The Study of Thin Film Coil with Thin Film Capacitor for Wireless Power Transmission based on Impedance Matching

Abstract

Recently, many electronic devices have been miniaturized by the development of integrated semi-conductor technology because of a convenience of use. Although wire-connections and batteries are mainly used as power supplying method for these devices, these are limited by some risks of specific applications or inconvenience. To solve these problems, wireless power transmission has been actively researched with downsizing effect. In order to apply wireless power transmission for a miniaturized de-vice, it is essential to develop the micro coil as the receiving antenna structure. As the size of micro coil antenna decreases, the inductance decreases, whereas the self-resonance frequency of the coil becomes several GHz ranges. When used in a self-resonant frequency of the receiving antenna, the efficiency drops sharply by the energy loss due to radiation, and the transmission of electromagnetic radiation is harmful to the human body. Therefore, this study focused on electromagnetic resonance methods to in-crease the efficiency of micro-coils, which did not affect the human body and interfere with other elec-tronic devices. In order to reduce the energy loss by lowering the resonance frequency, an appropriate capacitor was connected in series with the coil antenna for impedance matching in MHz band. However, when using commercialized capacitors, these are not suitable for micro devices because of the large dimension of the capacitor. Therefore, micro-coil and embedded micro-capacitor are needed to solve this challenge. In this concept, first, various structures of the micro coil were analyzed to understand its electromagnetic characteristics and to optimize the efficiency, and then, thin film capacitors were em-bedded together to match the impedance as micro receiver structure. The fabrication process of the mi-cro coil and thin film capacitor are based on silicon integrated microfabrication process. Therefore, we fabricated coil and capacitor together at the same process steps. Compared with the fabrication of micro coils, there is no additional process, which is one of important merits in various aspects. The capaci-tance of the thin film capacitor was controlled by adjusting the thickness since the size of the capacitor was limited and various resonance frequency in MHz range were required for applications. The oxide film in capacitor formed by chemical vapor deposition (CVD) was shown a very low level of leakage current in driving power range, so that, it was suitable to apply to the micro wireless antenna structure. In our concept of micro, since the Q-factor and the coupling factor are increased or decreased according to the width and the spacing, the efficiency of the coils having a diameter of 500 μm was optimized when the width and the space were respectively 10 μm and 10 μm, and the coils having a diameter of 1 mm was optimized when width and a spacing were respectively 25 μm and 10 μm. However, before matching, the transmission efficiency was -45.9dB and -47.9dB when the diameters were 1mm and 500um, because the power was transmitted by inductive coupling method at distance, which is shorter than diameter of micro coil. Therefore, in order to use the magnetic resonance method, thin film capaci-tors with diameter of 494um and 543um were connected to coils with diameters of 1mm and 500um, respectively, and designed to generate resonance at desired frequency band. As a result, the micro coil embedded with thin film capacitor shows that the transmission efficiency was -27.4dB and -37.5dB when the diameters were 1mm and 500um. Therefore, the micro coil embedded with thin film capacitor has higher efficiency than before matching and can transmit the power at desired frequency band. This study has significance in showing the possibility of being a power source for the next generation flexi-ble and implantable devices. ⓒ 2017 DGIST