Ultrahigh frequency ZnO silicon lens ultrasonic transducer for cell-size microparticle manipulation

Ultrahigh frequency ZnO silicon lens ultrasonic transducer for cell-size microparticle manipulation

Accepted Manuscript Ultrahigh frequency ZnO silicon lens ultrasonic transducer for cell-size microparticle manipulation Chunlong Fei, Hsiu-Sheng Hsu, ...

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Accepted Manuscript Ultrahigh frequency ZnO silicon lens ultrasonic transducer for cell-size microparticle manipulation Chunlong Fei, Hsiu-Sheng Hsu, Arash Vafanejad, Ying Li, Pengfei Lin, Di Li, Yintang Yang, EunSok Kim, K. Kirk Shung, Qifa Zhou PII:

S0925-8388(17)33153-5

DOI:

10.1016/j.jallcom.2017.09.113

Reference:

JALCOM 43169

To appear in:

Journal of Alloys and Compounds

Received Date: 5 June 2017 Revised Date:

8 August 2017

Accepted Date: 11 September 2017

Please cite this article as: C. Fei, H.-S. Hsu, A. Vafanejad, Y. Li, P. Lin, D. Li, Y. Yang, E. Kim, K.K. Shung, Q. Zhou, Ultrahigh frequency ZnO silicon lens ultrasonic transducer for cell-size microparticle manipulation, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.09.113. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Ultrahigh Frequency ZnO Silicon Lens Ultrasonic

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Transducer for Cell-size Microparticle Manipulation

3 Chunlong Fei1,2, Hsiu-Sheng Hsu2, Arash Vafanejad3, Ying Li2, Pengfei Lin1, Di Li1,

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Yintang Yang1, EunSok Kim3, K. Kirk Shung2, Qifa Zhou1,2,4,*

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1.

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University of Southern California, Los Angeles, California 90089, USA

School of Microelectronics, Xidian University, Xi’an 710071, China

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Department of Biomedical Engineering and NIH Transducer Resource Center,

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3.

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California, Los Angeles, California 90089, USA

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4.

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90089, USA

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*

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Chunlong Fei and Hsiu-Sheng Hsu contributed equally to this work.

Roski Eye Institutie, University of Southern California, Los Angeles, California

Abstract

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Corresponding author Email address: [email protected] (Q. Zhou)

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Department of Electrical Engineering-Electrophysics, University of Southern

In single beam acoustic tweezers application, the development of ultrahigh

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frequency ultrasonic transducer is crucial for trapping particles or cells with a size of

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few microns. In this study, A ZnO silicon-lens ultrasonic transducer with ultrahigh

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center frequency was designed and fabricated for single beam acoustic tweezers

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application. The silicon lens with small radius was developed through isotropic

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chemical wet etching technique. Finite element method was used for transducer

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design and evaluation. The transducer fabricated has a center frequency of 330 MHz,

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-6 dB bandwidth of 21 % and the insertion loss of -68 dB. The transducer was shown

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to be capable of manipulating individual 5 µm polystyrene microsphere which

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demonstrated the potential of this type of ultrahigh frequency transducer for

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biophysical and biomedical applications.

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Keywords

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Ultrahigh frequency ZnO ultrasonic transducer; silicon-lens; isotropic chemical wet

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etching technique; single beam acoustic tweezers

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Introduction

The ultrahigh frequency ultrasonic transducer (≥ 200 MHz) has attracted growing

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interests because its beam width approaches cellular dimension (micron size). As the

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core component of single beam acoustic tweezers (SBAT), the acquisition of ultrahigh

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frequency ultrasonic transducers makes it possible to manipulate micro-particles and

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biological cells contactlessly.[1-4] For the ultrasonic transducer, its beam width is

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inversely proportional to the center frequency and the degree of focus. It suggests that

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the key to trap smaller size particle or cell is to develop the tightly focused ultrahigh

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frequency transducers with higher center frequency. However, it is challenging and

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time consuming to build such ultrahigh frequency transducers using traditional

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approach in which piezoelectric ceramics or crystals are lapped down to the order of

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microns. [5, 6] To address these issues, piezoelectric film technology is preferred

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[7-10].

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Recently, a 200 MHz ZnO focused transducer based on a traditional design with a

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sapphire lens for focusing was used to realize cell immobilization. [11] Typically, a

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sapphire lens is made via the grinding method. With the increasing operation

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frequency of the transducer, the demand for small radius and high sphericity of the

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lens also increases. The traditional grinding method to make a concave radius in

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sapphire becomes much difficult as the radius decreases. Comparing with the sapphire

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lens fabrication, the small silicon lens can be made using the etching technique

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without machining. Several advantages can be expected using silicon as the lens

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material [12]: (1) Many lenses in a batch process with good uniformity can be utilized

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using MEMS lithography and etching techniques; (2) it is possible to make multi lens

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on a silicon lens body for advanced transducer configurations; (3) the silicon wafer is

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cheaper than the sapphire crystal. In this work, a ZnO transducer with a silicon lens for focusing was designed and

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fabricated for microparticle manipulation. The silicon lens with small radius was

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developed through isotropic chemical wet etching technique. Finite element method

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was used for transducer design and evaluation. Due to the high center frequency, this

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ultrahigh frequency transducer could be excited effectively and generate tightly

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focused ultrasound beam which makes it able to manipulate individual 5 µm

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microparticle.

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Silicon Based Lens

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Design and Fabrication of Silicon Lens

The isotropic chemical wet etching technique, as one of the standard techniques

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in micro-electro-mechanical systems (MEMS) micromachining, is selected to develop

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silicon lens in this study. This technique makes it possible to produce a spherical

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cavity with excellent sphericity, minimal surface roughness and a high degree of

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uniformity. The most common isotropic chemical etchant for silicon etching is HNA

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solution, a mixture of hydrofluoric acid (HF), nitric acid (HNO3), and acetic acid

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(CH3COOH). [13-16]

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Figure.1. Process steps used to fabricate the silicon lens

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The HNO3 in the solution oxides the silicon, while fluoride ions from the HF

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reacts with the silica to form the soluble silicon compound H2SiF6. The acetic acid

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helps prevent the dissociation of the nitric acid, thereby preserving the oxidizing

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power. The overall reaction of silicon in HNA etchant can be simplified as

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Si + ‫ܱܰܪ‬ଷ + 6‫ܪ → ܨܪ‬ଶ ܵ௜ ‫ ଺ܨ‬+ ‫ܱܰܪ‬ଶ + ‫ܪ‬ଶ ܱ + ‫ܪ‬ଶ The fabrication process flow of the silicon lens (Figure. 1) is simply composed of

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five steps: hard mask preparation, photolithography, RIE etching, HNA etching, and

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removal of mask layers. The process details are as follows. Firstly, a ~ 1 µm

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low-stress silicon nitride (SiNx) layer was deposited on a double-side polish (100)

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silicon wafer (400 µm in thickness) by low pressure chemical vacuum deposition as

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the hard mask for the HNA etching of silicon. The hard mask was used to protect the

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areas of the silicon wafer that should not be attacked by the HNA etchant. It should be

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noted that the low stress silicon nitride is required to avoid breaking of the mask in the

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zone of under-etching. The wafer was then patterned to open the areas for the etching

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via the photolithography technique. Next, the silicon nitride was selectively etched off

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for silicon etching using reactive ion etching (RIE). Once the exposed silicon nitride

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is etched, acetone is used to remove residual photoresist. Subsequently, the wafer was

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immersed in the HNA solution to form the silicon lens structure. The final step was to

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remove the residual silicon nitride mask by RIE.

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Table 1. Effect of etching recipes on etching rate and surface roughness of etched

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cavity. HNO3 (vol%)

HAC (vol%)

Temp. (°C)

A

20

30

60

25

B

25

50

80

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C

10

50

30

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2~3

Fair

D

10

50

30

50

3~4

Better

E

20

35

55

50

3~5

Best

Surface smoothness

2~3

Poor

4~5

Poor

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Etching rate (µm/min)

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HF (vol%)

In order to achieve tightly focused ultrasound beam, the lens is expected to have

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high sphericity and minimal surface roughness. Thus, the effect of etching parameters

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for lens structure was studied, various etchants and conditions were taken to find out

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the optimized parameters for making a silicon lens with high sphericity and minimal

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surface roughness. Table 1 lists some etching results with different etching recipes.

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The surface roughness of the etched cavity is present in Figure.2. As shown in

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Figure.2 (a)-(e), the optimum etching parameters (recipe E) to achieve smooth surface

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of the etched cavity is at an HF concentration of 20 vol%, HNO3 concentration of 35

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vol% and CH3COOH concentration of 55 vol% as well as operating temperature is at

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50°C. The earlier literature also reported that mirror-like surface could be obtained in

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high etching temperature.[12] Figure.2 (f) shows a SEM image of an etched acoustic

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lens for transducer fabrication, the cavity of silicon lens is 640 µm in diameter and

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160 µm in depth.

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Figure.2. (a-e) Effect of etching parameters (Table 1) on surface roughness of etched

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cavity: (a) Recipe A, (b) Recipe B, (c) Recipe C, (d) Recipe D , (e) Recipe E, and (f)

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Micrograph of the silicon etched cavity under recipe E.

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ZnO Silicon-lens Transducer

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Transducer Design and Fabrication

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Zinc oxide [17] was selected to fabricate the ultrahigh transducer because it

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exhibits relatively high electromechanical coupling coefficient (kt ~ 0.28), and a low

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relative dielectric constant (εs/ε ~ 8), which makes it an idea material for designing

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transducer with high center frequency. Specific design parameters and performance of 6

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the transducers were simulated through a finite element model-based simulation

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software PZFlex (Weidlinger Associates Inc, CA). The materials used for the

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simulation was listed at Table. 2. Table. 2. Materials used for the transducer simulation consideration. Function

c (m/s)

ZnO

Piezoelectric element

6400

Si

Lens

8430

Water

Front load

1540

EPO-TEK 301

Backing

2650

ρ(kg/m3)

Z(MRayl)

5680

36.3

2340

19.8

1000

1.54

1150

3.05

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Material

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Figure.3. a) Simulation structural schematic of the silicon lens transducer; b)

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Pulse-echo waveform and frequency spectrum of the ZnO silicon lens transducer

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achieved from the finite element simulation; c) Simulated result of focus beamplots; d)

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Lateral beam profile of the silicon lens transducer at the focused point.

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Figure.3 (a) shows the simulation structural schematic of the silicon lens 7

ACCEPTED MANUSCRIPT transducer. The designed center frequency was set to be 400 MHz, the thickness of

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ZnO was 7 µm. During the simulation process, piezoelectric material was connected

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series to a 50 Ohms resistor, and the transducer was driven by a sinusoidal signal

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under the following conditions: an excitation frequency of 400 MHz, and a driving

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voltage of 1V peak-to-peak. Box size was chosen to be 1/15 wavelength at both axial

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and lateral direction. Simulation time was set to be 1 µs for signal sending and

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receiving.

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The pulse-echo waveform and frequency spectrum were achieved from the finite element simulation (Figure.3 (b)). The center frequency (fc) and -6 dB bandwidth (BW)

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were acquired from the frequency spectra, which was calculated to be 399 MHz and

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28%. As illustrated in Figure.3 (c), the ultrasound beam was tightly focused at the

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focused point, and the lateral beam profile of the silicon lens transducer at the focused

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point (Figure.3 (d)) demonstrated the -6 dB beam width simulated to be around 13 µm.

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The finite element simulation results demonstrate that based on this silicon lens, it is

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possible to design and fabricate ultrasonic transducer with high center frequency and

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narrow -6 dB beam width.

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Figure.4 The fabrication process of the Si lens transducer.

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The fabrication process of the transducer is illustrated in Figure.4. Firstly, a thin 8

ACCEPTED MANUSCRIPT layer of Cr/Au (500 Å /1000 Å) was sputtered on the opposite side of the lens part as

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the bottom electrode. The ZnO piezoelectric thin film was then deposited on the

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bottom electrode using RF sputtering machine for the desired thickness. The

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sputtering conditions were set to O2: Ar (1:1) gas pressure of 10 mTorr at 300°C with

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an RF power of 300 W, as a result, the deposition rate was around 0.6 µm/h. Next, the

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top Cr/Au electrode was patterned and deposited on ZnO layer by a combination of

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lift-off process and sputtering technique. Subsequently, the wafer was diced to small

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chips (4*4 mm2) by the dicing saw. Figure.4 (a) & (b) shows a silicon wafer with the

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etched lenses on the front and backside of the ZnO layer with Cr/Au electrode after

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dicing. A diced chip was then electrical connected with the lead wires to bottom and

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top electrodes by conductive epoxy (E-solder 3022). To provide RF shielding, the

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device was encased in a brass tube. The gap between the cylindrical brass housing and

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the device was filled by insulating Epotek 301 epoxy. A 1 µm parylene layer was

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vapor deposited on the front face of the transducer, serving as a protection layer.

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Lastly, the silicon lens transducer was housed in a SMA connector. Figure.4 (c) shows

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the cross section of the deposited ZnO and Figure.4 (d) shows a picture of a

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completed silicon lens ultrahigh frequeny transducer.

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Figure.4 a) Produced silicon etched lenses on the front of the silicon wafer; b)

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backside of the ZnO layer with Cr/Au electrodes after dicing; c)Cross section of the

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silicon wafer with ZnO layer; and d) A photograph of the finished ultrahigh frequency

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silicon lens transducer.

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Transducer Characterization

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The performance of the fabricated transducer was evaluated at room temperature

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in a de-ionized water bath using a pulse-echo arrangement by reflecting the signal

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from a polished x-cut quartz target placed at the focus of the transducer. [18] The

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transducer was excited by a pulser/receiver (Panametrics model 5910R) with an

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electrical impulse at 200 Hz repetition rate and 50 Ohms damping. The reflected echo

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was received by the transducer and digitized by a 1GHz oscilloscope (LC534, LeCroy

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Group) with 50 Ohms coupling. The frequency spectrum was generated via Fast

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Fourier Transform (FFT) of the received waveform. The characteristics of the

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transducers were determined from the measured frequency spectrum. [19]

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The insertion loss (IL) was calculated by comparing the spectra of the

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transmitted and received responses. Compensation was applied for the attenuation in 10

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the water bath and the loss caused by the imperfect reflection from the quartz target.

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[18] The following equation was used for calculation: ‫ = ܮܫ‬20log

ܸ଴ + 1.9 + 2.2 × 10ିସ × 2݀ × ݂ ଶ ܸ௜

Where IL is the insertion loss, V0 and Vi are the transducer output and excitation

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voltage, respectively, d is the distance (mm) between the target and transducer surface,

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f is the center frequency. The attenuation compensation for quartz is 1.9 dB and the

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attenuation coefficient compensation for water is 2.2 × 10ିସ ݀‫ܤ‬/ܿ݉/‫ ݖܪܯ‬ଶ

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The measured pulse-echo waveform and normalized frequency spectrum of the

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fabricated transducer is shown in Figure.5. The measured transducer performance was

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list in Table.3. We can find that the center frequency of the transducer is 330 MHz,

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and -6 dB bandwidth is determined to be 21 % as well as -20 dB pulse length is 19 ns.

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Additionally, the insertion loss is measured and compensated to be -68 dB. The

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measured properties are closed to the simulated ones.

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Figure.5 Measured pulse-echo waveform (solid line) and normalized frequency

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spectrum (dashed line) for ultrahigh frequency silicon lens transducer.

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Table.3 Measured transducer performance. Performance

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330

-6 dB Bandwidth

21%

-20 dB Pulse length (nsec)

19

Insertion loss (dB)

-68

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Single Beam Acoustic Tweezer

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Setup

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The micro particle trapping device was set with the focused silicon-lens

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transducer in a customized mylar film dish of distilled water as shown in Figure.6. A

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customized LabVIEW program controlled three-axis motorized linear stage (LMG26

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T50 MM, OptoSigma, Santa Ana, CA) was used to manipulate the transducer

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perpendicularly to the beam axis at the focal distance. The trapped motions of the

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particles were recorded via a CMOS camera (ORCA-Flash 2.8, Hamamatsu, Japan)

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combined with an inverted microscope (IX-71, Olympus, Japan). The images as well

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as videos captured by the CMOS camera were recorded with a computer. To perform

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microspheres manipulation, the transducer was driven in a sinusoidal burst mode by a

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function generator (AFG3251, Tektronix, Anaheim, CA) and then amplified by a 50

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dB power amplifier (525 LA, ENI Rochester, MN) to achieve desirable peak-to-peak

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voltage amplitude, duty factor and pulse repetition frequency. Polystyrene

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microspheres (Polyscience, Warrington, PA) were used as targeted particles.

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Figure.6 Experimental setup to observe the trapping of microparticles using the

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fabricated silicon lens transducer.

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Due to the ultrahigh center frequency, the ZnO silicon-lens transducers were

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capable to be excited efficiently at frequency higher than 300 MHz and were able to

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trap and manipulate single microsphere with small size. An example of a single 5 µm

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microsphere manipulation using the ZnO silicon-lens transducer is demonstrated in

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Figure.7. In this case, the tightly focused transducer was driven by a sinusoidal burst

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under the following conditions: an excitation frequency of 330 MHz, a driving

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voltage of 32V, a duty cycle of 0.1% and a pulse repetition frequency of 1 kHz. A red

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dot was given as a reference point to show the location change of the microsphere. As

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can be seen, an individual 5 µm microsphere (a blue circle) was stably manipulated

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along with the movement of the transducer. As the size of the microsphere trapped is

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at the cellular level, such ultrahigh frequency silicon-lens transducers have great

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potential to be a single cell manipulator for wide range of applications in biomedical

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and biophysical science.

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FIG.6. A trapped 5 µm polystyrene microsphere motion is handled by the silicon lens

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ultrahigh frequency transducer. The trapped particle is present inside a blue circle

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while a red dot is given as a reference point to show the change in the particle location

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along with the movement of the transducer.

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Conclusions

In summary, this study demonstrated the design and fabrication of ultrahigh

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frequency ZnO silicon lens transducer as well as its application for single beam

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acoustic tweezers. The silicon based lens was fabricated through isotropic wet etching

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technique without machining. Using optimum etching parameters, we were able to

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make a silicon lens with smooth surface of the etched cavity. ZnO silicon-lens

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transducer fabricated has center frequency of 330 MHz with -6 dB bandwidth of 21 %,

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and insertion loss of -68 dB. The capability of this ZnO silicon-lens ultrahigh

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frequency transducer in manipulating an individual 5 µm microsphere has been

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demonstrated. As the size of the trapped particle is at the cellular level, this 330 MHz

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silicon lens ultrasonic transducer can offer wide range of applications in biomedical

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and biophysical science.

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Acknowledgements

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This work was supported by the National institutes of Health (Grant No.

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P41-EB002182); the National Key Project of Intergovernmental Cooperation in 14

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International Scientific and Technological Innovation (Grant No. 2016YFE0107900);

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and the National Natural Science Foundation of China (Grant No. 11604251).

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Highlights

1. Silicon based lens for ultrasound focusing was developed through isotropic chemical wet etching technique.

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2. Focused ZnO ultrahigh frequency (>300 MHz) ultrasonic transducer is fabricated and characterized.

3. The ZnO ultrasonic transducer enable the high frequency ultrasound to manipulate individual

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cell-size particles.