We survey progress over the past 25 years in the development of microscale devices for pumping fluids. We attempt to provide both a reference for micropump researchers and a resource for those outside the field who wish to identify the best micropump for a particular application. Reciprocating displacement micropumps have been the subject of extensive research in both academia and the private sector and have been produced with a wide range of actuators, valve configurations and materials. Aperiodic displacement micropumps based on mechanisms such as localized phase change have been shown to be suitable for specialized applications. Electroosmotic micropumps exhibit favorable scaling and are promising for a variety of applications requiring high flow rates and pressures. Dynamic micropumps based on electrohydrodynamic and magnetohydrodynamic effects have also been developed. Much progress has been made, but with micropumps suitable for important applications still not available, this remains a fertile area for future research.

https://microfluidics.stanford.edu/Publications/Micropumps_Cooling/Laser%20Review%20of%20Micropumps%20in%20JMM.pdf

A review of micropumps

Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has made possible the development of three-dimensional fluorescence imaging, optical data storage, and lithographic microfabrication. These applications take advantage of the fact that the two-photon absorption probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of order λ3 (where λ is the laser wavelength). Any subsequent process, such as fluorescence or a photoinduced chemical reaction, is also localized in this small volume. Although three-dimensional data storage and microfabrication have been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, such photopolymer systems exhibit low photosensitivity as the initiators have small two-photon absorption cross-sections (δ). Consequently, this approach requires high laser power, and its widespread use remains impractical. Here we report on a class of π;-conjugated compounds that exhibit large δ (as high as 1, 250 × 10−50 cm4 s per photon) and enhanced two-photon sensitivity relative to ultraviolet initiators. Two-photon excitable resins based on these new initiators have been developed and used to demonstrate a scheme for three-dimensional data storage which permits fluorescent and refractive read-out, and the fabrication of three-dimensional micro-optical and micromechanical structures, including photonic-bandgap-type structures.

Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication

This paper presents a review of silicon micromachined accelerometers and gyroscopes. Following a brief introduction to their operating principles and specifications, various device structures, fabrication, technologies, device designs, packaging, and interface electronics issues, along with the present status in the commercialization of micromachined inertial sensors, are discussed. Inertial sensors have seen a steady improvement in their performance, and today, microaccelerometers can resolve accelerations in the micro-g range, while the performance of gyroscopes has improved by a factor of 10/spl times/ every two years during the past eight years. This impressive drive to higher performance, lower cost, greater functionality, higher levels of integration, and higher volume will continue as new fabrication, circuit, and packaging techniques are developed to meet the ever increasing demand for inertial sensors.

/pdf/micromachined-inertial-sensors-esf6197vdc.pdf

Micromachined inertial sensors

Samples of 53 materials that are used or potentially can be used or in the fabrication of microelectromechanical systems and integrated circuits were prepared: single-crystal silicon with two doping levels, polycrystalline silicon with two doping levels, polycrystalline germanium, polycrystalline SiGe, graphite, fused quartz, Pyrex 7740, nine other preparations of silicon dioxide, four preparations of silicon nitride, sapphire, two preparations of aluminum oxide, aluminum, Al/2%Si, titanium, vanadium, niobium, two preparations of tantalum, two preparations of chromium, Cr on Au, molybdenum, tungsten, nickel, palladium, platinum, copper, silver, gold, 10 Ti/90 W, 80 Ni/20 Cr, TiN, four types of photoresist, resist pen, Parylene-C, and spin-on polyimide. Selected samples were etched in 35 different etches: isotropic silicon etchant, potassium hydroxide, 10:1 HF, 5:1 BHF, Pad Etch 4, hot phosphoric acid, Aluminum Etchant Type A, titanium wet etchant, CR-7 chromium etchant, CR-14 chromium etchant, molybdenum etchant, warm hydrogen peroxide, Copper Etchant Type CE-200, Copper Etchant APS 100, dilute aqua regia, AU-5 gold etchant, Nichrome Etchant TFN, hot sulfuric+phosphoric acids, Piranha, Microstrip 2001, acetone, methanol, isopropanol, xenon difluoride, HF+H/sub 2/O vapor, oxygen plasma, two deep reactive ion etch recipes with two different types of wafer clamping, SF/sub 6/ plasma, SF/sub 6/+O/sub 2/ plasma, CF/sub 4/ plasma, CF/sub 4/+O/sub 2/ plasma, and argon ion milling. The etch rates of 620 combinations of these were measured. The etch rates of thermal oxide in different dilutions of HF and BHF are also reported. Sample preparation and information about the etches is given.

/pdf/etch-rates-for-micromachining-processing-part-ii-2gdyzs6x1d.pdf

Etch rates for micromachining processing-Part II

Diffusion in silicon of elements from columns III and V of the Periodic Table is reviewed in theory and experiment. The emphasis is on the interactions of these substitutional dopants with point defects (vacancies and interstitials) as part of their diffusion mechanisms. The goal of this paper is to unify available experimental observations within the framework of a set of physical models that can be utilized in computer simulations to predict diffusion processes in silicon. The authors assess the present state of experimental data for basic parameters such as point-defect diffusivities and equilibrium concentrations and address a number of questions regarding the mechanisms of dopant diffusion. They offer illustrative examples of ways that diffusion may be modeled in one and two dimensions by solving continuity equations for point defects and dopants. Outstanding questions and inadequacies in existing formulations are identified by comparing computer simulations with experimental results. A summary of the progress made in this field in recent years and of directions future research may take is presented.

Point defects and dopant diffusion in silicon

Es wird ein Drehratensensor vorgeschlagen, der aus Silizium (Siliziumverbindungen oder Silizium/Glasverbindungen) oder anderen Halbleitermaterialien mit Techniken der Mikromechanik herausstrukturiert wird. Der Drehratensensor hat die Form einer Stimmgabel, deren Zinken in Ebenen parallel zu der Oberflache der Halbleiterwafer liegen. Diese Zinken werden in einer Ebene senkrecht zur Waferebene zu Schwingungen angeregt. Durch ein Sensorelement, das die Torsion der Stimmgabelaufhangung registriert, wird die Winkelgeschwindigkeit einer Drehung des Sensors um eine zur Stimmgabelaufhangung parallele Achse gemessen.

Mikromechanischer drehratensensor (drs)

Some experimental data on the anisotropy, selectivity, and voltage dependence of anisotropic silicon etchants are given. An attempt is undertaken to provide a general unifying model describing the reaction mechanism and the key features of all alkaline anisotropic etchants of silicon. It is shown that the reaction is electrochemical, comprising the transfer of four electrons between the electrolyte and the solid for the dissolution of one silicon atom. The crystallographic anisotropy can be attributed to small differences of the binding energy of surface atoms depending on their respective surface orientations. High boron concentrations induce the shrinking of a space charge layer on the silicon surface, which in turn leads to the fast recombination of electrons injected into the conduction band making them no longer available for the reduction of water. The electrochemical etch stop at positive potentials is due to the anodic oxidation of silicon. The finite etch rate observed at etch stop potentials was found to correspond well to the etch rate of SiO/sub 2/. >

The mechanism of anisotropic, electrochemical silicon etching in alkaline solutions

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.

A new capacitive type MEMS microphone

Effect of substrate properties and thermal annealing on the resistivity of molybdenum thin films

The invention relates to a micromechanical capacitive acceleration sensor for detecting the acceleration of an object. The sensor comprises a sensor housing (1), which is stationary with regard to the object, and an inert sensor mass (2), which is mounted in a manner that permits to elastically move around a starting position in relation to the sensor housing (1) and which is surrounded by said housing. The sensor also comprises a detection device for generating an output signal, which represents the acceleration of the object, based on the position of the sensor mass (2) relative to the sensor housing (1). The detection device has first capacitor electrodes, which are provided on the sensor mass (2), and second capacitor electrodes which, inside the sensor housing (1), are arranged opposite the first capacitor electrodes in a slightly interspaced manner while forming a narrow gap (3, 4). According to the invention, drainage structures (5; 6; 7; 8) are provided on one or more surfaces, which delimit the narrow gap (3, 4) between the sensor mass (2) and the sensor housing (1), and serve to accommodate gas particles that are present in the narrow gap (3, 4) when the gas particles are displaced due to the relative movement between the sensor mass (2) and the sensor housing (1).

Helmut Seidel

Papers

Mikromechanischer drehratensensor (drs)

The mechanism of anisotropic, electrochemical silicon etching in alkaline solutions

A new capacitive type MEMS microphone

Effect of substrate properties and thermal annealing on the resistivity of molybdenum thin films

Micromechanical capacitive acceleration sensor