The recent advancements in communication systems, embedded systems, automotive systems, etc., have developed many advanced technologies. But, still all these systems require devices or sensors that consume low power, small size with properties like linearity, affordability, reliability, and so on. All these requirement are provided by the advanced technology, namely MEMS technology. This article gives a brief description about RF power sensor based on MEMS technology. But, primarily one should know what MEMS technology is.
MEMS technology can be defined as technology of manufacturing small devices such as mechanical elements, sensors, etc., with miniature dimensions on a substrate using micro technology. MEMS devices weights are very less and sizes range from a few micrometers to millimeters. The optical networks systems, display technologies systems, sensor systems, etc., are a few systems in which MEMS technology is being used. MEMS technology is used in sensor systems like MEMS pressure sensors, MEMS accelerometers, MEMS inertial sensors, MEMS RF power sensors, MEMS chemical sensors, and so on.
RF Power Sensor Based on MEMS Technology
MEMS technology based RF power sensor works with new sensing principle that is proposed to measure the RF signal power. The attractive electrostatic force between the signal line and free standing electrode which is at a small distance from the signal line is sensed using this sensing principle.
The power sensor working on this principle exhibits some interesting properties that include, it is possible to operate the same sensor from low frequency AC through to microwaves as a true wideband device extending the frequency range of available instrumentation. Moreover, this power sensor doesn’t absorb the signal. As an alternative a through sensor is realized practically without absorbing the signal (almost).
The existing technologies for measuring the RF signals are mostly terminating devices with which the power is lost after measuring. This RF power sensor is not sensitive to the changes in ambient temperature, but care must be taken while designing the RF sensor to avoid self-heating due to dissipation of RF signal.
Principle of Operation of RF Power Sensor based on MEMS Technology
The RF power sensor works on the principle of attraction force between the voltage controlled capacitor’s two electrodes. A parallel plate capacitor with one fixed plate and one movable plate is shown in the figure. If voltage is applied to these plates, then the movable plate is attracted towards the fixed plate with electrostatic force. This consequence is very well-known and is used in several micro sensors and microo actuators.
This electrostatic force can be given as
The relation between voltage and charge is given by capacitance and is expressed as follows:
If AC voltage is applied between the capacitor electrodes, then this results in a time dependent, attractive and electrostatic force. But, voltage given in the equation is replaced by RMS voltage amplitude and only the average force is significant for all the frequencies which are above the mechanical resonance frequency. This AC signal (square of) RMS voltage amplitude can be obtained by measuring the displacement of the movable plate of capacitor or by measuring the force required to keep the movable plate in its original initial position. This effect has been used for realizing micro-mechanical RMS to DC converters. This can also be used for measuring power levels at very high frequencies (in the range of GHz) on transmission lines by replacing one of the capacitor electrodes with RF transmission line.
Basic Structure of Sensor Based on MEMS Technology
The basic structure of capacitive sensor with and without sensor structure is shown in the figure. If we consider an ideal transmission line, then the relation between inductance and capacitance is fixed and characteristic impedance is constant. Thus, the force of attraction due to the electric field is balanced by the force of repulsion due to magnetic fields. Hence, the RF energy transported is equally stored in the electric field and in a magnetic field as there is no force exists between conductors.
If we introduce a capacitive discontinuity in the transmission line, then a region is created in which magnetic fields are neglected compared to electric fields. If this capacitor has a movable plate, then the RF power can be sensed by detecting the deflection.
Fabrication of RF Power Sensor based on MEMS Technology
The overview of process of fabrication of RF power sensor based on MEMS technology is shown in the figure. Fabrication of RF power sensor based on MEMS technology involves the basic integrated circuit fabrication methods and also the process of removing silicon or other layers selectively called as micro machining process. The different fabrication technologies used in MEMS technology are fabricated using bulk micromachining, surface micromachining, and LIGA technique.
AF45 substrate having excellent RF properties is used for fabrication of RF power sensor based on MEMS technology and this fabrication can be done using aluminum surface micromachining fabrication method.
RF Power Sensing
RF power sensing based on MEMS technology uses RF sensor that delivers the signal (which is to be detected) for realizing the RF power detection measurements with the bridge sensors. A cable and RF probe are used for introducing the signal into the bridge sensor. Then, this signal passes through the power sensor and a second RF probe that is connected to another cable. Thus, the signal is absorbed by MEMS technology based RF power sensor that verifies the presence of the RF signal at the bridge sensor.
Power sensor based RF MEMS technology is based on detecting the electrostatic force between the RF signal line and a suspended membrane. The advanced technology has developed the ‘through sensors’ that do not absorb the signal. This detection of power can be realized with the capacitive measurement of movement of a grounded aluminum membrane that is suspended above transmission line which is carrying the RF signal. Hence, it is termed as capacitive MEMS technology based RF power sensor.
Furthermore, additional information regarding RF power sensor based on MEMS technology and its applications, MEMS sensors, and MEMS accelerometers can be provided based on your queries posted in the comments section below.