A Study of Liquid Crystal-based Phase Shifter in the Millimeter-wave Range for Utilizing Current Mass-production Technology of Display Industry

Abstract

Liquid crystals (LCs) are increasingly attracting attention in the radio-frequency (RF) technology. It is because of the several unique characteristics of LCs. Liquid crystals have features that decrease loss characteristics in high-frequency bands (milli-meter wave range), unlike other conventional RF devices. In addition, continuously tuning is possible, and a less bulky antenna structure can be implemented. Most importantly, the characteristic of liquid crystal is that it has a price advantage over conventional RF devices thanks to mass production technology in the LCD industry that has been developed for decades. However, to take advantage of these price advantages, it is necessary to apply mass production lines of modern LCD industries. The contemporary LC filling process (e.g., one drop filling, ODF) of an LCD mass-production line is specifically designed to produce LC cells with several microns (<10 μm). It is difficult for current equipment to manufacture devices with cell gap larger than 100 μm. Therefore, this work is mainly focus on the reducing the cell gap. ii In this work, I proposed and analyzed several new structures of LC-based coplanar waveguide (CPW) phase shifter. At first, I proposed an LC-based floating electrode (FE)-free CPW phase shifter with an additional LC layer on the signal electrode of the conventional model. This proposed model is operated as in-plane-switching mode of the LCD. I simulated the performance variations of the phase shifter in terms of Figure-of-merit (FoM), characteristic impedance, and driving voltage while sweeping the additional LC layer thickness up to 300 μm with each electrode condition at 28 GHz. In the case of electrode thickness variation, the improved FoM was shown to increase the electrode thickness regardless of the existence of the additional LC layer. However, in the case of the signal electrode width variation, I obtained an opposite FoM tendency depending on the presence of an additional LC layer. By this work, an efficient LC-based FE-free CPW phase shifter design can be possible for a given LC layer and electrode conditions. And the following work is the study about reducing the cell gap of Grounded CPW (GCPW) structure by using defected ground structure (DGS). By adding defects on the ground plane of the conventional GCPW model, I could reduce the shunt capacitance component in the transmission line under the same cell gap condition, reducing the characteristic impedance. In this paper, studies have been conducted on how far the DGS structure can lower the cell gap, mainly in the drive of liquid crystals for these DGS structures. And I considered what points should be considered more in order for this DGS structure to be used in liquid crystal phase shifters (LCPS).