Wolfgang Benecke

Microfabricated electrohydrodynamic (EHD) pumps for liquids of higher conductivity

A traveling-wave-driven electrohydrodynamic micropump without moving parts is discussed. The fundamental operating principles, such as high-frequency traveling waves, a self-stabilizing temperature gradient, and increased wave number, are outlined. The main advantages of the realized pump are its ability to move conductive liquids such as water and weak electrolyte solutions, the lack of any movable parts, and integration. A microfabricated structure demonstrating the pump operation is outlined, and quantitative results are described. Typical parameters characterizing the advantages and limitations of the pumping principle are discussed. Perspectives for optimization of the realized micropump can be seen in further miniaturization and increased number of electrodes. Possible applications are biological, medical, and chemical devices that can deliver accurately metered quantities of fluids in the nl/min and mu l/min range. >

A novel surface-micromachined capacitive porous silicon humidity sensor

Abstract The design, fabrication and characterisation of a novel humidity sensor are presented. The device consists of a capacitor with a porous silicon dielectric, two thermoresistors and a refresh resistor. The porous silicon is formed with a back-end process underneath a meshed metal electrode, which is fabricated in the same layer as the thermo- and refresh resistors. Due to this concept, very thin porous silicon could be formed with reproducible and stable metal contacts. At the same time, both the response time and the overall fabrication yield of the devices could be improved. The properties of the sensor are modelled and demonstrated with several experimental results.

Levitation, holding, and rotation of cells within traps made by high-frequency fields

Biological cells and other particles can be electrically manipulated by means of negative dielectrophoresis within microchambers whose electrode geometry is of the order of the cell size. Very-high-frequency fields (50 MHz and above) and media of increased relative permittivity are especially suitable for the purpose, as shown by experimental data on levitation and rotation. It appears to be possible to move and rotate cells or particles at will using this technology.

Linear motion of dielectric particles and living cells in microfabricated structures induced by traveling electric fields

Arrangements of microelectrodes as obtained by a microfabrication technique are found to be well suited for a linear transfer of microscopic particles such as biological cells and other objects of microscopic dimensions. The conditions for an effective manipulation of the particles are electrode geometries which correspond to the dimensions of the particle and adapted electrical excitation of the electrodes (traveling high-frequency waves). The motion of particles was found to be a super-position of dielectrophoresis and charge relaxation processes as they are dominant, e.g. in dielectric induction motors. Microparticle velocities of some hundreds mu /s could be achieved by applying phase-shifted rectangular pulses with amplitudes between 5 and 15 volts.<<ETX>>

Pumping of water solutions in microfabricated electrohydrodynamic systems

A microfabricated electrohydrodynamic pump without moving parts driven by low voltages with high-frequency traveling waves is presented. Micron-size scale systems without moving parts fabricated in planar silicon technology are presented and quantitatively described. The operating principle to pump water and weak electrolyte solutions is outlined. It is shown that conductive liquids such as water solutions can be pumped opposite to the direction of the traveling wave. Typical parameters characterizing the advantages and limitations of the pumping principle are discussed. Opportunities for optimization of the micropump in terms of further miniaturization and an increased number of electrodes are noted.<<ETX>>

A smart accelerometer with on-chip electronics fabricated by a commercial CMOS process

Abstract Piezoresistive accelerometers with a monolithically integrated operational amplifier were produced, the fabrication process based on a commercial 3 μm CMOS process. The mechanical structures were realized using wet anisotropic etching of silicon with KOH and the electrochemical etch-stop at p—n junctions. Measurements show that the integration of these necessary micromachining process steps into the IC process do not influence the parameters of the electronic devices. Also, the parameters of the mechanical structures are comparable to discrete devices. The realization of application-specific smart mechanical sensors and actuators using a standard CMOS process is now possible.

Vapor-Phase Self-Assembled Monolayers for Anti-Stiction Applications in MEMS

We have investigated the anti-stiction performance of self-assembled monolayers (SAMs) that were grown in vapor phase from six different organosilane precursors: CF₃(CF₂)₅(CH₂)₂SiCl₃ (FOTS), CF₃(CF₂)₅(CH₂)₂Si(OC₂H₅)₃ (FOTES), CF₃(CF₂)₅(CH₂)₂Si(CH₃)Cl₂ (FOMDS), CF₃(CF₂)₅(CH₂)₂Si(CH₃)₂Cl (FOMMS), CF₃(CF₂)₇(CH₂)₂SiCl₃ (FDTS), and CH₃(CH₂)₁₇(CH₂)₂SiCl₃ (OTS). The SAM coatings that were grown on silicon substrates were characterized with respect to static contact angle, surface energy, roughness, nanoscale adhesive force, nanoscale friction force, and thermal stability. The best overall anti-stiction performance was achieved using FDTS as precursor for the SAM growth, but all coatings show good potential for solving in-use stiction problems in microelectromechanical systems devices.

Thermal stability of vapor phase deposited self-assembled monolayers for MEMS anti-stiction

Six different source chemicals (organosilanes) were successfully used for deposition of self-assembled monolayers (SAMs) onto silicon substrates by a vapor phase process. Five different fluorocarbon coatings and one hydrocarbon coating were deposited. The thermal stability of the coatings was studied in detail with respect to degradation as a function of temperature, and for the fluorocarbon coatings also the degradation rate at 400 °C. For fluorocarbon coatings deposited from FDTS a useful lifetime of approximately 90 min at 400 °C was found allowing the coating to survive high temperature MEMS packaging operations, while fluorocarbon coatings deposited from FOTS, FOMDS, FOTES and FOMMS were less stable. The hydrocarbon coating deposited from OTS degrades already at approximately 200 °C. The thermal stability of the SAM coatings was found to be significantly reduced if aggregations from the deposition process are present on the coatings.

Thermally driven microvalve with buckling behaviour for pneumatic applications

The paper presents theoretical and experimental results of a new thermally driven microvalve. In contrast to earlier reported devices with bimorph composition this valve is based on the buckling effect of a heated pure silicon bridge structure. Simulation, fabrication and experimental results are presented. Analytical and finite-element calculations of the thermomechanical behaviour are performed. Prototypes of this first valve design operate with inlet pressures up to 1.0 bar showing flow rates of more than 700 ml/min. The measured switching time is about 15 ms which is extremely low for thermal principles. The power consumption of this valve is between 1 and 4 W.

Design and fabrication of a thermal infrared emitter

Abstract In this paper a thermal infrared (IR) emitter is described. It is part of a non-dispersive infrared (NDIR) gas-analysing system. The system is planned for multi-gas analysis. In particular, CO 2 should be detected from 100 ppm to 10%. The thermal heater is integrated with a Fresnel zone plate. The zone plate is a diffractive lens with dispersive properties. The number of components of the system is reduced by integration. A combination of surface and bulk micromachining is used for fabrication of the emitter. The cavity of the heating structure is made by direct bonding of two structured wafers. A new method for sealing the cavity by aluminium sputtering is used. Aluminium has bifunctional properties. Beside the metallization, it is used for sealing the cavity. The sputtering is carried out at a pressure below 6 mtorr so that the cavity is simultanously evacuated. An analytical model is developed to calculate the emitted power of the radiating structure and the thermal strain. For a 30 μm × 10 μm thermal emitter an emitted power of 430 nW at a maximum heating temperature of 1500 K has been calculated. The results are compared with finite-element method (FEM) analysis.