Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
1999-01-25
2001-11-27
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S678000, C324S686000
Reexamination Certificate
active
06323660
ABSTRACT:
The invention concerns an integrated device for capacitive measuring of nanometer distances which makes it possible to measure absolute distances being of order of magnitude of one nanometer.
The technical problem to be solved by the present invention is how to remove an influence of parasitic capacitances in a micromechanical sensor itself as well as in a pertaining electronic circuit of an integrated device for measuring nanometer distances so that an absolute measurement of such distances would be feasible.
Devices for capacitive measuring of nanometer distances comprise a micromechanical sensor and an electronic circuit producing input signals for the micromechanical sensor and processing output signals of the sensor.
Hitherto developed devices for capacitive measuring of nanometer distances are fabricated mainly by a hybrid technique. They distinguish themselves in that their micromechanical sensor comprises a movable plate of several square millimeters and in that their measuring capacitance is even from 5 pF to 30 pF (Sensors and Actuators A, 39 (1993) 209-217). With these devices good results are obtained. The movable plate in the micromechanical sensor, however, is fabricated by a technology differing from the technology for producing of the pertaining electronic circuit. This is reflected in a high price of such devices.
Recently, some devices for capacitive measuring of nanometer distances have appeared in which also the movable plate of the micromechanical sensor is integrated together with the pertaining electronic circuit. In supplementary technological steps the micromechanical sensor is fabricated above the rest of the device whereat these supplementary steps do not represent any substantial change in the technology of manufacturing the integrated circuit. Therefore these devices are considerably cheaper than the afore-described ones. The measuring capacitance of the sensor, however, is only from 0.1 pF to 1 pF. Therefore the features of these devices are not so outstanding as those of the hybrid devices, nevertheless they are good enough for an application in numerous measuring systems. With respect to the pick-up of the signals the following two embodiments are used.
Very common is an embodiment based on a differential distance measurement. A movable plate of a micromechanical sensor is fastened between two fixed plates and with them it forms two capacitors (Analog Devices ADXL50). They are connected to signal generator outputs of an opposite phase. When the movable plate is situated in the middle between the fixed plates the signals cancel each other. The sensor output is connected to a synchronized demodulator. A disadvantage of this embodiment is an exacting fabrication of the movable plate and of the two fixed plates. Moreover, the central movable plate must be insulated with respect to the substrate since it is connected to the input of the electronic circuit in the device.
Further, also an embodiment having two micromechanical sensors connected to a bridge has been known (Proc. IEEE Solid-State Sensor and Actuator Workshop, pp. 126-131, June 1992). Also in this embodiment the movable sensor plates must be insulated with respect to the substrate.
A common disadvantage of all integrated devices for capacitive measuring of nanometer distances exists in that parasitic capacitances of the sensor plates with respect to integrated circuit layers below are strongly pronounced. The sensor output signal is additionally reduced by the parasitic input capacitance of the pertaining electronic circuit.
Known integrated devices for capacitive measuring of nanometer distances do not render possible an absolute distance measurement. The influence of parasitic capacitances therein is not sufficiently reduced that by a signal at the output of the micromechanical sensor the distance between the movable plate and the sensing plate of the sensor could be determined. However, since knowing this distance is crucial for an evaluation of the sensor sensitivity, known integrated devices for capacitive measuring of nanometer distances need to be calibrated mechanically.
The said technical problem is solved by an integrated device of the invention for capacitive measuring of nanometer distances, which device is characterized in that above a well in a substrate, one below the other there are situated electrically conductive and electrically insulated plates, namely, a movable plate, a sensing plate and a plate building up the electric field and being electrically insulated from the well, and the well projects from below the sensing plate to the extent that the capacitance of the sensing plate with respect to the substrate is reduced to a minimum. The plate building up the electric field is connected to a pulsing generator and the sensing plate is connected to the input of a follower amplifier and the well is connected to the output of the follower amplifier. The movable plate is connected to the substrate. At the drain of an input transistor within the follower amplifier a potential is maintained, changing in the same way as a potential at the output of the follower amplifier.
The device of the invention is further characterized in that the plate building up the electric field is formed above a first oxide layer and the sensing plate is formed in an upper metal layer of the integrated circuit with the plate building up the electric field screening a large part of the sensing plate.
The device of the invention is further characterized in that the potential at the input of the follower amplifier within a first time slot of each cycle equals a potential of the sensing plate and within a second time slot of each cycle on the sensing plate a potential is built up which causes the sensing plate to attract the movable plate so that the latter is moved back to the sensing plate.
And finally, the device of the invention is characterized in that within the first time slot after the expiration of the clock signal a potential at a substrate of two switching transistors, whose drain and source, respectively, are connected to the sensing plate and which are provided for switching between the first time slot and the second time slot, and the potentials on the gates of the two transistors are raised parallelly with respect to potential at the output of the follower amplifier.
The most important advantage of the integrated device of the invention for capacitive measuring of nanometer distances with respect to other devices of this type exists in that it makes possible an absolute measurement of distances. By choosing an appropriate topology the parasitic capacitance of the micromechanical sensor is removed in a simple way and by an active circuit the influence of parasitic capacitances in the pertaining electronic circuit is eliminated. The device of the invention distinguishes itself also by a very simply realized attachment of the movable plate to the device substrate.
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Matsumoto et al., “Integrated Silicon Capacitive Accelerometer with PLL Servo Technique”, Sensors and Actuators, vol.. A39, No. 3, Dec. 1, 1993, pp. 209-217.
Analog Devices ADXL50, Monolithic Accelerometer With Signal Conditioning, Rev. 0, pp 1-16, Jun. 1993.
Surface Micromachined, Digitally Force-Balanced Accelerometer with Integrated CMOS Detection Circuitry, W. Yun et al., Proc. IEEE, Solid State Sensor and Actuator Workshop, pp. 126-131, Jun. 1992.
Kunc Vinko
Trontelj Janez
Austria Mikro Systeme Ing. AG
Hollington Jermele
Jacobson & Holman PLLC
Metjahic Safet
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