Miniaturized Self-Resonant Micro Coil Array with A Floating Structure for Wireless Multi-Channel Transmission

Abstract

Micro size antennas have significant merits due to the small size effect, enabling new device concepts. However, the low-quality factor (Q-factor), the large size of impedance matching components, and the poor selectivity of the multi-array design remain challenging issues. To solve these issues, a floating coil structure stacked on a loop micro-antenna is suggested. Various floating coil designs are prepared with appropriate matching conditions at specific target frequencies, using an easy fabrication process without the need for additional space. A simple one-loop antenna design shows a higher Q-factor than other, more complicated designs. The micro-sized loop antenna with the 80 µm trace width design exhibits the highest Q-factor, around 31 within 7 GHz. The 8 different floating coil designs result in high-frequency selectivity from 1 to 7 GHz. The highest selectivity contrast and WPT efficiency are above 7 and around 1%, respectively. Considering the size of the antenna, the efficiency is not low, mainly due to the good matching effect with the high Q-factor of the floating coil and the loop antenna. This micro-antenna array concept with high integration density can be applied for advanced wireless neural stimulation or in wireless pixel array concepts in flexible displays.

Keywords: high Q; impedance matching; micro antenna; wireless power transmission.

Figure 1

The wireless array system consisting of micro coils floated on loop antennas. a) Conceptual figure for the selective operation of each cell in the array system. b) Conceptual figure of the miniaturized system compared with the conventional impedance matching system. c) SEM cross‐section image of the micro‐coils floated on the loop antennas. d) SEM image of the floating coil. e) OM image of the micro‐coils floated on the loop antennas in the array system.

Figure 2

Frequency characteristics of micro spiral coils with conventional structures. a) Conceptual figure and b) DC resistances of the conventional structures of the micro spiral coils. The inductances and the real impedances of c) A loop antenna, d) Single layered structure, and e) Double‐layered structures. f) Scattering parameter S11 and g) Q‐factors according to the structures of the micro spiral coils.

Figure 3

Structure of the micro‐coil floated on the loop antenna. a) Conceptual figure of the micro‐coil floated on the loop antenna. b) Q‐factors according to the trace widths of the loop antennas. c) DC resistance comparison of loop antennas with and without the floating coil. d) H‐field and E‐field of a loop antenna without the floating coil. e) Inductance comparison of loop antennas with and without the floating coil and the mutual inductance. f) Real impedance comparison of loop antennas with and without the floating coil. g) E‐field and H‐field of a loop antenna with the floating coil. h) Matching point variations according to the fill factor of the floating coil and the trace width of the loop antenna. i) Scattering parameters S11 comparison of loop antennas with and without the floating coil.

Figure 4

Impedance matching process using structural parameters of the micro‐coils floated on the loop antennas. a) Block diagram of the impedance matching process while varying the PI gap and the trace width of the floating coil. b) Structural parameters in the micro coils floated on the loop antennas. c) Smith chart according to the variation in PI gap. d) Matching point and real impedance with the varying PI gap. e) Matching point and real impedance while varying the trace width of the floating coil. f) Equivalent circuit for the conventional impedance matching method. g) Comparison of WPT efficiency using the conventional matching method and floating coil. h) WPT efficiency according to the misalignment angle between TX and RX.

Figure 5

Frequency characteristics of the micro‐coils floated on the loop antennas in the array system. a) Inductance and b) Real impedance depending on the trace width of the floating coil. d) Crosstalk and e) Matching point variation of the floating antenna structure (floating trace width: 10 µm) when another floating antenna structure (floating trace width: 20 µm) with a similar matching frequency was close to the side. f) Variation in crosstalk and g) Matching point of the floating antenna structure (floating trace width: 10 µm) when another floating antenna structure (floating trace width: 80 µm) with a large difference in matching frequency was close to the side. h) Arrangement of the floating antenna structure in the array system. i) Crosstalk from channels 2–8 to channel 1 in the array system.

Figure 6

Selective operation of each cell in the array system using WPT with different operating frequencies. a) Scattering parameter S22 of TX. b) Scattering parameter S11 of the micro‐coils floated on the loop antennas in the RX array system. c) WPT efficiencies from TX to the micro‐coils floated on the loop antennas in the RX array system. d) Contrast comparison at each channel. e) H‐fields of the micro‐coils floated on the loop antennas in the RX array system while transmitting wireless power from the TX.