J. Held

Wafer-Scale Microtensile Testing of Thin Films

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 (SiNx) produced by plasma-enhanced chemical vapor deposition, and a ductile material, i.e., evaporated aluminum (Al), were characterized. The extraction of the Youngs modulus E, tensile strength sigmau, mean tensile strength sigmatildeu, and Weibull modulus m is demonstrated. Youngs moduli thus obtained for the poly-Si, SiNx, and Al films were 156.3plusmn 2.6, 112.2plusmn3.5, and 62.5plusmn 2.5 GPa, respectively. The SiNx layers, which have a mean tensile strength sigmatildeu of 2.084-0.177 +0.169 GPa and m=5.9-1.6 +1.8, are the strongest from the fracture point of view when compared to poly-Si with sigmatildeu=1.382-0.026 +0.023 GPa and m=17.3-3.2 +3.5 and Al with sigmatildeu=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 and Weibull statistics.

Microneedle arrays for intracellular recording applications

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 mm2. 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.

Simultaneous and Independent Measurement of Stress and Temperature Using a Single Field-Effect Transistor Structure

This paper reports on the simultaneous and independent measurement of mechanical stress and temperature using p- and n-channel field-effect transistor structures with multiple drain/source contacts fabricated in a commercial complementary metal-oxide-semiconductor technology. The respective average stress sensitivities of of three p-channel devices and 66 of two n-channel devices at room temperature originate from the shear piezoresistance effect, also termed pseudo-Hall effect, of the inversion layer carriers. The stress sensitivities show very small average temperature coefficients (TCs) of 1127 and 431 ppm/K for the p- and n-channel devices, respectively. Temperature values were obtained from the temperature-dependent threshold voltage . Robust values were extracted from the second-order transconductance smoothed using a new variant of Tikhonovs regularization method. With both types of devices, the obtained by this procedure was found to be stress insensitive in the range of stresses from 0 to 20.4 MPa. Between 25 and 150 , depends linearly on temperature, with average slopes of 1.67 and for p- and n-channel devices, respectively. Device-to-device variations of the temperature sensitivity and its slope suggest the use of a two-point calibration. With only a one-point calibration, a temperature uncertainty of less than 3 over a temperature range of 100 is achieved.

Design of experiment characterization of microneedle fabrication processes based on dry silicon etching

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.

Simultaneous and Independent Measurement of Stress and Temperature Using a Single Field Effect Transistor Based Sensor

This paper reports on the simultaneous and independent measurement of mechanical stress and temperature using a single field effect transistor with multiple source/drain contacts. A stress sensitivity of Sσ= -405 µ V/MPa V of the device originates from the shear piezoresistive effect, i.e. the pseudo-Hall effect. This sensitivity exhibits a very small temperature coefficient of only 914 ppm/K. Temperature values are acquired from the temperature dependence of the threshold voltage VTand extracted using a recently reported regularization method with a temperature sensitivity of SVT= -1.67 mV/K. We report VTto be stress independent in the measured range of 0 MPa to 15.6 MPa and in the temperature range of 25 ° C to 150 ° C.

Smart brush based on a high density CMOS stress sensor array and SU-8 microposts

This paper reports on a CMOS integrated stress sensor array combined with SU-8 posts fabricated on the chip surface. The chip contains an array of 1024 field effect transistor (FET)-based stress sensors occupying a total area of ca. 1 x 1 mm2. On-chip circuitry is used for sensor addressing, signal amplification, A/D conversion, signal processing, and serial bus communication. A novel scalable matrix-based sensor selection concept enables the integration of high-density stress sensor arrays with minimum wiring. The system significantly expands previous stress sensor arrays in terms of an increased number of sensors and reduced sensor pitch. An array of 8 x 8 circular SU-8 posts with a diameter of 62 mum and a height of 300 mum was fabricated on the CMOS chips. The new brush-like device was used for measuring vertical and horizontal force distributions.

High-throughput wafer-scale microtensile testing of thin films

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 (SiNx), and ductile materials such as aluminum (Al). The extraction of mechanical parameters such as the Youngs modulus E, mean tensile strength sigma tildeu, Weibull modulus m and 0.2% offset yield strength sigma0.2% is demonstrated.

FIB Preparation and SEM Investigations for Three-Dimensional Analysis of Cell Cultures on Microneedle Arrays

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 microneedles 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.

Development of Calibration Standards for the Optical Measurement of In-Plane Displacements of Micromechanical Components

The goal of this work is to develop miniaturized reference standards for the optical measurement of inplane displacements serving for the calibration of optical systems used in the production and characterization of MEMS. The proposed devices consist of SOI-based inplane microactuators. The displacements resulting from both mechanical and electrostatic actuations are measured by means of optical techniques such as stroboscopic illumination, laser-deflection method and digital speckle interferometry.

Hollow Microneedle Electrode Arrays for Intracellular Recording Applications

This paper reports on the fabrication of hollow microneedle electrodes with fluidic channels arranged in 8×8 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.