# Wireless Power Transfer via Subterahertz-Wave
## 3 Research Report on Millimeter-Wave High-Power Wireless Power Transmission
### 94 GHz Rectenna  
A 94 GHz rectenna was fabricated using semiconductor microfabrication technology. Small manufacturing errors strongly affect the RF-DC conversion efficiency because the wavelength is extremely short: about 3 mm at 94 GHz. The fabrication process used for this method is roughly divisible into two procedures. The first is sputtering of aluminum as a conductor on a Si substrate. Then one must draw a specified circuit pattern using photolithography, followed by etching to leave only the circuit pattern. This process is shown in Figure 13.  
1. Aluminum is sputtered or vapor-deposited on the Si wafer (considering the skin depth). 2. Apply a photosensitive resist on it. 3. Draw the circuit pattern using the electron beam drawing apparatus. 4. Develop with TMAH (Tetramethylammonium hydroxide)
5. The resist is peeled off. The mixed acid aluminum etching solution etches the exposed Al. 6. Only the Al circuit pattern remains. The second procedure is circuit pattern mask preparation by removing a circuit pattern itself by etching together with a Si substrate. Using this mask, cover another substrate and sputter again. This method is shown in Figure 14. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline No. & DiScience & \(\bigcirc\)A & \multicolumn{3}{c}{Mex.} & \multicolumn{3}{c}{Theoretical} & \\ \cline{3-10}  & No & DiScience & \(\bigcirc\)A & \multicolumn{3}{c}{Total} & \multicolumn{3}{c}{Total} & \multicolumn{3}{c}{Mex.} & \multicolumn{3}{c}{Total} \\ \hline \(\bullet\) & 900 mm & \(\times\) & 2.30\% & 2.35\% & 100\% & 32.6\% & 15.1\% & 47.7\% \\ \(\bullet\) & 900 mm & \(\bigcirc\) & 1.07\% & 3.45\% & 83.2\% & 57.5\% & 15.1\% & 47.7\% \\ \(\bigtriangleup\) & 1350 mm & \(\bigcirc\) & 0.12\% & 3.22\% & 83.2\% & 53.7\% & 15.1\% & 47.7\% \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison of measurement results, transmission efficiency to theoretical efficiency in respective conditions: 900 mm without CA, 900 mm with CA, and 1350 mm with CA  
Figure 13: Circuit fabrication method by semiconductor microfabrication 1. 1. Aluminum is sputtered or vapor-deposited on the Si wafer (considering the skin depth). 2. Apply a photosensitive resist on it. 3. Draw the circuit pattern using the electron beam drawing apparatus. 4. Develop with TMAH. 5. The resist is peeled off. Then the mixed acid aluminum etching solution etches the exposed Al. Only the circuit pattern is exposed Si. 6. Silicon is etched by deep silicon device to complete silicon mask. 7. Cover the dielectric substrate with a silicon mask and Al sputtering or vapor-evaporation. Using the former procedure, highly accurate patterns are producible directly. Consequently, fabrication with higher precision is possible. However, it is possible to use only Si or a substrate capable of semiconductor process as the substrate. A benefit of the latter is that sputtering and vapor-evaporation can be performed on low-loss dielectric substrates other than Si. The structure of the 94 GHz antenna and rectenna is shown in Figure 15. The Electromagnetic field simulators of EMPro and ADS (Keysight Technologies Co. Ltd., Santa Rosa, CA, USA) were used for designing the antenna and rectifier circuit. The antenna and the rectifier were designed with matching impedance respectively and integrated as a rectenna. A 94 GHz rectifier was fabricated by Al vapor-deposition on a PTFE (polytetrafluoroethylene) substrate using a Si mask. The PTFE substrate (NPC-F220A, Nippon Pillar Packing Co. Ltd., Osaka, Japan) has a low dielectric constant of 2.17. The diode (MA4E1310, Macom, Lowell, MA, USA) has high breakdown voltage and low junction capacitance. Therefore this diode can exhibit high RF-DC conversion efficiency with high power. For the connection between the diode and circuit, Ag conducting paste is used. The flip chip bonder was used to bond the diode while pressing the diode against the circuit. The rectifier was fabricated in a single series. On the input side, the notch filter is inserted to reapply the second harmonic to the diode.