Flexible Surface AcousticWave Device with AlN Film on Polymer Substrate

Surface acoustic wave device with c-axis-oriented aluminum nitride (AlN) piezoelectric thin films on polymer substrates can be potentially used for development of flexible sensors, flexible microfluidic applications, microsystems, and lab-on-chip systems. In this work, the AlN films have been successfully deposited on polymer substrates using the DC reactive magnetron-sputtering method at room temperature, and the XRD, SEM, and AFM methods reveal that low deposition pressure is beneficial to the highly c-axis-oriented AlN film on polymer substrates. Studies toward the development of AlN thin film-based flexible surface acoustic wave devices on the polymer substrates are initiated and the experimental and simulated results demonstrate the devices showing the acoustic wave velocity of 9000–10000 m/s, which indicate the AlN lamb wave.


Introduction
The surface-acoustic-wave-(SAW-) based microfluidic devices can be used not only for pumping, mixing, and droplet generation but also for biosensors and single-mechanismbased lab-on-a-chip applications [1].By now, all SAW devices are fabricated on the stiff substrates instead of flexible's, suah as LiNbO 3 [2], Piezoelectric (PE) thin films on Si wafer [3,4], diamond [5], Al 2 O 3 [6] and so on.SAW on flexible substrate is cheaper and can be bent easily, which is fitted for portable microfluidic applications, such as wrist health-care monitor.In this paper, a SAW device with aluminum nitride (AlN) piezoelectric thin films is fabricated on the polymer substrates, and its resonance response is also investigated.

Experimental Details
Kapton polyimide film 100H (Dupont-Toray Inc., thickness 250 μm) was chosen as the flexible substrate owing to its excellent mechanical and electrical properties, chemical stability, and wide operating temperature range (−269 • C to +400 • C).AlN was deposited by a home-made DC magnetron-sputtering system.The base pressure of the chamber was 1 × 10 −4 Pa before deposition.The aluminum (Al) target of purity was 99.999% and a diameter of 20 mm was used and water-cooled.The distance between the target and the substrate was fixed at 70 mm.Before depositing the AlN film, an Al underlayer was deposited on the polyimide substrate as a transition with deposition pressure of 0.27 Pa and DC power of 300 W. AlN was then deposited on the Al-coated polyimide substrate in an N 2 /Ar atmosphere.The effect of deposition pressure on the properties of the AlN films was investigated.The AlN deposition time for all samples is one hour, and the substrates were not intentionally heated.
The crystalline structure and crystal orientation of films were analyzed by X-ray diffraction (XRD-6000, Japan) using Cu-K α radiation and scanned angle of 2θ = 20 • ∼ 70 • .The degree of c-axis crystallization was examined by the full width at half maximum (FWHM) of the AlN (002) diffraction peak.The strain and crystallite size of the thin film were extracted from the XRD data by the standard method.Strain is calculated from ε z = (c − c 0 )/c 0 [7], where c 0 is the strain-free lattice parameter (4.979 Å) and the lattice constant c is equal to twice the interplanar spacing d, measured from the position of the (002) peak using Braggequation.Crystallite sizes were calculated from the Debye-Sherrer formula [8]: D = Kλ/(β cos θ), where K is the shape factor of the average crystallite with value of 0.94, λ the Xray wavelength (0.15406 nm for Cu target), β the FWHM in radians, θ the Bragg angle, and D the mean crystallite dimension normal to diffracting planes.For cross-sectional columnar structure observation, the microscopic film was observed using a Scanning Electron Microscope (SEM) (S4800, Hitachi, Ltd., Japan).The surface morphology and root mean square (rms) surface roughness of the AlN films were measured by atomic force microscopy (AFM) (SPI-3800N, Seiko, Japan).
To study the SAW propagation characteristics on flexible substrates, two-port resonators were designed and fabricated by conventional deep UV photolithography and lift-off process.Al was used as the electrodes with a thickness of 150 nm and each transducer consisted of 10 pairs of IDTs.The distance between the two transducers was 10λ, where the SAW wavelength λ is determined by the IDT pitch.The aperture was 80λ and the distance between the IDTs and the adjacent shorted reflecting gratings was designed as 5.375λ to ensure the formation of a standing wave at center frequency.The frequency characterization was carried out using Agilent 8722ES network analyzer.

Results and Discussion
Figure 1 shows the XRD pattern of the AlN films deposited at different deposition pressures.In Figures 1(a) and 1(b), AlN film shows a main XRD peak near 2θ = 36.1 • which corresponds to the AlN (002) crystal orientation.The results demonstrate that the AlN crystal structures are perpendicular to the polymer substrate with a good (002) orientation.The FWHM of the AlN (002) peak with 0.38 Pa is 0.321 • .The grain size is 274 Å and the strain is 0.4%.Figure 2 is an SEM micrograph of the cross-sectional structure of the AlN films deposited on the polymer substrate at the deposition pressure of 0.38 Pa.It is obvious that the film shows a neat arrangement and exhibits a typical (002)oriented columnar structure.The surface morphologies and Figures 5(a) and 5(b) present the measured S 21 spectra of the fabricated SAW resonators with the wavelengths of 7.128 μm and 6 μm, respectively.The measured S 21 spectra take the form of the Sinc function as expected for uniform IDT SAW devices [9].The resonance frequency of the SAW resonator with the wavelength of 7.128 μm is 1.355 GHz, corresponding to the acoustic wave velocity of 9658 m/s, whereas the resonance frequency of the SAW resonator with the wavelength of 6 μm shifts to 1.605 GHz, corresponding to the acoustic wave velocity of 9630 m/s, showing that the acoustic wave velocity is not affected by the device structure.The results have clearly demonstrated the piezoelectric effect of AlN film on polymer substrate.
To confirm whether the resonant frequency is generated through PE effect, the frequency response of the resonators has been modeled by the commercial software COMSOL Multiphysics.As the simulation diameters, the thicknesses of Al IDTs, AlN film, Al underlayer, and polyimide substrate are set to be the same as used in devices and are 150 nm, 1.23 μm, 70 nm, and 100 um, respectively.Figures 6(a the wavelength of 6 μm has the resonance frequency of 1.66 GHz, corresponding to the acoustic wave velocity of 9960 m/s.The experimental results are in good agreement with the simulated results.
Polymer substrates have a very low acoustic impedance (2 Mrayls) which is much smaller than that of the AlN layer (36 Mrayls).The Al underlay enhances the electromechanical coupling coefficient as an electric field can be induced between the IDT electrodes and the Al underlay [10].Moreover, the c-axis-oriented thin AlN films have relatively large thicknesses at about d = 0.2λ, where λ is the acoustic wavelength; therefore, the detected acoustic waves are the symmetrical S 0 Lamb waves which theoretically have the acoustic wave velocity near 10,000 m/s [11][12][13] in AlN.The velocity of the experimental results is slightly smaller than the simulated and theoretical results, possibly due to the defects of the AlN film and the slow-down effects by the Al-coated polyimide substrates.Figures 7(a the wavelength of 6 μm shifts to 1.81 GHz, corresponding to the acoustic wave velocity of 10860 m/s; both are higher than those in SAW with Al-coated polymer substrate.This result shows that Al-coated polyimide substrates would decrease the resonance frequency due to the low wave velocity of Alcoated polyimide substrate.

Conclusions
In this research, we have synthesized and characterized the AlN thin films on Dupont Kapton polyimide substrates by

Figure 1 :
Figure 1: XRD patterns of AlN thin films deposited on polymer substrates with Al electrodes under different deposition pressures.

Figure 2 :
Figure 2: The SEM micrograph of AlN thin films on polymer substrate.

Figure 5 :
Figure 5: The measured S 21 spectrum of the fabricated SAW resonators with a wavelength of (a) 7.128 μm and (b) 6 μm.

Figure 6 :Figure 7 :
Figure 6: The simulated resonance response of the SAW resonators with the wavelengths of (a) 7.128 μm and (b) 6 μm.