Qing-An Huang

2-D Micromachined Thermal Wind Sensors—A Review

This paper presents an overview and development of 2-D micromachined thermal wind sensors. It first compares different types of wind sensors, then gives a brief introduction to three basic measurement principles of the thermal wind sensors: hot-wire or hot-film, calorimetric, and time of flight. In order to have a good understanding of development of the 2-D micromachined thermal wind sensors, they are first categorized into thermoresistive, thermoelectric, and thermoelectronic wind sensors based on their operations. Then, each category is further divided into different subcategories, according to their measuring principles and sensor configurations. In Section IV, different materials, fabrication processes, packaging techniques, and control modes of the 2-D micromachined thermal wind sensors are analyzed. This paper also evaluates the core technologies on how to improve the sensitivity, measurement range, and power consumption of the thermal wind sensors.

Enhanced performance of a CMOS interdigital capacitive humidity sensor by graphene oxide

This paper presents a CMOS interdigital capacitive humidity sensor based on graphene oxide, and the sensitivity of this humidity sensor was improved significantly by using graphene oxide as sensing material. The passivation between the Al electrodes and the sensing material was used to ensure the reliability of the capacitive-type humidity sensor. The experimental results show that, compared with the sensor using polyimide as sensing material, the sensitivity of the sensor based on graphene oxide is 23 times larger, and the response time of such a sensor is also improved greatly.

LC Passive Wireless Sensors Toward a Wireless Sensing Platform: Status, Prospects, and Challenges

Inductor-capacitor (LC) passive wireless sensors use a transformer with loose coupling between an external readout coil and an inductor that receives power through this inductive coupling. Changes in the sensor are wirelessly and remotely detected by the readout coil, which makes them highly useful in applications that require the sensor to be powered remotely and to occupy a small volume, such as harsh and sealed environments, where physical access to the sensor is difficult. Although the sensor to accomplish this function dates from the 1960s, its rapid extension over the past decades has benefited from microelectromechanical systems. This paper provides an overview of the status and challenges in the LC passive wireless sensor toward a wireless sensing platform. The basic sensing principles are first categorized into detecting changes of the sensor in response to the capacitance, resistance, inductance, or coupling distance due to the parameter of interest through monitoring the impedance magnitude and phase spectrum. The present state of the art in sensor applications for pressure, strain, temperature, humidity, biochemical, gas, and so on is then reviewed and compared. For emerging applications from many Internet of Things scenarios, geometrical constraints, such as small and non-invasive coils, reduce the magnetic coupling between the sensor and the readout coil, resulting in a limited interrogation distance. Furthermore, an increasing number of applications also require the simultaneous measurement of multiple parameters. Recent efforts to increase the interrogation distance and to extend the measurement of single parameter to multiple parameters are finally outlined.

An LC -Type Passive Wireless Humidity Sensor System With Portable Telemetry Unit

This paper presents a high-sensitivity passive wireless humidity sensor system with a portable telemetry unit for applications in sealed environments. A complementary metal oxide semiconductor (CMOS) interdigital capacitive humidity sensor die was attached to an organic substrate (FR-4) on which a fixed planar spiral copper inductor was fabricated. The variable capacitor and the fixed inductor were wire bonded to form an inductor-capacitor (LC) tank circuit. The resonant frequency of the sensor tank is dependent on the sensor capacitance, which changes in response to the humidity. The sensitivity of the capacitive sensor was improved significantly using graphene oxide as a sensing material. The package-level integration was achieved by employing the embedded inductor on an organic packaging substrate. The LC-type sensor is interrogated wirelessly using our homemade portable telemetry unit, which is based on a standing wave ratio bridge to measure the real part of the readout coil impedance. Measurements show a sensitivity of -18.75 kHz/%RH over a range of 15%-95% RH. The implemented telemetry unit addresses the need for a low-cost, portable, and universal reader of the LC-type passive wireless sensors.

Humidity sensing properties of the sensor based on graphene oxide films with different dispersion concentrations

This paper presents the humidity sensing properties of the sensor based on graphene oxide (GO) films with different dispersion concentrations. A CMOS interdigital capacitance was used as the basic structure. The relationship between the sensing properties and dispersion concentrations is studied, by dripping four GO dispersions of different concentrations on the sensing area of the sensor chip, which was then baked for 2 hours at 50 °. The measured results imply that the sensor based on GO films with proper concentration shows monotonic relationship between the sensing capacitance and relative humidity, and is of fast response and good repeatability.

An online test microstructure for thermal conductivity of surface-micromachined polysilicon thin films

An online test structure for measuring the thermal conductivity of surface micromachined polysilicon thin films is presented. In the structure, a pair of microstructures, i.e., a reference structure and a test structure, are used. The surface micromachined structures are heated electrically. Heat dissipation by convection, radiation, and heat transfer through the air gap and into the substrate is considered in an electrothermal model. The model is confirmed by ANSYS Software. In experiments, current-voltage measurements are only required, and all measurements can be carried out in free air. The surface-micromachined and p-doped polysilicon thin films with a sheet resistance 116.25 /spl Omega//sq. are measured to have a thermal conductivity 28.7 W/mK at 300 K.

Implementation of Multiparameter Monitoring by an LC -Type Passive Wireless Sensor Through Specific Winding Stacked Inductors

A traditional LC-type passive wireless sensor has been used for the measurement in sealed environments. Due to the limitation of its operation principle, this method was only suitable for a single parameter monitoring. For most applications, it is desirable for multi-parameters to be monitored. In order to keep the chip area of the sensor as small as possible, multi-level inductors may be coaxially stacked. However, the transmitting signals affect each other due to strong mutual coupling between the stacked inductors. This paper presents a novel inductor structure by using a specific winding to achieve the minimized mutual inductance. Using the partial inductance theory, the mutual inductance of two stacked inductors is analyzed. Simulations of the two stacked inductors coaxially aligned, with each connected in a variable capacitor, show that the mutual inductance between the two inductors can be greatly suppressed. This phenomenon has also been verified through multi-layer PCB inductors. And capacitive temperature and pressure sensors were linked to the two stacked inductor to implement the simultaneous measurements of temperature and pressure. The measurement results indicate that the sensitivity of the temperature sensor is about 41.67 kHz/°C between -20°C to 100°C while the sensitivity of the pressure sensor is about -133.33 kHz/kPa between 50kPa to 110 kPa.

Study on vibration behavior of doubly clamped silicon nanowires by molecular dynamics

The vibration behavior of doubly clamped silicon nanowires with square cross sections is studied by molecular dynamics method. Silicon nanowires have lengths ranging from 4.888 to 12.491nm and cross sections ranging from 1.22nm × 1.22nm to 3.39nm × 3.39 nm. The size dependence of the resonant frequency is studied in detail. The results show that the vibration behavior of Si nanowire is quite different from the macroscopic beam, and the resonant frequency is much higher than the result based on the continuum theory, but close to the theoretical result based on the semicontinuum approach. Surface reconstruction can strongly affect on vibration behavior. These results demonstrate that the classic theory may not be suitable for analysis of performances of nanostructures, and the conclusion of the study has a certain practical significance on related fields.

Study on Size-Dependent Young’s Modulus of a Silicon Nanobeam by Molecular Dynamics Simulation

Young’s modulus of a silicon nanobeam with a rectangular cross-section is studied by molecular dynamics method. Dynamic simulations are performed for doubly clamped silicon nanobeams with lengths ranging from 4.888 to 12.491 nm and cros-sections ranging from 1.22 nm × 1.22 nm to 3.39 nm × 3.39 nm. The results show that Young’s moduli of such small silicon nanobeams are much higher than the value of Young’s modulus for bulk silicon. Moreover, the resonant frequency and Young’s modulus of the Si nanobeam are strongly dependent not only on the size of the nanobeam but also on surface effects. Young’s modulus increases significantly with the decreasing of the thickness of the silicon nanobeam. This result qualitatively agrees with one of the conclusions based on a semicontinuum model, in which the surface relaxation and the surface tension were taken into consideration. The impacts of the surface reconstruction with (2 × 1) dimmers on the resonant frequency and Young’s modulus are studied in this paper too. It is shown that the surface reconstruction makes the silicon nanobeam stiffer than the one without the surface reconstruction, resulting in a higher resonant frequency and a larger Young’s modulus.

A CMOS interdigital capacitive humidity sensor with polysilicon heaters

A CMOS interdigital capacitive humidity sensor with polysilicon heaters is presented. Polyimide was used as the sensing material. In order to find out the relationship between filling effect of deposited polyimide and the spacing distance of Al electrodes, another interdigital capacitive humidity sensor with no passivation around Al electrodes was also fabricated. The measured results show that good filling effect of deposited polyimide can be realized when the gap between interdigital electrodes is twice the thickness of deposited polyimide. By using the polysilicon heaters, the hysteresis of the humidity sensor is less than 2%RH.