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ABSTRACT: We report the investigation of the interfaces between microneedle arrays and cell cultures in patch-on-chip systems by using Focused Ion Beam (FIB) preparation and Scanning Electron Microscopy (SEM). First, FIB preparations of micro chips are made to determine the size and shape of the designed microneedles. In this essay, we investigate the cell-substrate interaction, especially the cell adhesion, and the microneedle's potential cell penetration. For this purpose, cross-sectional preparation of these hard/soft hybrid structures is performed by the FIB technology. By applying the FIB technology followed by high-resolution imaging with SEM, new insights into the cell-substrate interface can be received. One can clearly distinguish between cells that are only in contact with microneedles and cells that are penetrated by microneedles. A stack of slice images is collected by the application of the slice-and-view setup during FIB preparation and is used for three-dimensional reconstruction of cells and micro-needles.
Scanning 11/2011; 34(4):221-9. · 1.07 Impact Factor
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ABSTRACT: This paper presents a new experimental method to estimate the fatigue behavior of polysilicon thin films by using cyclic loading with gradually increasing stress amplitude. This method solves fundamental problems of conventional fatigue experiments with constant amplitude. The fatigue crack extension process determining fatigue lifetime was estimated by the well-known Paris law with two unknown parameters. These were fit to the results of two methods performed on specimens with two different situations of etching damage. It is shown that the new experimental method can be used to predict the fatigue lifetime in a more efficient manner.
Micro Electro Mechanical Systems (MEMS), 2010 IEEE 23rd International Conference on; 02/2010
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ABSTRACT: The fabrication, characterization and theoretical analysis of a novel two-dimensional silicon resonator with threefold rotational symmetry are described. The resonator consists of a 4 mm wide disk-shaped seismic mass having the full-wafer thickness of 525 µm and suspended by a system of concentric circular springs. The device is structured using two-sided deep reactive-ion etching of silicon. With its current spring thickness and height, the device has two closely spaced resonance frequencies at 370.5 and 373.9 Hz and a quality factor of 1800 at ambient pressure. The spring height and thus the resonance frequency of the device are easily tuned by simple adjustment of a single etch duration in the entire fabrication process. The dynamic response of the structure is modeled under the two assumptions that silicon is elastically (i) isotropic and (ii) anisotropic. In comparison with the isotropic model, the elastic anisotropy leads to a predicted mode splitting by 6.2 Hz, with oscillation directions aligned with the cubic crystal axes. Even small geometrical imperfections are found to significantly rotate the eigenmodes and to further modulate their frequency splitting. Experimental and numerical results corroborate these conclusions. Overall the present resonator design has the potential for a higher energy harvesting efficiency than a combination of two separate one-dimensional oscillators.
Journal of Micromechanics and Microengineering 02/2010; 20(3):035016. · 2.11 Impact Factor
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ABSTRACT: This paper reports on the characterization of dry etching-based processes for the fabrication of silicon microneedles using a design of experiment (DoE) approach. The possibility of using such microneedles as protruding microelectrodes able to electroporate adherently growing cells and record intracellular potentials motivates the systematic analysis of the influence of etching parameters on the needle shape. Two processes are characterized: a fully isotropic etch process and a three-step etching approach. In the first case, the shape of the microneedles is defined by a single etch step. For the stepped method, the structures are realized using the following sequence: a first, isotropic step defines the tip; this is followed by anisotropic etching that increases the height of the needle; a final isotropic procedure thins the microneedle and sharpens its tip. From the various process parameters tested, it is concluded that the isotropic fabrication is influenced mostly by four process parameters, whereas six parameters dominantly govern the outcome of the stepped etching technique. The dependence of the needle shape on the etch mask diameter is also investigated. Microneedles with diameters down to the sub-micrometer range and heights below 10 µm are obtained. The experimental design is performed using the D-optimal method. The resulting geometry, i.e. heights, diameters and radii of curvature measured at different positions, is extracted from scanning electron micrographs of needle cross-sections obtained from cuts by focused ion beam. The process parameters are used as inputs and the geometry features of the microneedles as outputs for the analysis of the process.
Journal of Micromechanics and Microengineering 01/2010; 20(2):025024. · 2.11 Impact Factor
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ABSTRACT: This paper reports on the mechanical characterization of thin films using the microtensile technique performed for the first time at the wafer scale. Multiple test structures are processed and sequentially measured on the same silicon substrate, thus eliminating delicate handling of individual samples. The current layout uses 26 test structures evenly distributed over a 4-in wafer, each of them carrying a microtensile specimen that bridges the gap between the fixed and movable parts of the micromachined wafer. A fully automated high-throughput setup makes possible the fast acquisition of data with statistical relevance for the reliable extraction of material properties. The technique was successfully applied to micrometer- and submicrometer-thick films. Two brittle materials, namely, polycrystalline silicon (poly-Si) obtained by low-pressure chemical vapor deposition and silicon nitride (SiN<sub>x</sub>) produced by plasma-enhanced chemical vapor deposition, and a ductile material, i.e., evaporated aluminum (Al), were characterized. The extraction of the Young's modulus E, tensile strength sigma<sub>u</sub>, mean tensile strength sigmatilde<sub>u</sub>, and Weibull modulus m is demonstrated. Young's moduli thus obtained for the poly-Si, SiN<sub>x</sub>, and Al films were 156.3plusmn 2.6, 112.2plusmn3.5, and 62.5plusmn 2.5 GPa, respectively. The SiN<sub>x</sub> layers, which have a mean tensile strength sigmatilde<sub>u</sub> of 2.084<sub>-0.177</sub> <sup>+0.169</sup> GPa and m=5.9<sub>-1.6</sub> <sup>+1.8</sup>, are the strongest from the fracture point of view when compared to poly-Si with sigmatilde<sub>u</sub>=1.382<sub>-0.026</sub> <sup>+0.023</sup> GPa and m=17.3<sub>-3.2</sub> <sup>+3.5</sup> and Al with sigmatilde<sub>u</sub>=0.347plusmn0.013 GPa. In each case, the best estimate of the mean and the corresponding 90% confidence interval were evaluated using maximum likelihood estimation and the likelihood ratio method, respectively, on the basis of Gaussian-
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Journal of Microelectromechanical Systems 11/2009; · 2.10 Impact Factor
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ABSTRACT: Strength and fatigue lifetime of polycrystalline silicon thin films with notches were successfully predicted from the information obtained from reference specimens with uniform stress distribution. The initial damage, which was induced by patterning process and supposed to be independent of specimen shapes, was characterized as an equivalent crack distribution from the strength distribution of reference specimens. Static strength of notched specimens was then estimated from a statistical point of view with stress distribution taken into account. Fatigue lifetime was estimated on the basis of fatigue crack extension process described by Paris' law, with the two parameters obtained from uniform stress specimens in the previous study as material inherent constants.
Solid-State Sensors, Actuators and Microsystems Conference, 2009. TRANSDUCERS 2009. International; 07/2009
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ABSTRACT: This paper reports on the fabrication of hollow microneedle electrodes with fluidic channels arranged in 8times8 arrays. Features of these electrodes include (i) an increased surface area for improved intracellular potential measurements with simultaneous membrane cell poration capabilities, (ii) their potential use in highly parallel patch-clamp applications and (iii) the ability to efficiently inject reagents and extract cytoplasm into and from the cell interior, respectively. Three different fabrication processes to realize hollow microneedle electrode arrays with incorporated microfluidic components, as well as initial experiments with cell cultures, are presented.
Micro Electro Mechanical Systems, 2009. MEMS 2009. IEEE 22nd International Conference on; 03/2009
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ABSTRACT: This paper reports on the extension of the wafer-scale microtensile technique to the piezoresistive characterization of thin-films, demonstrated for in-situ n-doped poly-Si layers. In addition to the reliable extraction of mechanical properties, this extended high-throughput method enables the acquisition of linear and, for the first time, nonlinear piezoresistive coefficients, namely the first and second order longitudinal parameters, pi<sub>l,1</sub> and pi<sub>l,2</sub>, respectively, with statistical significance. In contrast to previous studies, the data presented here are extracted for strains up to the fracture value, leading to meaningful linear and quadratic piezoresistive parameters. Along with the mechanical properties, these values are especially important for sensing applications where poly-Si films are subjected to significant levels of stress and strain.
Micro Electro Mechanical Systems, 2009. MEMS 2009. IEEE 22nd International Conference on; 03/2009
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ABSTRACT: This paper reports on the integration of several technologies to realize, to our knowledge, the first two-dimensional (2D) electrostatic resonant micro energy generator. In the generator, ambient vibrations lead to the oscillation of the movable parts of two micromachined and assembled substrates. Charged Cytop-based electret structures patterned on these parts and arranged in a variable capacitor configuration translate the oscillations into a displacement current whose energy is harvested. The device results from (i) the design, fabrication and characterization of low-frequency 2D resonators to extract energy from ambient vibrations, (ii) processing, charging and long-term characterization of Cytop as an electret material and (iii) development of a packaging technology based on adhesive bonding using Cytop as well.
Micro Electro Mechanical Systems, 2009. MEMS 2009. IEEE 22nd International Conference on; 03/2009
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ABSTRACT: This paper reports on the mechanical characterization of thin films using the microtensile technique performed for the first time at the wafer-scale. Multiple test structures are processed and sequentially measured on the same substrate, thus eliminating delicate handling of individual samples. The current layout uses 26 test structures evenly distributed on a 4-inch silicon wafer, each of them having microtensile specimens. A fully automated, high-throughput setup has been developed, which enables the fast acquisition of data with statistical relevance for the reliable extraction of material properties. The technique has been successfully applied to mum- and sub-mum-thick films. These include brittle materials, such as polycrystalline silicon (poly-Si) and silicon nitride (SiN<sub>x</sub>), and ductile materials such as aluminum (Al). The extraction of mechanical parameters such as the Young's modulus E, mean tensile strength sigma tilde<sub>u</sub>, Weibull modulus m and 0.2% offset yield strength sigma<sub>0.2%</sub> is demonstrated.
Micro Electro Mechanical Systems, 2008. MEMS 2008. IEEE 21st International Conference on; 02/2008
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ABSTRACT: This paper presents a scheme to predict the fatigue lifetime of polycrystalline silicon thin films under cyclic loading. The initial damage was characterized as an equivalent crack distribution from the results of quasi-static tensile tests, i.e., from the distribution of static strength. The fatigue crack extension process determining fatigue lifetime was then estimated by the well-known Paris law with two unknown parameters. These were fit to the results of fatigue tests performed on specimens with two different situations of etching damage. In spite of the difference in static strength, the parameters are consistent among the two cases. This implies that fatigue crack extension law is inherent to the material and that lifetime can be estimated on the basis of static strength.
Micro Electro Mechanical Systems, 2008. MEMS 2008. IEEE 21st International Conference on; 02/2008
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ABSTRACT: This paper reports on the fabrication and application of microneedles for the electroporation of adherently growing cells and intracellular recording with focus on the influence on external factors on the cell behavior. Patch-on-chip methods such as patch-clamp have been applied mostly to individual cells in suspension. However, in the human body most of cells are adherently growing cells, which motivated the development of a new chip design. The chip contains an array of 64 microneedles occupying a total area of approximately 1 mm<sup>2</sup>. The microneedles are fabricated using dry etching of silicon, followed by an insulation, metallization and passivation. The passivation layer is opened at the tip of the needles in order to expose the metal for cell positioning via dielectrophoresis, cell electroporation, as well as intracellular recording. Various needles with diameters in the sub-micron range and heights below 10 mum have been fabricated. Heart muscle cells, fibroblasts, and primary neuronal cells of mice were grown on these microneedle arrays. To electrically access the intracellular space, the cells were electroporated with a voltage of plusmn2 V. Preliminary tests show that more than 80% of the cells could successfully be porated.
Micro Electro Mechanical Systems, 2008. MEMS 2008. IEEE 21st International Conference on; 02/2008
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ABSTRACT: This paper reports on recent improvements of the bulge and microtensile techniques for the reliable extraction of material parameters such as the Young's modulus E, Poisson's ratio v, plane strain modulus E<sub>ps</sub> = E/(1-v<sup>2</sup>), prestress sigma<sub>0</sub>, fracture strength mu and Weibull modulus m, and on the direct comparison of these two methods. The bulge technique is extended to full wafer measurements enabling throughputs of data with statistical relevance whereas key improvements of a previous fabrication process of microtensile specimens lead now to much higher yields, approaching 100%. In view of its importance and wide use in MEMS, polycrystalline silicon is characterized with both methods. Values of E<sub>ps</sub> and sigma<sub>0</sub> of 160 GPa and -8 MPa, respectively, are extracted for films deposited at 625degC and fully annealed at 1050degC, with no thickness dependence. Fracture data reveal a decrease of the strength mu, normalized to a reference tensile volume V<sub>E</sub> of 32times10<sup>3</sup> mum<sup>3</sup> , from 2.17 to 1.38 GPa as the film thickness increases from 0.53 to 1.91 mum, with Weibull moduli m of the order of 20.
Solid-State Sensors, Actuators and Microsystems Conference, 2007. TRANSDUCERS 2007. International; 07/2007
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ABSTRACT: This paper describes the modeling, fabrication, and characterization of SOI-based micro- electro-mechanical transducers used for energy harvesting. The electrostatic transducers convert vibrations of a seismic mass by means of variable capacitors realized using comb structures. The seismic mass with comb electrodes is suspended by four 1-mm-long straight beam springs. Transducers with springs of different widths with theoretical resonance frequencies ranging from 96 to 1160 Hz were realized. The frequency behavior of the resonators is investigated using an in-plane vibrometer with nm resolution. Linear and non-linear responses are measured for weak and hard driving excitations, respectively. The effect of the capacitance variation on the harvested power is investigated as well as the application of different resistive loads to the devices.
Solid-State Sensors, Actuators and Microsystems Conference, 2007. TRANSDUCERS 2007. International; 07/2007
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ABSTRACT: This paper reports on the mechanical characterization of microtensile specimens made of silicon nitride (SiN<sub>x</sub>) thin-films with integrated 2D reflective gratings. By applying an axial force, the structures respond mechanically with an elongation and contraction in the longitudinal and transversal directions, respectively. The corresponding variations of both periods of the grating are monitored in real time by measuring the diffraction pattern resulting from the illumination of the grating with monochromatic light. The strain components are thus evaluated locally in the structure. By integrating such an optical technique with an efficient test structure previously developed, the extraction of materials' Young's modulus E, Poisson's ratio nu, residual strain epsiv<sub>res</sub> and stress sigma<sub>res</sub> and fracture strength sigma<sub>0</sub> is in principle made possible from the measurement of a single test structure. Here we demonstrate the extraction of E, epsiv<sub>res</sub>, sigma<sub>res</sub> and sigma<sub>0</sub> of the nitride films.
Micro Electro Mechanical Systems, 2007. MEMS. IEEE 20th International Conference on; 02/2007
