Numerical modeling and experimental investigation of resonance properties of microwave capacitors

Abstract

Investigation of the resonance properties of capacitors is very w x important for microwave applications 1 . As the operation frequency increases, the current flow in the interior of the capacitor is modified in a manner such that inductive effects are induced. In fact, at high frequencies, the capacitors behave like electromagnetic resonators, with undesirable resonances that degrade their performance. With the trend toward increasingly faster integrated circuits, it becomes necessary to search for better geometrical designs for capacitors in these circuits to push the resonant frequencies to higher values. Up to now, resonance processes in capacitors have not been analyzed using the advanced electromagnetic simulation techniques. Typically, capacitor designs are based on experience and approximating formulas that relate the geometry and material properties of a component to its characteristic parameters, such as capacitance, inductance, and resisw x tance 2 . Such design approaches leave much to be desired in terms of accurately predicting the high-frequency characteristics of the capacitors. It is desirable, therefore, to analyze them using the more reliable electromagnetic simulation techniques.The objective of this paper is to study resonance processes in single-layer planar capacitors and capacitor chips by using a powerful and general-purpose field solver, which is based Ž . on the finite-difference time-domain FDTD method. We compute the standing-wave patterns and determine associated resonance frequencies using the FDTD, and validate the results by comparing them with measured S spectra, which 11 confirm that the resonance absorption occurs at the frequencies predicted by the standing-wave simulations. Our simulation approach is useful not only for understanding the resonance processes in capacitors, but also for finding optimum design geometries that provide improved performance at high frequencies. SIMULATION PROCEDUREWe began by simulating the side-fed planar capacitor strucw Ž .x tures see Fig. 1 a described in more detail in Section 3, which were also fabricated and measured experimentally. The excitation for the FDTD simulation was a Gaussian-shaped pulse, with a bandwidth of 15 GHz, which was modulated by a sinusoidal wave of 7.5 GHz. The structures were excited by Ž .Ž . Figure 1 a Geometry of a planar capacitor. b Geometry of a capacitor chip applying the electric-field pulse between the microstrip feed-Ž . line and the ground plane. Perfectly matched layers PMLs were employed at all truncation boundaries of the computational domains, except at the bottom surfaces, which were perfectly conducting ground planes.In addition, we have simulated single-layer capacitor chips w Ž .x Fig. 1 b , with the same dimensions and dielectric material properties as those of the planar capacitors. The simulated capacitor chips comprise two metal plates, with a dielectric layer inserted in between. They are surrounded by air, and have two feedlines attached to the two metal plates from different sides. No ground p...