A Fully-Integrated RF LC Transponder Platform for Implantable Wireless Sensor Applications Sung-Hoon Cho
Jeong-Bong Lee
Department of Electrical Engineering, The University of Texas at Dallas, Richardson, Texas, U.S.A Email:
[email protected]
Department of Electrical Engineering, The University of Texas at Dallas, Richardson, Texas, U.S.A
Abstract—In this work, we report a SU-8-based fully integrated miniaturized inductively powered LC transponder for generic implantable wireless sensor applications. It consists of a 1 mm diameter octagonal spiral inductor and a micro fabricated MIM (metal insulator metal) capacitor. Polyvinylidene FluorideTrifluoroethylene (PVDF-TrFE) copolymer is applied as a dielectric material for the capacitor fabrication due to its high dielectric constant. The 1 mm diameter, 154 nH spiral inductor is built on top of the capacitor. The capacitor and the inductor are in parallel connection through SU8 via holes. SU8 is used as a packaging material due to its biocompatibility, and also it serves as an insulator between the capacitor and the spiral inductor. The operating frequencies of the LC tanks are decided by the sizes of the capacitors (45 × 45, 55 × 55, 95 × 95 and 100 × 100 µm), and measured operating frequency range is from 385 to 485 MHz. The fabricated LC tanks are held to the power transmitting coil coaxially at distances of 2, 5, 7 and 10 mm, and rectified induced voltage at the LC tank is 8.5 V with 29 dBm input power at a 5 mm distance.
I.
INTRODUCTION
In the past few decades, considerable scientific and technological works have been done in the area of wireless powering. It provides methods for power delivery from a power source to an electrical load without batteries and interconnecting wires. Wireless powering is becoming extensively used in RFID (radio frequency identification) systems, wireless sensors and access cards. The wireless power transmission can be a suitable method for instantaneous or continuous power transfer to the body and other toxic and hazardous environments that are inaccessible. Recently, inductively coupled wireless power delivery has been studied as one of the promising solutions to provide power to implantable medical devices [1], [2] because it reduces the risk of infection, injury and malfunction caused by wires passing through tissue and skin. As the size of the implantable devices becomes smaller, it is desired to have
compactly integrated devices, and MEMS (microelectromechanical system) technology is a logical choice to open a new pathway to realize compact implantable devices in sub-millimeter scale. PVDF-TrFE (polyvinylidenefluoride-trifluoroethylene) copolymer, which is well known ferroelectric material, was applied as a dielectric material of the capacitor. Recent studies have found that its dielectric constant can be up to over 50 [3]. Also, it is easily applicable to conventional MEMS surface micromachining technology due to ease of fabrication in thin film form by spin coating process at low temperature. Thin film planar form inductors, mostly in spiral configurations, have attracted steady attention due to relative ease of design, fabrication and integration to the conventional IC chip, and have played a very important role in the development of on-chip inductors. This type of inductor also has been used as RF antennas for inductive powering and data communications with implantable biosensors [4], [5]. However, the maximum dimensions of these coils are over 4 mm, which is relatively big for certain implantable devices. Any implantable device requires rigorous biocompatibility and biostability. It should not induce any toxic effect in the surrounding tissues, and also should not cause biofouling during long term exposure to the physiological environment. In this work, we employed SU-8, one of the most commonly used photoresist in MEMS fabrication, as a packaging material because it is a proven biocompatible material [6], [7]. In this study, we present the development of a MEMS based compact implantable LC tank as the candidate for providing required power to operate implantable medical devices through inductive coupling.
II.
DEVICE DESIGN
Fig. 1 shows a schematic diagram of our SU-8-based LC tank featuring an octagonal spiral inductor, a PVDF-TrFE based capacitor and a SU-8 platform. The overall length and
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Figure 2. Fabrication sequence for the parallel LC tank circuit: (a) deposition of SiO2 sacrificial layer; (b) creation of SU8 platform; (c) patterning thin gold layer as one of the parallel conductor plate of the capacitor; (d) spin coating and patterning PVDF-TrFE dielectric by O2 RIE with aluminum hard mask; (e) creation of SU8 via holes; (f) electroplating gold through SU8 via holes; (g) built up spiral inductor on top of SU8 insulation and (h) device encapsulated by SU8
Figure 1. Schematic diagram of SU8 based implantable LC tank
width of the device are both 1.5 mm, and thickness is 50 µm. The inductor and the capacitor were connected in parallel through SU-8 via holes. The whole device except for inductor contact pads was encapsulated by SU-8 due to biocompatibility issue. Theoretically, the induced voltage at the transponder coil at a given frequency and magnetic field intensity is given by:
Vind = 2πfNQAB
(1)
where f is frequency, N represents number of turn, Q denotes quality factor. A and B depict coil area and magnetic field, respectively. From the formula, enhancement in number of turn and Q factor is highly desirable for higher induced voltage at a given geometry of coil. Q factor can be expressed in the simplest form as
Q=
ωL Rs
(2)
where ω is the operating frequency, L is the inductance, and Rs represents the series resistance of the inductor. Hence, the higher inductance and lower series resistance are desired to achieve higher Q value. Series resistance strongly depends on conductor’s resistivity as well as skin effect and proximity effect at high frequency. In this work, gold was employed for our inductor due to its low resistivity (2.4 × 10-8 Ω·m) and biocompatibility nature. In order to minimize RF loss due to
the skin effect (estimated skin depth of 4.4 μm at 400 MHz for electroplated gold), the thickness of the inductor was decided to be 10 µm. Inductance value can be improved by increasing number of turn and mean diameter of the coil. For our study, we designed 19 turn, 1 mm diameter octagonal spiral inductor. The width of the conductor trace and pitch distance were 10 µm and 20 µm, respectively. Dimensions of the PVDF-TrFE based capacitor was decided by measured dielectric constant of PVDF-TrFE, 41.9 ~ 48.3. Four different sizes of capacitors, 45 × 45, 55 × 55, 95 × 95 and 100 × 100 µm with 615 nm thick dielectric layer, were devised for our target operating frequency of 400 MHz. III.
DEVICE FABRICATION
The LC tank was fabricated by conventional surface micromachining technology (Fig. 2). As the first step, 3-inch diameter silicon wafer was oxidized with thickness of 5 µm as a sacrificial layer. Then a 1.5 × 1.5 mm SU8 platform was created with thickness of 10 µm by UV lithography. A bottom conductor plate of a capacitor was deposited onto the SU8 platform by sputtering of thin chromium adhesion layer (200 Å) and gold (1500 Å). PVDF-TrFE copolymer solution, in the composition 60/40 mol. %, was prepared by melting 2.5 g of PVDF-TrFE powder (Ktech corp.) into 50 mL MEK (methyl ethyl ketone) which has better compatibility with SU-8 than any other soluble solvent [8]. The prepared solution was spin coated with thickness of 615 nm. Then, deposited PVDF-TrFE films were cured on the 150 °C hotplate for 30 minutes to enhance the crystallinity and to promote uniform chemical and mechanical properties across the surface. A 1500 Å thick aluminum was sputtered on top
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Figure 4. Released LC tank connected to SMA connector gold pads
through GSG
Another 10 µm thick SU-8 layer was applied on this fabricated capacitor as an insulator between the capacitor and the subsequent inductor. Via holes were created by pattering SU-8 by UV lithography. A seed layer was sputtered to form the electroplating base then gold was electroplated to fill up via holes through 10 µm thick SPR220-7.0 (Shipley) molds. A 10 µm thick gold spiral inductor was created on top of the SU-8 layer by the electroplating process. The electroplating seed layer was then removed by wet etching, and a complete LC tank was created. Finally, the whole device was encapsulated by SU-8 except for contact pads of the inductor. The LC tanks were released in 7:1 buffered oxide etch (BOE) using a timed etch process. The successfully fabricated LC tank was shown in Fig. 3. IV.
CHARACTERIZATION
A gold GSG (ground-signal-ground) pad was made on the glass substrate with sputtered gold (1500 Å). Then the released LC tank was wire bonded to the GSG pad by using K&S ball bonder. Next, a PCB edge mount SMA connector was attached to the GSG pad using conductive epoxy, shown in Fig. 4. The sample was cured in a 95 °C convection oven for 4 hours to completely evaporate solvent inside the applied conductive epoxy.
Figure 3. Fabricated LC tank: (a) optical image; (b) released LC tank on one cent coin; and (c) SEM image of cross section
of the cured copolymer as a hard mask for dry etching, followed by wet etching the aluminum hard mask with photoresist mask (S1813, Shipley). Capacitor fabrication was completed by patterning PVDF-TrFE dielectric layer using oxygen RIE (reactive ion etch) with 200 mTorr O2 and 200 Watt power for 10 minutes. The aluminum hard mask remained on top of the patterned PVDF-TrFE film also played a role as one of the parallel conductor plate of the capacitor.
A. Measurement of resonant frequency The fabricated LC tanks were measured by using Anritsu 37369A VNA (vector network analyzer) with Cascade Summit 12k on-wafer probing system. Prior to the characterization, VNA was calibrated using Agilent 85033E 3.5 mm calibration kit. The calibration was performed by using Short-Open-Load-Thru (SOLT) with Cascade Wincal software. The 1-port S-parameters were extracted in the frequency seep range from 40 MHz to 1.2 GHz. In order to enhance the accuracy of the final results, the parasitic effects induced by bonded wires, GSG pads and the SMA connector were subtracted from overall measurements with deembedding procedure. The measured 1-port S-parameters were converted to 1-port Z-parameters. The resonant frequency of the LC tank was found by examining the maximum peak impedance magnitude within the frequency sweep range. Resonant frequencies of four different types of LC tanks were measured by the above method. Each of them includes different sizes of capacitors, 45 × 45, 55 × 55, 95 ×
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Figure 7. Measured induced voltage at various distances Figure 5. Measurement data of impedance magnitude of LC tanks
95 and 100 × 100 µm with same dielectric thickness of 615 nm, and measured values were 490, 467, 418 and 393 MHz, respectively. Fig. 5 shows impedance spectrum and resonant frequencies of the LC tanks. It was shown that different sizes of capacitors resulted in variation of resonant frequencies. B. Measurement of induced voltage A power transmitting coil, 5-turn round spiral inductor with 5 mm diameter, was made by 300 µm diameter enamel insulated copper wire. A Rohde & Schwarz RF signal generator was used to apply RF power to the power coil, and applied power was amplified up to 29 dBm (~ 793 mW) by a RF signal amplifier. The rectifying circuit composed of two BAS28 high speed diodes (Aeroflex Corp.) and two metal clad SMT capacitors was built on a SMA connector (Fig. 6 a). The SMA connected LC tank was assembled with the rectifying circuit (Fig. 6 b). The LC tank was held to the power transmitting coil coaxially at distances of 2, 5, 7 and 10 mm (Fig. 6 d). The induced voltage was measured by
oscilloscope in the frequency range from 250 to 575 MHz. Induced voltage of 8.5 V was measured at a 5 mm distance, which is our target voltages for subcutaneous level wireless neural stimulation applications. Fig. 7 shows measured induced voltage at the LC tank at various distances. Acknowledgment We would like to express our appreciation to the UTD clean room staffs for their technical support. The work was sponsored by MicroTransponder, Inc. REFERENCES [1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
Figure 6. Test setup: (a) rectifying circuit built on a SMA connector; (b) LC tank assembled with rectifying circuit; (c) equivalent circuit diagram of LC tank and rectifying circuit; and (d) LC tanks coaxially aligned with power coil
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