Anton Leidl

Imprinted sol–gel materials for monitoring degradation products in automotive oils by shear transverse wave

Titania sol-gel layers imprinted with capric acid have been used as synthetic receptors for highly sensitive detection of oxidized products resulting from degradation of automotive engine oil. These layers have been applied as sensitive coating material on shear transverse wave (STW) resonators of frequencies ranging from 100 MHz to 430 MHz. A relatively small size of STW resonators, i.e. about 2 mm for 430 MHz makes these devices extremely useful while considering the concept of miniaturization. It has been proved experimentally that by increasing fundamental resonance frequency of these devices, a very high sensor response i.e. 22 kHz up to 460 kHz can be generated. The geometry of long chain capric acid fits best as recognition element in the synthesis of imprinted TiO(2) network. The thin titania layers coated on transducer surface provide excellent diffusion pathways to oxidized products of waste engine oil for selective and reversible re-inclusion i.e. recovery time of 30 min. Viscosity effects of oxidized engine oil can be minimized by shear waves which do not dissipate considerable amount of energy that ensure smooth liquid phase operation. Different oxidized products i.e. carbonic acids and esters can be characterized in lubricant via infra-red (IR) spectroscopy. The increasing IR absorbance of different waste oil samples is a clear indication of increasing concentration of carbonyl group. The IR absorbance of carbonyl groups is directly correlated to the age of respective waste engine oil samples and a quantitative relationship between sensor responses from STWs and IR absorbance was also developed.

A comparison of tunable ferroelectric Π- and T-matching networks

A comparison of tunable pi - and T-matching networks based on ferroelectric capacitors is presented. Both topologies are promising candidates for automated antenna tuners in mobile communication systems. Assets and drawbacks of both circuits are discussed comparing linear and nonlinear simulations and measurements of two exemplary circuits at 850 MHz. The requirements to achieve sufficient linearity are determined by means of simulations. Both matching networks cover an impedance range of approximately |Gammain| les 0.6 with a maximum transducer power loss of 4 dB and 6 dB, respectively.

Packaging of MEMS Microphones

To miniaturize MEMS microphones we have developed a microphone package using flip chip technology instead of chip and wire bonding. In this new packaging technology MEMS and ASIC are flip chip bonded on a ceramic substrate. The package is sealed by a laminated polymer foil and by a metal layer. The sound port is on the bottom side in the ceramic substrate. In this paper the packaging technology is explained in detail and results of electro-acoustic characterization and reliability testing are presented. We will also explain the way which has led us from the packaging of Surface Acoustic Wave (SAW) components to the packaging of MEMS microphones.

A Thin-Film BST Varactor Model for Linear and Nonlinear Circuit Simulations for Mobile Communication Systems

A simple model of thin film BST varactors for the use in commercial circuit simulators is presented. The model implies the varactors nonlinear capacitance as well as the frequency and voltage dependency of the losses. For the use of BST based components in mobile communication systems low tuning voltages are required. This leads unfortunately to high nonlinear distortion which is not acceptable. Another severe problem are the moderate to low quality factors and therefore high losses of the ferroelectric capacitors. The presented model allows estimating the losses and the distortion over a wide frequency range at every bias state with harmonic balance and envelope simulations. Further more it is now possible to predict the behavior of a designed circuit but also to deduce the requirements for BST varactors in order to achieve the demanded design goals. The model consists of a parallel circuit of equation based elements which represents the nonlinear capacitance and conductance. A constant series inductance and a constant resistance model the contacts. The required parameters can easily be extracted through measurements. To demonstrate the validity of the presented model a BST based SP2T switch at 1800 MHz was designed. An insertion loss of 1.4 dB at an isolation of 21 dB and a TOI of 35 dBm were achieved. The simulation and measurement results agree very good over a wide frequency range.

Flip-chip packaging of piezoresistive barometric pressure sensors

To miniaturize piezoresistive barometric pressure sensors we have developed a package using flip-chip bonding. However, in a standard flip-chip package the different coefficients of thermal expansion (CTE) of chip and substrate and strong mechanical coupling by the solder bumps would lead to stress in the sensor chip which is not acceptable for piezoresistive pressure sensors. To overcome this problem we have developed a new ultra low stress flip-chip packaging technology. In this new packaging technology for pressure sensors first an under bump metallization (UBM) is patterned on the sensor wafer. As the next step solder bumps are deposited. After wafer-dicing the chips are flip-chip bonded on copper springs within a ceramic cavity. As sources of residual stress we identified the copper springs, the UBM and the solder bumps on the sensor chip. Different CTEs of the silicon chip and the UBM/solder lead to creep strain in the aluminum metallization between UBM and chip. As a consequence a temperature hysteresis can be measured.

Chip scale package of a MEMS microphone and ASIC stack

Most MEMS microphone systems on the market are packaged by conventional chip bonding and wire bonding.. A significant step towards miniaturization was achieved earlier by applying flip-chip bonding to MEMS microphone packaging. This technology is called chip scale MEMS package (CSMP). Thereby the package size could be reduced to 2.8 × 2.05 × 0.9 mm³ compared to a standard size of 3.76 × 2.95 × 1.1 mm³. In this paper the next step in miniaturization is presented. A further reduction to a total size of only 2.05 × 2.05 × 0.95 mm³ was achieved by placing the ASIC directly under the microphone chip inside a cavity in the substrate. This design is called stacked CSMP.

Ceramic Materials for Mass-Sensitive Sensors - Detection of VOCs and Monitoring Oil Degradation

Inorganic frameworks obtained by the sol-gel route can be templated by a molecular imprinting (MIP) approach to generate functional cavities. Such MIP ceramics show highly appreaciable properties for chemical sensor applications, because they are inherently chemically and thermally robust. In combination with mass-sensitive devices (e.g. quartz crystal micro balance – QCM, surface transverse wave oscillator - STW), they yield highly selective and sensitive chemical sensors. Gas phase measurements with volatile organic compounds (VOCs) e.g. lead to sensitivities below 1 ppm. Sensitivity can be tuned by the sol-gel-precursor: when hydrolysing more bulky alkoxides, this leads to enhanced sensitivity by increasing porosity as a consequence of slower solvent evaporation. By adding products of oxidative oil degradation to the sol-gel mixture, we succeeded in generating sensors for degradation processes in these complex matrices. This allows parallelly monitoring both the chemical state of oil and changes in viscosity. Sensitivity is enhanced according to the Sauerbrey equation by going from 10 MHz QCM transducers to higher frequencies either by etching the quartz substrates and so reducing the resonator thickness or by applying STWs.

A method for improving the drop test performance of a MEMS microphone

Most micro electro mechanical system (MEMS) microphones are designed as capacitive microphones where a thin conductive membrane is located in front of a rigid counter electrode. The membrane is exposed to the environment to convert sound into vibrations of the membrane. The movement of the membrane causes a change in the capacitance between the membrane and the counter electrode. The resonance frequency of the membrane is designed to occur above the acoustic spectrum to achieve a linear frequency response. To obtain a good sensitivity the thickness of the membrane must be as small as possible, typically below 0.5 μm. These fragile membranes may be damaged by rapid pressure changes. For cell phones, drop tests are among the most relevant reliability tests. The extremely high acceleration during the drop impact leads to fast pressure changes in the microphone which could result in a rupture of the membrane. To overcome this problem a stable protection layer can be placed at a small distance to the membrane. The protective layer has small holes to form a low pass filter for air pressure. The low pass filter reduces pressure changes at high frequencies so that damage to the membrane by excitation in resonance will be prevented.