Wilco Keur

Progress in phosphors and filters for luminescent solar concentrators

Luminescent solar concentrators would allow for high concentration if losses by reabsorption and escape could be minimized. We introduce a phosphor with close-to-optimal luminescent properties and hardly any reabsorption. A problem for use in a luminescent concentrator is the large scattering of this material; we discuss possible solutions for this. Furthermore, the use of broad-band cholesteric filters to prevent escape of luminescent radiation from this phosphor is investigated both experimentally and using simulations. Simulations are also used to predict the ultimate performance of luminescent concentrators.

Ferroelectric Thin-Film Capacitors and Piezoelectric Switches for Mobile Communication Applications

Thin-film ferroelectric capacitors have been integrated with resistors and active functions such as ESD protection into small, miniaturized modules, which enable a board space saving of up to 80%. With the optimum materials and processes, integrated capacitors with capacitance densities of up to 100 nF/mm2 for stacked capacitors combined with breakdown voltages of 90 V have been achieved. The integration of these high-density capacitors with extremely high breakdown voltage is a major accomplishment in the world of passive components and has not yet been reported for any other passive integration technology. Furthermore, thin-film tunable capacitors based on barium strontium titanate with high tuning range and high quality factor at 1 GHz have been demonstrated. Finally, piezoelectric thin films for piezoelectric switches with high switching speed have been realized.

Oxide Films for Integrated Capacitors in Thin Film Functional Modules

Single oxide films as well as complex oxide films are intensively studied for multi-chip modules with integrated passive functions. In this paper the processing and properties of oxide films such as Ta 2 O 5 , TiO 2 , Nb 2 O5, Ta 2 O 5 -Al 2 O 3 , Ta 2 O 5 –Nb 2 O 5 as well as complex oxide films such as BiNbO 4 and Ba 1划x Sr x TiO 3 films will be discussed with respect to their integration into thin film functional modules.

New luminescent materials and filters for luminescent solar concentrators

In a Luminescent Solar Concentrator (LSC), short-wavelength light is converted by a luminescent material into longwavelength light, which is guided towards a photovoltaic cell. In principle, an LSC allows for high concentration, but in practice this is prevented by loss mechanisms like limited sunlight absorption, limited quantum efficiency and high self absorption. To tackle these problems, a suitable luminescent material is needed. Another important loss mechanism is the escape of luminescent radiation into directions that do not stay inside the light guide. To reduce this amount, wavelengthselective filters can be applied that reflect the luminescent radiation back into the light guide while transmitting the incident sunlight. In this paper, we discuss experiments and simulations of new luminescent and filter materials. We will introduce a phosphor with close-to-optimal luminescent properties. A problem for use in an LSC is the large scattering of this material; we will discuss possible solutions for this. Furthermore, we will discuss the use of broad-band cholesteric filters in combination with this phosphor.

Thin Film Piezoelectric MEMs Devices

Thin film piezoelectric devices, processed in Si‐related processes, are attractive for ultrasound transducers and piezoelectric switches. Thin film ultrasound transducer enable large bandwidth (> 100%), high frequency operation. In piezoelectric micromachined ultrasonic transducers (PMUTs) the ultrasonic waves are generated by flexural motion of the membrane, which is coupled to strain in the piezoelectric film. We have investigated the piezoelectric properties of thin films for ultrasound transducers and piezoelectric switches. Thin film piezoelectric ultrasound transducers as well as piezoelectric switches have been designed, processed and characterized. Thin film piezoelectric devices with excellent quality and reliability have been realized.

MI007: Galvanic piezoelectric RF MEMS switches

Galvanic piezoelectric RF MEMS switches are presented. The switches are completely processed and characterized mechanically and electrically. The mechanic deformation and switching times are discussed. Closing of the switch can be achieved within less than 10 ¿s. Electrical measurements yield contact resistances in the order of 50 ¿ and isolation resistance of more than 1 G¿.

A reliability study on high-breakdown integrated ferroelectric capacitors.

Capacitors, with a capacitance density of up to 100 nF/mm2 and a dielectric constant of 950¿1600 were produced by optimizing the ferroelectric material combined with stacking. Accelerated lifetime (ALT) tests under elevated temperatures of 210¿290°C and dc fields of 25¿250 kV/cm were performed and the lifetime criterion, common for ceramic multilayer capacitors, was employed: The increase of the current density by one order of magnitude is defined as the end of lifetime. The capacitor under these conditions is still functioning and therefore this criterion is more conservative than time dependent dielectric breakdown (TDDB). The activation energy and voltage dependence are determined from the ALT, to extrapolate to operation conditions. Activation energies of 1.1¿1.6 eV have been determined and a dependence on the applied dc voltage was observed. All capacitors showed a lifetime of more than 10 years at 85°C and 5 V.