Zhenxiang Yi

A capacitive power sensor based on the MEMS cantilever beam fabricated by GaAs MMIC technology

In this paper, a novel capacitive power sensor based on the microelectromechanical systems (MEMS) cantilever beam at 8?12?GHz is proposed, fabricated and tested. The presented design can not only realize a cantilever beam instead of the conventional fixed?fixed beam, but also provide fine compatibility with the GaAs monolithic microwave integrated circuit (MMIC) process. When the displacement of the cantilever beam is very small compared with the initial height of the air gap, the capacitance change between the measuring electrode and the cantilever beam has an approximately linear dependence on the incident radio frequency (RF) power. Impedance compensating technology, by modifying the slot width of the coplanar waveguide transmission line, is adopted to minimize the effect of the cantilever beam on the power sensor; its validity is verified by the simulation of high frequency structure simulator software. The power sensor has been fabricated successfully by Au surface micromachining using polyimide as the sacrificial layer on the GaAs substrate. Optimization of the design with impedance compensating technology has resulted in a measured return loss of less than??25?dB and an insertion loss of around 0.1?dB at 8?12?GHz, which shows the slight effect of the cantilever beam on the microwave performance of this power sensor. The measured capacitance change starts from 0.7 fF to 1.3 fF when the incident RF power increases from 100 to 200 mW and an approximate linear dependence has been obtained. The measured sensitivities of the sensor are about 6.16, 6.27 and 6.03 aF mW?1?at 8, 10 and 12?GHz, respectively.

Modeling of the terminating-type power sensors fabricated by GaAs MMIC process

In this paper, a two-dimensional (2D) model of the terminating-type power sensor is established under different input powers. The 2D heat transfer equation is applied to describe the temperature distribution, and Fourier series is used to obtain the solution based on the boundary conditions. In order to demonstrate the validity of the 2D model, finite-element method (FEM) simulation using ANSYS software was performed. The sensitivity from the 2D model and FEM is 0.25 mV mW−1 and 0.28 mV mW−1 respectively, while the sensitivity from the 1D model is 0.34 mV mW−1, which indicates that the presented 2D model is closer to the simulation than the 1D model. The terminating-type power sensor was designed and fabricated by MEMS technology and the GaAs MMIC process. The measured return loss is less than −26 dB for a frequency up to 10 GHz. The power measurement was performed and a good linearity of the output thermovoltage with respect to the input power is obtained. The measured sensitivity is close to 0.26 mV mW−1, 0.23 mV mW−1 and 0.16 mV mW−1 at 0.1, 1 and 10 GHz, respectively. Moreover, the frequency dependence measurement demonstrates that the measured thermovoltage decreases with increasing the frequency. The measurements demonstrate that the measured results agree with the presented 2D model for low frequency while the measured thermovoltage deviates from the expectation at high frequency. The reason is that the electromagnetic coupling loss of the coplanar waveguide and the parasitic loss of the load resistor become higher at high frequency.

Measurements on Intermodulation Distortion of Capacitive Power Sensor Based on MEMS Cantilever Beam

This letter presents an experimental study of intermodulation (IM) distortion of the capacitive power sensor based on MEMS cantilever beam for the first time. The return loss is less than -25 dB and the insertion loss is close to 0.1 dB at 8-12 GHz, indicating the good microwave performance. The input power dependence measurement implies that the third order intermodulation (IM3) increases with the input power and the slope of curve is ~ 3.39, 3.16, 2.94, and 2.93 under Δf=100, 200, 500 kHz, and 1 MHz, respectively. In addition, the frequency difference dependence measurement shows that a rapid decrease of the IM3 products is observed when Δf varies from 10 Hz to 200 kHz, while a slow decrease is observed from 200 kHz to 2 MHz. The experimental results demonstrate that the power sensor is easy to be disturbed by two signals with Δf less than 200 kHz. When Δf exceeds 200 kHz, the IM3 is small and significant IM distortion will not generate.

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 Frequency-Compensation-Type Microwave Power Sensor Fabricated by GaAs MMIC Process

In this letter, a novel microwave power sensor is proposed to accomplish frequency compensation for 1-20-GHz application. Compared with traditional power sensor, this design has two extra power compensation ports in order to adjust the output voltage. This device is designed and fabricated with GaAs MMIC process. The measured return loss of three ports is less than -26 dB over 1-20 GHz. The output voltage increases with the incident power and the sensitivity is close to 0.115, 0.111, and 0.106 mV/mW at 1, 10, and 20 GHz, respectively. Frequency compensation is performed and the output voltage is compensated to that of 10 GHz. Clearly, when the incident power is fixed, the curve after compensation is flat and the voltage does not change with the frequency of the signal anymore.

An Integrated Microwave Power and Frequency Sensor For 1–10 GHz Application

In this paper, an integrated sensor for microwave power and frequency measurement is proposed for 1-10 GHz application. The novelty is that this device cannot only measure microwave power but also can realize microwave frequency measurement at the same time. The incident power is dissipated and converted heat by two loaded resistors, and then detected indirectly by output voltage based on Seebeck effect. A microelectromechanical system (MEMS) coupler is designed over the coplanar waveguide transmission line and the coupled power varies with the frequency of the signal. Therefore, by measuring the coupled power, the frequency of the signal can be deduced. This integrated power and frequency sensor is fabricated by GaAs monolithic microwave integrated circuit process and MEMS technology. The measured return loss is about -26 dB at 1 GHz and -22 dB at 10 GHz. The power measurement indicates that the sensitivity is close to 0.116 mV/mW at 1 GHz and 0.110 mV/mW at 10 GHz. The frequency measurement is performed and the sensitivity is close to 0.035 mV/GHz under the incident power of 60 mW.