Ultra-compact dual-band smart NEMS magnetoelectric antennas for simultaneous wireless energy harvesting and magnetic field sensing

Ultra-compact wireless implantable medical devices are in great demand for healthcare applications, in particular for neural recording and stimulation. Current implantable technologies based on miniaturized micro-coils suffer from low wireless power transfer efficiency (PTE) and are not always compliant with the specific absorption rate imposed by the Federal Communications Commission. Moreover, current implantable devices are reliant on differential recording of voltage or current across space and require direct contact between electrode and tissue. Here, we show an ultra-compact dual-band smart nanoelectromechanical systems magnetoelectric (ME) antenna with a size of 250 × 174 µm2 that can efficiently perform wireless energy harvesting and sense ultra-small magnetic fields. The proposed ME antenna has a wireless PTE 1–2 orders of magnitude higher than any other reported miniaturized micro-coil, allowing the wireless IMDs to be compliant with the SAR limit. Furthermore, the antenna’s magnetic field detectivity of 300–500 pT allows the IMDs to record neural magnetic fields.

(ICP) etching using Cl2 based chemistry to define the shape of the resonant nano-plate. Next, a 23 100 nm thick gold (Au) film was evaporated and patterned by lift-off to form the top ground. 24 Finally, 500 nm thick FeGaB/Al2O3 multilayer layer was deposited by a magnetron sputtering and 25 patterned by lift-off. A 100 Oe in-situ magnetic field bias was applied during the magnetic 26 deposition along the width direction of the device to pre-orient the magnetic domains. Then, the 27 structure was released by XeF2 isotropic etching of the Silicon substrate. The silicon underneath 28 the rectangular resonators is completely etched and there is no leftover. Fig. S2f shows the device 29 layout of the ME FBAR antenna with the detailed dimensions. 30 The magnetic multilayer with the structure of [FeGaB (45 nm)/Al2O3 (5 nm)] ×10 was sputter-31 deposited on AlN thin film with a Ta (5nm) seed layer at the Ar atmosphere of 3 mTorr with a 32 background pressure of <1×10 -7 Torr. The Ta seed layer promoted the FeGaB thin film growth 33 exhibiting narrow resonance linewidth and close-to-bulk magnetic moment. The FeGaB layer was co-sputtered from FeGa (DC sputtering) and B (RF sputtering) targets. The Al2O3 layer was 35 deposited by RF sputtering using an Al2O3 target. The deposition rates are calibrated by X-ray 36 reflectivity. 37 The reason we have used FeGaB/Al2O3 multilayers (discussed in Appendix B, fabrication of ME 38 antenna) is that they demonstrate eddy-current loss reduction, lower out of plane anisotropy, and 39 enhance permeability in comparison with a single FeGaB layer with the same thickness. Therefore, 40 we are not worried about the eddy-current loss. We have been extensively researching ME 41 antennas and ME sensors optimization since 2015, and we are one of the leading teams on this 42 effort. As for the electrode, we have tried three different approaches: (1) Using FeGaB film as an 43 electrode; (2) Using gold between FeGaB and AlN for better conductivity; (3) Using gold as an 44 electrode on top of the FeGaB for better conductivity. The results always showed better 45 performance, sensitivity, and quality factors using FeGaB directly as the electrode. Therefore, in 46 this project, we used FeGaB directly instead of using other metal on top or in-between. 47

C-Misalignment and Rotation of the ME Antenna in other Directions (for Energy 48
Harvesting Tests) 49  shown and discussed in Section 2. In this appendix, we will discuss the magnetic flux density 52 distribution in several other orientations. Fig. S3a shows the spherical coordinate system used for 53 demonstrating different orientations, where the vector ҧ shows the direction towards which ME 54 antenna is sensitive. and are the angles between ҧ vector and the and axes, respectively. 55

D-Magnetic Field and Eddy Current Distribution Inside the Tissue and Air Mediums 61
As discussed in Section 2 of the article, the magnetic field distribution is different in air and 62 tissue mediums. According to simulation results, eddy current loops generated inside the tissue 63 impact the magnetic field distribution. These eddy current loops are generated by the initial 64 magnetic flux density generated in the Tx coil, which are flowing towards the z-axis. Fig. S4a and 65 S4b show the magnetic flux density on two different planes of air and tissue mediums, respectively. 66 The vertical planes are the same cross-section planes shown in Fig. S4c and S4d. They show that 67 the magnetic field distribution is normal in air but distorted in tissue medium. There are two 68 vortexes in tissue medium, which are due to eddy current loops created in the meninges layer 69 because of its higher conductivity. The horizontal planes show the magnetic flux density above 70 the coil at the interface of meninges and grey matter layers, where the mentioned vortexes are 71 created. Fig. S4c and S4d, horizontal planes, show the current density ( ⃗ ⃗ + , the first and second 72 terms are displacement current density and conduction current density, respectively) on the 73 interface of meninges and grey matter layers in air and tissue medium, respectively. The magnetic 74 flux Bz, shown in Fig. S4b, creates eddy current loops rotating around the B-fields in the tissue. 75 These eddy current loops in turn generate the magnetic field loops on the vertical plane which lead 76 to the vortexes shown with black-loops. There are no generated eddy current loops in the air, and 77 therefore the Tx coil field distribution is normal and not distorted. 78 The diagram of experimental setup for magnetic sensing using ME antenna is shown in Fig. S6. 96

E-Energy Harvesting and Magnetic Field Sensing Experimental Setup 81
The zoomed-in part on the right shows the device under test and its orientation with respect to the 97 Figure S6. Experimental setup used for magnetic sensing using an ME antenna, where the zoomed-in part on the right shows the device under test and its orientation with respect to external magnetic field. It is notable that this is the same direction as during the energy harvesting tests.

F-Transmitter Coil Design 105
A single turn TX coil on an FR4 PCB is designed to investigate the energy harvesting performance 106 and efficiency of ME antennas. The Sonnet simulation toolbox was used to optimize the Tx coil 107 in terms of size, Q-factor, self-resonance frequency (SRF), and trace width. The optimized and 108 fabricated Tx coil, shown in Fig. S7a, has a self-resonance frequency (SRF) of 4.1 GHz, Q-factor 109 and inductance of 110 and 10.49 nH (both @ 2.51 GHz and in air), length of 10 mm, and trace 110 width of 3 mm. An L-match capacitive network, also displayed in Fig. S7a, is used to match the 111 Tx coil at 2.51 GHz for maximization of the transmitted power. An SMP connector is soldered to 112 the PCB to connect the Tx coil to the VNA for impedance matching and characterization. Fig. S7b  113 shows the reflection coefficient (S11) of the TX coil matched at 2.51GHz, which is the FBAR 114 resonance frequency of the smart ME antenna as seen in Fig. 2b. Note that S11 in Fig. S7b was  115 measured when the Tx coil was surrounded by air. When the Tx coil is placed close to the skin or 116 tissue, the resonance frequency of the matched Tx coil would shift by 100-300 MHz compared to 117 the air medium; therefore, one has to re-match the Tx coil when tissue is present. For this reason, 118 Figure S7. (a) Diagram of the PCB traces of the Tx coil used in the energy harvesting experiment, and schematic of the L-matching network for impedance matching of the coil; (b) measured S11 of the Tx coil matched at 2.51 GHz, which is the FBAR resonance frequency of the smart ME antenna.

G-Electromagnetic FEM Simulations of Brain Tissue Layers 123
Electromagnetic simulations of brain tissue layers were performed using COMSOL Multiphysics 124 v5.4 software under AC/DC module [Magnetic and Electric fields (mef) physics]. A 10 mA at 2.51 125 GHz was applied to the Tx coil and the Maxwell equations were solved in the air and tissue layers. 126 The electric and magnetic field distributions in the tissue are within the reactive region given the 127 small distances from the Tx coil, and not in the radiation region. The electromagnetic properties 128 (At 2.51 GHz) and thickness of the brain layers are shown in Table S1 synthesize a local oscillator (LO) using the ME antenna operating at its contour mode resonator 162 (CMR) resonance frequency, and then to use this LO to apply a sinusoidal-like excitation current 163 at the ME antenna. This produces an amplitude-modulated output when there is a change in the 164 magnetic field due to neuronal activity. As visualized in Fig. S9, the device with the ME antenna 165 will require circuitry for energy harvesting at the FBAR resonance frequency, as well as a driver 166 circuit that generates a carrier signal at the CMR resonance frequency for transmission of the 167 sensed information. The magnetic modulation-based capability to transmit sensed data from the 168 ME antenna with the described approach has been assessed by simulating an ME antenna model 169 with the driver transistor stage (M1), for which our results are published in [28]. 170 With the above-mentioned transmission of the sensed information, the required external receiver 171 can perform standard operations for processing the amplitude-modulated signal generated in [28]. 172 The magnetically amplitude-modulated received signal has to be amplified and down-converted 173 by the external RF receiver front-end, and further processed with digital filtering to extract the 174 signal components that carries the sensed magnetic field information. For the first implementation 175 of these operations, we suggest the use of Universal Software Radio Peripheral (USRP) for 176 software-defined radio processing of the amplitude-modulated signal. 177 178 Using the proposed IC design we can also align the external Tx coil to the implanted device in 179 order to get the highest wireless power transfer efficiency. The device generates the largest power 180 Figure S9. Conceptual diagram of an ME antenna integrated with a chip for magnetic field sensing and wireless energy harvesting.