Measurement of evanescent wave properties of a bulk acoustic wave resonator

Acoustic wave fields in a thin-film bulk acoustic wave resonator are studied using a heterodyne laser interferometer. The measurement area is extended outside the active electrode region of the resonator, so that wave fields in both the active and surrounding regions can be characterized. At frequencies at which the region surrounding the resonator does not support laterally propagating acoustic waves, the analysis of the measurement data shows exponentially decaying amplitude fields outside the active resonator area, as suggested by theory. The magnitude of the imaginary wave vectors is determined by fitting an exponential function to the measured amplitude data, and thereby the experimentally determined dispersion diagram is extended into the region of imaginary wave numbers.     

Influence of the seed layer on structural and electro-acoustic properties of sputter-deposited AlN resonators

The growth of high-quality AlN films has been studied by reactive sputtering onto Mo electrodes with Ni, Ti, and TiW seed layers and subsequent integration into thin film bulk acoustic wave resonators. The crystalline structure and morphology of the Mo and c-axis oriented AlN films were found to vary strongly with seed layer material and thickness. The smoothest Mo electrodes were obtained on thin Ti films. Reactive sputtering of AlN on top of these optimized electrodes resulted in a dense columnar grain structure with a well-aligned (002) crystal orientation and good electro-acoustic properties, including an effective coupling coefficient of 6.89% and quality factor above 1000.     

Dispersion and mirror transmission characteristics of bulk acoustic wave resonators

A heterodyne laser interferometer is used for a detailed study of the acoustic wave fields excited in a 932-MHz solidly mounted ZnO thin-film BAW resonator. The sample is manufactured on a glass substrate, which also allows direct measurement of the vibration fields from the bottom of the acoustic mirror. Vibration fields are measured both on top of the resonator and at the mirror-substrate interface in a frequency range of 350 to 1200 MHz. Plate wave dispersion diagrams are calculated from the experimental data in both cases and the transmission characteristics of the acoustic mirror are determined as a function of both frequency and lateral wave number. The experimental data are compared with 1-D and 2-D simulations to evaluate the validity of the modeling tools commonly used in mirror design. All the major features observed in the 1-D model are identified in the measured dispersion diagrams, and the mirror transmission characteristics predicted for the longitudinal waves, by both the 1-D and the 2-D models, match the measured values well.     

A low noise 0.9 GHz FBAR clock

A low noise 0.9 GHz FBAR clock consisting of an oscillator and divider circuit for single-sided-to-differential conversion for a high-speed interleaved pipeline A/D-converter was designed, realized with an in-house FBAR and a commercial 0.35 μm CMOS process, and tested. The circuit showed very good jitter and phase noise performance. A temperature coefficient of –47 ppm/K was measured.     

Piezoelectric ZnO films by r.f. sputtering

Piezoelectric zinc oxide films are used in microelectromechanical systems (MEMS) applications, where they can be used in sensors to detect, e.g., pressure or acceleration. Beside sensors, ZnO films are applied in activation devices, where force is needed. Conductive-doped zinc oxide (most often with aluminum) is also used in optoelectronics. Piezoelectric films including AlN and ZnO are more difficult to produce than the corresponding conductive materials. In order to achieve good piezoelectricity in ZnO films, they have to possess high purity, a (0 0 1) orientation (ZnO has hexagonal crystal structure), high resistivity, and fine columnar microstructure perpendicular to the substrate. We have used r.f. magnetron (13.56 MHz) sputtering from a ZnO target in an oxygen atmosphere to achieve the piezoelectric ZnO. The aim of this work has been to develop an r.f. sputtering process for ZnO to achieve highly piezoelectric thin films. As a test vehicle to measure the piezoelectricity of the ZnO films we have fabricated resonators and passband filters in the 1–2 GHz range using standard microelectronics photolithography, deposition, and etching techniques on 100-mm diameter Corning glass or silicon wafers. The influence of the sputtering-process parameters on the film properties has been studied by X-ray diffraction, scanning electron microscopy, atomic force microscopy, and electrical measurements. In this study, the effects of the process parameters on the final material properties of the ZnO film are discussed in detail.     

Experimental investigation of acoustic substrate losses in 1850-MHz thin film BAW resonators

After optimizing for electromechanical coupling coefficient K², the main performance improvement in the thin film bulk acoustic wave resonators and filters can be achieved by improving the Q value, i.e., minimizing the losses. In Braggreflector- based solidly mounted resonator technology, a significant improvement of Q has been achieved by optimizing the reflector not only for longitudinal wave, the intended operation mode, but also for shear waves. We have investigated the remaining acoustic radiation losses to the substrate in so-optimized 1850-MHz AlN resonators by removing the substrate underneath the resonators and comparing the devices with and without substrate by electrical characterization before and after the substrate removal. Several methods to extract Q-values of the resonators are compared. Changes caused by substrate removal are observed in resonator behavior, but no significant improvement in Q-values can be confirmed. Loss mechanisms other than substrate leakage are concluded to dominate the resonator Q-value. Difficulties of detecting small changes in the Q-values of the resonators are also discussed.     

Import control of irradiated foods by the thermoluminescence method

Summary A thermoluminescence (TL) method was applied for the import control of irradiated foods. The method is based on the determination of the TL of mineral contaminants in foods. Detection of irradiation was incorporated in official Finnish control procedures in spring 1990. For foodstuffs with a reduced microbe content and in which no fumigant residues are found, possible irradiation is investigated by the TL method. The minerals are separated from the foods in different ways: picking is used for spices; water rinsing for herbs, spices, berries and mushrooms; high-density liquid to separate the organic material from the mineral fraction in seafood; and carbon tetrachloride for foods forming gels with water. To date about 140 food samples have been analysed for control purposes: 50 samples of herbs and spices, 25 samples of berries and mushrooms and 65 samples of seafood. Of these, 14 samples of herbs and spices and 5 samples of seafood were shown to have been irradiated. Differences in TL intensity between irradiated and unirradiated samples were at least 1 and usually 3–4 orders of magnitude.

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Einfuhrkontrolle bestrahlter Lebensrnittel mit Hilfe der Thermoluminescenz-Methode

Zusammenfassung Eine Thermoluminescenz-Methode (TL) wurde für die Einfuhrkontrolle bestrahlter Lebensmittel verwendet. Diese Methode basiert auf der Messung der Thermoluminescenz von in den Lebensmitteln vorhandenen mineralischen Verunreinigungen. Diese Methode, eine Bestrahlung festzustellen, wurde im Frühjahr 1990 in das offizielle finnische Kontrollverfahren aufgenommen. Lebensmittel mit reduziertem Mikrobengehalt, ohne feststellbare Begasungsrückstände werden mit der TL-Methode auf eventuelle Bestrahlung überprüft. Die mineralischen Stoffe werden mit verschiedenen Verfahren aus den Lebensmitteln abgetrennt. Dabei werden bei Gewürzen die mineralischen Partikel manuell entnommen, bei Kräutern, Gewürzen, Beeren und Pilzen werden diese mit Wasser abgespült, bei Meeresfrüchten wird mit hochkonzentrierter Lösung behandelt und das organische Material wird von der anorganischen Fraktion bei Lebensmitteln, die mit Wasser gelieren, mit Kohlenstofftetrachlorid abgetrennt. Bis jetzt sind zu Kontrollzwecken etwa 140 Lebensmittelproben analysiert worden: 50 Kräuter- und Gewürzproben, 25 Beeren- und Champignonproben und 65 Proben von Meeresfrüchten. Dabei konnte bei 14 Kräuter- und Gewürzproben sowie bei 5 Proben von Meeresfrüchten eine Bestrahlung nachgewiesen werden. Zwischen den bestrahlten und den unbestrahlten Proben zeigten sich in der TL-Intensität Unterschiede von mindestens einer, meist jedoch von drei bis zu vier Größenordnungen.

     

Estimating materials parameters in thin-film BAW resonators using measured dispersion curves

The dispersion curves of Lamb-wave modes propagating along a multilayer structure are important for the operation of thin-film bulk acoustic wave (BAW) devices. For instance, the behavior of the side resonances that may contaminate the electrical response of a thin-film BAW resonator depends on the dispersion relation of the layer stack. Because the dispersion behavior depends on the materials parameters (and thicknesses) of the layers in the structure, measurement of the dispersion curves provides a tool for determining the materials parameters of thin films. We have determined the dispersion curves for a multilayer structure through measuring the mechanical displacement profiles over the top electrode of a thin-film BAW resonator at several frequencies using a homodyne Michelson laser interferometer. The layer thicknesses are obtained using scanning electron microscope (SEM) measurements. In the numerical computation of the dispersion curves, the piezoelectricity and full anisotropy of the materials are taken into account. The materials parameters of the piezoelectric layer are determined through fitting the measured and computed dispersion curves.