive property of p-type single crystalline 3C-SiC (p-3C-SiC), which was epitaxially grown on a silicon (Si) wafer, and then transferred to a glass substrate using a Focused Ion Beam (FIB) technique. A negative and relatively large temperature coefficient of resistance (TCR) up to -5500 ppm/K was observed. This TCR is attributed to two activation energy thresholds of 45 meV and 52 meV, corresponding to temperatures below and above 450 K, respectively, and a small reduction of hole mobility with increasing temperature. The large TCR indicates the suitability of p-3C-SiC for thermal-based sensors working in high-temperature environments.3 In other work, the Lab has identified the gauge factor of the p-type nanocrystalline SiC was found to be 14.5, which is one order of magnitude larger than that in most metals. This result indicates that mechanical strain has a significant influence on the electrical conductance of p-type nanocrystalline SiC, which is promising for mechanical sensing applications in harsh environments.4

Polymeric micromachining

Poly(methyl methacrylate) (PMMA) has been used as a basic substrate material for fabrication of microfluidic devices due to its high mechanical stability, good chemical properties and excellent optical clarity. The manufacturing process of PMMA microchips includes several fabrication methods such as hot embossing, injection moulding, LIGA (German acronym for x-ray lithography electro deposition and moulding), and excimer laser (ultra-violet laser) ablation. For replication methods such as hot embossing and injection moulding, the costly and time-consuming mould fabrication process hampers the rapid turnaround of new design although the technology is intrinsically suitable for mass production. The implementations of x-ray lithography and excimer laser ablation are expensive because of the high cost of the x-ray and excimer laser system and the corresponding masks. More recently, the cheaper CO2 laser has been used to create microchips in PMMA. This technique has been proven to be rapid and effective for fabricating microfluidic devices, especially for scientific trials and small-scale production.1

The NTNLab presents a laser-micromachined polymeric membraneless fuel cell. The membraneless fuel cell, constructed with three PMMA layers, takes advantage of two laminar flows in a single micro channel to keep the fuel and oxidant streams separated yet in diffusional contact. The packed fuel cell has been electrochemically characterized by an electrochemical analyser. The Lab demonstrated for the first time the use of hydrogen peroxide in sulfuric acid as the oxidant. The new oxidant composition allows a simple recycling process and better fuel utilization.2

PMMA due to its cost, transparency and mechanical and chemical properties, is of interest to the microfluidic research community. Meanwhile, the more flexible polydimethylsiloxane (PDMS) is well suited for pneumatic actuation. However, PDMS is permeable to gases and absorbs molecules from the sample liquids. The NTNLab has developed a simple and reliable technique for bonding PMMA to PDMS. A 25 µm thick adhesive was laminated onto a clean PMMA surface. The PDMS/adhesive membrane acts as the pneumatic actuator for the micropump. Pressurized air was switched to the three pneumatic actuators by solenoid valves and control electronics. The micropumps can achieve a flow rate as high as 96 µ/min, allowing the integration of microfluidic components made of both PMMA and PDMS in a single device.3

Low-cost alternative devices

Flexible and multifunctional electronic devices show potential for various applications including human-motion detection and wearable thermal therapy. The key advantages of these systems are (1) highly stable, sensitive and fast-response devices, (2) fabrication of macroscale devices on flexible substrates, and (3) integrated (lab-on-chip) and multifunctional devices. However, their fabrication commonly requires toxic solvents, as well as time-consuming and complex processes. The NTNLab is successfully researching low-cost, rapid-prototyping and user-friendly fabrication of flexible transducers using recyclable, water-resistant poly(vinyl chloride) films as a substrate, and ubiquitously available pencil graphite as a functional layer without using any toxic solvents or additional catalysts. The flexible heaters showed good characteristics such as fast thermal response, good thermostability (low temperature coefficient of resistance) and low power consumption. The heaters with their capability of perceiving human motion were shown to be effective. Results from the Lab indicate that a wide range of electronic devices fabricated from the environmental-friendly material by this simple and user-friendly approach could be utilized for cost-effective, flexible and low power consuming thermal therapy, health monitoring systems and other real-time monitoring devices without using any toxic chemicals or advanced processes.1

Paper-based microfluidics and sensors have attracted great attention as low-cost alternative devices. Work in the NTNLab examines the development of a self-sensing paper-based magnetic-actuator. The actuating component of the device was formed by employing nanoferromagnetic particles absorbed into porous paper. A graphite layer was used to create the mechanical sensing element for the paper-based actuator. For the proof of concept, a cantilever-type device was fabricated using a clean-room free process, demonstrating the actuating and self-sensing functions integrated in a single paper-based device.2

NTNLab is also investigating graphite on paper (GOP), in which graphite layers are deposited on paper by either graphite-ink printing techniques or manual pencil drawing techniques.  The excellent electrical conductivity of graphite and the low thermal conductivity of porous paper make GOP an attractive material for thermoresistive sensors. Consequently, GOP, as a transducing element for thermal-based sensors, has remarkable advantages such as low cost, cleanroom-free fabrication and high sensitivity.3. The Lab is also investigating solvent-free fabrication of a biodegradable hot-film flow sensor for noninvasive respiratory monitoring. Our research shows that the solvent-free and cleanroom free fabrication of a degradable flexible sensor could be affixed to the upper lip to noninvasively monitor human respiration in real-time with a high sensitivity.  The sensors offer high sensitivity, low power consumption, high signal-to-noise ratio (SNR) and good long-term stability. Results indicate that sensors fabricated by this simple but effective approach can be utilized to produce low-cost, environment-friendly, sustainable, flexible and wearable sensors for noninvasively monitoring human respiration in realtime.4

NTNLab is also investigating flexible and stretchable electronics that have a wide variety of wearable applications in portable sensors, flexible electrodes/heaters, flexible circuits and stretchable displays. Spinnable carbon nanotubes (CNTs) constructed on flexible substrates are a potential material for wearable sensing applications owing to their high thermal and electrical conductivity, low mass density and excellent mechanical properties. The Lab is exploring wearable thermal flow sensor for healthcare using lightweight, high strength, flexible CNT yarns as hotwires, pencil graphite as electrodes, and lightweight, recyclable and biodegradable paper as flexible substrates, without using any toxic chemicals.5