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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS

1

Liquid-Loaded Piezo-Silicon Micro-Disc Oscillators
for Pico-Scale Bio-Mass Sensing
Hakhamanesh Mansoorzare , Graduate Student Member, IEEE, Sarah Shahraini , Member, IEEE,
Ankesh Todi, Graduate Student Member, IEEE, Nilab Azim , Swaminathan Rajaraman , Member, IEEE,and Reza Abdolvand , Senior Member, IEEE

Abstract— In this work, we report on the implementationof a closed-loop liquid-loaded resonant mass sensor as a test
vehicle for the realization of highly sensitive miniaturized biomarker assays. Using thin-film piezoelectric-on-silicon (TPoS)
contour-mode disc resonators that are specifically designed andoptimized for liquid-phase operation, a liquid-loaded low power(∼1.6 mW) oscillator at ∼17 MHz is demonstrated with asingle transistor. By proper placement of the electrodes and thesupporting tethers, a high liquid-loaded quality factor (∼380) anda low insertion loss (∼19 dB) are concurrently achieved whichenable the accurate frequency tracking of the potential mass
microbalance. The frequency shifts due to the liquid-loading andthe preliminary characterized frequency stability of the oscillator
imply a minimum mass detection resolution in the picograms
range.
[2020-0177]
Index Terms— Liquid-loaded oscillator, liquid mass-sensing,microelectromechanical disc resonator, piezoelectric transducer.

I. I NTRODUCTION

OVERCOMING the global health issues calls for platforms that enable large scale testing and diagnosis as
well as extensive drug and vaccine discovery. Such platforms require rapid, high throughput, and yet precise test
vehicles that are capable of detecting specific biomarkers
that often reside within a biological fluid. The enzyme-linked
immunosorbent assay (ELISA) [1], is established as a clinicalgold standard for detecting a diversity of biomarkers. However,ELISA in its conventional form, does not meet the aforementioned requirements and is considered to be resource intensive [2]. Therefore, enhancement of this scheme is desired,
Manuscript received May 15, 2020; revised July 3, 2020; accepted
July 6, 2020. This work was supported by the National Science Foundation
under Grant 1711632. The work of Swaminathan Rajaraman was supportedby the University of Central Florida (UCF) Startup Funding. Subject Editor
M. Rais-Zadeh. (Corresponding author: Hakhamanesh Mansoorzare.)Hakhamanesh Mansoorzare, Sarah Shahraini, Ankesh Todi, and Reza
Abdolvand are with the Department of Electrical and Computer Engineering, University of Central Florida, Orlando, FL 32816 USA (e-mail:hakha@knights.ucf.edu).
Nilab Azim is with the Department of Chemistry, University of CentralFlorida, Orlando, FL 32816 USA, and also with the NanoScience Technology
Center, University of Central Florida, Orlando, FL 32816 USA.
Swaminathan Rajaraman is with the Department of Materials Science andEngineering, University of Central Florida, Orlando, FL 32816 USA, alsowith the Department of Electrical and Computer Engineering, University ofCentral Florida, Orlando, FL 32816 USA, and also with the NanoScience
Technology Center, University of Central Florida, Orlando, FL 32816 USA.
Color versions of one or more of the figures in this article are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JMEMS.2020.3008319

especially through automating the readout mechanism andminiaturizing the apparatus. In this regard, resonant sensors
with a frequency output that could be directly interfaced withdigital electronics are well suited. More specifically, piezoelectric resonators have been employed for sensing a varietyof physical properties such as mass, density and viscosity since
the establishment of the relationship between their adsorbed
mass and resonance frequency. As of today, quartz crystal
microbalances (QCM) invented by Sauerbrey remain one of
the most widely used methods of accurate mass sensing [3].
Although, MEMS resonators offer miniaturization and integration beyond the limits possible with QCMs, utilization of suchdevices in liquid environment (i.e. biological media) has beenrather limited. This is mainly because of the signal degradationthat subsequently results in lower signal to noise ratio (SNR).
The losses associated with the liquid-loading of the resonatorare largely determined by its boundaries with the liquid and themode-specific pattern of the radiation of energy into the liquid.
As such, we previously studied several resonance modesof liquid-loaded thin-film piezoelectric-on-silicon (TPoS) discresonators to identify those with minimum quality factor (Q)degradation. Also the TPoS backside release step was modified to form a liquid-tight reservoir underneath the resonatorby depositing a very thin (∼200 nm) layer of Parylene-C.
Accordingly, a compound contour resonance mode with
4 nodal diameters and 2 nodal circles, conventionallynamed S(4,2) , was identified to exhibit unprecedented liquidphase Q [4]. Further, such resonators were embedded withina microfluidic testbed to prepare for the assembly of aclosed-loop liquid-loaded sensing platform [5]. In this work,
we have enhanced our earlier results by improving the coupling efficiency for the S(4,2) mode which has enabled implementation of a simple and low-power pierce oscillator for highresolution liquid-phase frequency tracking. From the results
reported in this work the practical detection resolution limits
that could be achieved using this system is estimated. The conceptual schematic of the proposed sensor is depicted in Fig. 1.
II. D ESIGN AND C HARACTERIZATION OF THE S ENSOR
A. TPoS Disc Resonator
The electrode design guideline to achieve maximum coupling and minimum insertion loss (IL) in a TPoS resonator isto pattern the top electrode so that each connected piece of1057-7157 © 2020 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
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2JOURNAL OF MICROELECTROMECHANICAL SYSTEMS

Fig. 1. The schematic of the MEMS microfluidic platform showing theTPoS resonator embedded within the LUT reservoir. Sustained oscillation isachieved by the addition of a simple electronic gain component.

Fig. 3. Transmission response of DR1 having different clamping and reflector
configurations.

Fig. 4. Transmission response of DR1 and DR2 before and after insertionof DI water with the corresponding Q and IL.

Fig. 2. (a), (b) The simulated lateral stress of the S(4,2) mode in a disc withradius of 400 μm and (c), (d) the spurious mode for two different clamping
conditions.

conductor covers surface areas under which the stress polarity
is similar. The lateral stress profile (vector sum of the stress inx and y direction) of the S(4,2) mode of a TPoS disc resonatorwith a radius of 400 μm is displayed in Fig. 2 (a) and (b).
The former is clamped to the substrate from two spots along
the [100] plane of silicon (Si) while the latter has fouranchors along the 22.5◦ off [100] plane. Accordingly, twoelectrodes covering the opposite stress regions are considered.
However, there exists a spurious mode that exhibits a stresspattern very similar to that of S(4,2) mode; as shown in
Fig. 2 (c) and (d) respectively for the two clamping conditions.
If not suppressed properly, this mode could cause unintended
oscillations and false frequency tracking. Here we demonstratethat the clamping configuration with four anchors resolves
such issue as for such design, all four tethers are locatedat maximum displacement node of the spurious mode as
demonstrated in Fig. 2 (d) and substantially dampen the Qof the mode.
Two sets of TPoS disc resonators (DR) with a radius of
200 μm (DR1 ) and 400 μm (DR2 ) are fabricated on a stackof 1 μm scandium aluminum nitride (Sc0.2 Al0.8 N provided
by AMSystems Inc.) that is sandwiched between 100 nm
thick molybdenum (Mo) electrodes and sputtered on a 16 μm
silicon-on-insulator (SOI) substrate using a process flow thatis elaborated in [5]. Liquid-phase operation is enabled once

Fig. 5. Process flow for the integration of the microfluidics platform showing
the PDMS casting, routing of formed microchannels (respective to DRs), andbonding of the PDMS structure underneath the MEMS substrate.

the resonators are embedded within a microfluidic device(elaborated in the following section) that is isolated from theresonator with a ∼200 nm layer of Parylene-C. The insulating film is shortly treated with oxygen plasma to conditionhydrophilic surfaces.
Prior to inserting the liquid under test (LUT), the RF characteristics of the disc resonators are measured by a Rohde &Schwarz ZNB 8 network analyzer. As expected and evidenced
in Fig. 3, the four-anchor configuration exhibits superior performance (higher Q and lower IL) with ∼24.5 dB suppressionof the spurious mode. Additionally, the efficiency of utilizing
in-plane acoustic reflectors [6], in such designs is evident.
In the next step, the microfluidic channels are filled with
deionized (DI) water and from the liquid-loaded responses,
the sensitivity of the devices is determined. Fig. 4 displays
the |S21 | of DR1 (34.39 MHz) and DR2 (17.17 MHz) as wellas their Q, extracted from their modified Butterworth-Van
Dyke equivalent circuit, and IL before and after inserting

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MANSOORZARE et al.: LIQUID-LOADED PIEZO-SILICON MICRO-DISC OSCILLATORS FOR PICO-SCALE BIO-MASS SENSING3Fig. 6. (a) schematic of the single stage oscillator circuit; (b) the implemented oscillator circuit comprising the bonded MEMS-microfluidic along with theoscillator PCB; (c) the liquid-loaded output waveform of the oscillator.

DI water. Since the penetration of shear acoustic waves in
the water approximately results in a virtual added mass [7]of 11.5 ng and 65 ng for DR1 and DR2 , respectively, the measured liquid-loaded frequency shifts of 50 kHz and 20 kHz
could be attributed to mass sensitivities of 128 ppm.ng−1(4350 Hz.ng−1) and 18 ppm.ng−1 (308 Hz.ng−1 ), respectively. While DR1 offers much higher mass sensitivity due
to the smaller size and higher frequency, here, the preliminary
oscillator is built with DR2 as it offers substantially lower
liquid-loaded IL and higher Q.
B. Microfluidic Platform
As an expandable platform for future implementation of abiomarker assay through functionalizing the Parylene-C layer
with proper binding agents, arrays of disc resonators areintegrated with the microfluidic channels. The microchannels
that guide the LUT to the desired reservoirs underneath theresonators are fabricated using polydimethylsiloxane (PDMS)based soft lithography. The process flow for the integration of
the microfluidic structure is shown in Fig. 5. A 3D printed
mold is sputtered with gold to enable easy detachment of thecasted PDMS; the resulting cured structure is aligned with
the MEMS substrate, so that the microchannels are properly
routed underneath the resonators and the two are bonded to
one another after a short oxygen plasma treatment.
C. Oscillator Circuit
The low liquid-loaded IL attained by DR2 allows forimplementation of the oscillator circuit with a single stagetransistor-based sustaining amplifier. Consequently, a proofof-concept pierce oscillator is demonstrated using a bipolartransistor (Avago’s AT-41486 BJT) as schematically shown in
Fig. 6 (a). The bonded MEMS-microfluidic substrate alongwith the oscillator’s printed circuit board (PCB) are placed andsecured on a mounting substrate (Fig. 6 (b)). The electrodesof the DR2 are wire bonded to the circuit and DI water
is injected into the LUT reservoir that is underneath DR2through the corresponding LUT microchannel inlet. The output
waveform of the liquid-loaded oscillator (Vout ) is displayedin Fig. 6 (c). With only 2 V supply voltage and 0.8 mA bias
current the oscillator consumes 1.6 mW; considerably lower
than the reported values in prior art [8]–[10].
Since the detection resolution of the sensor is governed by
the frequency stability of the sensing oscillator, after insertion

Fig. 7. Frequency stability characterization of the liquid-loaded oscillator
showing the measured (a) phase noise and (b) Allan deviation; a frequencyresolution of 1.5 Hz at τ = 1 s sampling time is estimated.

of an intermediate buffer stage, the liquid-loaded oscillator ischaracterized by a Rohde & Schwarz FSUP8 signal source
analyzer; the measured phase noise and Allan deviation areplotted in Fig. 7 (a) and (b), respectively. While at a long sampling time (τ ) prior to onset of random-walk, i.e. ∼700-1000 s,
the theoretical limit of frequency resolution (δ f ) is attainable,
real-time mass sensing requires choosing small (<1 s) valuesof τ . Accordingly, δ f at τ = 1 s is estimated to be 1.5 Hz(9E-8 × 17.15 MHz); therefore, considering the derived mass
sensitivity of 308 Hz.ng−1, the mass resolution of the sensor
is calculated to be 4.8 pg (1.5 Hz / 308 Hz.ng−1 ) which as areference is roughly four times the mass of a single Escherichia
coli (E. coli) bacterium. As a side note, a confidence factor of∼3 is conventionally applied to the mass detection resolution
to determine the limit of detection of the sensor (i.e. 14.4 pg).
III. C ONCLUSION
A liquid-loaded MEMS microbalance platform was demonstrated that enables simple closed-loop operation for accurate
readout. Future work would focus on functionalization of themicrobalance with proper receptors for detection of biomarkers and single cell analysis within an assay.
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