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

We report on the design, fabrication and performance of a silicon microvalve for small gas flows. It consists of two micromachined components which are bonded together. One part contains the gas flow inlet, the other part a deflectable silicon membrane. This membrane may be activated by a combination of electromagnetic and electrostatic forces. This principle is especially useful for minimizing the power consumption. The main experimental results are as follows: The maximum pressure against which the valve can fully operate is 160 mbar. With this pressure, a flow of up to 3 ml/min can be controlled. Typically, current pulses of 200 mA and voltage amplitudes of 30 V are applied for electromagnetic and electrostatic actuation, respectively. The switching time is below 0.4 ms.

A silicon microvalve with combined electromagnetic/electrostatic actuation

The lattice strain in highly boron‐doped, highly germanium‐doped, and simultaneously boron‐and‐germanium‐doped epitaxial layers was investigated by means of x‐ray techniques. The lattice‐contraction coefficient was determined for both impurities, and was found to be for boron and for germanium doping. A simple linear superposition of calculated or experimentally determined lattice‐contraction coefficients obtained for layers doped with boron vs. those doped with germanium does not yield satisfactory results in estimating the germanium concentration necessary for strain compensation in highly boron‐doped layers. Strain‐compensated dislocation‐free epitaxial layers were fabricated with a boron concentration of , using a germanium concentration of .

X‐Ray Investigation of Boron‐ and Germanium‐Doped Silicon Epitaxial Layers

In this work a silicon angular rate sensor for automotive applications with a new architecture is presented. It is based on the vibrating tuning fork principle with excitation direction of the tines perpendicular to the wafer surface. This arrangement allows the design of tines with significant inertial masses which lead to substantial signal. The oscillation of the tines is excited by a piezoelectric drive using an AlN thin film layer. The angular rate to be measured causes a torsional oscillation of the stem. The torsional amplitude is proportional to the angular rate and is measured by a piezoresistive read-out structure. We use silicon bulk micromachining based on a new twofold SOI-technique.

Silicon angular rate sensor for automotive applications with piezoelectric drive and piezoresistive read-out

A systematic design procedure to tailor the modal response of micro-resonators based on flexible plates with piezoelectric films is demonstrated. Sensors/actuators were designed by optimizing the surface electrode shapes in the plane dimensions. A numerical finite element procedure, which considers the effective surface electrode covering the piezoelectric film as a binary function on each element, has been implemented. Two design goals are considered: (i) optimized response (actuation or sensing) in a given mode; (ii) implementation of a modal transducer by filtering specific modes. For a given mode in a plate with arbitrary boundary conditions, our calculations allowed us to predict the top electrode layout reaching higher displacement in resonance than any other electrode design for the same structure. Microcantilevers and microbridges were fabricated and their modal response characterized by laser Doppler vibrometry. In comparison to a conventional square-shaped electrode, our experiments show that the implemented designs can increase the response in any desired resonant mode and simultaneously attenuate the contributions from other unwanted modes, by simply shaping the surface electrodes. Enhancement ratios as high as 42 dB, relative to a full-size electrode case, are demonstrated. The limitations imposed by the fabrication are also discussed.

Modal optimization and filtering in piezoelectric microplate resonators

This paper reports on the design and analysis of a capacitive vibration-to-electrical energy converter. A theoretical design model of a parallel-plate electrostatic spring-mass-system is presented, based on state space equations. The charging of the parallel-plate capacitor takes place by utilizing materials with different work functions for the electrodes. Numerical simulations are performed in order to optimize design parameters targeting a maximum output power. The shown Micro-Electro-Mechanical System (MEMS) based capacitive energy converter is able to provide an output power of 4.28 μW at an external vibration with a frequency of 1 kHz and an amplitude of 1.96 m/s2 (0.2 g). A basic electronic circuit for storing the electrical energy is also introduced. Furthermore, a MEMS design and a process flow are presented.

Helmut Seidel

Papers

A silicon microvalve with combined electromagnetic/electrostatic actuation

X‐Ray Investigation of Boron‐ and Germanium‐Doped Silicon Epitaxial Layers

Silicon angular rate sensor for automotive applications with piezoelectric drive and piezoresistive read-out

Modal optimization and filtering in piezoelectric microplate resonators

Power MEMS—A capacitive vibration-to-electrical energy converter with built-in voltage