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

Gas Sensing by SnO2 Thin Films Prepared by Large-area Pulsed Laser Deposition☆

In this study, a novel design concept for an oxygen sensor based on the paramagnetic effect is presented. To evaluate the potential of miniaturisation, an analytical model is formulated to compute the trajectories of oxygen molecules in an inhomogeneous magnetic field under various boundary conditions. A multi-layer ceramic technology is proposed for the 3D integration of the magnetic coil into the fluidic system of the gas analysis system.

Theoretical considerations on the design of a miniaturised paramagnetic oxygen sensor

Due to their chemical inertness, hydrogenated amorphous silicon carbide (a-SiC:H) thin films exhibit excellent properties as passivation layers for micro-sensors and micro-actuators operating in harsh environmental applications. They are typically synthesized with a plasma-enhanced chemical vapor deposition process at moderate temperatures, providing an excellent backend compatibility. To demonstrate their suitability and their signal stability for hot film anemometers in high pressure loaded automotive applications, molybdenum and titanium/platinum micro-heaters without and with a-SiC:H passivation were operated in an oil medium for up to 380 h at nominal thin film temperatures of 60 and 160 °C. As expected, the drift of the electrical resistance increased with rising temperature. In the unpassivated case, the molybdenum metallization exhibited an increased long-term stability when operated as the flow sensitive thin film. A protection of the platinum based thin film element with a 1 μm thick a-SiC:H layer reduced the drift in sensor resistance by more than one order of magnitude compared to a bare hot film probe. As a result of the tensile stress in the metallic thin film and a compressive stress in the a-SiC:H coating, a 30 h pre-aging is recommended in order to stabilize the micro-heater structure. At a high-pressure hydraulic test bench, an a-SiC:H/Pt/Ti test structure on a low temperature co-fired ceramics substrate was exposed to the dynamic pressure fluctuations with a maximum value of 135 MPa (1350 bar) for 1 h. No cracks or delaminations were found in the a-SiC:H surface passivation layer, as determined by optical inspection and scanning electron microscopy.

Enhanced stability of Ti/Pt micro-heaters using a-SiC:H passivation layers

A multi-axial monolithic acceleration sensor has the following features. The acceleration sensor consists of plural individual sensors with respectively a main sensitivity axis arranged on a common substrate. Each individual sensor is rotatably moveably suspended on two torsion spring elements and has a seismic mass with a center of gravity. Each individual sensor has components that measure the deflection of the seismic mass. The acceleration sensor preferably consists of at least three identical individual sensors. Each individual sensor is suspended eccentrically relative to its center of gravity and is rotated by 90°, 180° or 270° relative to the other individual sensors.

Helmut Seidel

Papers

Gas Sensing by SnO2 Thin Films Prepared by Large-area Pulsed Laser Deposition☆

Theoretical considerations on the design of a miniaturised paramagnetic oxygen sensor

Enhanced stability of Ti/Pt micro-heaters using a-SiC:H passivation layers

Multiaxial monolithic acceleration sensor

Efficient Nanostructure Formation on Silicon Surfaces and in Indium Tin Oxide thin Films by sub-15 fs pulsed near-infrared Laser Light