Balanceable Coupled Piezoelectric NEMS Resonators

This project aims at designing, fabricating and characterising balanceable coupled NEMS resonators, in order to verify a theoretical model. In the field of sensing, coupled NEMS resonators benefit from an immensely increased sensitivity towards changes in mass or in stiffness as compared to single resonators, but they are limited by the fabrication imprecisions, which may yield unbalanced resonators. As mode localisation is crucial to maximise the sensitivity, the fabricated resonators need to be balanceable, which is here to be achieved through the reverse piezoelectric effect, with the application of a direct voltage on one of the clamped-clamped beam resonators. While the mathematical model predicted a large tunability of the resonant frequencies per unit voltage, the fabricated resonators presented a smaller tunability. In fact, it has been discovered that the resonators are bending out of the plane to varying extents depending on the voltage that is applied for tuning, which partially invalidates the mathematical model by introducing a countereffect. Coupled resonators have nevertherless been able to be balanced, in spite of a weaker response to tuning.

Micromachined tunable devices based on silicon integrated BaxSr1-xTiO3 thin films

Andreas Nöth

Thin Film Bulk Acoustic Wave Resonators (TFBARs) had been developed a decade ago and since then were implemented extensively in mobile communications devices. The "heart" of a TFBAR consists of a piezoelectric film that operates as an acousto-electric transducer, stabilizing the transmission at a given predetermined frequency. For reasons such as space economy in hand-held devices, it is of interest to make these TFBARs tunable, so that a single TFBAR is multi-band responsive. This thesis demonstrates for the first time electrically tunable, single-component TFBARs. A theory describing the tuning behavior of dc bias induced acoustic resonances was developed. Then we made the hypothesis that dc bias induced piezoelectric BaxSr1-xTiO3 (BST) thin films – namely, paraelectric, non-piezoelectric films operating under dc bias – can be used to make electrically tunable TFBARs and that the devices can be switched on or off depending on the dc bias state. The devices were then fabricated: We integrated BST lms onto silicon substrates, micromachined the substrate to create the TFBARs and a new type of suspended planar capacitor which were then characterized, analyzed, and modeled, demonstrating successfully the new concept: We developed a theory describing the electrical tuning behavior of the dc bias induced acoustic resonances in paraelectric thin lms in terms of material parameters. The field dependent constitutive piezoelectric equations were derived from the Landau free energy P-expansion by taking the linear and nonlinear electrostrictive terms as well as the background permittivity into account. We considered two modes of excitation for the tuning of the acoustic resonances, namely the thickness excitation (TE) mode and the lateral field excitation (LFE) mode. The tuning behavior of the two types of resonators based on BST thin films was modeled and discussed. For the modeling we calculated the relevant tensor components controlling the tuning of the BST resonators from the available literature data. The fabrication of the membrane-type TFBARs was realized by integrating BST thin films onto silicon substrates and using micromaching technologies. We showed that the developed TFBARs can be switched on or off with a dc bias. At a dc electric field of 615 kV/cm we observed a tuning of -2.4% (-66 MHz) and -0.6% (-16 MHz) for the resonance and antiresonance frequencies of the device, while the resonance frequency at a dc electric field extrapolated to 0 kV/cm was 2.85 GHz. The effective electromechanical coupling factor k2eff of the device increased up to 4.4%. The tuning was non-hysteretic. The Quality-factor (Q-factor) of the device was about 200. The developed micromaching processes for the TFBARs were used to fabricate coplanar BST capacitors on silicon. Micromaching was used to remove the Si substrate under the active area of the device. Comparing this new micromachined coplanar capacitor with conventional non-micromachined capacitors, we demonstrated that the removal of the substrate from the active device area resulted in a reduction of parasitic effects. The micromachined coplanar capacitor showed an increased tunability and a reduced loss tangent in comparison to the non-micromachined capacitor. The micromachined capacitor showed a relative tunability of 37% at a dc electric field of 1100 kV/cm. The loss tangent was 0.08 and 0.06 at zero and maximum dc bias, respectively, at a measurement frequency of 20 GHz. The integration of epitaxial BST thin films on silicon was studied as well because of its potential improvement of the device performance in comparison to devices based on polycrystalline films. Two different electrode/buffer layer systems, YBa2Cu3O7-x (YBCO) / CeO2 / YxZr1-xO2-x/2 (YSZ) and TiN were used for the integration. Epitaxial BST thin films were grown with both structures. For the BST / YBCO / CeO2 / YSZ / Si structure, the BST unit cell was rotated by 45° in the in-plane dimension with respect to the substrate. The BST layer exhibited good structural quality as indicated by a Full Width at Half Maximum (FWHM) of 0.5° of the rocking curve of the BST(002) diffraction peak. For the BST / TiN / Si structure, the BST layer was grown with a cube-on-cube epitaxial relationship on the silicon substrate, thus demonstrating epitaxially grown BST on silicon using conventional bottom electrode that is easily acceptable by the microelectronic industry. However, in this case, the structural quality of the BST layer was reduced in comparison to the BST / YBCO / CeO2 / YSZ / Si structure. The FWHM of the rocking curve of the BST(002) diffraction peak was 2.2°. We established the temperature (T=550 to 600 °C) and pressure (p ≈ 10-7 to 5 × 10-4 Torr) conditions for the growth of epitaxial BST thin films on TiN-buffered Si. At too high temperatures and/or oxygen pressures epitaxial BST thin film growth was impeded due to the oxidation of the TiN layer. By introducing experimental results from the electrical characterization of the micromachined devices with the polycrystalline BST into our developed theory, we could successfully model the tuning behavior of our fabricated TFBARs. The modeling allowed us to de-embed the intrinsic electromechanical properties of a freestanding BST layer. The effect of increasing mechanical load on the tuning performance of the device was modeled and studied experimentally. Under strong mechanical load, the tuning of both resonance and antiresonance frequency was reduced. The effect was attributed to a reduction in the tuning of k2eff of the device and of the sound velocity of the BST layer.

EPFL2009

Carbon Nanotube Electromechanical Devices for Radio Frequency Applications

Anupama Arun

Carbon nanotubes (CNT) have been investigated extensively in the recent years for application in nano-electromechanical systems (NEMS). CNTs have extraordinary electrical and mechanical properties. They are stiff, with low density, high electrical conductivity and hence are the ideal candidates for NEMS. Beyond miniaturization, NEMS can also offer better performance than its predecessor micro-electro mechanical systems (MEMS). NEMS find application in ultra-sensitive mass and force detection, RF signal processing, low power operated switches with fast switching speeds, to name a few. Despite tremendous promise the main challenge in CNT research has been the fabrication technology to realize CNT based devices with precise control of location and properties of CNT. CNTs are one-dimensional structures with high aspect ratio. To realize devices for practical application with high signal-to-noise-ratio, it becomes necessary to operate many of them in parallel. To fabricate an array of CNTs in parallel posse's additional challenge to the fabrication technology but the resulting structures could provide benefits compared to both NEMS fabricated with one dimensional structures and MEMS. During the course of this thesis, different fabrication technologies were investigated to realize CNT array based devices. A self assembly method, dielectrophoresis and CVD growth were optimized to realize CNT based devices. A novel technique using a modified dielectrophoresis method was setup to be able to trap a single CNT. A CNT crossed junction was fabricated using the same. The electrical properties of the junction were studied. A Suspended Gate CNT array Field Effect Transistor (SG-CNT-FET) based resonator was demonstrated for the first time. SG-CNT-FET combines the excellent mechanical properties of CNTs with the gain of the solid state field effect transistor. The use of an array configuration ensures better signal-to-noise-ratio compared to device implemented with a single CNT. A novel process flow combining the fabrication of a FET and deposition of CNTs by dielectrophoresis was developed. Resonance frequency up to 150 MHz with a quality factor of approximately 50 was obtained. The resonance frequency is tuneable by modulating the tension in the CNT array by applying a DC bias. Dependence of the resonance frequency on the applied bias has been studied under the frame work of a continuum mechanics model. The small signal circuit parameters have been extracted, enabling future circuit design. Vertically grown CNT array were grown on metal lines to realize a tuneable MEM capacitor. The CNT array forms a membrane like structure which can be electro-statically actuated. Morphological and electrical characterizations were performed on the fabricated devices. DC characterization revealed the pull-in behavior like conventional MEMS. The Young's modulus of the grown vertical CNT array was extracted from the measured pull-in voltage. An effective Young's modulus of 30-100 MPa was obtained. An equivalent electrical model was developed to extract the capacitance from the S-parameters measurements, measured up to 6 GHz. The model was validated by ADS circuit simulations. A voltage controlled oscillator at 2.45 GHz has been implemented using the thus realized tuneable MEM capacitor.

EPFL2010

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

Evgeny Milyutin

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.

EPFL2011