Photo-processable Polyimide is widely used as a thin sensitive layer (less than 4µm) for relative humidity or as a passivation layer (higher than 4µm) for power semiconductor devices (Si, SiC, GaN, etc.). In this work, an interdigitated transducer (IDT) was produced by standard photolithography on glass substrate. Each IDT consists of two interdigitated gold electrodes with filling area of 2×2 mm2 and two wires around the interdigitated electrodes, one to heat the sensor and the other to measure the local temperature. Then the polyimide material was spin coated homogeneously on top of the planar electrodes to obtain different layer thicknesses ranging from 1 to 11 µm.The electrical properties of the sensors (coated IDT with polyimide) were monitored by impedance spectroscopy using a HP4192 4192A LF impedance analysis controlled by a PC, permitting automated data collection. The advantage of our sensor is the possibility to carry out the impedance measurements at different temperatures (from RT to 350°C) by locally heating up the sensor using the integrated heater around the electrodes. The design of the realized sensor is adequate to study the sensitivity of the polyimide layer to relative humidity as well as the DC-bias and temperature effect on its dielectric properties.First, impedance measurements were performed from 1 MHz to 100 Hz to obtain the optimum frequency at which the sensor capacitance changes linearly with relative humidity. Next, capacitive measurements of the sensor were recorded at 100 kHz under different relative humidity levels. The detected signal, the capacitance, follows a linear dependence on the relative humidity once it is varied from 5 to 80%. The coating of the sensor with an optimal thickness of polyimide allows the best performance of the humidity sensor in terms of response and recovery times.Second, we studied the effect of temperature and DC bias on the change in sensor capacitance at a constant frequency of 100 kHz and under a constant flux of 250 sccm N2. The temperature and DC bias were increased from 20 to 350°C and from 0 to 35 V, respectively. Only when the sensor temperature exceeds 280°C does the DC bias begin to have an effect on the measured capacitance. The sharp increase in capacitance is due to the increase in charge carriers. This peak immediately decreases exponentially even before the bias is removed. The decrease can be referred to the loss of energy, due to the transport of charge carriers. Since electromigration is active at temperatures above 200 °C, this could explain why an effect starts to occur when sensors are heated above this threshold. The large increase in capacitance at higher temperature and higher DC current is generally reproducible in terms of shape but not in terms of magnitude. At higher temperature and under higher DC bias, a sharp increase in capacitance usually appears and then followed by an exponential decrease to a constant value even though the sensor is still under the same DC bias.Finally, we performed impedance measurements of our sensors over the frequency range 100 to 1MHz. From these measurements, we deduced the complex electric modulus, which corresponds to the dielectric relaxation processes inside a medium, if an electric field is applied and which is inversely proportional to the complex permittivity. This gives an alternative approach to analyze the electrical response of the Polyimide material and allows to study the relaxation phenomena, to study the long-range conductivities, as well as to identify the localized dielectric relaxation phenomena. The real part of the electrical modulus shows a decrease with the increase of the DC bias (from 0 to 35V) for frequencies below 30 kHz. Its value tends to zero for low frequencies and reaches a plateau for high frequencies. But not affected by the applied bias. This suggests a decrease in relaxation processes by an applied electric field, especially for low frequencies. At frequencies below a few kHz, space charge polarizations are the most dominant mechanism. In the case of a strong electric field obtained by a higher DC polarization, the relaxation process may be prohibited, resulting in a lower value of the electric modulus. It reaches a constant plateau at high frequencies, suggesting that the relaxation mechanism is mainly frequency dependent rather than DC bias dependent, which is in good agreement with the literature. The measurement of the electrical modulus suggests that the relaxation mechanisms are mainly frequency dependent, rather than dependent on the applied DC bias.
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Oct 21, 2021
Xiangyun Xiao + 7
Open Access