Craig R. Friedrich

Development of the micromilling process for high-aspect-ratio microstructures

At the macroscale, the milling process is very ver- satile and capable of creating three-dimensional features and structures. Adaptation of this process at the microscale could lead to the rapid and direct fabrication of micromolds and masks to aid in the development of microcomponents. This task has been undertaken, and results of the process indicate it can become an increasingly useful method. The micromilling process is characterized by milling tools that are currently in the range from 22-100 pm in diameter and made by the focused-ion beam machining process. The tools are used in a specially designed, high-precision milling machine. Results are comparable to other processes currently used to fabricate mold and mask features. The micromilling process can create trench-like features with nearly vertical sidewalls and good smoothness. External corners are sharp and stepped features can be machined simply by programming those shapes. The process is direct, alnd therefore dimensional errors do not accumulate as can occur with serial fabrication processes. (159)

Wettability changes of TiO2 nanotube surfaces.

This study examines the effect of environmental and experimental conditions, such as temperature and time, on the wettability properties of titania nanotube (TNT) surfaces fabricated by anodization. The fabricated TNTs are 60-130 nm inner diameter and 7-10 µm height. One-microliter water droplets were used to define the wettability of the TNT surfaces by measuring the contact angles. A digital image analysis algorithm was developed to obtain contact angles, contact radii and center heights of the droplets on the TNT surfaces. Bare titanium foil is inherently less hydrophilic with approximately 60°-80° contact angle. The as-anodized TNT surfaces are more hydrophilic and annealing further increases this hydrophilic property. Furthermore, it was found that the TNT surface became more hydrophobic when aged in air over a period of three months. It is believed that the surface wettability can be changed due to alkane contamination and organic contaminants in an ambient atmosphere. This work can provide guidelines to better specify the environmental conditions that changes surface properties of TNT surfaces and therefore affect their desirable function in specific applications such as orthopedic implants.

Micrometer-scale machining: tool fabrication and initial results

Conventional milling techniques scaled to ultrasmall dimensions have been used to machine polymethyl methacrylate (PMMA) with micrometer-sized milling tools. The object of this work is to achieve machining of a common material over dimensions exceeding 1 mm while holding submicrometer tolerances and micrometer size features. Fabricating the milling tools themselves was also an object of the study. A tool geometry for nominal 25 micrometer diameter cutting tools was found that cuts PMMA with submicrometer tolerances over trench lengths of 2 mm. The tool shape is a simple planar facet cut by focused ion beam milling on ground and polished 25 micrometer diameter steel tool blanks. Pairs of trenches 24 micrometers wide, 26 micrometers deep, 2.3 mm long, with a 14 micrometer separation were milled under various machining conditions. The results indicate that the limits of the machining process in terms of speed, pattern complexity, and tolerances have not been approached. This is the first demonstration of a generic method for microtool making by focused ion beam machining combined with ultraprecision numerically controlled milling. The method is shown to be capable of producing structures and geometries that are considered inaccessible by conventional materials removal techniques, and generally regarded as applications for deep X-ray lithography.

Micro heat exchangers fabricated by diamond machining

Abstract Machining of thin metal foils with specially contoured diamond cutting tools allows the production of small and very smooth fluid microflow channels for micro heat exchanger applications. Heat exchanger plate wall thickness, as well as fin dimensions, may be carefully controlled and machined to dimensions on the order of tens of micrometers. The plates are stacked and bonded with the vacuum diffusion process to form a cross-flow, plate-type heat exchanger. These fabrication techniques allow the production of small heat exchangers with a very high volumetric heat transfer coefficient and inherent low weight. The design and fabrication process for a copper-based, cross-flow micro heat exchanger has been developed. The micro heat exchanger provided a volumetric heat transfer coefficient of nearly 45 MW/m 3 K under very conservative deisgn and operating conditions. This corresponds to a volumetric capacity nearly 20 times that of more conventional compact heat exchangers. High thermal capacity, coupled with low cost and ease of production, make these devices practical in areas where high thermal flux in a small volume is required. The methods and procedures for this type of micromachining closely parallel those for precision machining.

Direct Compressive Measurements of Individual Titanium Dioxide Nanotubes

The mechanical compressive properties of individual thin-wall and thick-wall TiO(2) nanotubes were directly measured for the first time. Nanotubes with outside diameters of 75 and 110 nm and wall thicknesses of 5 and 15 nm, respectively, were axially compressed inside a 400 keV high-resolution transmission electron microscope (TEM) using a new fully integrated TEM-atomic force microscope (AFM) piezo-driven fixture for continuous recording of the force-displacement curves. Individual nanotubes were directly subjected to compressive loading. We found that the Youngs modulus of titanium dioxide nanotubes depended on the diameter and wall thickness of the nanotube and is in the range of 23-44 GPa. The thin-wall nanotubes collapsed at approximately 1.0 to 1.2 microN during axial compression.

Micromilling development and applications for microfabrication

Abstract In conventional machining, milling is the most versatile of the cutting processes. Micromechanical milling has also been shown to be a very versatile and repid method for the removal of material and the creation of microstructures. These microstructures range form direct fabrication of molds in polymethyl methacrylate (PMMA) to direct fabrication of x-ray lithography masks using a machinable carrier and one of several metallic absorbers, or various combinations of absorbers to better suit the machining environment. The micromilling tools are commercially available in diameters larger than 50 micrometers and custom-fabricated tools 22 micrometers in diameter are made at the Institute for Micromanufacturing (IfM). The custom-fabricated tools are made using the focused ion beam process and the resulting microstructures are machined on a very high precision, custom-built milling machine. The focused ion beam process has also been used to fabricate very small probe tips for biomedical use and microscalpels with extremely sharp cutting edges. These devices are currently under study and development for research applications.

High-density cochlear implants with position sensing and control

Silicon-based thin-film technology has been used to develop high-density cochlear electrode arrays with up to 32 sites and four parallel channels of simultaneous stimulation. The lithographically-defined arrays utilize a silicon-dielectric-metal-parylene structure with 180 microm-diameter IrO sites on 250 microm centers. Eight on-board strain gauges allow real-time imaging of array shape during insertion, and a tip sensor measures forces on any structures contacted in the scala tympani (e.g., the basilar membrane). The array can be pre-stressed to hug the modiolus, which provides position reference. Tip position can be resolved to better than 50 microm. Circuitry mounted on the base of the array generates stimulating currents, records intra-cochlear responses and position information, and interfaces with a custom microcontroller and inductively-coupled wireless interface over an eight-lead ribbon cable. The circuitry delivers biphasic 500 microA current pulses with 4 microA resolution and a minimum pulse width of 4 micros. Multiple sites can be driven in parallel to provide higher current levels. Backing structures and articulated insertion tools are being developed for dynamic closed-loop insertion control.

Development of chitosan–vancomycin antimicrobial coatings on titanium implants

Techniques for titanium surface modification have been studied for applications in orthopedic implants specifically for local drug delivery. The extensive research in surface modification is driving the development of devices that integrate infection prevention, osseointegration, and functionality in a structural role. In this study, vancomycin was applied to modified titanium surfaces to determine the effect of surface morphology on drug loading and release profiles. The antimicrobial effectiveness of the released vancomycin was evaluated and found to have a similar effect as the standard vancomycin. The engineered surfaces included sandblasted, sandblasted acid etched, electrochemically etched, and sandblasted electrochemically etched. The antibiotic release was observed to be independent of the measured surface parameters of the engineered surfaces. The development of an implantable device in which the surface morphology can be tailored for an application with no effect on the total drug released would be beneficial to more precisely control the biological response while maintaining local drug delivery for infection prevention.

Direct fabrication of deep x-ray lithography masks by micromechanical milling

Micromechanical milling has been shown to be a rapid and direct method for fabricating masks for deep x-ray lithography with lateral absorber features down to 10 micrometers. Conventional x-ray mask fabrication requires complex processes and equipment, and a faster and simpler method using micromechanical milling was investigated for larger microstructures for mesoscale applications. Micromilled x-ray masks consisting of a layered architecture of gold and titanium films on graphite yielded exposures in PMMA with accuracy and repeatability suitable for prototype purposes. A method for compensating milling tool radial runout was adapted, and the average accuracy of mask absorber features was 0.65 micrometers, with an average standard deviation of 0.55 micrometers. The milling process leaves some absorber burrs, and the absorber wall is tapered, which introduces an additional process bias. Mask fabrication by micromilling is fast and, therefore, less costly than conventional mask fabrication processes.

The micromilling process for high aspect ratio microstructures

High aspect ratio microstructures are currently created by several processes which include lithography (X-ray, deep ultraviolet, etc.) and mechanical machining (diamond machining, microdrilling, etc.) The lithographic processes require more extensive processing equipment such as an energy source, mask/mask holder/mask aligner, photoresist and substrate, and chemical development capacity. In addition, these processes are serial in nature and each adds to the tolerances of the finished structure. The current mechanical processes provide for the direct removal of the substrate material in a single step but are more limited in the geometric patterns which can be created. In conventional machining, the process which provides the most versatility in geometric patterns is milling. The micromilling process has two basic components. The first is the fabrication of small milling cutters with very sharp cutting edges. The second is the actual removal of the workpiece material with a very precise and repeatable machine tool. Several basic cutter designs have been fabricated using focused ion beam micromachining and are undergoing testing. The cutter diameters are nominally 100 micrometers and 22 micrometers. Results have been obtained which show that this process can be very effective for the rapid fabrication of molds and mask structures.