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AlN-based Electro-Acoustic Sensors for Analytics in Liquids

From Material Development to the Device
: Reusch, Markus
: Ambacher, Oliver; Fiederle, Michael

Volltext urn:nbn:de:bsz:25-freidok-1496156 (7.9 MByte PDF)
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Erstellt am: 5.9.2019

Freiburg/Brsg., 2018, 167 S.
Freiburg/Brsg., Univ., Diss., 2018
Dissertation, Elektronische Publikation
Fraunhofer IAF ()

Due to their outstanding properties flexural plate wave (FPW) devices based on piezoelectric aluminum nitride (AlN) are especially well suited as acoustic wave micro sensors operating in contact with liquids. The sensing mechanism of such devices is based on the sensitivity to target substances (e.g. one component in a complex mixture) which is achieved by biologically or chemically active agents, such as antibodies or enzymes, bonding the target substances to the plate surface. Consequently, due to the extra mass the characteristics of the path, over which the acoustic wave travels, are altered and can be used as sensor output signal. FPW sensors utilizing the antisymmetric A0 Lamb-wave mode of a thin plate, first discussed by White et al. in 1987 [1], can serve as an acoustic wave guide and only the evanescent field interacts with the surrounding liquid, while in other acoustic wave devices the viscous damping typically reduces the quality factor, resulting in a strong attenuation of the electrical signal, making the operation in contact with liquids practically impossible. Therefore, FPW sensors have great potential to replace traditional sensing methods, for example in mobile care applications, as well as in environmental surveillance or in-line monitoring for biological or chemical process control, and even the quality check in foods and beverages. However, to compete with the traditional methods like high performance liquid chromatography (HPLC) or techniques based on mass spectrometry, which are usually time-consuming, require large laboratories, and use expensive equipment, the sensitivity of the micro sensors has to be maximized, whereas the cross-sensitivity should be minimized. State-of-the-art FPW sensors are designed as unimorph transducers, consisting of an elastic support layer and the piezoelectric film, where the interdigital transducers (IDTs) for the excitation and detection of the acoustic waves are placed on top of the piezoelectric film. The absence of an electrical shielding of the IDT against the surrounding liquid is one of the main drawbacks of the conventional FPW sensor design as only one side of the plate can be used for sensing, making the FPW sensor prone to pressure induced cross-sensitivities. In this work, a new FPW sensor design, where the IDTs are designed as buried electrode and are thus electrically shielded from the surrounding liquid, was experimentally investigated for the first time. This design enables to completely immerse the sensor into the liquid reducing the pressure cross-sensitivity and additionally allowing the use of both sides of the plate for sensing, enhancing the sensitivity. Most important component of the FPW sensors is the piezoelectric AlN, which can be deposited by means of reactive sputtering at relatively low temperatures on various substrates and the film properties can be fine-tuned by adjusting the growth conditions. Since the FPW devices investigated in this thesis are based on thin plates with total thickness of 2.4 µm and large lateral size of up to 12.3 mm2, residual film stress in the AlN layer was carefully tailored. High compressive stress results in unwanted undulation of the thin plates, whereas high tensile film stress can lead to film or plate cracking. However, FPW device performance also strongly depends on the film crystal quality, piezoelectric response, surface morphology, and mechanical properties and it is necessary to optimize these device-relevant properties simultaneously. Within this dissertation new growth models were developed, considering the mean free path of the plasma particles during sputtering and explaining the evolution of intrinsic stress gradients, which helps to understand the growth process and allowing tuning the film stress over a wide range. It is shown that AlN film stress can be precisely adjusted from strong compressive stress values of 1000 MPa to tensile stress of +500 MPa by tuning the bombardment of the growing film surface by the energetic plasma particles. In order to adjust the film bombardment the total process pressure, the nitrogen concentration in the argon/nitrogen (Ar/N2) reactive gas mixture, the sputtering input power, and the target-to-substrate distance (TSD) were finely tuned. It is shown that the optimization of the film stress by means of the N2 concentration in the sputter gas mixture is beneficial as the process window is larger. The mean free path of the plasma species has strong influence on the crystal quality and if the TSD is larger than the mean free path of the plasma particles the formation of additional, randomly oriented AlN grains is favored. The strong intrinsic stress gradient caused by the growing in-plane grain size along film thickness was minimized by gradually increasing the N2 concentration in the Ar/N2 gas mixture during the growth process. Due to the increased process window for the N2 concentration changing process conditions did not degrade the device-relevant material properties. After additionally adjusting the sputtering power and growth temperature, the AlN films exhibited low residual film stress between 350 MPa and +260 MPa, surface roughness of below 2 nm (root-mean-square), nanoindentation hardness of 21 GPa and piezoelectric response of d33 = 5.2 pC/N. Finally, the AlN-based FPW sensors with buried IDTs were designed, fabricated, and characterized with respect to their application as sensors operating in liquids. The functionality of the proof-of-concept sensors was demonstrated by means of laser Doppler vibrometry and analysis of the electrical scattering parameters in air, water, and ethanol. The fabricated devices showed a mass sensitivity of 240 cm²/g for loading with de-ionized water which was in good agreement with the theoretically determined mass sensitivity and significantly higher than previously reported for FPW sensors. The increased sensitivity was mainly achieved by carefully designing the acoustic wavelength of the FPW sensors and by decreasing the total mass of the thin plate. Moreover, the temperature cross-sensitivity is a critical issue for FPW sensors and it is shown, that the temperature coefficient of frequency (TCF) can be significantly improved by fabricating devices with initially compressive film stress in the AlN instead of tensile stress. Devices with compressive film stress exhibited a TCF of 62 ppm/K to 28 ppm/K, while the devices with tensile film stress showed an increased TCF of 391 ppm/K to 72 ppm/K. It is shown that variations in in-plane tension are mainly accountable for the increased TCF, while the lower TCF is primarily caused by material softening. It is thus concluded that by fabricating the FPW sensors with buried IDTs, stress tailored AlN thin films, and carefully defining the acoustic wavelength, the sensor cross-sensitivity to pressure fluctuations can be improved, temperature drifts can be reduced, and the sensitivity can be increased, making this sensor design very attractive for future chemical and biological sensing applications.