AIN Thin Film Structures for High-Q Resonators

The development of wireless and mobile communication in the last decade led to the elaboration of new components based on thin films with continually increasing performance and reduced fabrication cost. A fundamental element for mobile communication is the passband filter defining the total bandwidth, the so-called RF filter. Thanks to outstanding characteristics, thin film bulk acoustic wave resonators (TFBAR's) based on piezoelectric AlN thin films have conquered parts of this market against surface acoustic wave devices based on piezoelectric crystals. AlN is today the most favoured material for such these devices, thanks to its excellent electro-acoustic properties. This thesis explores paths for a new type of application in oscillators for time and frequency control in frequency range of several GHz. In portable applications, this domain is largely dominated by quartz crystals. Their resonance frequency – typically around 33 kHz – is multiplied to reach higher frequency domains. For such applications, the main issues are high quality factors and temperature stability of the resonators. This exploration implied two research directions, the first focused on AlN thin film property-process relationships for films grown on amorphous substrates, and the second on the property-design relations of such new devices. High quality factors and temperature stability are key issues in oscillator applications. We proposed to apply symmetric composite resonators (composite TFBAR) including two amorphous SiO2 thin films sandwiching high quality piezoelectric thin films of AlN. This required the growth of AlN on amorphous and insulating surfaces. We thus studied the growth of AlN on such substrates to evaluate the impact on film microstructure and morphology, mechanical stress, quality of orientation, piezoelectric constant d33,f , and polarization uniformity. Substrate roughness and substrate bias power (voltage) were identified as crucial growth parameters. In parallel, we also optimized Mo-electrode thin films for optimal AlN/Mo thin film structures. This was necessary to derive properties from pure AlN TFBAR's, and to produce HBAR's for comparison with our composite BAR's. The impact of the general device design, comprising active area surface, apodization and lateral edge design, was investigated concerning the Q-factor, kt2 and parasitic behaviour of devices. The important role of excitation of parasitic resonances was highlighted. We could show that also the microstructure of AlN plays a key role in their excitation and propagation. AlN exhibits a polar structure with one polar axis (c-axis). The growth process has to provide mechanisms to align the polar axis of all grains. No reorientation is possible by external means. The stability of the piezoelectric AlN layer with applied electric field guarantees a linear response for sensors and actuators. The high stiffness, combined with low density leads to a high longitudinal acoustic velocity of AlN, thus enabling the use of very high application frequencies. Almost all the reports so far have concentrated on depositing AlN on metallic layers, such as Mo, or Pt. The ability to grow high quality AlN thin film on amorphous substrates increases the possibility of developing new devices and applications. The orientation and stress properties, as well as the piezoelectric constant and the polarization of polycrystalline AlN thin films deposited on amorphous substrates, were investigated. Thin sputtered amorphous silicon layers of defined nanoscale roughness were assessed for their ability to manipulate the mechanical stress in AlN thin films that need to be grown on thermal oxide. With a roughness variation from 1 to 11 Å in the Si film, a stress variation from -700 to +200 MPa was observed in the AlN film, which shows the high impact of substrate roughness on the stress in AlN films. The rocking curve width of the 1.2 µm AlN deposited on the Si layers showed a pronounced, linear increase as a function of Si layer roughness. Low density grain boundaries appeared, and the overall stress changed to positive values. A study of the piezoelectricity and the grain polarity of AlN thin films grown on amorphous SiO2 thin films has been done. d33,f of 5.0 pm/V were achieved at low substrate roughness and low mechanical AlN film stress. This value is comparable to values reached on oriented Pt. Increasing substrate roughness and stress leads to a deterioration of d33,f which is correlated with a higher density of opposite polarity grains detected by piezoresponse force microscopy (PFM). Extrapolating to 100% uniform polarity, a maximum d33,f value of 6.1 pm/V is derived. High-Q, bulk acoustic wave composite resonators based on a symmetric layer sequence of SiO2-AlN-SiO2 sandwiched between electrodes have been developed. The SiO2 film thickness was varied while the piezoelectric AlN film had a constant thickness of 1.2 µm. Experimental results of quality factors (Q) and coupling coefficients (kt2) are in agreement with finite element calculations. Q-factors of about 2000 were observed for the first harmonic of the 310 nm oxide devices. Third harmonic resonances were very intense for devices with 770 nm oxide double layers. The temperature drift reveals the impact of the SiO2 layers, which is more pronounced on the first harmonic, reducing the TCF to about 0 ppm/K. Hybrid harmonic bulk acoustic resonators (HBAR) based on an AlN/Si structure were fabricated. In order to increase the coupling kt2 of the simple device, an additional film of AlN was deposited on the top of the HBAR to optimize the excitation energy localization in the stack. The gain of the coupling was more than a factor of 2. A general study of the TFBAR performance is given in this work. The performance of circular, polygonal and elliptical electrodes were compared according to the area. The figure of merit Q · kt2 is higher in the case of elliptical design electrodes in comparison with others. Due to its high acoustic impedance and low electrical resistivity molybdenum is the best electrode material used in this work. The use of the lateral edge design on symmetric SiO2-AlN-SiO2 composite resonators slightly reduces the strength of spurious resonances of the third harmonic, but decreases the Q-factor of the resonances. This composite design has the advantage of attenuating the spurious resonances. In order to study the impact of the microstructure of AlN films on electroacoustic properties of TFBARs, simple Mo-AlN-Mo devices with different values of AlN in-plane stress (from -500 MPa to +500 MPa) were developed. These devices showed a clear correlation between the stress and the intensity of the spurious resonances, devices with tensile stressed AlN films reached Q-factors of about 1800. The impact of the microstructure on the transmission of in-plane acoustic waves is responsible for this phenomena. Voided grain boundaries, that are correlated with tensile films, reduce this transmission. These devices showed that the use of amorphous SiO2 thin films in the stack of resonators increased the Q-factor and the figure of merit Q · f to (∼8.0 ·1012 Hz), which is close to the ultimate value of Quartz (∼1.6·1013 Hz). The reached value compares also well with a novel improved HBAR structure reaching a Q · f of (∼2.7·1012 Hz). The composite resonator in addition exhibits a much higher coupling coefficient, which facilitates the implementation of the electronics. The admittance curves of the composite resonators could be well simulated by combining finite element modelling with simple circuits including parasitic elements. The derived materials quality factors of SiO2 and for the Al/AlN/Pt stack amounted to 50'000 and 3'000, respectively. Finally, composite resonators working at 3.4 GHz, corresponding to the half ground state hyperfine separation of rubidium atoms, were developed. The Q-factor was equal to 2300 and the TCF equal to 1.5 ppm/K. A figure of merit Q · f of ∼0.8 ·1013 Hz and a kt2 of 1% were reached. This device will be implemented in an oscillator as planned to realize an atomic clock control circuit.

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

Highly piezoelectric AlN thin films grown on amorphous, insulating substrate

Alvaro Artieda, Paul Muralt, Silviu Cosmin Sandu

AlN thin films were grown by reactive sputtering on amorphous SiO2 thin films. Film texture, x-ray rocking curve width, mechanical stress, and the clamped piezoelectric constant d33,f were studied as a function of rf bias power and substrate roughness. A high d33,f of 5.0 pm/V was achieved at low substrate roughness and low mechanical AlN film stress. Increasing substrate roughness and stress leads to a deterioration of d33,f, which is correlated with a higher density of opposite polarity grains detected by piezoresponse force microscopy. Extrapolating to 100% uniform polarity, a d33,f of 6.1 pm/V is derived as highest possible value, probably corresponding to the d33,f=e33/cof perfect single crystalline material. Growth mechanisms are proposed and underlined by high resolution transmission electron microscopy to explain the observed phenomena.

2010

High coupling materials for thin film bulk acoustic wave resonators

Janine Conde

Radio frequency (RF) filters based on bulk acoustic wave resonances in piezoelectric thin films have become indispensable components in mobile communications. The currently used material, AlN, exhibits many excellent properties for this purpose. However, its bandwidth is often a limiting factor. In addition, no tuning is possible with AlN. Ferroelectrics would offer both larger coupling to achieve larger bandwidths, and tunability. However, their acoustic properties are not well known, especially in the thin film case. The goal of this thesis is to investigate the potential and identify the limitations of ferroelectric thin films for thickness mode resonators in the 0.5 - 2 GHz range. The Pb(Zrx,Ti1-x)O3 (PZT) solid solution system is the main candidate, since it is known for its large piezoelectric constants and its growth is already well studied. As a main test vehicle, free standing thin film bulk acoustic resonator (TFBAR) structures with Pt/PZT/Pt/SiO2 membranes were successfully fabricated using silicon micro-machining techniques. The main drawback of ferroelectrics is the damping of acoustic waves by domain wall motion both in the RF electric field and in the pressure wave. For this reason films with varying orientations and compositions were investigated. From the device structures the electro-mechanical coupling constants kt2, the quality factors (Q-factors) and several materials parameters have been obtained. High coupling constants have been found for sol-gel Pb(Zr0.53,Ti0.47)O3 films with a {100} texture, kt2 is found to be 0.4 for a 1 µm thick film and 0.8 for a 3.8 µm thick film. However, the Q-factors of these films are low, 18 for the first film and 3 for the second film. The increase of kt2 and the decrease of the Q-factor with frequency indicates that the domains present in these films contribute to these characteristic parameters. It was generally observed that high coupling constant are associated to low Q-factors. This became evident when comparing films with 53/47 composition, where both tetragonal and rhombohedral phases are present, to tetragonal films as well as when comparing {100} textures with (111) textures. Both for the 53/47 composition and for the {100} texture, ferroelastic domain walls are thought to play a bigger role than for tetragonal compositions and (111) textures. The highest figure of merit (FOM) of about 15 was found when combining the composition leading to a high coupling constant (53/47) and the orientation leading to lower losses (111). However the losses even in this film are too high for RF-filter applications. On the other hand, films with low Q-factors but high coupling could prove very useful as transducers for ultrasonic imaging applications, where low Q-factors are desired. The stiffness coefficients of the studied PZT films were shown to be higher than expected from ceramics data. Most likely the stiffness of ceramics always contains domain contributions leading to softening. In contrast, in textured films the variety of domain orientation is very much reduced. In order to reduce losses due the presence of ferroelastic domains three different potential solutions were explored. The first idea was to manipulate the domain populations of the films deposited on silicon by using heat and vacuum treatments. Silicon substrates are known from previous works to be unfavourable for high c-domain fractions. It was discovered that an anneal in vacuum at 550 °C lead to a significant reduction of c-domains in tetragonal 30/70 PZT ({100}). On the contrary, if the sample was subjected to a compressive stress during cooling, the c-domain fraction could be increased. Analysis of the film stress versus temperature curves revealed a trend consistent with theoretical predictions, i.e. a phase boundary between the c/a/c/a and the a1/a2/a1/a2 domain patterns between room temperature and the Curie temperature θC. However, even though this method reveals interesting results, it can not be exploited as a method to achieve a sufficient c-domain population. The second idea explored was the implementation of a high thermal expansion material as a substrate. PZT films deposited on MgO are known to be compressive due to the difference in thermal expansion of the two materials. The compressive stress leads to highly c-axis oriented PZT films. Devices using MgO substrates were fabricated, however difficulties in the micro-machining of the MgO substrate inhibited a complete liberation of the membrane. Nevertheless, preliminary measurements indicate these devices could lead to both high coupling and high Q-factors, suggesting that further detailed study of this method is worthwhile. As a third method for avoiding ferroelastic domains, the uniaxial ferroelectric potassium lithium niobate (KLN) was explored. The unique ferroelectric axis in this material means that only 180° domain walls are present, which can theoretically be removed by poling. This material has been deposited in thin film form using pulsed laser deposition (PLD). KLN thin films with a {001} texture were deposited successfully on Pt/Si substrates. The films were piezoelectric with a d33,f value of around 10 pm/V and a dielectric constant of 250. This is the first time that piezoelectric properties were measured on KLN thin films. A columnar structure has been observed, however the small grain size and the rough surface currently make it difficult to apply this material to TFBAR's.