Mark M. Crain

Design and development of a MEMS capacitive bending strain sensor

The design, modeling, fabrication and testing of a MEMS-based capacitive bending strain sensor utilizing a comb drive is presented. This sensor is designed to be integrated with a telemetry system that will monitor changes in bending strain to assist with the diagnosis of spinal fusion. ABAQUS/CAE finite-element analysis (FEA) software was used to predict sensor actuation, capacitance output and avoid material failure. Highly doped boron silicon wafers with a low resistivity were fabricated into an interdigitated finger array employing deep reactive ion etching (DRIE) to create 150 ?m sidewalls with 25 ?m spacing between the adjacent fingers. The sensor was adhered to a steel beam and subjected to four-point bending to mechanically change the spacing between the interdigitated fingers as a function of strain. As expected, the capacitance output increased as an inverse function of the spacing between the interdigitated fingers. At the unstrained state, the capacitive output was 7.56 pF and increased inversely to 17.04 pF at 1571 ?? of bending strain. The FEA and analytical models were comparable with the largest differential of 0.65 pF or 6.33% occurring at 1000 ??. Advantages of this design are a dice-free process without the use of expensive silicon-on-insulator (SOI) wafers.

Design, modeling, fabrication and testing of a MEMS capacitive bending strain sensor

Presented herein are the design, modelling, fabrication and testing of a MEMSbased capacitive bending strain sensor utilizing a comb drive. This sensor is designed to be integrated with a telemetry system that will monitor changes in bending strain to assist orthopaedic surgeons with the diagnosis of spinal fusion. ABAQUS/CAE version 6.5 finite element analysis (FEA) modelling software was used to predict sensor actuation, capacitance output and the avoidance of material failure. Highly doped boron silicon wafers with a low resistivity were fabricated into an interdigitated finger array employing deep reactive ion etching (DRIE) to create 150 µm sidewalls with 25 µm spacing between the adjacent fingers. For testing, the sensor was adhered to a steel beam, which was subjected to four-point bending. This mechanically changed the spacing between the interdigitated fingers as a function of strain. As expected, the capacitance output increased as an inverse function of the spacing between the interdigitated fingers, beginning with an initial capacitance of 7.56 pF at the unstrained state and increasing inversely to 17.04 pF at 1571 µe of bending strain. The FEA and analytical models were comparable with experimental data. The largest differential of 0.65 pF or 6.33% occurred at 1000 µe.

Alternative fabrication methods for capillary electrophoretic device manufacturing

This work represents research that explores the development of novel manufacturing methods to create microcapillary electrophoretic (CE) devices. Nontraditional substrates that were investigated include polymers such as SU-8, poly dimethylsiloxane (PDMS), acetate, Riston, Kapton, polyimide, and polyester. Hot embossing, chemical etching, micro-molding, wafer level bonding, chemical treatment, and lamination techniques were developed for these substrates. The purpose of this paper is to explore the feasibility of micromachining a select group of alternative materials.

Selective plasma nitridation and contrast reversed etching of silicon

A new method of selectively patterning a silicon substrate with silicon dioxide and silicon nitride is demonstrated. An oxide patterned silicon substrate is directly nitrided using a microwave generated nitrogen plasma. Upon subsequent selective wet chemical etching using KOH, the oxide is removed and etching proceeds into the silicon, revealing a contrast reversed pattern of the oxide. The etch resistance of the nitrided surface is maximized by increasing the microwave power, pressure, and nitridation duration. The etch rate of silicon dioxide is negligibly affected and its etch rate is nearly the same as before nitridation. Compositional analysis of the nitride and the nitrided oxide using x-ray photoelectron spectroscopy confirms that microwave plasma nitridation produces Si–N covalent bonds.

Maskless Direct Write Grayscale Lithography for MEMS Applications

Grayscale lithography is one area of lithography that has been relatively underutilized. There are several reasons for this, but one of the most prominent is the difficulty of modern techniques of grayscale exposure. However, this paper discusses a relatively novel approach to grayscale exposure using mask writing technology. Traditional lithography is characterized by the binary exposure of photoresist: meaning that some areas are exposed while other areas remain completely unexposed. The goal of grayscale lithography is to expose a gradient of intensities to photoresist. The result of this is a topography of photoresist that is potentially much more complicated than its binary counterpart. This paper will discuss direct write grayscale lithography using the Heidelberg DWL 66FS Laser Pattern Generator. Traditionally the Heidelberg DWL 66FS is used for binary exposure, but it also has the ability to vary laser intensity during an exposure. By varying the intensity of the focused laser beam, the user is able to expose photoresist differently in various regions of the substrate, generating the grayscale structure.

Fabrication of a Micro/Nanofluidic Platform Via Three-Axis Robotic Dispensing System

The purpose of this work is to introduce a new fabrication technique for creating a fluidic platform with embedded microor nanoscale channels. This new technique includes: (1) a three-axis robotic dispensing system for drawing micro/nanoscale suspended polymer fibers at prescribed locations, combined with (2) dry film resist photolithography, and (3) replica molding. This new technique provides flexibility and precise control of the microand nano-channel location with the ability to create multiple channels of varying sizes embedded in a single fluidic platform. These types of micro/nanofluidic platforms are attractive for numerous applications, such as the separation of biomolecules, cell transport, and transport across cell membranes via electroporation. The focus of this work is on the development of a fabrication technique for the creation of a nanoscale electroporation device. [DOI: 10.1115/1.4034611]

Fabrication of a Glass Capillary Electrophoresis Microchip With Integrated Electrodes

In this chapter, a detailed outline delineating the processing steps for microfabricating capillary electrophoresis (CE) with integrated electrochemical detection (ECD) platforms for performing analyte separation and detection is presented to enable persons familiar with microfabrication to enter a cleanroom and fabricate a fully functional Lab-on-a-Chip (LOC) microdevice. The processing steps outlined are appropriate for the production of LOC prototypes using easily obtained glass substrates and common microfabrication techniques. Microfabrication provides a major advantage over existing macro-scale systems by enabling precise control over electrode placement, and integration of all required CE and ECD electrodes directly onto a single substrate with a small footprint. In the processing sequences presented, top and bottom glass substrates are photolithographically patterned and etched using wet chemical processing techniques. The bottom substrate contains seven electrodes required for CE/ECD operation, whereas the top substrate contains the microchannel network. The flush planar electrodes are created using sputter deposition and lift-off processing techniques. Finally, the two glass substrates are thermally bonded to create the final LOC device.

Dual capillary electrophoresis devices with electrochemical detection on a single platform

The purpose of this paper is to demonstrate the feasibility of developing a single lab-on-a-chip (LOC) platform capable of performing dual, simultaneous separation and detection of multiple analytes. Computational modeling was performed to determine optimum device geometry and performance. The soda-lime glass-based device was fabricated using traditional microtechnology processes, including UV photolithography, buffered oxide etch (BOE), electrode deposition and compression thermal bonding. The device was characterized with a mixture of dopamine (2mM) and catechol (2mM) in a phosphate buffer (20mM, 6.5 pH). Modeling results yielded migration velocities of 0.6 mm/s and 0.42 mm/s for dopamine (electrokinetic (EK) mobility=60,000 /spl mu/m/sup 2//V/spl middot/s) and catechol (EK mobility=42,000 /spl mu/m/sup 2//V/spl middot/s), respectively. Experimental results obtained from microchips exhibiting the same EK mobilities demonstrated identical electropherograms in both detection channels with migration velocities of 0.58 mm/s for dopamine and 0.41 mm/s for catechol.

Design and Fabrication of Microtacks for Retinal Implant Applications

To adhere an artificial retinal implant onto the epiretinal surface of the eye, our group has designed retinal microtacks. The microtacks were fabricated using two different micromachining techniques: 1) deep reactive ion etching (DRIE) and 2) ultrahigh precision micromilling. The DRIE process consisted of machining a double-sided polished three-inch silicon wafer using ICP with the Bosch process. For the ultra-high precision micromilling technique, titanium foil was bonded to a silicon wafer and precision machined with a 150-μm end-mill using PMAC code interfaced to a machine motion controller. Due to fabrication limitations, the tip of the DRIE fabricated Si tack was chisel-shaped, whereas versatility of the micromilling technique allowed a partially conical, tapered tip to be added to the Ti tack, which created a sharper point. For the Si tacks, the average overall length and width were measured to be within 7% and 2%, respectively, of the design while the Ti tacks were found to be within 1% and 6%, respectively. Additionally, the grip width, stop thickness, and the tip taper angle of the Ti tacks were within 3%, 9%, and 4%, respectively, of the design.Copyright

Microfabrication activities at the University of Louisville

This paper describes the new microfabrication effort presently underway at the University of Louisville. A 1500 square foot class 100/1000 cleanroom facility was completed in the 1997 Spring semester. The cleanroom contains a wide spectrum of microfabrication processing and test equipment. The new lab supports a broad range of interdisciplinary research projects and will be utilized in the upcoming 1997 Fall semester for a new NSF-sponsored undergraduate course in microfabrication. Details of the cleanroom and ongoing fabrication activities at the University of Louisville are presented.