Resonators are sensitive to physical inputs such as pressure and force. The parameters of interest are measured by the shift in resonance frequency. The ease of measurement and their high sensitivity make resonators an attractive means of measuring many physical and chemical parameters.
Resonators are also used in communications systems. As the spectrum gets more crowded with the increase in wireless communications, high performance filters are needed to protect receivers from adjacent channel interference. Since the advent of the IC revolution, digital circuits in wireless systems have become more dense. Frequency control components, however, have remained relatively bulky and expensive. Many of these components are mechanical resonators. Mechanical resonance is more desirable than electrical resonance due to high Q's, low loss, and good temperature and aging stabilities of mechanical components.
Micromechanical (MEMS) resonators, which can be fabricated using the same process technologies used to manufacture IC's, are therefore an attractive replacement. Their small size and low cost make them appealing for various sensing applications, such as measuring pressure' and force.
An important issue with regards to the fabrication of resonant sensors is vacuum packaging. MEiMS are sensitive to particulate contamination and moisture. Micromachined resonant structures may be subject to excessive viscous damping when operated at ambient temperature. Sensors with a very small output signal require vacuum to minimize the effects of Brownian noise. Brownian noise arises from the bombardment of a microstructure by fluid molecules. A spurious signal will be detected due to the movement of the structure.
From a commercial point of view, it is desirable to package hermetically at the wafer level. The parallel nature of this method of packaging will result in higher throughput and lower production cost. However, to ensure the quality of the product, it is necessary to measure the pressure within each sealed chip
May 31, 2001
Oh, S. R. (2001). Microresonators as Vacuum Gauges. United States: University of California, Berkeley.