Showing papers by "Helmut Seidel published in 2010"

The influence of sputter deposition parameters on piezoelectric and mechanical properties of AlN thin films

Abstract: Aluminium nitride (AlN) reactively sputter-deposited from an aluminium target is an interesting piezoelectric thin film material with high CMOS compatibility. A good c -axis orientation is essential for obtaining high piezoelectric coefficients. Therefore, the influence of different sputtering conditions on the microstructure of AlN thin films with a typical thickness of about 500 nm was investigated. In this study it is demonstrated that highly c -axis oriented AlN thin films can be deposited on nominally unheated (1 0 0) silicon substrates, most preferentially when using a pure nitrogen atmosphere. The degree of c -axis orientation increases with higher nitrogen concentration and with decreasing the sputtering pressure, whereas the influence of plasma power on the microstructure was found to be negligible. A low sputtering pressure is also useful for minimizing the amount of oxygen contaminations in the deposition chamber and hence for reducing the incorporation of impurities into the AlN films. Intrinsic stress values of AlN thin films were determined by wafer bow measurements and were found to be between −3.5 and 750 MPa depending on choice of deposition parameters. Finally, the piezoelectric coefficients d 33 and d 31 were determined experimentally by laser scanning vibrometry in conjunction with a theoretical model. Effective values in c -axis oriented 500 nm films with FWHM of 0.33° are 3.0 and −1.0 pm/V. For a film of 2.4 μm thickness, values of 5.0 and −1.8 pm/V were measured, which are near the bulk values.

Modal optimization and filtering in piezoelectric microplate resonators

Abstract: A systematic design procedure to tailor the modal response of micro-resonators based on flexible plates with piezoelectric films is demonstrated. Sensors/actuators were designed by optimizing the surface electrode shapes in the plane dimensions. A numerical finite element procedure, which considers the effective surface electrode covering the piezoelectric film as a binary function on each element, has been implemented. Two design goals are considered: (i) optimized response (actuation or sensing) in a given mode; (ii) implementation of a modal transducer by filtering specific modes. For a given mode in a plate with arbitrary boundary conditions, our calculations allowed us to predict the top electrode layout reaching higher displacement in resonance than any other electrode design for the same structure. Microcantilevers and microbridges were fabricated and their modal response characterized by laser Doppler vibrometry. In comparison to a conventional square-shaped electrode, our experiments show that the implemented designs can increase the response in any desired resonant mode and simultaneously attenuate the contributions from other unwanted modes, by simply shaping the surface electrodes. Enhancement ratios as high as 42 dB, relative to a full-size electrode case, are demonstrated. The limitations imposed by the fabrication are also discussed.

A new capacitive type MEMS microphone

Abstract: A new capacitive type of MEMS microphone is presented. In contrast to existing technologies which are highly specialized for this particular type of application, our approach is based on a standard process and layer system which has been in use for more than a decade now for the manufacturing of inertial sensors. For signal conversion, a mixed-signal ASIC with digital sampling of the microphone capacitance is used. The MEMS microphone yields high signal-to-noise performance (58 dB) after mounting it in a standard LGA-type package. It is well-suited for a wide range of potential applications and demonstrates the universal scope of the used process technology.

Influence of a chip scale package on the frequency response of a MEMS microphone

Abstract: This paper describes the influence of a chip scale MEMS package (CSMP) on the acoustic behaviour of a silicon microphone. The influence was calculated using an electro-mechanical–acoustical equivalent circuit. Standard packaging of microphones using die bonding and wire bonding leads to a large front volume which acts as a Helmholtz resonator. This can dramatically influence the frequency response of the microphone system by adding a second resonance. In the worst case this second resonance is in the acoustic frequency range, thus degrading its performance in an unacceptable way. In case of the CSMP only a small front volume is generated between the substrate and the flip-chipped microphone chip. Thus the resonance step-up is very small compared to standard packages. Furthermore the frequency response can be flattened by optimizing the geometry of the small sound holes in the substrate. By choosing an appropriate geometry of these sound holes the package can act as a low pass filter where the cut-off frequency can be placed to the desired value of the acoustic spectrum.

Characterization and displacement control of low surface-stress AlN-based piezoelectric micro-resonators

Abstract: Micro-cantilevers and micro-bridges actuated by sputter-deposited aluminium nitride (AlN) thin films were measured with a scanning laser Doppler vibrometer up to 6 MHz, covering more than 10 resonance modes of different nature. A finite element model (FEM) was used to simulate the modal response of the micromachined structures. The comparison between experiment and simulation, regarding modal shapes and frequencies, resulted in an excellent agreement. An interferometric microscope was also used to study the static deflection of the structures. These measurements revealed a very low surface stress for the different micro-resonators. Finally, we point out how the amplitude of a given resonant mode can be controlled depending on the piezoelectric charge collected by the top electrode layout.

High-Frequency Characterization of Porous Low-Temperature Cofired Ceramics Substrates

Abstract: In this work, the permittivity of porous low-temperature cofired ceramics (LTCC) DP 951 is measured and evaluated. Porosification is performed at originally dense LTCC substrates in the fired state by a wet chemical etching procedure using hot phosphoric acid. Choosing this approach, areas with tailored permittivity can be generated in one single LTCC layer. The etch time and the bath temperature precisely control the penetration, and hence, the porosification depth. Therefore, the decrease in dielectric constant of the LTCC substrate can be correlated to the thickness of the porous layer. The dielectric constant is measured using a ring resonator in the microstrip configuration. From the resonances occurring in the transmission S-parameter |S21| spectrum between 1 and 10 GHz, the relative dielectric constant can be determined. Using 820-μm-thick substrates, a relatively low reduction from ɛr=7.8 to 6.45 is achieved when a porosification depth of about 35 μm is reached. Applying a fitting procedure, the dielectric constant of the pure porous layer is deduced to ɛr=2.3. Based on numerical simulations, the effective dielectric constant for a 100-μm-thick glass–ceramic layer, which is modified to a depth of 35 μm is calculated to 5.2, whereas the thickness represents a lower limit for commercially available tapes being typically implemented into the fabrication process of monolithic LTCC systems with integrated metallization planes. Compared with commercially available low-K LTCC materials, this value is lower than all other commonly used tape systems.

Chip scale package of a MEMS microphone and ASIC stack

Abstract: 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.

Two-photon lithography and nanoprocessing with picojoule extreme ultrashort 12 femtosecond laser pulses

Abstract: A compact ultra-broadband femtosecond laser scanning microscope with 12 femtoseconds pulse width at the focal plane of a high NA objective has been employed in material nanoprocessing. The laser works at 85 MHz with an M-shaped emission spectrum with maxima at 770 nm and 827 nm. Different motorized setups based on the introduction of chirped mirrors, flint glass wedges, and glass blocks have been realized to vary the in situ pulse length from 12 femtoseconds up to 3 picoseconds. Nanoprocessing was performed in silica, photoresists, glass, polymers, and biological structures. Mean powers as low as 2 mW were sufficient to realize plasma-mediated cutting effects in human chromosomes with sub-80 nm cut width. Using a mean power of 7-9 mW, transient nanoholes were "drilled" in the cellular membrane for targeted transfection of stem cells and the introduction of μRNA probes. Region of interest (ROI) scanning have been used for optical cleaning of human adult stem cell populations and blood cell suspensions. 3D two-photon nanolithography based on the ultrabroad band laser pulses was realized with the photoresist SU-8. Multiphoton sub-20fs microscopes may become novel non-invasive 3D tools for highly precise nanoprocessing of inorganic and organic targets.

Digital potentiostat for electrochemical bio sensor chips

Abstract: A rail-to-rail potentiostat circuit suitable for CMOS based electronic bio sensors is presented. The design approach enables robust operation far a large range of electrochemical conditions. Measurement results prove proper operation of the potentiostatic control circuitry.

Robust miniaturized amplification unit for piezoelectric actuators: comparison of single crystal silicon and superelastic nickel titanium as membrane materials

Abstract: In this work, amplification units made of robust metallic nickel titanium (NiTi) and single crystal silicon are compared and evaluated for the application in miniaturized piezoelectric actuators for flow control purposes. The amplification mechanism with a sliced membrane structure is based on a mechanical lever in order to amplify the low piezoelectrically induced deformation. Therefore, an enhanced output stroke can be provided up to high frequencies. The different membrane fabrication processes using laser ablation for the NiTi alloy and deep reactive ion etching for the silicon substrate, as well as the results of finite element simulations and experimental measurements are reported. An amplification factor of 9 has been achieved for an optimized load transmission point position. The dynamic response shows a quality factor of 25 and 494 at the first fundamental mode for NiTi and silicon membranes, respectively. Compared to silicon, NiTi shows enhanced properties against failure and facilitates the integration process.

Fast cell immobilization by using non-immunological method for cell based biosensor

Abstract: This paper reports a concept of a micro cell analysis system for fast cell immobilization by using a non-immunological method which is composed of a cell/drug inlet, a sheath flow inlet, a micro chamber and a outlet to biosensor/waste. Micro cell wells were located in the micro chamber. To define optimized parameters of micro cell analysis system, cell adhesion experiment using Hela at 0.5×106, 1.0×106 or 2.0×106 cells/ml in micro wells with 10µm depth and 25µm, 50µm and 100µm size respectively was performed. Different tendencies of cell adhesion between 0.5×106 cells/ml and 1.0×106 cells/ml concentration were not detected. The highest concentration gave the best performance of cell immobilization, even though cell congregation appeared.