Weiwen Feng

Temperature can reflect vital activities, and researchers have attempted to guide Chinese medicine diagnosis and treatment by observing acupoint temperature changes. Integrating a temperature sensor at the needle tip enables in situ acupoint temperature measurement. However, the sensor needles for acupoint temperature monitoring designed in previous studies were fabricated by manually soldering thermistor beads and metal wires, making mass production difficult. In this work, using MEMS manufacturing technology, a flexible temperature sensor that can be integrated at the needle tip is proposed and can be mass-produced on silicon wafers. The sensor uses a Pt thermistor as the temperature-sensing element and has a slender flexible structure with dimensions of 125 μm width by 3.2 cm length. As the sensor is inserted into a hollow needle, the Pt thermistor is glued to the needle tip. In the temperature range of 30 °C to 50 °C, the fabricated temperature sensor has a sensitivity of 5.00 Ω∙°C⁻¹, a nonlinearity of ±0.39%FS, and a repeatability error of ±2.62%FS. Additionally, the sensor has been applied to in vivo acupoint temperature monitoring experiments in rats and demonstrated good performance, suggesting its promise for future research on acupoint temperature.

Flexible pressure sensors and temperature sensors are widely used in various fields because of their advantages in high flexibility, good shape retention and extremely small thickness. However, it is quite challenging to fabricate ultra-thin flexible pressure sensors with reliable sensing performance. In this work, we propose a new type of silicon–polymer heterogeneously integrated MEMS flexible sensor with an ultra-thin silicon-based absolute pressure sensing element and a thermistor. In the study, a flexible MEMS fabrication process is developed, which enables simultaneous fabrication in two different substrates and self-release of the thin and slim flexible sensor. The front-end section of the flexible sensor is with the width as 125 μm, length as 3.2 cm and total thickness as 12 μm, where the integrated silicon substrate thickness is only 3 μm. The sensor takes a slender shape to allow for medical invasive measurement by inserting it into a slim medical catheter or a syringe needle-tube. The sensitivity of the fabricated ultra-thin absolute pressure sensor is tested as 45.2 μV kPa⁻¹ under 3.3 V supplied voltage, with the nonlinearity as only ±0.16% FS. The sensitivity of the thermistor is 10.4 Ω °C⁻¹ in the range of 0 °C–100 °C. Moreover, the polysilicon thermistor can also serve as a micro-heater, where an electric heating power of 107 μW results in a temperature increase of 13.5 °C. With ultra-thin slim structure and satisfactory performance, the MEMS flexible sensor is promising in various fields like biomedical applications.

This paper presents an ultra-small absolute pressure sensor with a silicon-micromachined TSV backside interconnection for high-performance, high spatial resolution contact pressure sensing, including flexible-substrate applications. By exploiting silicon-micromachined TSVs that are compatibly fabricated with the pressure sensor, the sensing signals are emitted from the chip backside, thereby eliminating the fragile leads on the front-side. Such a design achieves a flat and fully passivated top surface to protect the sensor from mechanical damage, for reliable direct-contact pressure sensing. A single-crystal silicon beam–island structure is designed to reduce the deflection of the pressure-sensing diaphragm and improve output linearity. Using our group-developed microholes interetch and sealing (MIS) micromachining technique, we fabricated ultra-small piezoresistive pressure sensors with the chip size as small as 0.4 mm × 0.6 mm, in which the polysilicon-micromachined TSVs transfer the signal interconnection from the front-side to the backside of the wafer, and the sensor chips can be densely packaged on the flexible substrate via the TSVs. The ultra-small pressure sensor has high sensitivity of 0.84 mV/kPa under 3.3 V of supply voltage and low nonlinearity of ±0.09% full scale (FS) in the measurement range of 120 kPa. The proposed pressure sensors with backside-interconnection TSVs hold promise for tactile sensing applications, including flexible sensing of wearable wristwatches.

Ultra-responsive single-crystal silicon MEMS thermopiles for differential thermal analysis (DTA) are developed. Facilitated by a unique “microholes interetch and sealing (MIS)” technique, pairs of suspended thermopiles are batch fabricated in a differential form, with high-density (54 pairs) n-type/p-type single-crystal silicon thermocouples integrated within each thermopile (sample area ~0.045 mm²). The fabricated MEMS thermopile sensors exhibit outstanding power responsivity of 99.5 V/W and temperature responsivity of 27.8 mV/°C, which are more than 4 times higher than those reported for material thermal analysis. The high-responsivity MEMS DTA chips allow us to accurately measure the indium melting point at different heating rates of ~1–100 °C/s. We also perform DTA measurement of the dehydration process of CuSO₄·5H₂O and the crystals show three stages of losing water of crystallization before becoming anhydrous copper sulfate salt. Our high-performance, cost-effective MEMS sensing chips hold promise for rapid and accurate DTA characterization for a wide range of applications.