Theoretical and Experimental Study of Piezoelectric Modulated AlN Thin Films for Shear Mode BAW Resonators

The use of acoustic resonators for sensors application has opened a new branch in research and applications of piezoelectric materials and devices. The first generation of such sensors is constituted by the quartz crystal microbalance (QCM) based on AT-cut mono-crystalline quartz. High sensitivities of gravimetric sensing in both air and liquids were demonstrated. Since the 1980s, when the first QCM-based sensor was demonstrated for the detection of silver in a liquid solution, many other application in chemical, bio-medical and environmental sensing were realized using the same concept. QCM's exhibit a very good thermal stability. The AT-cut quartz plate leads to the excitation of shear waves when used in parallel capacitor geometry. This is important for achieving high quality factors in the immersed operation of the sensor. A second generation of gravimetric sensors is based on surface acoustic wave (SAW) structures, working for instance with Love waves in a SiO2 layer on top of a LiTaO3 single crystal. SAW devices are mainly used as RF filters in television and mobile phones. SAW sensors are a side product of the much larger telecommunication market. The evolution of thin film and MEMS technology has lead to a third generation of gravimetric sensors that is based on bulk acoustic wave (BAW) resonances in piezoelectric thin films. Again, such sensors are a side product from the large telecommunication market where such resonators are used for RF filters. With every generation, the oscillation frequency increased. While QCM's operate typically at 5 MHz, the SAW resonators work typically at a few 100 MHz, and the thin film BAW resonators (TFBAR) operate typically around 2 GHz. The increase of the frequency goes together with an increase in sensitivity, and a decrease of the thickness and mass range that can be measured. The TFBARs are in some sense miniaturized analogs of QCM's operating at much higher frequencies. They are very promising as they reach higher sensitivities. This attracted the attention of researchers, and experimental evidence of the potential of TFBARs as sensors was delivered. In addition to the higher sensitivity, the miniaturization allows for using arrays of sensors with different immobilization layers as needed for drug screening. However, the application of the same TFBARs as used in telecommunications would not allow a good performance in immersed operation. It would only be good for operation in air. The principal characteristic of TFBARs used in mobile phones is the value of the electromechanical coupling and not the resonance mode at which this coupling is achieved. But for the in-liquid operated sensors, it's rather the opposite – the mode of the resonance should be such that the surface of the resonator, which tis in contact with liquid, should move parallel to the surface (in-plane motion, or transverse motion) to minimize emission of acoustic waves into the liquid. The coupling coefficient is of secondary importance in this case. Therefore, the development of shear mode TFBARs became an interesting task that was challenged by several research groups. One of the most successful solutions is the use of c-axis inclined AlN thin films. Inclination of c-axis in a parallel plate capacitor structure enables the coupling of the electric field to the shear strain, which enables the excitation of a shear mode. Such devices were successfully applied for selective sensing of organic species, such as DNA molecules, suspended in a liquid. Even if the process of deposition of c-axis inclined AlN films doesn't require hardware modification, the quality of the process is still far from the one for deposition of (0001)AlN films used in telecommunications. So the goal of this thesis was to find a solution for the shear mode TFBAR's based on (0001)AlN films. In (0001)AlN films, the shear strain cannot be induced by the electric field produced in a parallel plate geometry as the coefficient e35 and e34 of the piezoelectric tensor are zero. But there are e15 and e24 coefficients that are not zero, meaning that in-plane electric field can be used to excite the shear waves. In the frame of this work, this concept was studied theoretically and experimentally. The in-plane electric field was generated via inter-digitated electrodes (IDE). A first device type was realized in solidly mounted resonator (SMR) design and based on uniform (0001)- oriented AlN thin films. The anti-phase of the electric field in adjacent half-periods of the IDE resulted in an anti-phase movement of the corresponding regions of the film. Finite element modeling and boundary element modeling (FEM-BEM) were carried out to clarify the kind of vibrations present in the device. A kind of shear/lingitudinal mode with elliptic motions was obtained. A device was fabricated and tested both in liquid and air. The resonance of the device was observed in the expected frequency range (1.86 GHz) and high quality factors under operations in air (Q=870) and silicon oil (Q=270) were obtained. The drop of the quality factor was explained by the up and down motion of the regions of the film located directly under the IDE electrodes. Such a motion is due to anti-phase motion of different regions of the film as mentioned above. To prevent this effect, a second device type was studied. It is again based on the use of (0001)AlN thin films, but with modulated piezoelectric properties. Having different piezoelectric properties in the regions corresponding to adjacent half-periods of IDE, breaks the mirror symmetry of the device and allows for coupling the electric field to a pure shear mode. Analytical and numerical models explaining such a device were established and evaluated. The optimal situation is found when perfect Al-polar and N-polar regions of AlN are combined. This maximizes the coupling coefficient k that is derived as being proportional to the difference of e15 coefficients. Finding a way to fabricate the AlN thin film with different piezoelectric properties was thus the next objective. Growth features to decrease the piezoelectric effect were first studied. Providing rougher regions on the otherwise smooth substrate allowed to modify locally the quality of AlN thin films. Device based on such films were fabricated and characterized. The resonance of a pure shear mode was found at the expected frequency (roughly 2GHz) when the piezoelectric effect was modulated. Devices without this modulation failed to show the resonance at the exact frequency, exactly as the theory predicted. We managed to reduce two times the d33 coefficient of the film by inducing a increased roughness to the substrate – from 5.0 pm/V, corresponding to 1.5 degree rocking curve, down to 2.4pm/V, corresponding to 7 degree of rocking curve. The final step of the thesis was the process development for the simultaneous growth of Al-polar and N-polar regions within sputter deposited (0001)AlN, in order to achieve the maximal possible "piezomodulation" effect. As the sputter deposition yielded only N-polar films, we included Al-polar films from another source as seed layers. High temperature epitaxial growth methods of GaN and AlN on Si(111) and Si(100) lead to Ga- and Al-polarities. On such films, the sputter deposited AlN copies polarity from the growth substrate. The selective polarity was then obtained by preventing the expitaxy locally through a patterned oxide layer. Wet etching tests together with PFM measurements were performed to prove the dual polarity in the sputter deposited film. Finally, the integration of this process into the process flow for device fabrication was investigated.

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