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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.

Micro-fabricated solid oxide fuel cells (µSOFCs) are finding an increasing interest as potential power sources for portable devices such as MP3 players or laptops. The aim of this work was to fabricate a µSOFC demonstrator that works at 500°C and is fuelled by hydrogen. This thesis was divided into two parts. The first one was devoted to the development of an electrolyte and electrodes in form of sputtered thin films with electrical and mechanical properties suitable for the implementation in a real cell. YSZ and CGO electrolyte have been reactively sputtered from metallic targets. Both films are dense and have a columnar microstructure. The ionic conductivity of these films was of 0.5 S/m at 550°C for the CGO and of 5.5 x 10-3 S/m at 500°C for the yttria stabilized zirconia (YSZ) . Albeit the ceria doped gadolinia (CGO) was a better ionic conductor at low temperature, it was not possible to obtain an open circuit voltage (OCV) with a CGO electrolyte film. Most likely, the reduction of the Ce+4 ion into Ce+3 in a hydrogen atmosphere creates an electrical leakage. Better results were obtained with YSZ layers. Single, (111) textured columnar films showed OCV's of 200 mV. Best results were obtained with a double layer of two different microstructures. The first one exhibited a dense, columnar microstructure with (111) texture. The second layer was porous with nanocrystalline grains and preferential (200) orientation. The improved properties are ascribed to the absence of film crossing grain boundaries. Of special interest is the mechanical stress behaviour upon heating to the operation temperature. The stress was investigated as a function of temperature up to 700°C. An anomalous, hysteretic behaviour was found during the first heating cycle in YSZ as well as CGO thin films. This phenomenon could be modelled as an oxygen uptake to fill up excess oxygen vacancies created during the sputtering process. The model allowed to derive a diffusion activation energy of 0.6 eV for these excess vacancies in YSZ. Annealing in air at 700 °C permits to reduce stress and to stabilize the YSZ membrane. As electrode materials, sputter deposited, porous platinum, porous Ni-CGO composites and dense LaxSr1-xCoO3-y (LSCO) thin films were developed and characterized. The PEN (Positive electrode-Electrolyte-Negative electrode) layer processes were combined with Micro Electro Mechanical System (MEMS) process technology to fabricate µSOFC test devices. The PEN membranes were liberated by deep silicon dry etching. The cell diameter was varied between 0.5 and 5 mm, the electrolyte thickness between 500 and 700 nm. A nickel grid grown by electroplating was used to support the electrolyte layer and to serves as current collector for the anode. The cell with a 5 mm diameter shows a very good mechanical stability up to 600°C in SOFC operating conditions and for several heating cycles. The functionality of the fuel cell with two 20 nm thick porous platinum electrodes and a YSZ bilayer electrolyte (500 nm) has been demonstrated. An OCV of 850 mV was measured at 500°C with hydrogen fuel. Unfortunately, a too high cathode contact resistance reduced the current to very low values. The achieved maximal power density was only 19 µW/cm2. A simple design change should remedy the problem.

Piezoelectric micro-electro-mechanical systems (MEMS) are finding an increased interest for applications requiring high frequency operation and high mechanical quality. The aim of this work was to improve piezoelectric MEMS along two main research directions. The first one was devoted to the strongest piezoelectric thin film material so far established, namely to the solid solution system lead zirconate titanate (PZT), whose properties are peaking at the morphotropic phase boundary at 53/47 Zr to Ti ratio. Properties of such films were lagging behind the ones of bulk ceramics. In sol-gel deposited films, one of deteriorating factors was earlier identified in the compositional gradient preventing to hit the 53/47 composition throughout the whole film volume. In this work, this gradient could be lowered by the preparation of a new set of solutions, reducing the Zr concentration fluctuations from ±12 to ±2.5at%. In combination with a new sol-gel PbTiO3(100) seed layer, and an optimized lead excess and heating scheme, the transverse piezoelectric coefficient |e31.f| could be increased from 12 to 17 C/m2 in 2µm {100}-textured PZT thin films deposited on plane 100mm wafers. In parallel the dielectric constant increased from 1200 to 1500 with a decreasing loss tangent. It could be shown that the dielectric loss must be entirely due to domain wall motions. The second research direction was devoted to the introduction of new piezoelectric shapes by means of deposition into cavities. A free calotte membrane, consisting of a sputtered PZT thin film and its electrodes, has been successfully developed and fabricated as demonstrator of a novel three-dimensional transducer. Calotte profile holes in silicon were prepared by wet etching, and were used as micromould for the PZT membrane. The quasi-static vibration amplitude was measured by means of atomic force microscopy (AFM) yielding a responsivity of 3.5 nm/V, i.e. about 20 to 50 times the d31 value. The second device was a piezoelectric PZT coated active AFM cantilever. A PZT layer was sputter deposited on patterned (111) platinum bottom electrodes into deep cavities (>300µm) prepared by anisotropic wet chemical etching. The photolithography was made possible using spray coating techniques and correcting masks for aberration errors. The study of the basic properties of such a structure has shown promising results. The maximum deflection at the end of the beam was measured as 5.3 nm/V in quasi-static operation. The so derived e31,f value in the cavity corresponded well to the value obtained on flat wafers. Micro-structural analysis showed that PZT grains grow always perpendicular to the local surface inclination, showing no effect of directional sputter flux. The texturing scheme worked as well on inclined surfaces, i.e. the Pt(111) planes and the PZT {100} planes always stay parallel to the local surface.