Silicon carbide based MEMS

Microelectromechanical systems sensors have been intensively developed utilizing various physical concepts, such as piezoresistive, piezoelectric, and thermoresistive effects. Among these sensing concepts, the thermoresistive effect is of interest for a wide range of thermal sensors and devices, thanks to its simplicity in implementation and high sensitivity.1

Silicon carbide (SiC) is one of the most promising materials for applications in harsh environments due to its excellent electrical, mechanical, and chemical properties. Among several large band gap materials, SiC is one of the most promising semiconductors for MEMS transducers used in harsh environments due to its large energy band gap of 2.3 to 3.4 eV, excellent chemical inertness, and large Young’s modulus.2 Further, NTNLab research examines the piezoresistive effect of p-type single crystalline 3C-SiC characterized at high temperatures, using an in-situ measurement method. Experimental results show that the highly doped p-type 3C-SiC possesses a relatively stable gauge factor of approximately 25 to 28 at temperatures varying from 300 K to 573 K. The in-situ measurements also show that the combination of the piezoresistive and thermoresistive effects can increase the gauge factor of p-type 3C-SiC to approximately 20% at 573 K.3

The NTNLab has also found that the orientation dependence of the pseudo-Hall effect in p-type 3C–SiC four-terminal devices under mechanical stress. Research results indicate that the offset voltage of p-type 3C–SiC four-terminal devices significantly depends on the directions of the applied current and stress.4

The thermoresistive effect for advanced thermal sensors, including fundamental research, design and application development of high-temperature thermal sensors are being investigated in the NTNLab employing large band gap semiconductors such as silicon carbide. The development of thermal sensors could be driven towards miniaturization of a wide range of measurement without signal saturation, large working bandwidth and capability of detecting multi-directions of physical signals. An example from the NTNLab is the effect in pencil graphite, and a proof of concept of thermal flow sensors has been successfully demonstrated.5

The NTNLab has devised an innovative nano strain-amplifier employed to significantly enhance the sensitivity of piezoresistive strain sensors. Inspired from the dogbone structure, the nano strain-amplifier consists of a nano thin frame released from the substrate, where nanowires were formed at the centre of the frame. Analytical and numerical results indicated that a nano strain-amplifier significantly increases the strain induced into a free standing nanowire, resulting in a large change in their electrical conductance. The proposed structure was demonstrated in p-type cubic silicon carbide nanowires fabricated using a top down process. The experimental data showed that the nano strain-amplifier can enhance the sensitivity of SiC strain sensors at least 5.4 times larger than that of the conventional structures. This result indicates the potential of the proposed strain-amplifier for ultra-sensitive mechanical sensing applications.6

Silicon based MEMS

Over several decades, electronic sensing devices have been significantly developed, with an emphasis on miniaturization and high sensitivity with many successful applications. Nanowires (NWs), as an important achievement of the miniaturization process, are of interest, since these one-dimensional nanostructures offer extremely small sizes with great integration ability into micro/ nano systems. Recent research facilitating thermal-sensing nano-systems has focused on highly thermosensitive NWs, which are particularly useful for monitoring temperature variations within a narrow range.  Research from the NTNLab shows highly thermosensitive silicon nanowires (SiNWs) for thermal-sensing applications. Crystalline Si was amorphized by Focused Ion Beam in the fabrication process of the SiNWs, and subsequently recrystallized by a thermal annealing process to improve their electrical conductivity. A temperature coefficient of resistance (TCR) from -8000 ppm/K to -12,000 ppm/K was measured for the SiNWs. This large negative TCR is attributed to the boundary potential barrier of 110 meV between silicon crystallites in the poly crystalline SiNWs.1

The piezoresistive effect in silicon nanowires (SiNWs) has attracted a great deal of interest for NEMS devices.  The NTNLab is examining the piezoresistive effect of p-type silicon nanowires fabricated by a top-down process using focused ion beam (FIB) implantation and wet etching.  In this work our Lab looks at characterization of the piezoresistive effect of SiNWs fabricated by localized amorphization through FIB implantation, selective wet etching, and re-crystallization by thermal annealing process. As the properties of SiNWs are changed due to amorphization and post-FIB thermal treatment, the piezoresistive effect of Si NWs fabricated using this process is expected to be different from that of bulk single crystalline Si.  Our research examines the piezoresistive effect in released SiNWs fabricated by a top-down process using the FIB followed by wet etching of Si and thermal annealing. A relatively large gauge factor found in this top-down fabricated SiNWs indicates its high potential for nano electro mechanical systems (NEMS).2

Polymer based MEMS

Cells are well known to have the ability to sense and respond to mechanical stimuli, a process is referred to as mechanotransduction. Greater understanding of mechanotransduction is of interest to clinicians and scientists to improve clinical diagnosis and to understand medical pathology. However, the complexity involved in in vivo biological systems creates a need for better in vitro technologies, which can closely mimic the cells’ microenvironment using induced mechanical strain. This has motivated the NTNLab in the development of cell stretching devices for better understanding of the cell response to mechanical stimuli.1

Research from the NTNLab examines the design, fabrication and characterisation of a cell stretching device based on the side stretching approach. A unique PDMS-based micro fabrication process was developed that achieved parallelisation, controlled membrane thickness and an ultra-thin bottom layer. The platform was tested for cell growth under cyclic stretching, and preliminary results show that the device is compatible with all standard microscopes. Two types of cells were cultured and stretched in the device. The effective transferral of strain from the device membrane to the cells was demonstrated.2 Our research has found that the heterogeneity of the device provided an ideal platform for establishing strain requirement for the OEC culture. The cell stretching system developed may serve as a tool in exploring the mechanobiology of OECs for future SCI transplantation research.3

In further work, the NTNLab’s novel electromagnetic cell stretching platform can introduce a cyclic and static strain pattern on a cell culture. The platform was tested with fibroblasts, and the experimental results are consistent with cytoskeleton reorganisation and cell reorientation induced by strain. Our observations suggest that the cell orientation is highly influenced by external mechanical cues. Cells reorganise their cytoskeletons to avoid external strain and to maintain intact extracellular matrix arrangements.4