Epoxy Cure Monitoring With an Interdigitated Gate Electrode Field Effect Transistor (IGEFET)
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niques require the analysis to be accomplished in a well-equipped laboratory, destruction of the sample, and they are usually complex and costly to implement. However, the most serious limitation posed by these diagnostic techniques is that they cannot readily be utilized to evaluate a product's in situ epoxy cure on a real-time basis. Since epoxy resins and their curing agents are contaminated with ionic impurities, the electrical current flow through these materials consists of the conventional electrically polarized component, as well as an ionic component. It has been suggested that the ionic current conductivity will decrease as the curing process progresses because increasing polymerization will act to impede ion mobility [5]. As a result, a prominent solid-state sensor, referred to as the charge-flow transistor (CFT) or microdielectrometer, has been developed to monitor the cure of epoxy resins [6]. The monitoring of epoxy cure with the CFT is based on measuring the change in the material's complex dielectric function as it cures. Usually, the epoxy cure data is reduced to a series of plots involving the dielectric loss factor (or alternating current conductivity) and the relative dielectric permittivity. An alternative perspective developed in this paper is to interpret the electrical conductivity associated with epoxy cure by utilizing the IGEFET and analyzing the sensor's temporally-dependent response in the frequency-domain. IGEFET SENSOR THEORY As illustrated in Figure 1(a), the IGEFET sensor consists of an interdigitated electrode structure which is coupled to the gate contact of a conventional metal-oxide-semiconductor fieldeffect transistor (MOSFET). The interdigitated gate electrode structure is composed of a drivenelectrode which envelopes the entire sensor and functions as a guard ring to minimize stray surface-leakage currents. The corresponding floating-electrode component is used to establish electrical contact with the MOSFET's gate connection. Electrical isolation between the drivenand floating-electrodes (greater than 100 MQ2) is accomplished by supporting them on a thick (at least 1 micron), high-quality (resistivity greater than 1014 ohm.cm) silicon dioxide layer. The IGEFET's epoxy cure sensing capability is realized by depositing the epoxy mixture on the surface of, and between, the interdigitated electrode components. As the epoxy's molecular cross-linking reaction proceeds, its electrical impedance is perturbed. These impedance perturbations can be observed and quantified when the IGEFET's driven-electrode is excited with a voltage pulse. As a consequence of the floating-electrode being electrically connected to the MOSFET's gate terminal, charge is transferred through the curing epoxy and manifests itself as a temporally-dependent potential applied to the high-input impedance gate oxide contact of the MOSFET [6]. The judicious specification of the MOSFET's materials, geometry, and operating bias conditions establishes its amplification characteristics (principally, its gain, linearit
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