Fabrication and Characterization of Thickness Mode Piezoelectric Devices for Atomization and Acoustofluidics

Summary

Fabrication of piezoelectric thickness mode transducers via direct current sputtering of plate electrodes on lithium niobate is described. Additionally, reliable operation is achieved with a transducer holder and fluid supply system and characterization is demonstrated via impedance analysis, laser doppler vibrometry, high-speed imaging, and droplet size distribution using laser scattering.

Abstract

We present a technique to fabricate simple thickness mode piezoelectric devices using lithium niobate (LN). Such devices have been shown to atomize liquid more efficiently, in terms of flow rate per power input, than those that rely on Rayleigh waves and other modes of vibration in LN or lead zirconate titanate (PZT). The complete device is composed of a transducer, a transducer holder, and a fluid supply system. The fundamentals of acoustic liquid atomization are not well known, so techniques to characterize the devices and to study the phenomena are also described. Laser Doppler vibrometry (LDV) provides vibration information essential in comparing acoustic transducers and, in this case, indicates whether a device will perform well in thickness vibration. It can also be used to find the resonance frequency of the device, though this information is obtained more quickly via impedance analysis. Continuous fluid atomization, as an example application, requires careful fluid flow control, and we present such a method with high-speed imaging and droplet size distribution measurements via laser scattering.

Introduction

Ultrasound atomization has been studied for almost a century and although there are many applications, there are limitations in understanding the underlying physics. The first description of the phenomenon was made by Wood and Loomis in 1927¹, and since then there have been developments in the field for applications ranging from delivering aerosolized pharmaceutical fluids² to fuel injection³. Although the phenomenon works well in these applications, the underlying physics is not well understood^4,5,6.

A key limitation in the field of ultrasonic atomization is the choice of material used, lead zirconate titanate (PZT), a hysteretic material prone to heating⁷ and lead contamination with elemental lead available from the inter-grain boundaries^8,9. Grain size and mechanical and electronic properties of grain boundaries also limit the frequency at which PZT can operate¹⁰. By contrast, lithium niobate is both lead-free and exhibits no hysteresis¹¹, and can be used to atomize fluids an order of magnitude more efficiently than commercial atomizers¹². The traditional cut of lithium niobate used for operation in the thickness mode is the 36-degree Y-rotated cut, but the 127.86-degree Y-rotated, X-propagating cut (128YX), typically used for surface acoustic wave generation, has been shown to have a higher surface displacement amplitude in comparison with the 36-degree cut¹³ when operated in resonance and low loss. It has also been shown that thickness mode operation offers an order of magnitude improvement in atomizer efficiency over other modes of vibration¹³, even when using LN.

The resonance frequency of a piezoelectric device operating in the thickness mode is governed by its thickness t: the wavelength λ = 2t/n where n = 1, 2,... is the number of anti-nodes. For a 500 µm thick substrate, this corresponds to a wavelength of 1 mm for the fundamental mode, which can then be used to calculate the fundamental resonance frequency, f = v/λ if the wave speed, v, is known. The speed of sound through the thickness of 128YX LN is approximately 7,000 m/s, and so f = 7 MHz. Unlike other forms of vibration, particularly surface-bound modes, it is straightforward to excite higher-order thickness mode harmonics to much higher frequencies, here to 250 MHz or more, though only the odd-numbered modes may be excited by uniform electric fields¹⁴. Consequently, the second harmonic (n = 2) near 14 MHz cannot be excited, but the third harmonic at 21 MHz (n = 3) can. Fabrication of efficient thickness mode devices requires depositing electrodes onto opposing faces of the transducer. We use direct current (DC) sputtering to accomplish this, but electron-beam deposition and other methods could be used. Impedance analysis is useful to characterize the devices, particularly in finding the resonance frequencies and electromechanical coupling at these frequencies. Laser Doppler vibrometry (LDV) is useful to determine the output vibration amplitude and velocity without contact or calibration¹⁵, and, via scanning, the LDV provides the spatial distribution of surface deformation, revealing the mode of vibration associated with a given frequency. Finally, for the purposes of studying atomization and fluid dynamics, high-speed imaging can be employed as a technique to study the development of capillary waves on the surface of a sessile drop^16,17. In atomization, like many other acoustofluidic phenomena, small droplets are produced at a rapid rate, over 1 kHz in a given location, too quickly for high-speed cameras to observe with sufficient fidelity and field of view to provide useful information over a sufficiently large droplet sample size. Laser scattering may be used for this purpose, passing the droplets through an expanded laser beam to (Mie) scatter some of the light in reflection and refraction to produce a characteristic signal that may be used to statistically estimate the droplet size distribution.

It is straightforward to fabricate piezoelectric thickness mode transducers, but the techniques required in device and atomization characterization have not been clearly stated in the literature to date, hampering progress in the discipline. In order for a thickness mode transducer to be effective in an atomization device, it must be mechanically isolated so that its vibration is not damped and it must have a continuous fluid supply with a flow rate equal to the atomization rate so that neither desiccation nor flooding occur. These two practical considerations have not been thoroughly covered in the literature because their solutions are the result of engineering techniques rather than pure scientific novelty, but they are nonetheless critical to studying the phenomenon. We present a transducer holder assembly and a liquid wicking system as solutions. This protocol offers a systematic approach to atomizer fabrication and characterization for facilitating further research in fundamental physics and myriad applications.

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Protocol

1. Thickness mode transducer fabrication via DC sputtering

1. Wafer preparation
   1. Place a 100 mm 128YX LN wafer in a clean glass dish of at least 125 mm diameter. Sonicate the wafer in at least 200 mL of acetone for 5 min.
   2. Repeat sonication with isopropyl alcohol and again with deionized water for 5 min each.
   3. Remove visible water from the surface using dry nitrogen.
   4. Completely remove water from the surface by placing the wafer on a hotplate at 100 °C for 5 min. Ensure that there is a sheet of aluminum foil on the hotplate as this helps in dissipation of charge buildup on the wafer.
2. Electrode deposition
   1. Place the wafer in the vacuum chamber of the sputter deposition system and pump down the chamber to 5 x 10^-6 mTorr. Set the argon pressure to 2.3 mTorr and the rotation speed to 13 rpm.
      NOTE: If parameters for the specific instrument being used have been established that result in high quality films, then use those instead.
   2. Deposit 5−10 nm of titanium at 1.2−1.6 A/s.
      NOTE: Before beginning this process with the intended wafer, test the deposition rate with the plasma power set to 200 W and depositing for 1 min. Then measure the height of the layer with a profilometer. Do this separately for each metal. Set the power according to this test in order to achieve the stated deposition rate.
   3. Deposit 1-1.2 µm of gold at 7−9 A/s.
      NOTE: Deposition at a higher rate due to increased plasma power or increased argon partial pressure may reduce film quality.
   4. Remove the wafer and repeat steps 1.2.1−1.2.3 for the second side of the wafer.
3. Dicing
   1. Use a dicing saw to dice the entire wafer as needed.
      NOTE: A protective resist can be applied on the substrate prior to dicing, and the system (Table of Materials) used here applies a UV curable film just before the samples are loaded on the dicing stage. It is found that dicing the samples with an automated dicing saw does not compromise the integrity of the samples. Hand-scribe dicing of LN is possible, though tedious and prone to inconsistencies.

2. Making electrical and mechanical contact with the transducer

NOTE: Several methods are described below (steps 2.1−2.4), and it is highlighted later in the protocol which method is most appropriate for each subsequent step.

1. Place a diced transducer flat on a magnetic steel plate. Mount one pogo-probe in contact with the plate and another pogo-probe in contact with the top surface of the transducer. Hereafter this will be referred to as pogo-plate contact.
2. Place the transducer between two pogo-probes. Hereafter referred to as pogo-pogo contact.
3. Solder wire to each face of the transducer. Hereafter referred to as solder contact.
4. Assemble a custom transducer holder.
   1. Order the custom printed circuit boards (PCBs) whose Gerber files have been provided.
   2. Solder two surface mount spring contacts(Table of Materials) to each custom PCB. Press fit the spikes into the plated holes on the custom PCBs such that they point away from each other.
   3. Connect the two custom PCBs with board spacers and screws so that the contacts are just in contact with each other. Adjust the spacing with plastic washers if necessary.
   4. Slide a 3 mm x 10 mm transducer in between the inner pair of contacts. Clip the outer contacts so they do not short the circuit.
      NOTE: Figure 1 shows the entire assembly.

3. Resonance frequency identification via impedance analysis

1. Ensure that a port calibration has been performed according to the manufacturer’s instructions for the specific contact method being used.
2. Connect a transducer to the open port of the network analyzer (Table of Materials) with one of the contact methods described in steps 2.1−2.4.
   NOTE: It can be instructive to repeat this analysis with multiple electrical contact methods and compare the results.
3. Select the reflection coefficient parameter, s11, via the user interface of the network analyzer, choose the frequency range of interest, and perform the frequency sweep.
   NOTE: s11 is the input reflection coefficient and has a minimum value at the resonance frequency of operation. For a typical 500 µm thick 128YX LN wafer, the primary resonance frequency will be near 7 MHz and the second harmonic will be near 21 MHz, as illustrated in Figure 2. The impedance plot in frequency space displayed on the instrument will exhibit local minima at the resonance frequencies.
4. Export the data by selecting Save/Recall | Save Trace Data on the user interface for closer inspection using data processing software to identify the precise minima locations.

4. Vibration characterization via LDV

1. Place a transducer in pogo-plate contact on the LDV stage. Connect the pogo-probe leads to the signal generator. Ensure that the objective in use is selected in the acquisition software (Table of Materials) and focus the microscope on the surface of the transducer.
2. Define the scan points by selecting Define scan points or proceed to step 4.3 if performing a continuous scan.
3. Select the Settings option and under the General tab, select either the FFT or Time option depending on whether the scan is being performed in frequency or time domain. Select the number of averages in this section.
   NOTE: The number of averages affects scan time. Five averages for the transducers described in this protocol have shown to give sufficient signal/noise ratio.
4. In the Channel tab, make sure that the Active boxes are checked, which correspond to the reference and reflected signal from the transducer. Adjust the reference and incident channels by selecting a voltage value from the drop-down menu in order to obtain maximum signal strength from the substrate.
5. In the Generator tab, if the measurement is carried out under single frequency signal, select Sine from the Waveform pull down list; if it is under a band signal, select MultiCarrierCW.
6. Change the bandwidth and FFT lines in the Frequency tab to adjust the scan resolution for a frequency domain scan. Similarly, change the Sample Frequency in the Time tab when performing time domain measurements.
   NOTE: The bandwidth typically used is 40 MHz and the number of FFT lines is 32,000. The presentation software (Table of Materials) can be used to process and analyze the data obtained from the scan. A typical displacement spectrum is provided in Figure 3.

5. Fluid supply

1. Obtain a 25 mm long, 1 mm diameter wick composed of a bundle of fibers of a hydrophilic polymer designed to transport aqueous liquid across its length such as those available for plug-in air fresheners. Trim one end such that an off center point is formed.
2. Insert the wick into a syringe tip with an inner diameter that provides a snug fit and a length that allows the wick to extend 1−2 mm beyond each end. Lock the tip onto a syringe with the desired capacity (1−10 mL).
3. Mount the wick/syringe assembly such that the wick is 10°−90° from horizontal (depending on the desired atomization rate, which also depends on the applied voltage) and the tip of the wick is just in contact with the edge of the transducer as shown in Figure 1C.
4. Fill the syringe with water and apply a continuous voltage signal (starting with 20 Vpp) at the resonance frequency determined using the impedance analyzer. Adjust the voltage level until the liquid is atomized continuously without the device flooding or drying out.

6. Dynamics observation via high-speed imaging

1. Rigidly mount a high-speed camera horizontally on an optical table, place a transducer in either pogo-pogo contact or pogo-plate contact on an x-y-z stage near the focal length of the camera, and position a diffuse light source at least one focal length on the opposite side of the transducer from the camera.
2. For pogo-pogo contact, position the fluid supply so that it does not block the camera view or the light source. For pogo-plate contact, apply fluid directly to the substrate with a pipette.
3. Adjust the camera focus and the x-y-z position to bring the fluid sample into sharp focus.
4. Estimate the frequency of the specific phenomenon to be studied based on literature. Choose a frame rate at least twice as large as this frequency according to the Nyquist rate in order to avoid aliasing.
   NOTE: For example, consider capillary waves that occur on a sessile drop at a range of frequencies. Cameras limited in spatial resolution can only distinguish waves with a minimum amplitude. In this case the minimum amplitude occurs around 4 kHz so a frame rate of 8,000 frames per second (fps) is chosen.
5. Adjust the light intensity, the camera shutter, or both in order to optimize contrast between the fluid and the background.
   NOTE: An opaque dye can be added to the fluid in order to increase the contrast.
6. Connect alligator clips from the amplified signal generator to the pogo-probes leads.
7. Capture video in the camera software simultaneously with actuation via the voltage signal either by manually triggering both at the same time or by connecting a trigger output from the signal generator to the camera.
   NOTE: The typical frame rate used is 8,000 fps and a CF4 objective is used.
8. Save only the frames containing the phenomenon to avoid wasted storage, which is particularly relevant at large frame rates, to produce a result as shown in Figure 4.
   NOTE: Make sure to save the file in a format that is compatible with the image processing software of choice so that useful data can be extracted.

7. Droplet size measurement via laser scattering analysis

1. The laser scattering system (Table of Materials) has a module that transmits the laser and one that receives the scattered laser signal. Position the modules along the rail provided with the system, with a 20−25 cm gap between them.
2. Rigidly mount a platform in this gap such that, when the transducer and fluid supply assemblies are placed on it, atomized mist will be ejected into the laser beam path. Facilitate this alignment by turning on the laser beam via selecting Tools | Laser Control... | Laser on as a visual indicator.
3. Fix the transducer holder to the platform and fix the fluid supply assembly to an articulated arm (Table of Materials). Position the fluid supply assembly so that the tip of the wick is just in contact with the edge of the transducer.
4. Create a standard operating procedure (SOP) in the software by clicking the New SOP icon. Configure the SOP with the following settings: template = Default continuous, sampling period (s) = 0.1, under Data handling, click Edit... and set Spray profile | Path length (mm) to 20.0, click Alarms to uncheck Use default values and set Min transmission (%) to 5 and 1 and set Min scattering to 50 and 10. Leave all other settings as defaults.
   NOTE: Consult the software manual that came with the instrument.
5. Start the measurement within the software by clicking Measure | Start SOP and selecting the SOP created in step 7.4. Wait for background calibrations to complete. Fill the fluid supply reservoir, the syringe, with water up to the desired level and note the volume. Turn on the voltage signal to begin atomizing the fluid. Start the stopwatch and start the measurement by clicking Start.
6. The software generates a size distribution based on the scattered laser signal at the receiver due to Mie theory and a multiple scattering algorithm. Once the desired volume of fluid has been atomized, turn off the voltage signal, stop the stopwatch, and record the final volume, and stop recording data by clicking Stop.
   NOTE: The laser scattering system is capable of measuring as little as 1 μL of fluid and does not have an upper limit for fluid volume. The atomization flow rate can simply be calculated by dividing the volume by the time duration.
7. In the measurement histogram, select the portion of the data during which the atomization was occurring as expected and the signal at the receiver was strong enough to be statistically significant. Click Average | Ok to generate a distribution based on the selected data.
   NOTE: All measurements with this technique are statistical averages and thus, if there are too few droplets, then the scattered signal will be weak, and the measurement will be statistically insignificant.
8. Save the average distribution by selecting the window and clicking Edit | Copy text then pasting the result in a text file and saving with an appropriate name.
   NOTE: This distribution data can now be used with other software (e.g., MATLAB) to create the plot in Figure 5.

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Results

Thickness mode piezoelectric devices were fabricated from 128YX lithium niobate. Figure 1 shows a complete assembly to hold the transducer in place with a custom transducer holder used with the passive fluid delivery system developed for continuous atomization. The characterization steps for these devices include determination of the resonant frequency and harmonics using an impedance analyzer (Figure 2). The fundamental frequency of the devices was found ...

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Discussion

The dimensions and aspect ratio of a transducer affects the vibration modes it produces. Because the lateral dimensions are finite, there are always lateral modes in addition to the desired thickness modes. The above LDV methods can be used to determine dominant modes in the desired frequency range for a given transducer. A square with dimensions below 10 mm typically gives a close approximation to a thickness mode. Three by ten millimeter rectangles also work well. Movie 1 and Movie 2

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Materials

Name	Company	Catalog Number	Comments
Amplifier	Amplifier Research, Souderton, PA, USA	5U1000
Articulating arm	Fisso, Zurich, Switzerland
CF4 Objective	Edmund Optics, Barrington, NJ, USA		Objective used for high speed imaging
Dicing saw	Disco, Tokyo, Japan	Disco Automatic Dicing Saw 3220
Fiber Fragrance Diffuser Wick	Weihai Industry Co., Ltd., Weihai, Shandong, China	https://www.weihaisz.com/Fiber-Fragrance-Diffuser-Wick_p216.html
High Speed Camera	Photron, San Diego, USA	Fastcam Mini
Laser Doppler Vibrometer	Polytec, Waldbronn, Germany	UHF120	Non-contact laser doppler vibrometer
Laser Scattering Droplet size measurement system	Malvern Panalytical, Malvern, UK	STP5315
Lithium niobate substrate	PMOptics,Burlington, MA, USA	PWLN-431232	4” double-side polished 0.5 mm thick 128°Y-rotated cut lithium niobate
Luer-lock syringes	Becton Dickingson, New Jersey, USA
Nano3 cleanroom facility	UCSD, La Jolla, CA, USA		Fabrication process is performed in it.
Network Analyzer	Keysight Technologies, Santa Rosa, CA, USA	5061B
Oscilloscope	Keysight Technologies, Santa Rosa, CA, USA	InfiniiVision 2000 X-Series
PSV Acquistion Software	Polytec, Waldbronn, Germany	Version 9.4	LDV Software
PSV Presentation Software	Polytec, Waldbronn, Germany	Version 9.4	LDV Software
Signal generator	NF Corporation, Yokohama, Japan	WF1967 multifunction generator
Single Post Connector	DigiKey, Thief River Falls, MN	ED1179-ND
Sputter deposition	Denton Vacuum, NJ, USA	Denton 18	Denton Discovery 18 Sputter System
Surface Mount Spring Contacts	DigiKey, Thief River Falls, MN	70AAJ-2-M0GCT-ND
Teflon wafer dipper	ShapeMaster, Ogden, IL, USA	SM4WD1	Wafer Dipper 4"
XYZ Stage	Thor Labs, Newton, New Jersey, USA	MT3	Optical table stages