Fabrication of Surface Acoustic Wave Devices on Lithium Niobate

Summary

Two fabrication techniques, lift-off and wet etching, are described in producing interdigital electrode transducers upon a piezoelectric substrate, lithium niobate, widely used to generate surface acoustic waves now finding broad utility in micro to nanoscale fluidics. The as-produced electrodes are shown to efficiently induce megahertz order Rayleigh surface acoustic waves. 

Abstract

Manipulation of fluids and particles by acoustic actuation at small scale is aiding the rapid growth of lab-on-a-chip applications. Megahertz-order surface acoustic wave (SAW) devices generate enormous accelerations on their surface, up to 10⁸ m/s², in turn responsible for many of the observed effects that have come to define acoustofluidics: acoustic streaming and acoustic radiation forces. These effects have been used for particle, cell, and fluid handling at the microscale—and even at the nanoscale. In this paper we explicitly demonstrate two major fabrication methods of SAW devices on lithium niobate: the details of lift-off and wet etching techniques are described step-by-step. Representative results for the electrode pattern deposited on the substrate as well as the performance of SAW generated on the surface are displayed in detail. Fabrication tricks and troubleshooting are covered as well. This procedure offers a practical protocol for high frequency SAW device fabrication and integration for future microfluidics applications.

Introduction

Relying on the well-known inverse piezoelectric effect, where the atomic dipoles create strain corresponding to the application of an electric field, piezoelectric crystals such as lithium niobate LiNbO₃ (LN), lithium tantalite LiTaO₃ (LT), can be used as electromechanical transducers to generate SAW for microscale applications^1,2,3,4,5,6. By enabling the generation of displacements up to 1 nm at 10-1000 MHz, SAW-driven vibration overcomes the typical obstacles of traditional ultrasound: small acceleration, large wavelengths, and large device size. Research to manipulate fluids and suspended particles has recently accelerated, with a large number of recent and accessible reviews^7,8,9,10.

Fabrication of SAW-integrated microfluidic devices requires fabrication of the electrodes—the interdigital transducer (IDT)¹¹—on the piezoelectric substrate to generate the SAW. The comb-shape fingers create compression and tension in the substrate when connected to an alternating electric input. The fabrication of SAW devices has been presented in many publications, whether using lift-off ultraviolet photolithography alongside metal sputter or wet etching processes¹⁰. However, the lack of knowledge and skills in fabricating these devices is a key barrier to entry into acoustofluidics by many research groups, even today. For the lift-off technique^12,13,14, a sacrificial layer (photoresist) with an inverse pattern is created on a surface, so that when the target material (metal) is deposited on the whole wafer, it can reach the substrate in the desired regions, followed by a “lift-off” step to remove the remaining photoresist. By contrast, in the wet etching process^15,16,17,18, the metal is first deposited on the wafer and then photoresist is created with a direct pattern on the metal, to protect the desired region from “etching” away by a metal etchant.

In a most commonly used design, the straight IDT, the wavelength of the resonant frequency of the SAW device is defined by the periodicity of the finger pairs, where the finger width and the spacing between fingers are both /4¹⁹. In order to balance the electric current transmission efficiency and the mass loading effect on the substrate, the thickness of the metal deposited on the piezoelectric material is optimized to be about 1% of the SAW wavelength²⁰. Localized heating from Ohmic losses²¹, potentially inducing premature finger failure, can occur if insufficient metal is deposited. On the other hand, an excessively thick metal film can cause a reduction in the resonant frequency of the IDT due to a mass loading effect and can possibly create unintentional acoustic cavities from the IDTs, isolating the acoustic waves they generate from the surrounding substrate. As a result, the photoresist and UV exposure parameters chosen vary in the lift-off technique, depending upon different designs of SAW devices, especially frequency. Here, we describe in detail the lift-off process to produce a 100 MHz SAW-generating device on a double-sided polished 0.5 mm-thick 128° Y-rotated cut LN wafer, as well as the wet etching process to fabricate the 100 MHz device of identical design. Our approach offers a microfluidic system enabling investigation of a variety of physical problems and biological applications.

Protocol

1. SAW device fabrication via the lift-off method

1. Perform wafer solvent cleaning in a Class 100 clean room facility by immersing the 4” (101.6 mm) LN wafer into acetone, followed by isopropyl alcohol (IPA), then deionized water (DI water), each in a sonication bath for 5 min. Pick up the wafer and blow the surface dry with nitrogen (N₂) gas flow to remove the remaining DI water from the wafer. 
   CAUTION: Perform the acetone and IPA immersions in a fume hood. Avoid inhalation and skin contact with IPA. Avoid skin and eye contact with acetone. Do not swallow.
   NOTE: Do not allow any fluid to evaporate upon the wafer; if any dust or debris on the surface, start this step over. 
2. Place the wafer onto a hotplate at 100 °C to prebake for 3 min.
   NOTE: Because of the pyroelectric property of LN, it will generate static charges and associated stress within the wafer during heating and cooling. It is recommended to put the wafer onto a piece of aluminum (Al) foil after removing it from the hot plate to release the static charges and avoid breaking it. 
3. Place the wafer onto a spin coater. Using a dropper, cover about 75% of the wafer’s surface with negative photoresist (NR9-1500PY). Program a speed of 500 rpm with an acceleration of 3,000 rpm/s for 5 sec and then a speed of 3,500 rpm with an acceleration of 3,000 rpm/s for 40 sec, to produce a layer of photoresist around 1.3 µm.
   CAUTION: Perform spin coating in a fume hood. Inhalation of photoresist fumes can cause headaches. 
   NOTE: The thickness may vary depending on the condition of the photoresist and the spin coater used, even with the same spin settings. The photoresist may be spun beyond the edge and onto the wafer’s obverse edge; this must be removed by using an acetone-doused swab. Left present, the photoresist will stick the wafer to the hotplate during the soft bake.
4. To soft bake, place the wafer onto a hotplate at 100 °C, ramp the temperature up to 150 °C, hold it at 150 °C for 1 min. Then move the wafer from the hotplate, and let the wafer cool in the air to room temperature (RT).
   NOTE: Due to the pyroelectric effect, if the temperature of the LN wafer is suddenly changed, for example, by directly transferring the LN wafer onto the hotplate or Al foil at 150 °C, the thermal shock within the wafer will likely shatter it. The presence of nonuniform metal on the surface, such as electrodes, significantly enhances this risk. In applications where the transparency of the LN is not important, consider using so-called “black” LN or more accurately reduced LN, which is dark brown and translucent but has negligible pyroelectricity.
5. Transfer the wafer to the mask aligner (MLA150) for ultraviolet exposure. Expose the photoresist with an energy dose of 400 mJ/cm² at 375 nm. The dose required may vary depending on the mask design and the age and condition of the photoresist. 
   NOTE: The wave propagation direction induced by IDTs should be along the X-propagating direction in order to effectively generate SAW. In other words, this implies the “fingers” of the IDT should be perpendicular to the X-axis direction. Typical LN wafer manufacturers place the primary (larger) wafer flat (straight edge alongside of wafer) perpendicular to the X-axis, so your IDT fingers should be parallel to this flat. Some manufacturers introduce a second (smaller) wafer flat to help indicate the Y- and Z-axis directions, but this detail is unimportant for SAW generation. Manufacturers often request specifications for the surface finish of the wafer; if you require a transparent wafer, request double-sided optically polished wafers. However, keep in mind that LN is birefringent, so any object illuminated with standard laboratory light and seen through the material will produce not one but two images. Overcoming this problem is discussed later. Single-side polished LN is a better choice for SAW generation if you do not need to see through the wafer, because spurious acoustic waves are diffused by the rough back surface.
6. Place the wafer onto a hotplate at 100 °C for 3 min for a post-exposure bake. Then transfer it onto Al foil and allow it to cool to RT.
   NOTE: The patterns should be visible after the post-exposure bake. If not, consider stripping the photoresist and restarting the process over from step 1.1 above.
7. Develop the wafer by placing it in a beaker filled with pure RD6 developer for 15 sec. Gently shake the beaker during development. Immerse the wafer into DI water for 1 min, and then rinse the wafer under DI water flow. Finally, use dry N₂ flow to remove the remaining DI water from the wafer. Never let any fluid evaporate on the wafer surface. 
   CAUTION: Develop the wafer in a fume hood. Avoid breathing in vapors or contacting the developer with eyes and skin.
   NOTE: The photolithography is complete after this step. The protocol can be paused here.
8. Hard bake the wafer on a hotplate at 100 °C for 3 min. Then transfer it onto Al foil and allow it to cool to RT. 
   NOTE: This step is to remove any moisture from the wafer and photoresist to prevent later outgassing during sputtering.
9. For electrode sputter deposition, place the wafer into a sputter deposition system. Vacuum the chamber to 5 x 10^-6 mTorr. Use a 2.5 mTorr argon flow, sputter chromium (Cr) with a power of 200 W for 5 nm as an adhesion layer, followed by sputtering Al with a power of 300 W for 400 nm to form the conductive electrodes.
   NOTE: Deposition time should be calculated from the expected thickness and the deposition rate. Titanium (Ti) can be used instead of chromium, though the removal process is more difficult, because Ti is tougher. Gold (Au) is also commonly deposited as electrodes. However, for higher frequency SAW devices, Al should replace Au to avoid the mass loading effects of the Au IDT fingers, which reduce the local SAW resonant frequency under the IDT, forming an acoustic cavity from which the SAW can only escape with significant loss.
10. For the lift-off process, transfer the wafer into a beaker and immerse in acetone. Sonicate at medium intensity for 5 min. Rinse with DI water and dry the wafer with N₂ flow.
    CAUTION: Use acetone in a fume hood. Avoid inhalation and skin or eye contact with acetone. Do not swallow.
    NOTE: The protocol can be paused here.
11. Use a dicing saw to dice the entire wafer into small pieces of chips as SAW devices for further applications. 
    NOTE: The process is complete. The protocol can be paused here. 
    NOTE: Instead of a saw, a diamond-tipped wafer scribe (or even a glass cutter) can be used to dice the LN wafer with some practice, though due to the anisotropy of LN it is important to scribe and break the wafer first along scribe lines perpendicular to the X-axis, followed by those lines along the X-axis.

2. SAW device fabrication via the wet etching method

1. Wafer solvent cleaning: In a Class 100 clean room facility by immersing the 4” (101.6 mm) LN wafer in acetone, followed by IPA, then DI water, each in a sonication bath for 5 min. Pick up the wafer and dry the surface using N₂ to remove the remaining DI water from the wafer. 
   CAUTION: Use acetone and IPA in a fume hood. Avoid inhalation and skin contact with IPA. Avoid acetone contact with skin and eyes. Do not swallow.
2. Place the wafer onto a hotplate at 100 °C for thermal treatment for 3 min. Then transfer it onto Al foil to cool down to RT.
3. Place the wafer into a sputter deposition system. Vacuum the chamber to 5 x 10^-6 mTorr. Use argon flow at 2.5 mTorr, sputter Cr with a power of 200 W for 5 nm as an adhesion layer, followed by sputtering Au with a power of 300 W for 400 nm to form the conductive electrodes.
   NOTE: The protocol can be paused here.
4. Place the wafer onto a spin coater. Using a dropper, cover about 75% of the wafer’s surface with positive photoresist (AZ1512). Program a speed of 500 rpm with an acceleration of 3,000 rpm/s for 10 sec and then a speed of 4,000 rpm with an acceleration of 3,000 rpm/s for 30 sec, ultimately producing a layer of photoresist around 1.2 µm. 
   CAUTION: Perform spin coating in a fume hood. Inhalation of photoresist fumes can cause headaches.
5. To soft bake, place the wafer onto a hotplate at 100 °C for 1 min. Then transfer it onto Al foil and allow it to cool to RT.
6. Transfer the wafer to the mask aligner (MLA150) for ultraviolet exposure. Expose the photoresist with an energy dose of 150 mJ/cm² at 375 nm. The dose required may vary depending on the mask design and the age and condition of the photoresist.
7. Place the wafer into a beaker filled with pure AZ300MIF developer for 30 sec. Gently shake the beaker during development. Immerse the wafer into DI water for 1 min, then rinse the wafer under DI water flow. Finally, use dry N₂ flow to remove the remaining DI water from the wafer. Never let any fluid evaporate on the wafer surface.
   CAUTION: Avoid contacting AZ300MIF with skin or eyes. Do not swallow.
8. Immerse the wafer into a beaker filled with Au etchant for 90 sec, gently shaking the beaker. After rinse the wafer under DI water flow, dry with N₂ flow to remove the remaining DI water from the wafer. Never let any fluid evaporate on the wafer surface.
   CAUTION: Gold etchant can be hazardous to the eyes and skin, and will cause respiratory irritation. This step requires more personal protective equipment (PPE), such as safety glass, black neoprene gloves, apron, etc.
9. Immerse the wafer into a beaker filled with Cr etchant for 20 sec, gently shaking the beaker. After rinse the wafer under DI water flow, dry with N₂ flow to remove the remaining DI water from the wafer. Never let any fluid evaporate on the wafer surface.
   CAUTION: Chromium etchant can cause eye, skin, and respiratory irritation. This step also requires more PPE.
10. Clean the (sample) wafer, by putting it into acetone, followed by IPA, and DI water in a sonication bath for 5 min each. Pick up the wafer and dry with N₂ gas flow over the surface of the wafer to remove the remaining DI water from the wafer.
    CAUTION: Use acetone in a fume hood. Avoid inhalation and skin contact acetone with skin and eyes. Do not swallow.
    NOTE: This step is to remove the undesired photoresist on the wafer. The protocol can be paused here.
11. Use a dicing saw to dice the entire wafer into discrete SAW devices for further use. 
    NOTE: The process is complete. The protocol can be paused here.

3. Experimental setup and testing

1. Observe the SAW device under bright-field optical microscopy.
   NOTE: There are possibly scratches across the metal layers on the LN. Generally they will not cause a notable influence of the device performance, as long as the scratches are not deep enough to result in an open circuit.
2. For SAW actuation, attach absorbers at both ends along the propagation direction of the SAW device to prevent reflected acoustic waves from the edges. 
3. Use a signal generator to apply a sinusoidal electric field to the IDT at its resonant frequency of around 100 MHz. An amplifier should be connected to amplify the signal. 
4. Use an oscilloscope to measure the actual voltage, current and power applied onto the device. The amplitude and frequency response of the SAW are measured by a laser Doppler vibrometer (LDV); the SAW-actuated droplet motion is recorded using a high-speed camera attached to the microscope.

Results

The IDT to be measured is designed to have a resonant frequency at 100 MHz, as the the finger width and the spacing between them are 10 μm, producing a wavelength of 40 μm. Figure 1 shows the SAW device and IDT fabricated using this method.

Using an oscillating electrical signal matched to the resonant frequency of the IDT, SAW can be generated across the surface of the piezoelectric material. The LDV measures the vibration via the Doppler effect on the surface,...

Discussion

SAW devices fabricated from either method are capable of generating useful traveling waves on the surface, and these methods underpin more complex processes to produce other designs. The resonant frequency is usually a little lower than the designed value, due to the mass loading effect of the metal deposited on top. However, there still some points worth discussing to avoid problems. 

Lift-off method
The choice of photoresist is important. It is possible to use a p...

Materials

Name	Company	Catalog Number	Comments
Absorber	Dragon Skin, Smooth-On, Inc., Macungie, PA, USA	Dragon Skin 10 MEDIUM
Amplifier	Mini-Circuits, Brooklyn, NY, USA	ZHL–1–2W–S+
Camera	Nikon, Minato, Tokyo, Japan	D5300
Chromium etchant	Transene Company, INC, Danvers, MA, USA	1020
Developer	Futurrex, NJ, USA	RD6
Developer	EMD Performance Materials Corp., Philidaphia, PA, USA	AZ300MIF
Dicing saw	Disco, Tokyo, Japan	Disco Automatic Dicing Saw 3220
Gold etchant	Transene Company, INC, Danvers, MA, USA	Type TFA
Hole driller	Dremel, Mount Prospect, Illinois	Model #4000	4000 High Performance Variable Speed Rotary
Inverted microscope	Amscope, Irvine, CA, USA	IN480TC-FL-MF603
Laser Doppler vibrometer (LDV)	Polytec, Waldbronn, Germany	UHF-120	4” double-side polished 0.5 mm thick 128°Y-rotated cut lithium niobate
Lithium niobate substrate	PMOptics, Burlington, MA, USA	PWLN-431232
Mask aligner	Heidelberg Instruments, Heidelberg, Germany	MLA150	Fabrication process is performed in it.
Nano3 cleanroom facility	UCSD, La Jolla, CA, USA
Negative photoresist	Futurrex, NJ, USA	NR9-1500PY
Oscilloscope	Keysight Technologies, Santa Rosa, CA, USA	InfiniiVision 2000 X-Series
Positive photoresist		AZ1512	Denton Discovery 18 Sputter System
Signal generator	NF Corporation, Yokohama, Japan	WF1967 multifunction generator	Wafer Dipper 4"
Sputter deposition	Denton Vacuum, NJ, USA	Denton 18
Teflon wafer dipper	ShapeMaster, Ogden, IL, USA	SM4WD1