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MOde-localized mass Sensors with Thermal Actuation and Piezoresistive DEtection

Periodic Reporting for period 1 - MOSTAPDE (MOde-localized mass Sensors with Thermal Actuation and Piezoresistive DEtection)

Reporting period: 2020-12-01 to 2022-11-30

Micro/nano-electromechanical system (M/NEMS) resonators have been widely applied in the fields of sensing, communication, and biomedical diagnosis. Frequency-modulated resonant sensors that transduce a physical or chemical quantity to be measured into a shift in resonant frequency are extremely attractive. Resonant mass sensors have been employed for biochemical applications for weighing single molecules, proteins, and nanoparticles, and even monitoring the growth of living cells. Great challenges for MEMS resonant mass sensors include (i) Improving the sensors’ quality factor (Q-factor) at ambient atmospheric pressure and realizing a real-time monitoring system with high sensitivity are the two great challenges that resonant sensors face. The project “MOSTAPDE” aims to develop a thermal-piezoresistive resonant mass sensor with a mode localization phenomenon to address the above-mentioned challenges.

The following milestones are approached during the progress:
(1) Developed the dicing-free fabrication process for thermal-piezoresistive resonators, demonstrating the fundamental basis and the feasibility of this project.
(2) Simulated the mechanical characteristics of the thermal-piezoresistive resonators, and observed physical self-oscillation when being excited by the current.
(3) Fabricated single degree-of-freedom thermal-piezoresistive resonators as well as coupled thermal-piezoresistive resonators, and constructed the measurement setup.
(4) Characterized the fabricated devices and obtained some exploitable results which have not been reported yet, and published several papers on this topic.

The objectives listed in the DoA include:
(1) Establish a theoretically analytical framework and a finite element model (FEM) of the WCRs, with multi-physics simulations coupling the mechanical, thermal and electrical fields using COMSOL Multiphysics and CoventorWare, as design tools for the sensor and circuit developments.
(2) Develop the mass sensor as well as its fabrication process, to integrate on a silicon wafer substrate weakly coupled resonators, thermal actuation elements and piezoresistive detection gauges.
(3) Design a closed-loop control circuit for the proposed mass sensor to realize real-time monitoring at ambient pressure with high resolution and short response time.
In this project, we have approached the following exploitable results:
(1) We revealed the intrinsic principle of the enhancement of the quality factor of thermal-piezoresistive resonators, and an effective quality factor as high as 1.06×106 can be obtained when the bias direct current is higher than a specific threshold.
(2) Relations between the dynamic range, bias current, effective quality factor, and the frequency stability of the thermal-piezoresistive resonators are explored, and concluded that the increase of the effective quality factor is not accompanied by the improvement of the frequency stability, i.e. sensing resolution. There should exist an optimized bias current for the best resolution but not for the highest effective quality factor.
(3) Combing the mode localization phenomenon, we observed that the amplitude sensitivity of the coupled thermal-piezoresistive resonant mass sensors is two orders of magnitude higher than that of the frequency readout, however, the resolutions based on amplitude ratio readout and frequency readout are quite approximate, which means that the proposed idea of mode-localization phenomenon improves the performance of mass sensors is only partially correct, i.e. correct for sensitivity/responsivity but is not solid for resolution.
(4) A mass sensor with the best mass resolution of ~80fg was achieved.
For the first time, the researcher has demonstrated the following contributions to thermal-piezoresistive based mass sensors:
(1) Tuning the bias current, the effective quality factor can be improved, and as a result, the start time of the oscillator can be significantly decreased.
(2) A higher effective quality factor is not always accompanied with the improvement of the frequency stability, i.e. the mass sensing resolution.
(3) Mode-localized mass sensors with high sensitivity based on thermal-piezoresistive resonators have been demonstrated.
Principle of TPR