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3D-printed microelectronics for integrated circuitry and passive wireless sensors

Sung-Yueh Wu, Chen Yang, Wensyang Hsu, Liwei Lin

Microsystems & Nanoengineering 1, Article number: 15013 (2015)



Three-dimensional (3D) additive manufacturing techniques have been utilized to make 3D electrical components, such as resistors, capacitors, and inductors, as well as circuits and passive wireless sensors. Using the fused deposition modeling technology and a multiple-nozzle system with a printing resolution of 30 μm, 3D structures with both supporting and sacrificial structures are constructed. After removing the sacrificial materials, suspensions with silver particles are injected subsequently solidified to form metallic elements/interconnects. The prototype results show good characteristics of fabricated 3D microelectronics components, including an inductor–capacitor-resonant tank circuitry with a resonance frequency at 0.53 GHz. A 3D “smart cap” with an embedded inductor–capacitor tank as the wireless passive sensor was demonstrated to monitor the quality of liquid food (e.g., milk and juice) wirelessly. The result shows a 4.3% resonance frequency shift from milk stored in the room temperature environment for 36 h. This work establishes an innovative approach to construct arbitrary 3D systems with embedded electrical structures as integrated circuitry for various applications, including the demonstrated passive wireless sensors.

Fig. 1 Schematic diagram of the additive 3D manufacturing process including the filling of liquid metal paste for producing basic microelectronic components, integrated circuitries, and a passive wireless sensor. (a) The 3D fabrication process with embedded and electrically conductive structures. (b) 3D microelectronics components, including parallel-plate capacitors, solenoid-type inductors, and meandering-shape resistors. (c) A 3D LC tank, which is formed by combining a solenoid-type inductor and a parallel-plate capacitor. (d) A wireless passive sensor demonstration of a “smart cap,” containing the 3D-printed LC-resonant circuit. The degradation of the liquid food inside the liquid package can cause the changes of the dielectric constant and the shift of the resonance frequency of the LC circuity. A wireless inductive reader is used to monitor the signals in real time.

Fig.2 (a) An optical image showing fabricated microelectronics components produced using the 3D printing process without the embedded conductive structure compared with a one-cent US coin. (b) Fabricated 3D components, including resistors, inductors, and capacitors, and an LC tank after the liquid metal paste filling and curing process. (c) The cross-sectional view of a 4-turn solenoid coil. The overall size of the prototype resistor, inductor, and capacitor are 10 × 10 × 2.4, 10 × 10 × 6.4, and 10 × 10 × 6.4 mm3, respectively, whereas the size of the LC tank is 10 × 20 × 6.4 mm3.

Fig.3 Measurement results of 3D microelectronic components and an LC tank. (a) DC I-V curves of 3D-printed resistors. (b) Total inductance L and (c) quality factor QL of 3D-printed RF inductors with different numbers of turns. (d) Total capacitance C, (e) quality factor QC and (f) calculated relative permittivity εr of 3D-printed RF capacitors with different overlapping areas.

Fig.4 (a) Measured inductance enhancements of 3D solenoid inductors with magnetic cores compared to the reference air-core inductors. (b) Impedance magnitude vs. frequency and (c) phase vs. frequency of a 3D-printed RF LC-resonant circuit with the inductor and capacitor connected in parallel.

Fig.5 The proposed “smart cap” for rapid detection of liquid food quality featuring wireless readout: (a) the smart cap with a half-gallon milk package, and the cross-sectional schematic diagram; (b) sensing principle with the equivalent circuit diagram.

Fig.6 Fabricated devices. (a) Cross-sectional view of a fabricated smart cap; (b) optical image of a completed 3D cap structure with a one-cent coin; (c) the fabricated cap after the liquid metal filling process; and (d) magnified optical image showing the spiral inductor around the top surface of the cap. Test results of wireless LC tank sensors from the RF reader: (e) magnitude versus frequency curves with milk at 22 °C after 0, 12, 24, and 36 h; (f) resonance frequency versus time for a milk sample at 4 °C and a milk sample at 22 °C.

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