Jukka Kyynäräinen

Microelectromechanical systems in electrical metrology

Microelectromechanical systems (MEMS) will have an important role in metrology. The essential features of a MEMS are (1) a piece of single crystal silicon forming a spring; (2) metallized surfaces of silicon structures that define an electrode geometry; (3) electrostatic forces between surfaces in a vacuum. With an electrostatic drive and readout such a system will dissipate very little power. In addition, compared to semiconducting devices, microelectromechanical components are large in size, and hence a low 1/f noise level is expected. We show that a MEMS can be used, in principle, to realize both a DC and an AC voltage reference, an AC/DC converter, a DC current reference, a low frequency voltage divider, a microwave and millimeter wave detector, etc. Unfortunately, existing MEMS technologies, where uncoated silicon structures form the electrodes, cannot be used due to trapped charges in silicon dioxide or on its surface. Thus, metallization of the surface is needed. We report preliminary results of our DC voltage reference showing a relative fluctuation level below 1 /spl mu/V/V.

Stability of microelectromechanical devices for electrical metrology

Microelectromechanical systems (MEMS) have been recently proposed for realizing several references in electrical metrology. Such devices are formed from micromachined electrodes of which at least one is supported by a compliant structure such that an electrostatic force between two electrodes displaces the moving electrode. The properties of these electromechanical devices can be very stable if they are fabricated from single-crystalline silicon and sealed hermetically in a low-pressure atmosphere. In comparison to several semiconducting reference devices, micromechanical components are large in size and consume a negligible power. Thus, a low 1/f noise level is expected. The proposed MEMS electrical references include a DC and an AC voltage reference, an AC/DC converter, a low-frequency voltage divider, a microwave and millimeter-wave detector, a DC current reference, etc. Measurements on a prototype for a MEMS DC reference are discussed. The stability is presently limited by charge fluctuations on the native oxides of electrode surfaces. Preliminary results show relative fluctuations below 1 /spl mu/V/V.

Optimized design and process for making a DC voltage reference based on MEMS

A micromechanical moving plate capacitor has been designed and fabricated for use as a dc voltage reference. The reference is based on the characteristic pull-in property of a capacitive microelectromechanical system (MEMS) component. The design is optimized for stability. A new silicon-on-insulator (SOI) process has been developed to manufacture the component. We also report on improved feedback electronics and the latest measurement results.

Increasing the Dynamic Range of a Micromechanical Moving-Plate Capacitor

Large electrostatic forces on a micromechanical capacitor plate can be obtained if the capacitor is tuned by using an inductor. Such an LC circuit can be used to control the position of a micromechanical capacitor plate over a large dynamic range. The pull-in phenomenon of capacitor plates does not occur because the LC drive is intrinsically stable. The LC drive can be implemented either by sweeping the frequency or the amplitude of the driving AC voltage. In both cases relatively good linearity can be obtained. It is found that the LC drive can tolerate large parasitic capacitances. Measurements done on a dual capacitive acceleration sensor verify the calculated results. A drive AC voltage rms amplitude of 10% of the DC pull-in voltage deflected the moving plate by about 60% of the nominal gap, limited only by a mechanical stopper.

A micromechanical DC-voltage reference

Prototype of a novel DC voltage reference has been constructed. A characteristic voltage of an electrostatically actuated micromechanical silicon device provides the reference voltage. Its stability relies on elastic properties of monocrystalline silicon. The device is suited for batch fabrication and may prove more stable than Zener voltage references.

Stability of Electrostatic Actuation of MEMS

The increased electrostatic stability of MEMS sensors enables new application areas for the sensors, and decreases the manufacturing costs of existing products. Especially in the applications where the MEMS component is operated under bias voltage close to the pull-in point, the undesired instability phenomenon becomes a major source of inaccuracy. We demonstrate that biasing the sensor to the pull-in point using AC voltage is significantly more stable than the conventionally used DC voltage biasing.

Increasing the dynamic range of a micromechanical moving-plate capacitor

The LC series resonant circuit can be used to obtain large electrostatic forces at relatively low AC voltages. This makes LC derive attractive for electrostatic actuation and force feedback. It can also be used for achieving large displacements of a micro mechanical plate capacitor either by sweeping the frequency or the amplitude of the driving AC voltage. In both cases relatively good linearity can be obtained. The minimum driving voltages and maximum driving speeds are discussed. It is found that the LCR drive can tolerate relatively large parasitic capacitances. measurement done on a dual capacitive acceleration sensor verify the calculated results. A drive AC voltage rms amplitude of 10 percent of the DC pull-in voltage deflected the moving plate by about 60 percent of the nominal gap, limited only be a mechanical stopper.

A micromechanical RMS-to-DC converter

A micromechanical silicon device has been designed for use as a precise batch-fabricated AC voltage to DC voltage converter. The converter measures the difference of the electrostatic RMS forces induced by AC and DC voltages.

Wideband Microwave Power Sensor Based on MEMS Technology

An optimized transmission-type MEMS RF power sensor is presented. The design is based on microwave filter theory. It is shown that the thermal resolution of the sensor can be reduced below -50 dBm for RF bandwidths up to les 40 GHz

AC Voltage Reference Based on a Capacitive Micromechanical Component

The design and characterization of a high stability capacitive MEMS device intended for an AC voltage reference at 100 kHz or higher frequencies is presented. Preliminary results from a first prototype device show that the pull-in voltage of the device is stable to within 10 ppm over 60 hours. We discuss an optimised device design which is expected to show greater stability