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MEMS-Based Capacitive RF Power Sensor

June 1, 2006 By: Peter Adrian

This content is excerpted from Sensor Technology Alert and Newsletter, a sensor intelligence service published by the Technical Insights unit of Frost & Sullivan.

Sensor Technology Alert

Micro-electro mechanical systems (MEMS) can integrate electronics, mechanical elements, actuators, and sensors on a common silicon substrate through microfabrication technology. The electronics are fabricated by integrated circuit (IC) process sequences (for example, complementary metal oxide semiconductor [CMOS], BiCMOS, and bipolar processes); and the micromechanical components are fabricated by compatible "micromachining" processes--patterning through lithography techniques followed by etching where parts of the silicon wafer are selectively etched away or new structural layers are added to form mechanical and electromechanical devices. Although they are fabricated by batch micromachining and planar processing or surface micromachining techniques, considerably similar to those used in IC design, they differ significantly from ICs as they incorporate miniature mechanical components, such as gears, cantilevers, and diaphragms. Combined with microelectronics of this kind, signals are generated by the moving structures to give perception and control capabilities, thus creating a new generation of sensors.

MEMS are an outcome of the silicon processing industry whose work is based solely upon the mechanical properties of silicon, which is only a third of the weight of steel but much stronger than steel. Silicon has significant advantages because of its material properties. When silicon is flexed in single crystal form, no hysteresis is observed and hence there is almost no energy dissipation. Silicon also suffers very little fatigue, making it highly suitable for repeatable motion with service lifetimes exceeding billions and trillions of cycles.

L. Fernandez and a team of researchers at the University of Twente, Netherlands, have designed a MEMS-based RF power sensor. Modern communication systems require devices with low power consumption, low weight, volume and a high level of integration with electronics. Most methods for measuring the power of RF signals is based on terminating devices, such as bolometers, thermister mounts and microcalorimeters, where the power is lost after the measurement. Moreover, such devices can have a limited dynamic range, take long settling times and are highly sensitive to ambient temperature changes. Even diode sensors can suffer temperature sensitivities and can generate impendence matching problems. The MEMS-based RF power sensor has an important advantage that it can be operated from low-frequency AC through to microwaves as a true wideband device. Furthermore, the sensor has very small losses and is basically a 'through' sensor, meaning it can be used in the middle of the measurement chain.

The basic operation of the RF power sensor is based on the attractive force between the two electrodes of a voltage-controlled capacitor. Hence, in principle, the power sensor is based on sensing the attractive electrostatic force between the signal line and a free standing electrode suspended at a small distance from the signal line. "A power sensor based on this principle has very interesting properties, as mentioned above," Fernandez told Sensor Technology.

The sensor consists of a movable metal plate suspended above a coplanar waveguide (CPW) carrying the RF signal, comparable to a parallel plate capacitor with one fixed and one movable plate. The voltage difference between the plate and the signal line of the CPW results in a electrostatic force that deflects the plate to a degree proportional to the square of the rms value of the RF signal. Capacitance changes between the plate and the sense electrodes below it give the deflection value. A linear relation between the power level of the signal and the deflection of the plate is observed. Hence, the wideband 100 kHz to 4 GHz power sensors work based on RFMEMS technology. The dynamic range is in principle comparable to diode sensors; however, in practice it will be limited by the accuracy at which the membrane displacements can be detected.

The sensor is fabricated by aluminum surface micromachining on AF45 glass wafers. To improve the reflection and transmission coefficients, the waveguide is slightly adjusted just before and after the sensor capacitance. "An optimized bandwidth design resulted in return loss lower than minus 28 dB, an insertion loss lower than 0.15 dB at 4 GHz and a sensitivity of the order of 90 aF mW-1. For the realized devices, a resolution in the order of 0.5 mW was obtained, which is far from the theoretical noise limit," Fernandez added.

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