Low Phase Noise, Dual-Frequency Pierce MEMS Oscillators with Direct Print Additively Manufactured Amplifier Circuits

Abstract

This paper presents the first demonstration and comparison of two identical oscillator circuits employing piezoelectric zinc oxide (ZnO) microelectromechanical systems (MEMS) resonators, implemented on conventional printed-circuit-board (PCB) and three-dimensional (3D)-printed acrylonitrile butadiene styrene (ABS) substrates. Both oscillators operate simultaneously at dual frequencies (260 MHz and 437 MHz) without the need for additional circuitry. The MEMS resonators, fabricated on silicon-on-insulator (SOI) wafers, exhibit high-quality factors (Q), ensuring superior phase noise performance. Experimental results indicate that the oscillator packaged using 3D-printed chip-carrier assembly achieves a 2-3 dB improvement in phase noise compared to the PCB-based oscillator, attributed to the ABS substrate's lower dielectric loss and reduced parasitic effects at radio frequency (RF). Specifically, phase noise values between -84 and -77 dBc/Hz at 1 kHz offset and a noise floor of -163 dBc/Hz at far-from-carrier offset were achieved. Additionally, the 3D-printed ABS-based oscillator delivers notably higher output power (4.575 dBm at 260 MHz and 0.147 dBm at 437 MHz). To facilitate modular characterization, advanced packaging techniques leveraging precise 3D-printed encapsulation with sub-100 μm lateral interconnects were employed. These ensured robust packaging integrity without compromising oscillator performance. Furthermore, a comparison between two transistor technologies-a silicon germanium (SiGe) heterojunction bipolar transistor (HBT) and an enhancement-mode pseudomorphic high-electron-mobility transistor (E-pHEMT)-demonstrated that SiGe HBT transistors provide superior phase noise characteristics at close-to-carrier offset frequencies, with a significant 11 dB improvement observed at 1 kHz offset. These results highlight the promising potential of 3D-printed chip-carrier packaging techniques in high-performance MEMS oscillator applications.

Keywords: MEMS; additive manufacturing; advanced packaging; oscillator; phase noise; piezoelectric; quality factor; resonators; silicon-on-insulator (SOI); zinc oxide (ZnO).

Figure 1

Step-by-step fabrication process of piezoelectric ZnO-on-SOI MEMS resonators: (a) resonator pre-release; (b) bottom electrode deposition and patterned by lift-off; (c) sputtering deposition of the piezoelectric ZnO film; (d) etch vias through ZnO for the bottom electrodes access; (e) deposition and patterning of top electrodes; and (f) ZnO dry etch followed by silicon dry etch.

Figure 2

The top-view photos of a 250 µm × 91 µm ZnO-on-SOI rectangular plate resonator along with its key dimensions, including (a) Keyence VHX-6000 digital microscope image, where G and S represent the ground and signal probe pads, respectively; and (b) SEM photo of this resonator device.

Figure 3

(a) A MEMS-based common-emitter oscillator circuit designed and milled on a PCB board, and (b) a 3D-printed chip-carrier assembly for a MEMS oscillator using ABS as the substrate material.

Figure 4

A conceptual illustration of a MEMS resonator-based oscillator circuit diagram, where green and orange color areas represent the piezoelectric ZnO film and the IDT electrode scheme of the piezoelectrically-transduced MEMS resonator, respectively.

Figure 5

A conceptual illustration of the 3D-printing process used to implement a MEMS resonator-based Pierce oscillator in the form of system-in-package, including key steps: (a) the 3D-printing of the ABS substrate layer by fused deposition modeling (FDM), followed by microdispensing of CB028 ink (a silver paste)-based RF ground plane that is then covered by a FDM 3D-printed ABS layer with the thickness matching that of the MEMS and IC chips; (b) the laser milling of the cavity for embedding the MEMS/IC chips and needed via holes; (c) the insertion of the MEMS and IC dies into the designated cavities by the pick-and-place function; (d) the microdispensing interconnects and via hole refilling with the CB028 ink; and (e) the final structure of the assembled MEMS resonator-based Pierce oscillator.

Figure 6

(a) A comparison between measured frequency responses (S₂₁ in dB) and simulated results based on a full dual-mode equivalent circuit model of the ZnO-on-SOI resonator at 259.7 MHz and 436.4 MHz; and (b) the implemented electrical equivalent circuit model for dual-mode resonances of the ZnO-on-SOI resonator, which is employed to obtain a simulated frequency response (S₂₁ in dB).

Figure 7

A complete Pierce oscillator circuit diagram with a single transistor (a BJT for this example) in a closed-loop configuration with two capacitors (C₁ and C₂) and a MEMS resonator tank circuit. During circuit simulation, an electrical equivalent circuit model for the dual-mode MEMS resonator with specifically chosen resonance frequencies was employed.

Figure 8

A comparison between measured time-domain waveforms generated by a MEMS oscillator operating at 260 MHz versus two types of simulated time-domain oscillator output waveforms.

Figure 9

HBT transistor-based dual-mode Pierce oscillator’s output waveforms measured by an oscilloscope: (a) at 260 MHz with a peak-to-peak voltage of 0.664 V and (b) at 437 MHz with a peak-to-peak voltage of 0.175 V.

Figure 10

E-pHEMT transistor-based dual-mode Pierce oscillator’s output waveforms measured by an oscilloscope: (a) at 260 MHz with a peak-to-peak voltage of 4.62 V and (b) at 437 MHz with a peak-to-peak voltage of 2.36 V.

Figure 11

Measured broadband frequency responses of the MEMS oscillator showing two output signals: (a) at 260 MHz with a signal voltage of 46.13 dBmV and (b) at 437 MHz with a signal voltage of 41.32 dBmV, which are measured by using a spectrum analyzer.

Figure 12

Open-loop frequency responses of the (a) magnitude and (b) phase of the serially cascaded MEMS resonator-sustaining amplifier circuit by ADS simulation and measurement.

Figure 13

Comparison of the open-loop gain vs. frequency of the resonator-sustaining amplifier by ADS simulation and RF measurements for both FR4 PCB and 3D-printed ABS oscillator circuits.

Figure 14

Comparison of measured phase noise performance of two oscillator designs at 260 MHz implemented on PCB boards with HBT and E-pHEMT transistors under different bias conditions.

Figure 15

Comparison of the output amplitude measured for two 260 MHz oscillator design implementations: (a) PCB board with an HBT transistor with an output power of 5.152 dBm and (b) 3D-printed assembly with an HBT transistor with an output power of 4.575 dBm.

Figure 16

Comparison of the output amplitude measured for two 437 MHz oscillator design implementations: (a) PCB board with an HBT transistor with an output power of 0.753 dBm and (b) 3D-printed assembly with an HBT transistor with an output power of 0.147 dBm.

Figure 17

(a) The measured phase noise performance of two HBT-based 260 MHz Pierce oscillators implemented using PCB and 3D-printing technologies, and (b) the measured phase noise performance of two HBT-based 437 MHz Pierce oscillators implemented using PCB and 3D-printing technologies.

Figure 18

(a) Comparison of measured phase noise performance of three oscillator designs at oscillation frequency of 260 MHz, and (b) comparison of measured phase noise performance of three oscillator designs at oscillation frequency of 437 MHz.

Figure 19

A schematic diagram to illustrate electron-hole transfer between the valence band and conduction band with a ZnO layer as a high bandgap semiconductor material, which can typically be excited by external energy sources such as UV radiation.