Capacitive Force Sensor Research Articles

Equivalent circuit representation of electromechanical transducers: II. Distributed-parameter systems

Distributed-parameter electromechanical transducers are examined theoretically with special regard to their dynamic electromechanical behaviour and equivalent circuits used to represent them. The circuits are developed starting from basic electromechanical transduction principles and the electrical and mechanical equations of equilibrium. Within the limits of the assumptions on boundary conditions, the theory presented is exact with no restrictions other than linearity. The link with lumped-parameter transducers, that were the subject of a previous publication will be indicated. Elementary electrostatic, electromagnetic, electrodynamic and piezoelectric transducers are used to illustrate the basic theory. Exemplary devices include a capacitive force (displacement) sensor, several types of a mechanical resonator, electromechanical filters, a piezoelectric thickness monitor and a piezoelectric acceleration sensor.

An electret microphone as a force sensor for combined scanning probe microscopy

The operation of an electret microphone as a sensitive force sensor in scanning probe microscopy is described. The behavior of the electret microphone used as a capacitive force sensor is characterized and the experimental setup for a combined STM/SFM is described. First applications of this hybrid microscope to biological objects are presented. Unlike in conventional force microscopy, the sample can be placed on the top of the force sensor. This allows for a free choice of the near-field probe. Hence, the electret microphone is well suited for combined scanning microscopies where the probe-to-sample distance is kept constant by force gradient measurement while the near-field probe measures some local properties of the specimen.

Scanning force microscopy in the dynamic mode using microfabricated capacitive sensors

We report on the first successful operation of a scanning force microscope using microfabricated capacitive force sensors. The sensors, which are made from single crystal silicon on insulator wafers, consist of a cantilever spring with integrated tip at the free end and an electrically insulated counter electrode. Dynamic force gradient sensing is the preferred operating mode. Here, tip–sample interactions are detected by letting the sensor act as a resonator in a phase controlled oscillator setup and measuring corresponding shifts of the oscillation frequency. Experiments were performed in vacuum using a standard tunneling microscope. A Cr grating on a quartz substrate served as the test sample. Topographic images showing details on a 10 nm scale were obtained operating at a constant force gradient of the order of 0.01 N/m. In addition, critical design parameters are discussed based on an analysis of the electromechanical properties of the sensors.

In situ force calibration of high force constant atomic force microscope cantilevers

A miniature capacitive force sensor which fits in the sample position of an atomic force microscope (AFM) has been used to calibrate the force applied by the scanning tip during nanohardness measurements. The sensor is simple, physically robust, and easily fabricated from readily available materials. It can be operated with commonly available electronic instrumentation. The device described here is optimized for tip-sample forces between 10−4 and 10−2 N, a range useful for micrometer-scale mechanical modification of surfaces. With minor design changes, sensors of this general type should be capable of much greater sensitivity. The sensor is calibrated outside the AFM by the application of free weights. Calculated estimates of AFM cantilever force constants are difficult for all but the simplest geometries and may rely on uncertain values for certain critical dimensions and material properties. These estimates can thereby be replaced by a simple and direct measurement.

New design of micromachined capacitive force sensor

Anisotropic etching of a (110) oriented silicon wafer has been used to realize a differential capacitive force sensor. This design is based on a simple concept allowing force to be measured in the plane of the sensor, giving a new and original product in the field of small force measurements. These sensors can be adjusted to measure force in the range 0.01 N to 10 N or, in a low rigidity version, to act as a high precision displacement sensor with an expected resolution of 20 nm. These sensors are planed to be used with a commercial capacitance measurement chip (nominal capacitance: 1 pF, resolution: 1 fF).

Design considerations for a silicon capacitive tactile cell

This paper describes the concept of a new miniature silicon capacitive force sensor. The small size of the device and its realization on a silicon wafer make it potentially useful as a cell in a tactile sensor with a readout circuit integrated near every sensing element. The basic structure of the force sensor is a polysilicon bridge which is fabricated on top of a silicon wafer using surface micromachining. Using this basic structure, two possible realizations of the force sensor are considered. The first design consists of a single bridge. In this case, the detection of a force of a few mN requires a bridge with a thickness of 4 μm. The cell has a maximum size of 300 μm × 200 μm and, theoretically, a typical sensitivity of 2–12 fF/mN and a nominal capacitance of 140 fF. The thickness can be reduced to less than 1 μm with the second design, in which several bridges are placed in a matrix. Every bridge now has to support only a part of the force that acts on the sensor. This offers special freedom in optimizing the beam dimensions because of the ability to select the number of bridges within one force sensor. Some calculations are carried out concerning a cell composed of nine bridges, each of which has a typical size of 60 μm × 60 μm. The sensitivity of a cell in the second design is 16 fF/mN, and the nominal capacitance is 300 fF.

A high-performance silicon tactile imager based on a capacitive cell

An 8 × 8-element silicon-based tactile imager fabricated using integrated-circuit process technology is described. The imager consists of an X-Y organized array of capacitive force sensors on 2-mm centers. Each transducer has a zero-pressure capacitance of 1.6 pF, an average sensitivity of 60 mV/g, and a maximum operating force of about 10 g/element. The operating force can be scaled over a wide range without changing the process or lateral array dimensions. The array is read out using a switched-capacitor charge integrator which has been shown capable of a resoiutionof about 1 fF(5 fC), giving a resolution for the imager itself (pad excluded) of more than 8 bit. The readout scheme permits off-chip electronics to be used so that the fabrication sequence requires only five noncritical masks. The array is addressed as a memory, with an access time of less than 20 µsec for 8 pixels. Scaling of the array size to other dimensions is examined and performance limitations are discussed.