Jian-Qiu Huang

Simultaneous Remote Sensing of Temperature and Humidity by LC-Type Passive Wireless Sensors

This paper presents an integrated wireless passive sensor for remotely monitoring both temperature and relative humidity. The sensor consists of a coil and a capacitor to form an inductor-capacitor (LC) resonant circuit, which oscillates electrically at its resonant frequency. The inductor was a singlelayer planar spiral copper inductor and the capacitor was fabricated by the silicon-on-glass process, which utilizes graphene oxide films as the sensing material. The change of the capacitance due to environmental humidity variation shifts the resonant frequency, while environmental temperature affects the resistance and capacitance of the LC circuit and changes the resonant frequency and quality factor. By monitoring the real portion magnitude maximum of the impedance and the resonant frequency for the sensor, it is possible to get the capacitance and resistance from which the temperature and humidity can be extracted. The results presented here show that the sensitivity of the passive wireless sensor is about -17.80 kHz/%RH and 7.32 Q/%RH at 25 °C from 55%RH to 95%RH, and it is about -7.69 kHz/°C and 6.27 Ω/°C at 65%RH from 10 °C to 40 °C.

Development of a self-packaged 2D MEMS thermal wind sensor for low power applications

This article describes the design, fabrication, and testing of a self-packaged 2D thermal wind sensor. The sensor consists of four heaters and nine thermistors. A central thermistor senses the average heater temperature, whereas the other eight, which are distributed symmetrically around the heaters, measure the temperature differences between the upstream and downstream surface of the sensor. The sensor was realized on one side of a silicon-in-glass (SIG) substrate. Vertical silicon vias in the substrate ensure good thermal contact between the sensor and the airflow and the glass effectively isolates the heaters from the thermistors. The substrate was fabricated by using a glass reflow process, after which the sensor was realized by a lift-off process. The sensors geometry was investigated with the help of simulations. These show that narrow heaters, moderate heater spacing, and thin substrates all improve the sensors sensitivity. Finally, the sensor was tested and calibrated in a wind tunnel by using a linear interpolation algorithm. At a constant heating power of 24.5 mW, measurement results show that the sensor can detect airflow speeds of up to 25 m s−1, with an accuracy of 0.1 m s−1 at low speeds and 0.5 m s−1 at high speeds. Airflow direction can be determined in a range of 360° with an accuracy of ±6°.

Sensitivity Improvement of a 2D MEMS Thermal Wind Sensor for Low-Power Applications

In this paper, we report a novel and low-cost method to improve the sensitivity of a low-power 2D microelectromechanical systems thermal wind sensor by using HF wet etching. After wet etching, the thickness of the glass substrate decreases, so that the sensors thermal vias become more exposed to the wind. As a result, the conductive heat transfer is weakened and the convective heat transfer is enhanced in sensor operation. Finite-element method simulations verify this analysis. Moreover, the sensor chips with different lengths of silicon vias above the substrate are successfully fabricated and tested. Measurement results show that the wet etching has no influence on the metal film sensing and heating elements of the sensor. Besides, before and after wet etching for 7 and 14 min, at the wind speed of 5 m/s, the measured sensitivities of the sensor with a total power consumption of 24.5 mW are 77.2, 98.6, and 164.1 mK/(m/s). Measurement results also show that the improved sensitivity of the sensor chip can provide a more accurate measurement in wind speed but has little effect on the wind direction measurement. Instead, the accuracy of wind direction measurement is mainly related to the structural and thermal symmetries of the wind sensor. After compensation, the proposed thermal wind sensor can detect the wind direction in a full range of 360° with an mean error of 2.3° and a maximum error of 6°.

A MEMS capacitive pressure sensor compatible with CMOS process

This paper reports a capacitive pressure sensor. Like a common capacitor, it consists of three parts: the top electrode, the dielectric layer and the bottom electrode. The dielectric layer consists of the silicon oxide and an air gap. The fabrication process of this structure combines the CMOS process with the post-CMOS MEMS process. The bottom electrode made of polysilicon is formed during the CMOS process. the gap is formed by sacrificial layer release and the Al vapor process is used to seal the gap and form the top electrode, which is deformable and used to sense the pressure change. The sensor is tested under the pressure ranges from 100hPa to 1100hPa. The results show that the capacitance value increases with the pressure applied. For the structures with the membrane size of 100μm, 130μm and 150μm, their average sensitivity is about 0.085fF/hPa, 0.104fF/hPa and 0.099fF/hPa, respectively. For the latter two structures, the membrane contacts the substrate when the pressure is large enough.

Strain Effect of the Dielectric Constant in Silicon Dioxide

The effect of mechanical stress on the dielectric constant of is experimentally studied. A beam-bending method is used to extract the strain effect coefficient M₁₂. According to the measurements, the dielectric constant changes linearly with the stress. The value of is shown to be -(0.19 ± 0.01) × 10^-21 m²/V². The mechanism underlying the phenomena is discussed.

A GaAs MMIC-based dual channel microwave phase detector at X-band

A microwave phase detector at X-band with dual channels fabricated using Au surface micromachining technology is presented. The detector is composed of power dividers, power combiners, phase shifters and thermoelectric power sensors. Besides the initial phase difference, the detector introduces additional 45° phase lead and 45° phase lag in two channels. Microwave power sensors are employed to detect the output powers, and the initial phase difference can be finally calculated from anti-trigonometric function of the normalized output in -180° to +180°. The fabrication is compatible with GaAs MMIC process. The measured S-parameters show that the detector has reflection loss less than -12dB over a bandwidth of more than 66% centered at 10GHz and isolation less than -13dB, and the normalizations of measured output voltages agree well with the calculated results in three situations.

Piezocapacitive effect of a sandwich structure in a microfabricated cantilever

This paper gives insight into the origin of the piezocapacitive effect of a sandwich structure (metal-dielectric-heavily doped silicon) in a microfabricated cantilever. It implies that the mechanical stress changes both the sizes of the capacitor and the dielectric constant of the dielectric. A beam bending method was used to study the effect of the mechanical stress on the dielectric constant of Si3N4. The piezocapacitive effect can be used as a basic sensing technology in the MEMS devices. As an application, a novel flow rate sensor with a piezocapacitive sensing element is reported.

A Fully Integrated Capacitive Pressure Sensor with High Sensitivity

A fully integrated absolute capacitive pressure sensor is presented. The sensing structure consisting of poly Si/gate oxide/n well Si sandwich is a solid-state capacitor, which changes under applied pressure due to the variations of the permittivity of the dielectric, the area and distance between the electrodes. The on-chip interface circuit based on capacitance-frequency conversion is also introduced. The device was fabricated by CMOS process with some post-processing. The typical pressure response of the structure shows the sensitivity is about 43.6 fF/hPa and the nonlinearity is less than 3.3% over the range from 800 hPa to 800 hPa. The resolution of the interface circuit is about 3.2 Hz/hPa.

A piezoresistive pressure sensor with improved sensitivity in low pressure condition

In this paper, a high sensitive piezoresistive pressure sensor is designed, fabricated and characterized. The designed structure is composed of 4 diagrams separated by two crossed thick beams. There are two piezoresistors located at two ends of the beam, which are used to measure the largest stress in the structure during stressed. Four piezoresistors are connected into a Wheatstone Bridge. The crossed thick beams are covered with a 2μm thick Al layer resulting more residual stress on piezoresistors. This structure has shown improved sensor sensitivity in low pressure condition. The sensor is characterized under the pressure ranges from 5-400hPa. As a reference, a conventional flat membrane pressure sensor is also fabricated. The testing results show that the sensitivity of crossed beam structure is 32.9μV/hPa, which is 3.8 times higher than that of the conventional structure. Finally, the stress simulation results of the crossed beam membrane (CBM) and the conventional flat membrane (CFM) are given to verify the testing results.

A self-packaged self-heated thermal wind sensor with high reliability and low power consumption

A self-packaged self-heated thermal wind sensor was designed, fabricated and measured for the first time in this paper. To achieve a low power and reliable sensor, a newtype silicon-in-glass (SIG) substrate with anisotropic thermal conductivity was introduced. In this substrate, the embedded vertical silicon vias are used to realize the thermal interconnections between the sensor and the wind, while the horizontal thermal conduction between the thermistors is isolated effectively by the glass. The substrate is based on a glass reflow process and the sensor was fabricated on this substrate by using a lift-off process. The whole process only need three masks. At last, we performed a wind tunnel test in constant voltage (CV) mode, and the measurement results show that the thermal wind sensor can measure wind speeds up to 17.5 m/s, and the measured sensitivities of the sensor with different applied voltages of 0.8 V, 0.9 V, and 1 V are respectively 6.3 mV/(m/s), 9.52 mV/(m/s), and 14.17 mV/(m/s) at zero-flow point. The corresponding power consumption of the sensor with different voltages are respectively 12.3 mW, 15.57 mW and 19.23 mW. Measurement results also show that wind direction in a full range of 360° with an error less than 6° could be obtained.