## 4 Flat-Panel Beam-Steering Antennas with Low-Profile Phase Shifters with Fast Response
### Fast Tuning Low-Profile Planar Delay Line Phase Shifter
As expected, the decay time for this modified loaded-line topology filled with GT7 is much faster than that with TUD-566. It is in the range of 10 to 35 ms for \(s=2\) to 4 \(\upmu\)m. Note that the decay time given on the top axis is slightly different to the switch-off response time derived from the parallel capacitor cell in Figure 8, because the effective LC layer height \(h_{LC}\) is a bit larger than \(s\). For a wider width \(w\), the FoM increases, but the line impedances decrease. Analogous to the analysis with the IMSL in Figure 18, the FoM is always better for the same design parameter at 60 GHz than for at 30 GHz. As already mentioned, important requirements for phase shifters implemented in electrically steerable planar phased array antennas is the provision of a 360\({}^{\circ}\) differential phase shift and an overall size not exceeding \(\lambda_{0}\)/2 \(\times\)\(\lambda_{0}\)/2 to fit behind a single antenna element. Even the length \(\ell\) of a 360\({}^{\circ}\)-phase shifter exceeds \(\lambda_{0}\)/2, it can be meandered or spiraled within this squared area. However, each bend of a line could cause additional losses. Therefore, a straight delay line phase shifter is always a preferred solution. This might be achieved by utilizing (1) phase shifters with less differential phase shift, i.e., physical length, or (2) by using the slow-wave effect as above, by which the phase velocity of the line can be reduced to miniaturize it by factor of two to five. First, a 90\({}^{\circ}\)-phase shifter with reduced length compared to a 360\({}^{\circ}\)-phase shifter was used in combination with a MEMS switch at the end, as shown in Figure 28, which means a 0\({}^{\circ}\) or 180\({}^{\circ}\) phase shift of the reflected wave for OPEN or SHORT of the MEMS switch. Hence, this reflection-type phase  
Figure 23.3 Plots for designing CPW loaded-line structure given in the upper right picture. The varactors are filled with second-generation LC TUD-566 (**upper left**) and third-generation LC GT7-29001 (**below**) for 30 GHz (**left**) and 60 GHz (**right**). As the top and bottom layer, use is made of fused silica with height \(h_{s}=300\)\(\upmu\)m, relative permittivity of \(e_{rs}=3.82\), and a dielectric loss factor of \(\tan\delta_{s}=5\cdot\)10\({}^{-5}\). The electrical conductivity of the metal parts is set to \(o=4\cdot\)10\({}^{7}\) S/m. A pre-orientation of the LC in the propagation direction was chosen. These plots exhibit curves of constant phase shifter figure-of-merit (FoM, full red lines), orientation effectivity \(\tau_{\varphi}\) (dashed blue lines), and line impedance \(Z_{Line}\) (dotted gray lines), depending on the strip line width \(w\) (vertical axis) and the distance \(s\approx h_{LC}\) (lower horizontal axis) between the metallic conductors and the decay time \(T_{10}^{90}\) (upper horizontal axis). shifter requires a 90\({}^{\circ}\) tunable delay line only to tune the overall phase shift continuously between 0\({}^{\circ}\) and 360\({}^{\circ}\). In order to reduce the response time at the same time, this reflection-type phase shifter uses the loaded-line concept in Figure 24 with an LC layer height in between 4.5 and 5 \(\upmu\)m.  
This concept was proposed for the first time in [267; 268] and realized in the W-band. The CPW line was processed on a 300 \(\upmu\)m thick fused silica substrate, where the ground planes are made of a 5 \(\upmu\)m thick gold layer, while the signal line is made of a 600 nm thick multi-metal Ti-TiN-Al-Ti-TiN layer with a 150 nm thick gold layer on top, creating a floating metal bridge. To realize the moveable bridge, and thus to form the thin LC cavity, photoresist was deposited above and around this multi-metal gold layer, before the gold parts such as the bridge, the pillars, or the grounds have been processed by using an electroplating process. Finally, the photoresist has been removed by using oxygen plasma. The LC has been filled under the MEMS bridges by using a micro-injector, which was kept in due to the capillary forces.