Wolfram Wersing

Microwave ceramics for resonators and filters

Abstract High-permittivity ceramics make it possible to noticeably miniaturize passive microwave devices. These ceramics must fulfil the requirements of high permittivity, very low dielectric losses and an extremely low temperature dependence of permittivity to yield temperature-stable resonators. Recent progress has been made in understanding the physics of microwave ceramics, particularly the property-structure relationship, in developing new materials with higher permittivity, better temperature stability and considerably lower sintering temperature, and in making co-fired multilayer devices possible, as well as in developing new microwave devices such as oscillators, antennas and band-pass filters.

LTCC glass-ceramic composites for microwave application

Advanced materials of rare earth derived glass and reactive bonded glass-ceramic composites are exceptionally interesting for 1C packaging, radar, antennas and wireless technologies for the next generation of miniature electronic devices. Glass ceramic composites in the system 1:1:4 BaO-Nd 2 O 3 -TiO 2 and modified rare earth glasses based on boron oxide for passive integration in LTCC demonstrate excellent dielectric properties in the middle permittivity F range of 20/70 with high quality factor Q and low temperature coefficient TCf at microwave frequencies. Depending on the glass-ceramic system, concentration, significant processing parameters e.g. powder preparation techniques and sintering of dense composite at temperatures < 900 C were achieved. Dielectric properties were studied by a cavity resonator method at frequencies 1-6 GHz in correlation of the crystalline microstructure. This work was supported by tridimensional modeling systems to estimate dielectric behavior of multiphase glass ceramic composites.

First results on label-free detection of DNA and protein molecules using a novel integrated sensor technology based on gravimetric detection principles.

A novel integrated bio-sensor technology based on thin-film bulk acoustic wave resonators on silicon is presented and the feasibility of detecting DNA and protein molecules proofed. The detection principle of these sensors is label-free and relies on a resonance frequency shift caused by mass loading of an acoustic resonator, a principle very well known from quartz crystal micro balances. Integrated ZnO bulk acoustic wave resonators with resonance frequencies around 2 GHz have been fabricated, employing an acoustic mirror for isolation from the silicon substrate. DNA oligos have been thiol-coupled to the gold electrode by on-wafer dispensing. In a further step, samples have either been hybridised or alternatively a protein has been coupled to the receptor. The measurement results show the new bio-sensor being capable of both, detecting proteins as well as the DNA hybridisation without using a label. Due to the substantially higher oscillation frequency, these sensors already show much higher sensitivity and resolution comparable to quartz crystal micro balances. The potential for these sensors and sensors arrays as well as technological challenges will be discussed in detail.

Dielectric, elastic and piezoelectric properties of porous pzt ceramics

Abstract Porous ceramics with type 3–3 connectivity were prepared and investigated with a view to their application for ultrasound transducers. Ceramics with a porosity in the order of 0.4 to 0.5 exhibit sufficiently high permittivity (≈ 500), a thickness coupling factor equalling that of dense material (≈ 0.5), a low transverse coupling factor, a low vibrational Q (≈ 20) and low acoustic impedance (≈ 9·106kg/m2·s), all of which indicates their eminent suitability as a material for the fabrication of transducers as used in medical diagnostics. The experimentally determined influence of the porosity on the dielectric constants of these materials was used for the critical testing of various theories for calculating the constants of multiphase materials. It was found that the dielectric and elastic constants of porous piezoelectric ceramics can be very satisfactorily described on the basis of Bruggemans theory, which has fallen somewhat into obscurity. Their piezoelectric properties are discussed with refer...

Electro-acoustic hysteresis behaviour of PZT thin film bulk acoustic resonators

Abstract Thin film bulk acoustic resonators (FBARs) based on Pb(Zr x Ti (1− x ) )O 3 (PZT) with varying compositions, ranging from x =0.25 to 0.6, were fabricated to investigate hysteresis-like dependencies of the resonance frequency and electro-mechanical coupling constant on bias voltage. The resonators, formed by a simple sandwich structure consisting of bottom electrode, PZT thin film and top electrode, arranged on a planar acoustic mirror, were designed to give a resonance frequency of about 2 GHz. PZT thin films were deposited in a planar multi target sputtering system using three metallic targets in a reactive Ar/O 2 mixture. For low Zr-content, where PZT is grown in the tetragonal phase, the parallel resonance frequencies are strongly dependent on the applied electric field, while the series resonance frequency is practically unaffected. This behaviour is completely different for rhombohedral PZT at higher Zr-content. Here the series resonance frequency becomes strongly field dependent, which can be attributed to 109°/71° domain switching. As a potential application based on the observed strong field dependence of the acoustic properties, a bandwidth-tuneable or programmable RF filter based on PZT FBARs is proposed.

Novel integrated FBAR sensors: a universal technology platform for bio- and gas-detection

In this paper the feasibility of thin film bulk acoustic resonators (FBAR), for applications in bio- and gas-detection, is shown for the first time. Solidly mounted, ZnO FBARs with frequencies around 2 GHz have been fabricated on silicon substrates. The dependence of the FBAR mass sensitivity on the design of the layer stack has been investigated exhibiting an optimized sensitivity of 2.5 Hz cm/sup 2//pg. Using a common protein assay the capability of detecting bio-molecules has successfully been proved. Gas sensing has been demonstrated by coating the FBAR with a humidity absorbing polymer. A strong non-linear dependence of the humidity sensitivity on the thickness of the polymer coating has been found. When the polymer thickness is far less than the acoustic wavelength, a pure mass dependent response occurs, leading to a negative shift in resonance frequency. Moreover, as the polymer thickness becomes significant, acoustic influences affect the response and the shift becomes large and positive. A sensitivity to humidity of up to two orders of magnitude higher than that of comparably coated quartz crystal micro-balances has been observed.

Excimer‐laser‐induced etching of ceramic PbTi1−xZrxO3

The etching of ceramic PbTi1−xZrxO3 (PZT) by XeCl excimer laser radiation has been investigated. In air, the threshold fluence for etching was about 2 J/cm2. At fluences of 10 J/cm2, etch rates of 0.1 μm/pulse were observed. The geometry of etched structures can readily be defined by choosing suitable experimental conditions, suggesting potential applications of this process to the production of devices.

PZT-based multilayer piezoelectric ceramics with AgPd-internal electrodes

Abstract The advantages of piezoelectric actuators are often opposed by one decisive handicap: their high control voltage. Piezoelectric multilayer ceramics do not show this disadvantage. Their highest possible deflection can be gained with voltages between 5 and 15 V which are common in todays electronics.

Anisotropic piezoelectric effect in modified PbTiO/sub 3/ ceramics

Experimental and theoretical investigations of the electromechanical anisotropy of ceramics modified with Ca, Ni-Nb, and Mn are presented. It is demonstrated that the large anisotropy of these ceramics is neither a bulk property of the PbTiO/sub 3/-crystallites nor a domain wall effect. The anisotropy is caused by the statistical orientation of the crystallites in the ceramics and by the material properties due to this orientation. The ratio of electrostrictive coefficients and the ratio of single-crystal permittivities in these ceramics also play a decisive role. The result enables one to comprehend easily the dependence of the ceramics electromechanical anisotropy on the material composition, the degree of poling, and the temperature.<<ETX>>

Deposition of self-polarized PZT films by planar multi-target sputtering

Abstract A planar multi target sputtering approach was used to deposit self polarized PZT films on TiO2/Pt bottom electrodes for the use in thin film pyroelectric IR detector arrays. By using elevated substrate temperatures of about 450°C “in situ” growth of tetragonal PZT could be achieved. The films exhibited pyroelectric currents without poling. The pyroelectric coefficient was 2×10−4 C/m2K, the dielectric constant was 300 and dielectric loss tan δ was 0.01. The self polarization disappears after heating the sample to 600°C. Stresses were studied in the thin film processing for the bottom electrode and the PZT film. The TiO2/Pt electrode is under high tensile stress of 900 MPa after preparation. PZT has a small compressive stress of -60 MPa, the whole TiO2/Pt/PZT stack has a tensile stress of +80 MPa. This low stress level together with the self polarization and the good electrical properties makes the films suitable for the use in pyroelectric detector arrays.