POTENTIAL OF Al 2 O 3 /GaN/SAPPHIRE LAYERED STRUCTURE FOR HIGH TEMPERATURE SAW SENSORS

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1 POTENTIAL OF Al 2 O 3 /GaN/SAPPHIRE LAYERED STRUCTURE FOR HIGH TEMPERATURE SAW SENSORS Sergei ZHGOON 1,*, Ouarda LEGRANI 2,3, Omar ELMAZRIA 4, Thierry AUBERT 2,3,5, Meriem ELHOSNI 4, Hamza MERSNI 2,3, Laetitia LEFEVRE 2,3, Sami HAGE-ALI 4 1 National Research University, Moscow Power Engineering Institute, Moscow, Russian Federation, CentraleSupélec, LMOPS EA 4423, Metz, France 3 Université de Lorraine, LMOPS EA 4423, Metz, France 4 Institut Jean Lamour, UMR 7198 Université de Lorraine-CNRS, Vandœuvre-lès-Nancy, France 5 Laboratoire SYMME, Université Savoie Mont Blanc, Annecy-le-Vieux, France * Corresponding author, zhgoon@ieee.org; Tel.: This paper describes the investigation of the potential of packageless structures for acoustic wave sensor applications based on Al 2 O 3 /IDT/GaN/Sapphire structure. The finite elements modeling predicts the possibility of acoustically isolated waveguiding layer acoustic wave (WLAW) to propagate inside this structure with very low displacement at the top surface of the topmost Al 2 O 3 layer. This layer serves as a protecting layer against oxidation of internal layers as well as an acoustically isolating layer, providing a mean for packageless packaging of this structure. The modeling results get confirmation in experiments with the precursor IDT/GaN/Sapphire structure thus supporting hopes on the predicted correct operation of the final Al 2 O 3 /IDT/GaN/Sapphire structure. Keywords: Layered structure; High temperature; SAW; Sensor 1. INTRODUCTION Surface acoustic wave (SAW) wireless, batteryless, passive sensors have demonstrated operability in harsh environment up to C. However, packaging at such temperatures is a serious issue. Indeed, even when the package survives mechanically at high temperature, the hermeticity is often altered. Gas leakage inside the package or the package internal degassing damage the SAW device substrate and its electrodes. That is why additional protection is required, and if it can finally substitute the packaging procedure itself, it becomes even more useful, as the fabrication costs drop significantly. Previous studies have pointed to AlN/sapphire layered structure as a good candidate for high temperature sensor [1-3]. Other studies of AlN/ZnO/Si and AlN/ZnO/Diamond structures have demonstrated the possibility of acoustical isolation of a wave propagating mainly in a low velocity ZnO layer surrounded by materials with higher velocity [4, 5]. A waveguiding layer acoustic wave (WLAW) can exist in these structures and the top high velocity layer eliminates the need for packaging, as the WLAW is immune to mechanical dumping on its top surface [6]. In this study, the potential of packageless structures for acoustic wave sensor applications based on Al 2 O 3 /IDT/GaN/Sapphire structure is investigated. 2. MODELING DETAILS 2.1. Model description We have used a commercial finite element method (FEM) software package Comsol Multiphysics, notably the version 4.3a for 2D modeling of an infinite periodic interdigital transducer (IDT). More details of the theoretical aspects have been previously reported [7]. The studied structures are shown Fig. 1: - Air/IDT/GaN/Sapphire, is used to study the SAW structure and to validate the model and physical constants considered. Validation is made by comparison with experimental characterization. - Al 2 O 3 /Pt-IDT/GaN/Sapphire FEM model is used to study the eligibility of this approach for creation of a packageless structure and then for optimization of the Al 2 O 3 overlayer thickness /12/$ IEEE 106

2 3. EXPERIMENTAL Figure 1. Schematic 2D representation of the studied structures structure 2.2. Modeling results An example of the frequency response of an IDT/GaN/Sapphire infinite periodic SAW structure is shown in Fig 2. The electromechanical coupling factor K 2 is close to 0.4%. Initial extensive modeling of Al 2 O 3 /IDT/GaN/Sapphire structure parameters in Comsol Multiphysics has defined the thickness ranges that provide the existence conditions for the WLAW. The effect of normalized thicknesses of GaN on structure performance was calculated. The FEM modeling predicts sufficient acoustical isolation for WLAW traveling inside a GaN waveguide. The electromechanical coupling factor K 2 is relatively low and it reaches about 0.06%. Such values of K 2 are common to quartz substrates and they do not pose any complications to the design of SAW resonators. Increasing the GaN thickness releases the requirements to the top layer thickness and increases the K 2. For the experimental part, 0.4 and 2 µm-thick GaN (002) was grown on C-plane sapphire by MOCVD technic. Subsequently SAW delay lines were fabricated by photolithography with 100 nm thick Al electrodes. The number of IDT finger pairs was 50, the IDT aperture and inter-idt distance were fixed, respectively, at 2 mm and 1 mm. Taking into account the thickness of the GaN film and the various spatial period of the IDTs considered (8, 10, 12, 14 and 16 µm), the normalized thickness value (kh GaN ) of our samples varied from to 1.57, where k = 2π/s the wave vector modulus and h GaN is the GaN film thickness. The IDT metallization ratio was fixed at 33% in order to generate simultaneously the third, fifth and seventh harmonics [8]. Thus, larger values of the normalized thickness could be obtained, because when considering higher harmonics (of the order n), the kh GaN value increases by a factor n. The S 21 frequency response was measured using a vector network analyzer and RF probes. The GaN acoustic parameters were fitted in the model in order to match the measured data and the modeled response. Thick Al 2 O 3 films were fabricated by RF magnetron sputtering. The influence of temperature, pressure, power and Ar/O2 gas mixture on the film growth was investigated. The microstructure, phase composition and morphology were determined by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM), respectively. The measurements of GaN/sapphire SAW delay lines have shown good agreement with the modeling results thus confirming the feasibility of future use in WLAW packageless structures. The SAW velocity dispersion curves were obtained for the first, third and fifth spatial harmonics and they correspond to previously published calculations. Concerning the optimization of Al 2 O 3 deposition conditions, the EDS and TEM analysis revealed that the alumina films obtained were congruent, amorphous and nano-granular. 4. RESULTS AND DISCUSSION Figure 2. Frequency response of a GaN/Sapphire infinite periodic SAW structure Fig. 3 shows the wide range frequency response (0.3 to 1.8 GHz) of a SAW device fabricated with spatial IDT period P = 14 μm (the pitch = 7 μm). We can observe several peaks related to different Rayleigh wave modes and to corresponding different harmonics. The frequency value, the propagation-mode order and the harmonic order are indicated for each peak (for example M1h3 means mode 1 and harmonic 3). 107

3 Note that usually the mode 0 is called Rayleigh mode and mode 1 - Sezawa mode. S 21 (db) h 1 Bulk wave h 3 h 3 h 5 h 5? P = 14µm h 9 h 7 figure the shape of this deformation clearly indicates the harmonic and mode orders. The experimental verification was also done by measuring the delay time at each peak to determine the corresponding group velocity. Fig. 5 shows the velocity dispersion curves in the structure GaN/Sapphire calculated and measured. One can observe good correlation between the experimental and theoretical results indicating the accuracy of the model and of the physical constants used in this study Frequency (MHz) Figure 3. Frequency response of a GaN/Sapphire SAW delay line obtained with P = 14 μm. For each peak (M k h i ) means mode k and harmonic i P hase V elocity(m /s) Experimental = 8 µm = 10 µm = 12 µm = 14 µm = 16 µm Simulation = 8 µm = 10 µm = 12 µm = 14 µm = 16 µm The phase velocities (v i ) were determined from the peak s frequency f i using the following equation: v i = f i * with = P/i and i is the order of considered harmonic. In order to determine precisely the acoustic wave propagation properties in the GaN/Sapphire structure and thus to avoid any confusion between the mode s order and the harmonic s order, we have plotted in Fig. 4 the deformation along the Y direction. As shown in this kh GaN Figure 5. Phase-velocity dispersion curves of the GaN/Sapphire structure. Compilation of experimental and theoretical results h 1 h 3 h 3 h 5 h 5 h 7 f exp: MHz f exp: GHz f exp: GHz f exp: unmeasurable f exp: 2.65GHz f exp: unmeasurable f theo: MHz f theo: MHz f theo: GHz f theo: GHz f theo: GHz f theo: GHz Figure 4. Mechanical deformation of the elastic waves propagating in the GaN/Sapphire structure. The results show the total displacement in the X and Y directions. The considered spatial period is P=12 μm (λ =12 μm for the fundamental peak). The GaN thickness is h GaN =2 μm 108

4 5. INVESTIGATION OF THE PACKAGLESS SAW SENSOR FEASIBILITY In this part we investigate the potential of Al 2 O 3 /IDT/GaN/Sapphire structure as a packageless structures for acoustic wave sensor applications. With GaN thickness and acoustic wavelength fixed, the Al 2 O 3 thickness is a key parameter to obtain the acoustic isolated wave. At this step, we need to define a threshold value of the Al 2 O 3 thickness, which provides a sufficient isolation of the WLAW acoustic field. In our study we fixed the GaN thickness at 2 μm, acoustic wavelength at 8 μm and we vary Al 2 O 3 thickness from 0 to 20 μm the latter value corresponds to 2.5λ. Fig. 6 shows selected results obtained at different Al 2 O 3 thicknesses when considering the harmonic 0 of the mode 0 in the Al 2 O 3 /GaN/Sapphire structure. Fig. 6-a shows the 2D Y-displacement shape for GaN/Sapphire structure (h Al2O3 =0). The top layer being air, the displacement is mainly localized in the GaN layer and in the top surface of the Sapphire. Fig. 6-b shows the effect of 6 μm of Al 2 O 3 on top of the GaN layer, where the beginning of the reduction of the displacement amplitude toward the topmost surface of Al 2 O 3 layer is observed. The acoustic wave Y-displacement expands into the Al 2 O 3 film and decreases slowly but remains important at the surface. For Fig. 6-c the Al 2 O 3 thickness is equal to two wavelengths (16 μm) and the acoustic wave is now confined between the Sapphire and the Al 2 O 3 materials as the displacement amplitude at the surface equals to 3% of the maximum Y-displacement in the layered structure. Fig. 2-d, corresponding to the thickness of 20 μm of Al 2 O 3, shows no visible change of the Y-displacement shape compared to Fig. 2-b. For a quantitative comparison, in order to determine directly the wave confinement and the minimum Al 2 O 3 thickness required, Fig. 7 shows the profile of the Y-displacement (μm) in Al 2 O 3 /GaN/Sapphire. These data are very useful to determine the wave confinement and the minimum Al 2 O 3 thickness required. One can observe that the surface amplitude decreases rapidly as a function of the Al 2 O 3 thickness. With an Al 2 O 3 top film of 18 μm, the Al 2 O 3 surface displacement is about 1.4% while it is only 0.6% for h Al2O3 =20μm and 1.2ppm for h Al2O3 =30μm. A reasonable definition of the threshold may be based on the decision that the surface displacement amplitude must be less than 1% of the maximum displacement amplitude in the structure. In this case we can conclude that 20 μm of Al 2 O 3 is sufficient for the wave confinement. h Al2O3 = 0 µm Al 2 O 3 GaN Saphhire 6 µm 16 µm 20µm a b c d Figure 6. Y-displacement profile as a function of the Alumina (Al 2 O 3 ) thickness for one half-period. The GaN thickness (h GaN ) is equal to 2 μm and the wavelength (λ) is 8 μm. Figure 7. Y-displacement amplitude dependencies on the vertical coordinate for different Al 2 O 3 thicknesses. One can note that the thickness of 20 μm of Al 2 O 3 is so large and the deposition of this layer can degrade the piezoelectric properties GaN. However, it is also important to note that the minimum required Al 2 O 3 109

5 thickness is proportional to considered acoustic wavelength and if we increase the operating frequency of the WLAW device the thickness will be decreased. Fig. 8 shows the Y-displacement profile for Al 2 O 3 /IDT/GaN/Sapphire structure when the acoustic wavelength is reduced to 2 μm. The GaN thickness still equals to 2 μm, and the thickness of Al 2 O 3 is fixed at 6 μm. Fig. 8 clearly shows that the acoustic wave is completely confined in the GaN layer and thinner layer of Al 2 O 3 is required for confinement. Figure 8. Y-displacement profile. The GaN (h GaN ) and Al 2 O 3 (h Al2O3 ) thicknesses are respectively (h GaN ) is equal to 2 μm and 6μm, and the wavelength (λ) is 2 μm. 6. CONCLUSION The modeling of Al 2 O 3 /GaN/sapphire layered structures and preliminary experiments with GaN/sapphire SAW structures predicts the WLAW packageless structures feasibility together with useful values of electromechanical coupling coefficient. The expectations are that Al 2 O 3 /GaN/sapphire structures may help to improve the robustness of SAW sensors and to increase the time and the service interval at higher temperatures. ACKNOWLEDGEMENTS This research was supported in part by the Ministry of Education and Science of Russian Federation (Grant No. 201/ ). REFERENCES [1] Aubert T, Bardong J, Legrani O, et al. J. Appl. Phys, 114: art , [2] Aïssa KA, Elmazria O, Boulet1 P, et al. Correlation between crystalline and acoustic properties of SAW structures based AlN/Sapphire measured in situ and in a wide temperature range; IEEE Trans. on UFFC. 62: pp , [3] Legrani O, Elmazria O, Bartasyte A, et al. AlN/IDT/AlN/Sapphire as Packageless Structure for SAW Applications in Harsh Environments. In: Proc. IEEE IU Symp, pp : , Dresden, [4] Legrani O, Elmazria O, Pigeat P, et al. Deposition of Crack-Free 30 μm AlN on IDT/ZnO/Si for Wave Guiding Layer Acoustic Wave Applications. In: Proc. IEEE IU Symp., pp , Orlando, [5] Elmazria O, Zhgoon S, et al. AlN/ZnO/Diamond structure combining isolated and surface acoustic waves. Appl Phys. Lett, 95, pp (1-3), [6] Legrani O, Elmazria O, Zhgoon S, et al. AlN/ZnO/Si Structure: a Packageless Solution for Acoustic Wave Sensors. IEEE Sensors Journal, 13, pp , [7] L. Le Brizoual, O. Elmazria,; FEM modeling of AlN/diamond surface acoustic waves device. Diam. & Related Mater., 16(4-7): pp , [8] Campbell CK, Obtaining the fundamental and harmonic radiation conductances of a reflective SAW interdigital transducer. In: Proc. IEEE IU Symp., pp , Sendai,