This review paper focuses on interdigital electrodes-a geometric structure encountered in a wide variety of sensor and transducer designs. Physical and chemical principles behind the operation of these devices vary so much across different fields of science and technology that the common features present in all devices are often overlooked. This paper attempts to bring under one umbrella capacitive, inductive, dielectric, piezoacoustic, chemical, biological, and microelectromechanical interdigital sensors and transducers. The paper also provides historical perspective, discusses fabrication techniques, modeling of sensor parameters, application examples, and directions of future research.

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Interdigital sensors and transducers

All dielectric measurements involve the determination of the electrical polarization and conduction properties of a sample subjected to a time-varying electric field. Section 2 addresses dielectric measurement methods, the various instruments and electrodes, and their calibrations. Section 3 examines the microscopic mechanisms giving rise in the observed microscopic dielectric properties, and Section 4 explores in detail the effects of temperature and cure on these properties. Finally, Section 5 contains a selected bibliography of applications of dielectric analysis to the study of thermoset cure.

Dielectric Analysis of Thermoset Cure.

A modular materials characterization apparatus includes a sensor array (10) disposed on a substrate (16), with a standardized array and contact pad (14) format; electronic test and measurement apparatus (54) for sending electrical signals to and receiving electrical signals from the sensor array (10); an apparatus for making electrical contact (50) to the sensors in the standardized array format; an apparatus for routing signals (129) between one or more selected sensors and the electronic test and measurement apparatus and a computer (52) with a computer program recorded therein for controlling the operation of the apparatus. The sensor array (10) is preferably arranged in a standardized format used in combinatorial chemistry applications for rapid deposition of sample materials on the sensor array. An interconnection apparatus (40) and sensor array (10) and contact pad (32) allow measurement of many different material properties by using substrates carrying different sensor types.

Sensor array-based system and method for rapid materials characterization

This paper presents general rules and principles for designing multichannel fringing electric field (FEF) sensors. A detailed analysis on how the design parameters, especially sensor geometry, affect the performance of FEF sensors is provided. Tradeoffs among different design objectives are explained, and qualitative design rules for balancing these tradeoffs are presented. The rules are illustrated with the design examples of two concentric FEF sensors. The effects of shielding electrode width and substrate thickness on sensor performance were evaluated through finite element simulations. In addition, the performance of the two sensors was compared based on the numerical calculations of penetration depth and signal strength. The comparison proves that the addition of shielding electrodes can improve the penetration depth of FEF sensors.

Design principles for multichannel fringing electric field sensors

A sensor-array based materials characterization apparatus for characterizing one or more material properties of at least five test samples, wherein the apparatus includes a sensor array disposed on a substrate; electronic test circuitry for sending electrical signals to and receiving electrical signals from the sensor array; electronic circuitry for routing signals between selected sensors and the electronic test circuitry; and a computer or processor electrically coupled to electronic test circuitry and electronic circuitry.

Sensor array for rapid materials characterization

An impedance measuring apparatus having a measuring transistor with its gate electrode adapted to form a two electrode, interdigitated capacitor with the material to be measured forming the dielectric, a second reference transistor connected in differential configuration to the measuring transistor so that their drain currents are constrained to be equal, a time-varying voltage generator connected to one electrode of the interdigitated capacitor and a gain-phase meter connected to the gate of the reference transistor.

Methods and apparatus for microdielectrometry

: Integrated circuit technology has been used to develop a miniature dielectric cure monitor probe that combines small size with built in amplification to achieve good sensitivity down to 1 Hz, making the probe more sensitive to physical property changes than conventional dielectrometers operating at 1000 Hz. This paper describes the design and operation of the microdielectrometer chip, and the model used to determine the complex dielectric constant of the material under study. Two typical applications are illustrated by experiments. First, results of a postcure study of a Versamide 140, Epon 828 mixture are presented and compared to the results of a similar study using a conventional parallel plate capacitor. In a second experiment, several probes were placed in a 100 ml mold and used to monitor the cure of a sample of the same resin system. The data show the nonuniformity of cure within the sample, and can be explained on the basis of gelation and the temperature dependence of the dielectric properties as determined in the post-cure experiment. (Author)

Steven L. Garverick

Papers

Methods and apparatus for microdielectrometry

Microdielectrometry: A New Method for In Situ Cure Monitoring.

Monolithic integrated circuit implementations of the charge-flow transistor oscillator moisture sensor