A rapid and cost-effective metallization technique for 3C-SiC MEMS using direct wire bonding

This paper presents a simple, rapid and cost-effective wire bonding technique for single crystalline silicon carbide (3C–SiC) MEMS devices. Utilizing direct ultrasonic wedge–wedge bonding, we have demonstrated for the first time the direct bonding of aluminum wires onto SiC films for the characterization of electronic devices without the requirement for any metal deposition and etching process. The bonded joints between the Al wires and the SiC surfaces showed a relatively strong adhesion force up to approximately 12.6–14.5 mN and excellent ohmic contact. The bonded wire can withstand high temperatures above 420 K, while maintaining a notable ohmic contact. As a proof of concept, a 3C–SiC strain sensor was demonstrated, where the sensing element was developed based on the piezoresistive effect in SiC and the electrical contact was formed by the proposed direct-bonding technique. The SiC strain sensor possesses high sensitivity to the applied mechanical strains, as well as exceptional repeatability. The work reported here indicates the potential of an extremely simple direct wire bonding method for SiC for MEMS and microelectronic applications

Electrically stable carbon nanotube yarn under tensile strain

We report a highly stable electrical conductance of a compact and well-oriented carbon nanotube yarn under tensile strain. The gauge factor of the yarn was found to be extremely small of approximately 0.15 thanks to the improvements in the dry spinning process, includingmultiweb spinning and heat treatment. The threshold strain εs, below which the yarn retains its electrical conductance stability, has also been determined to be approximately 15 × 103 ppm. Owing to its highly stable resistance under mechanical strain, the yarn has a good potential as a wiring material for niche applications,where lightweight and resistance stability are required

Highly sensitive 4H-SiC pressure sensor at cryogenic and elevated temperatures

The slow etching rate of conventional micro-machining processes is hindering the use of bulk silicon carbide materials in pressure sensing. This paper presents a 4H-SiC piezoresistive pressure sensor utilising a laser scribing approach for fast prototyping a bulk SiC pressure sensor. The sensor is able to operate at a temperature range from cryogenic to elevated temperatures with an excellent linearity and repeatability with a pressure of up to 270 kPa. The good optical transparency of SiC material allows the direct alignment between the pre-fabricated piezoresistors and the scribing process to form a diaphragm from the back side. The sensitivities of the sensor were obtained as 10.83 mV/V/bar at 198 K and 6.72 mV/V/bar at 473 K, which are at least a two-fold increment in comparison with other SiC pressure sensors. The high sensitivity and good reliability at either cryogenic and elevated temperatures are attributed to the profound piezoresistive effect in p-type 4H-SiC and the robust p-n junction which prevents the current from leaking to the substrate. This indicates the potential of utilising the laser scribing approach to fabricate highly sensitive bulk SiC pressure sensors for harsh environment applications

Giant piezoresistive effect by optoelectronic coupling in a heterojunction

Enhancing the piezoresistive effect is crucial for improving the sensitivity of mechanical sensors. Herein, we report that the piezoresistive effect in a semiconductor heterojunction can be enormously enhanced via optoelectronic coupling. A lateral photovoltage, which is generated in the top material layer of a heterojunction under non-uniform illumination, can be coupled with an optimally tuned electric current to modulate the magnitude of the piezoresistive effect. We demonstrate a tuneable giant piezoresistive effect in a cubic silicon carbide/silicon heterojunction, resulting in an extraordinarily high gauge factor of approximately 58,000, which is the highest gauge factor reported for semiconductor-based mechanical sensors to date. This gauge factor is approximately 30,000 times greater than that of commercial metal strain gauges and more than 2,000 times greater than that of cubic silicon carbide. The phenomenon discovered can pave the way for the development of ultra-sensitive sensor technology

3C-SiC on glass: an ideal platform for temperature sensors under visible light illumination

This letter reports on cubic silicon carbide (3C–SiC) transferred on a glass substrate as an ideal platform for thermoresistive sensors which can be used for in situ temperature measurement during optical analysis. The transfer of SiC onto an insulating substrate prevents current leakage through the SiC/Si junction, which is significantly influenced by visible light. Experimental data shows that the 3C–SiC on glass based sensor possesses a large temperature coefficient of resistance (TCR) of up to −7508 ppm K−1, which is about 10 times larger than that of highly doped Si. Moreover, the 3C–SiC based temperature sensor also outperforms low doped Si in terms of stability against visible light. These results indicate that 3C–SiC on glass could be a good thermoresistive sensor to measure the temperature of cells during optical investigations

High-temperature tolerance of the piezoresistive effect in p-4H-SiC for harsh environment sensing

4H-silicon carbide based sensors are promising candidates for replacing prevalent silicon-based counterparts in harsh environments owing to their superior chemical inertness, high stability and reliability. However, the wafer cost and the difficulty in obtaining an ohmic contact in the metallization process hinders the use of this SiC polytype for practical sensing applications. This article presents the high-temperature tolerance of a p-type 4H-SiC piezoresistor at elevated temperatures up to 600 °C. A good ohmic contact was formed by the metallisation process using titanium and aluminium annealed at 1000 °C. The leakage current at high temperatures was measured to be negligible thanks to a robust p–n junction. Owing to the superior physical properties of the bulk 4H-SiC material, a high gauge factor of 23 was obtained at 600 °C. The piezoresistive effect also exhibits good linearity and high stability at high temperatures. The results demonstrate the capability of p-type 4H-SiC for the development of highly sensitive sensors for hostile environments

3C-SiC/Si Heterostructure: An excellent platform for position-sensitive detectors based on photovoltaic effect

Single-crystalline silicon carbide (3C-SiC) on the Si substrate has drawn significant attention in recent years due to its low wafer cost and excellent mechanical, chemical, and optoelectronic properties. However, the applications of the structure have primarily been focused on piezoresistive and pressure sensors, bio-microelectromechanical system, and photonics. Herein, we report another promising application of the heterostructure as a laser spot position-sensitive detector (PSD) based on the lateral photovoltaic effect (LPE) under nonuniform optical illuminations at zero-bias conditions. The LPE shows a linear dependence on spot positions, and the sensitivity is found to be as high as 33 mV/mm under an illumination of 2.8 W/cm2 (635 nm). The structure also exhibits a linear dependence of the LPE over a large distance (7 mm) between two electrodes, which is crucial for PSDs as the region with a linear dependence of LPE is only usable for PSDs. The LPE at different spot positions and under different illumination conditions have been investigated and explained based on the energy-band analysis. The temperature dependence of the LPE and position sensitivity is also investigated. Furthermore, the two-dimensional mapping of the lateral photovoltages reveals the potential for utilizing the 3C-SiC/Si heterostructure to detect the laser spot position precisely on a plane

Photoresponse of a highly-rectifying 3C-SiC/Si heterostructure under UV and visible illuminations

In this letter, we report the photo-characteristics of an n-3C-Silicon carbide (SiC)/p-Si heterojunction under ultraviolet (UV) and visible illuminations. The 3C-SiC thin film has been grown on Si (100) substrate by a low pressure chemical vapor deposition technique at 1000 °C. The as-grown structure shows an excellent rectification ratio (IF/IR) of 1.8 × 10 6 at ±2 V and a reverse bias current of 5.5 × 10 -9 A at 2V in dark conditions. The heterojunction exhibits good sensitivity simultaneously to both UV (375 nm) and visible (637 nm) illuminations. The results indicate that the fabricated structure could be an excellent platform where detection of a wide spectral illumination is vital. The insight of electron-hole pairs generation and carrier transport mechanism at different illumination conditions is explained in detail via an anisotype heterojunction energy band diagram

Optoelectronic Enhancement for Piezoresistive Pressure Sensor

Pressure sensing is a critical task of microelectromechanical systems with a wide range of applications. Enhancing the performance such as sensitivity, durability, reliability, linearity and response time of pressure sensors has always been a top priority for researchers and technology developers. This paper demonstrates a new method for enhancing the performance of a piezoresistive pressure sensor. By using light illumination combined with controlling supply current, the performance of a 3C-SiC/Si heterojunction piezoresistive pressure sensor is significantly improved. The sensitivity of the pressure sensor is enhanced a few hundred thousand times under bright condition in comparison with that under dark condition. This enhancement is unprecedented for a micromachined pressure sensor. In addition, the durability, signal to noise ratio and measurement range are substantially improved

Opto-electronic coupling in semiconductors: Towards ultrasensitive pressure sensing

The discovery of a giant piezoresistive effect in a semiconductor heterojunction by optoelectronic coupling can open a new era for mechanical sensors. This paper develops a novel concept of opto-electronic coupling in semiconductor heterojunctions for pressure sensing. We employ non-uniform illumination of visible light on a SiC/Si heterojunction to generate a gradient of charge carriers in the SiC nanofilm. These charge carriers are then manipulated by a tuning current, producing giant relative resistance changes in the material under applied pressure. We successfully demonstrated the enhancement by opto-electronic coupling in a SiC/Si heterojunction pressure sensor of sensitivity up to 185[thin space (1/6-em)]000 times compared to the unilluminated condition. In addition, the opto-electronic coupling enables significantly improved repeatability, stability, signal-to-noise ratio and detectable range of the pressure sensor. The ultrahigh sensitive pressure sensing mechanism by opto-electronic coupling will pave a way for development of extremely sensitive mechanical sensor