Microelectronics An Integrated Approach

[Alfie Overacre]

Thistext describes device physics and circuit design in the context of modern microelectronics integrated circuit technology. It introduces approaches to learning the core device physics and analog/digital circuit concepts that make the subject more accessible to the current generation of students. The authors have designed a concise, concentrated presentation, limiting coverage to only those concepts necessary for the understanding of devices and circuits.

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Figure 1 illustrates the proposed 3D design and the fabrication steps. First, functional 3D structures are designed and constructed using the 3D printing technique. The hollow microchannels and cavities are designed in the 3D structures to be filled later with liquid metal paste. A hollow solenoid-shaped channel is formed as shown in Figure 1a1. To facilitate the liquid metal paste filling step, injecting holes are designed as the inlet/outlet ports for the solenoid channels, as shown in Figure 1a2. For direct frequency characterizations of the designed RLC circuitry, the solenoid-inductor structure has designated cavities as the ground-signal-ground (G-S-G) pads on the top surface for contact pads. After the 3D printing process, liquid metal paste is injected to form conductive electrical structures, as shown in Figure 1a3. The overflow of the liquid metal paste at the outlets on the top surface are flattened and used as the contact pads. The final solidification process cures the liquid metal paste to form solid structures, while the top surface of the device is planarized to remove the injecting holes.

The prototype fabrication process uses the 3D printing machine, ProJet HD 3000, based on the fused deposition modeling technology16 with a printing resolution of 30 μm. During the printing process, polymer materials are heated and ejected from the nozzles of the inkjet printer. Building (VisiJet EX 200, 3D Systems Inc., Rock Hill, SC, USA)17 and sacrificial materials (VisiJet S100, 3D Systems Inc., Rock Hill, SC , USA)18 are deposited alternatively from the dual nozzles to form the printed samples, in which the building material defines the molding structure, while the sacrificial material occupies the hollow channels19. Afterwards, a post-printing process is conducted to remove the sacrificial materials. First, the whole 3D-printed sample is immersed in a mineral oil bath at 80 C to dissolve the sacrificial material. Second, the residual mineral oil is removed by thoroughly washing with detergent and water in sequence. The liquid metal paste comprised of a silver suspension (Pelco 16040-30, Ted Pella Inc., Redding, CA, USA)20 is then injected into the channels and cavities. Next, the as-filled sample is kept at room temperature for 2 h for the solidification process. The detailed fabrication method flow can be found in the Supplementary Information. These components can be scaled up or down based on the capabilities of the specific type of 3D printer. However, the liquid metal paste filling process has practical limits. Specifically, smaller channels (diameter of 400 μm or smaller in our experiments) have large flow resistance that prevents the filling process, and the 600-μm diameter design is the optimal channel size in this work using the ProJet HD 3000 printer, Hewlett-Packard Company, Palo Alto, CA, USA.

The prototype 3D-printed microelectronics components without the liquid metal paste are fabricated as shown in the optical photo of Figure 2a, with a one-cent US coin shown for reference. After removing the sacrificial materials and injecting the liquid metal paste, the resulting functional components and the LC circuitry are shown in Figure 2b. Note that the volume of the silver suspension shrank after the solidification process, which could leave voids inside the metal traces. By repeating the filling operations, these voids are minimized, thereby improving the electrical conductivity. With five times repeated filling, the measured average volume-filling ratio reaches 68.7%, as characterized in the cross-section views of these components shown in Figure 2c. The electrical performances of the fabricated passive components were characterized as follows. The DC I-V curves of the resistors are measured using a semiconductor parameter analyzer (HP 4145B, Hewlett-Packard Company, Palo Alto, CA, USA). The two-port RF S-parameter spectra of the inductors, capacitors, and LC tank are measured using Cascade Microtech ACP40-GSG-200, Cascade Microtech Inc., Beaverton, OR, USA, probes and a network analyzer (Agilent E5071B, Agilent Technologies, Santa Clara, CA, USA). The parasitic effects of G-S-G pads are de-embedded accordingly.

Figure 3b and 3c show the measured inductance and quality factor of the inductors with different numbers of coil turns, N. These solenoid-shaped inductors have a designed diameter of 4 mm. The cross-sectional shape of the metal traces is circular with a diameter of 600 μm. The line spacing between adjacent windings is 400 μm. In Figure 3b, the measured total inductance L increases as N increases. For example, the inductances at 0.4 GHz are 23 nH, 51 nH, and 92 nH for inductors with 2, 4, and 6 turns, respectively. For each inductor, the L increases first as the frequency increases and then reaches a maximum value due to self-resonance. For example, the inductance of the 6-turn inductor increases from 92 nH at 0.4 GHz to over 350 nH approximately 0.67 GHz. For frequencies above the resonance, the inductance rapidly decreases. The frequency at which the L drops to zero, i.e., the self-resonance frequency f0 is 1.49 GHz, 0.93 GHz, and 0.71 GHz for the inductors with 2, 4, and 6 turns, respectively. Note that larger N corresponds to smaller f0 due to the larger inductance. Figure 3c shows the measured quality factors. The quality factor first increases as the frequency increases and then decreases to zero due to the high loss at the self-resonance frequency. Note that higher inductance leads to higher quality factor, which proves that 3D inductors are helpful in reducing the energy losses. For example, the 6-turn inductor has the highest QL of approximately 24 at 0.19 GHz, whereas the 2-turn inductor shows a maximum QL of 5.6 at 0.70 GHz. During the magnetic energy storage cycles in the inductors, the energy loss mechanisms mainly include the skin-effect induced ohmic losses in the conductor and the electric field energy losses due to parasitic capacitance.

Magnetic materials can be easily integrated into the prototype 3D solenoid inductors by designing openings inside the coils to allow for the placements of magnetic materials to enhance the inductance. For example, one can insert a magnetic bar (Fair-Rite 3061990861, Fair-Rite Products Corp., Wallkill, NY, USA)24 into a 3D solenoid inductor manually. Figure 4a shows the preliminary results on the prototype 3D-printed inductors with inserted magnetic bars. The inductance is found to increase due to the addition of the magnetic core, and larger number of coil turns is found to lead to higher increases of the inductance. Specifically, the 6-turn coil device exhibits inductance enhancements over the reference air-core inductor are in the range of approximately 550% to 734% over the frequency range of 0.01 GHz to 0.2 GHz.

The measured frequency responses of the prototype device consisting of a 4-turn solenoid and a parallel-plate capacitor are shown in Figure 4b and c. In this case, the cross-sectional shape of the metal traces is circular, with a diameter of 600 μm, and the line spacing between the adjacent winding wires of the solenoid is 400 μm. The rectangular-shaped parallel-plate capacitor has an area of 8.36 mm2, with a gap of 400 μm. As the frequency increases, the magnitude of the impedance increases and reaches its maximum at 1.75 kΩ at the resonant frequency of 0.53 GHz, as shown in Figure 4b. The bandwidth is extracted as 40.72 MHz, based on 1 / 2 of the peak impedance value. The calculated quality factor Q is 13, which could be further increased by improving the material conductivity. Figure 4c shows the phase versus frequency plot.

Figure 5a illustrates the sensing architecture of the proposed cap with the embedded LC tank sensor. In this design, the circuit is composed of an inverted-cone-shape capacitor and a planar spiral-shaped inductor to form the LC-resonant circuit. By flipping the food package upside down, the liquid food is trapped inside the capacitor gap of the LC tank and acts as the dielectric material. The LC tank's fres, is determined by the dielectric constant of the liquid food. When the liquid deteriorates, the value of fres can shift as a result of the change of dielectric constant. The value of fres can be detected wirelessly using an RF reader, as shown in Figure 5b. By imposing a frequency-swept electrical field in the reader coil, the LC tank stores energy due to near-field inductive coupling and exhibits electrical oscillation. The most pronounced oscillation occurs when the driving frequency matches the LC tank's resonance frequency because, at this point, the LC tank absorbs the most electromagnetic energy. This resonance induces a negative peak in reader coil's reflection coefficient S11 spectrum, as measured by a network analyzer33. By recording this peak, fres is tracked wirelessly, and the quality of the liquid food is detected in real time without the need to open the package.

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