Martin A. Schmidt

Gaseous slip flow in long microchannels

An analytic and experimental investigation into gaseous flow with slight rarefaction through long microchannels is undertaken. A two-dimensional (2-D) analysis of the Navier-Stokes equations with a first-order slip-velocity boundary condition demonstrates that both compressibility and rarefied effects are present in long microchannels. By undertaking a perturbation expansion in /spl epsiv/, the height-to-length ratio of the channel, and using the ideal gas equation of state, it is shown that the zeroth-order analytic solution for the streamwise mass flow corresponds well with the experimental results. Also, the effect of slip upon the pressure distribution is derived, and it is obtained that this slip velocity leads directly to a wall-normal migration of mass. The fabrication of wafer-bonded microchannels that possess well-controlled surface structure is described, and a means for accurately measuring the mass how through the channels is presented. Experimental results obtained with this mass-flow measurement technique for streamwise helium mass flow through microchannels 52.25-/spl mu/m wide, 1.33-/spl mu/m deep, and 7500-/spl mu/m long for a pressure range of 1.6-4.2 atmospheres (outlet pressures at atmospheric) are presented and shown to compare favorably with the analysis.

Characterization of a Time Multiplexed Inductively Coupled Plasma Etcher

We report the experimentally obtained response surfaces of silicon etching rate, aspect ratio dependent etching (ARDE), photoresist etching rate, and anisotropy parameter in a time multiplexed inductively coupled plasma etcher. The data were collected whi le varying eight etching variables. The relevance of electrode power, pressure, and gas flow rates is presented and has been found t o agree with observations reported in the literature. The observed behavior presented in this report serves as a tool to locate a nd optimize operating conditions to etch high aspect ratio structures. The performance of this deep reactive ion etcher allows the tai loring of silicon etching rates in excess of 4 mm/min with anisotropic profiles, nonuniformities of less than 4% across the wafer, and ARDE control with a depth variation of less than 1 mm for trenches of dissimilar width. Furthermore it is possible to prescribe the slope of etched trenches from positive to reentrant.

Wafer-to-wafer bonding for microstructure formation

Wafer-to-wafer bonding processes for microstructure fabrication are categorized and described. These processes have an impact in packaging and structure design. Processes are categorized into direct bonds, anodic bonds, and bonds with intermediate layers. Representative devices using wafer-to-wafer bonding are presented. Processes and methods for characterization of a range of bonding methods are discussed. Opportunities for continued development are outlined.

Mass flow and tangential momentum accommodation in silicon micromachined channels

High-precision experimental results are reported showing the tangential momentum accommodation coefficient (TMAC) for several gases in contact with single-crystal silicon to be less than unity. A precise and robust experimental platform is demonstrated for measurement of mass flows through silicon micromachined channels due to an imposed pressure gradient. Analytic expressions for isothermal Maxwellian slip flows through long channels are used to determine the TMAC at a variety of Knudsen numbers. Results from experiments using nitrogen, argon and carbon dioxide are presented. For all three gases the TMAC is found to be lower than one, ranging from 0.75 to 0.85.

A microfluidic electroporation device for cell lysis

We demonstrate a micro-electroporation device for cell lysis prior to subcellular analysis. Simple circuit models show that electrical lysis method is advantageous because it is selective towards plasma membrane while leaving organelle membrane undamaged. In addition, miniaturization of this concept leads to negligible heat generation and bubble formation. The designed microdevices were fabricated using a combination of photolithography, metal-film deposition, and electroplating. We demonstrate the electro-lysis of human carcinoma cells in these devices to release the subcellular materials.

A six-wafer combustion system for a silicon micro gas turbine engine

As part of a program to develop a micro gas turbine engine capable of producing 10-50 W of electrical power in a package less than one cubic centimeter in volume, we present the design, fabrication, packaging, and experimental test results for the 6-wafer combustion system for a silicon microengine. Comprising the main nonrotating functional components of the engine, the device described measures 2.1 cm/spl times/2.1 cm/spl times/0.38 cm and is largely fabricated by deep reactive ion etching through a total thickness of 3800 /spl mu/m. Complete with a set of fuel plenums, pressure ports, fuel injectors, igniters, fluidic interconnects, and compressor and turbine static airfoils, this structure is the first demonstration of the complete hot flow path of a multilevel micro gas turbine engine. The 0.195 cm/sup 3/ combustion chamber is shown to sustain a stable hydrogen flame over a range of operating mass flows and fuel-air mixture ratios and to produce exit gas temperatures in excess of 1600 K. It also serves as the first experimental demonstration of stable hydrocarbon microcombustion within the structural constraints of silicon. Combined with longevity tests at elevated temperatures for tens of hours, these results demonstrate the viability of a silicon-based combustion system for micro heat engine applications.

Design and calibration of a microfabricated floating-element shear-stress sensor

A microfabricated floating-element shear-stress sensor for measurements in turbulent boundary-layers is reported. Using surface micromachining of polyimide, a 500- mu m*500- mu m probe has been fabricated incorporating a differential-capacitor readout circuit. A model for the sensor response is described and is used for the design of an element to measure shear stresses of 1 Pa in air. The sensor is packaged for calibration in laminar flow, and electrical results obtained match the expected response. >

Design and fabrication of microfluidic devices for multiphase mixing and reaction

Using silicon microfabrication technology, microchemical devices have been constructed for the purpose of conducting heterogeneously catalyzed multiphase reactions. The motivation behind the design, the fabrication approach, and the experimental characterization are presented for two classes of devices. The first design involves multiple parallel channels with integrated filter structures to incorporate standard catalytic materials. These catalysts are in the form of finely divided porous particles in a packed-bed arrangement. The second device involves the incorporation of porous silicon as a catalyst support, in the form of a thin layer covering microstructured channels. These microstructured channels simulate the structure of a packed bed and enhance mass transfer relative to an open channel. The ability to incorporate features at the tens-of-microns scale can reduce the mass-transfer limitations by promoting mixing and dispersion for the multiple phases. Directly integrating the catalyst support structures into the channels of the microreactor allows the precise definition of the bed properties, including the supports size, shape and arrangement, and the void fraction. Such a design would find broad applicability in enhancing the transport and active surface area for sensing, chemical, and biochemical conversion devices. Reaction rates for the gas-liquid-solid hydrogenation of cyclohexene using the integrated catalyst with porous silicon as a support compare favorably to those rates obtained with the packed-bed approach. In both cases, the mass transfer coefficient is at least 100 times better than conventional laboratory reactors.

Molding of Deep Polydimethylsiloxane Microstructures for Microfluidics and Biological Applications

Here we demonstrate the microfabrication of deep (> 25 microns) polymeric microstructures created by replica-molding polydimethylsiloxane (PDMS) from microfabricated Si substrates. The use of PDMS structures in microfluidics and biological applications is discussed. We investigated the feasibility of two methods for the microfabrication of the Si molds: deep plasma etch of silicon-on-insulator (SOI) wafers and photolithographic patterning of a spin-coated photoplastic layer. Although the SOI wafers can be patterned at higher resolution, we found that the inexpensive photoplastic yields similar replication fidelity. The latter is mostly limited by the mechanical stability of the replicated PDMS structures. As an example, we demonstrate the selective delivery of different cell suspensions to specific locations of a tissue culture substrate resulting in micropatterns of attached cells.

Microfluidic systems with on-line UV detection fabricated in photodefinable epoxy

This paper describes a method for fabricating microfluidic devices in a photodefinable epoxy (SU-8). This technique is compatible with, and complementary to, conventional fabrication techniques. It allows microstructures formed in SU-8 to be bonded to produce sealed microfluidic channels. A micromixer fabricated entirely in SU-8, using this technique, for performing liquid-phase reactions is shown to be suitable for visible spectroscopy. This fabrication method also allows the incorporation of materials that are often difficult to integrate. By fabricating hybrid devices that incorporate quartz windows, we demonstrate that these devices are compatible with organic solvents and that in situ ultraviolet detection in a microfluidic system is possible.