Scott D. Cambron

Maskless Grayscale Lithography Using a Positive-Tone Photodefinable Polyimide for MEMS Applications

The novel use of a positive-tone photosensitive polyimide for the rapid production of grayscale features using a maskless lithography system is demonstrated. The removal rate of the polyimide, HD-8820, is characterized as a function of exposure dose. A broad contrast curve is found that is suitable for grayscale lithography. Three-dimensional polyimide structures up to 22 μm thick are demonstrated, and the surface roughness after the final cure is Ra = 4.4 nm, which is suitable for many microelectromechanical systems (MEMS) applications, including many optical applications. Tensile testing of 63 polyimide samples shows excellent mechanical properties for four different polyimide thicknesses produced with grayscale lithography. The modulus of elasticity is found to be 1.92 GPa, the yield strength to be 103 MPa, the fracture strength to be 133 MPa, and the percent elongation to be 51%. The test results show that the mechanical properties are consistent and do not change due to a partial exposure to UV light. The entire fabrication sequence, from computer-aided design file to cured structure, can be performed in less than 4 h, making this a fast low-cost method of producing polymer MEMS devices with excellent mechanical properties.

Fabrication of an insulated probe on a self-assembled metallic nanowire for electrochemical probing in cells

Conductive nanowire probes that are stiff enough too be inserted inside individual cells could provide unprecedented detail in real-time about the chemistry of living cells and cell compartments including cell membranes, cytosolic organelles and the nucleus. Localization also requires that the probe is insulated except at its end. To enable simple interconnection to an experimental apparatus, the nanowire would need to be attached to a larger platform, e.g. a MEMS device or cantilever (such as used in atomic force microscopes-AFM.) Such a device with 100 nm or less diameter can be fabricated with only a few processing steps by taking advantage of a previously reported technique for self-assembling metal alloy nanowires at selected locations and with desired orientations with respect to the surface [1]. In this report such a concept device and its fabrication process is reported. All steps have been individually demonstrated, although the complete fabrication from end-to-end has yet to be demonstrated at the time of this writing.

Custom fabrication of freestanding and suspended three-dimensional polymer structures

An atomic force microscope (AFM) is used as a micromanipulator to fabricate freestanding micron diameter wires and bridges in a matter of minutes by pulling polymer materials into fibers. The fabrication procedure appears to have significant application in easier and more rapid prototyping of micro-, nano- and MEMS devices. While fiber pulling technology has advanced to high degrees of perfection, our process represents the first time that a nano-positioning tool has been used to fabricate three-dimensional microstructures with a degree of flexibility and simplicity far exceeding traditional MEMS and microfabrication processing methods. Preliminary efforts at demonstrating the use of the fibers in device fabrication and applications are also presented.

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]

Prescribed 3-D Direct Writing of Suspended Micron/Sub-micron Scale Fiber Structures via a Robotic Dispensing System.

A 3-axis dispensing system is utilized to control the initiating and terminating fiber positions and trajectory via the dispensing software. The polymer fiber length and orientation is defined by the spatial positioning of the dispensing system 3-axis stages. The fiber diameter is defined by the prescribed dispense time of the dispensing system valve, the feed rate (the speed at which the stage traverses from an initiating to a terminating position), the gauge diameter of the dispensing tip, the viscosity and surface tension of the polymer solution, and the programmed drawing length. The stage feed rate affects the polymer solution’s evaporation rate and capillary breakup of the filaments. The dispensing system consists of a pneumatic valve controller, a droplet-dispensing valve and a dispensing tip. Characterization of the direct write process to determine the optimum combination of factors leads to repeatedly acquiring the desired range of fiber diameters. The advantage of this robotic dispensing system is the ease of obtaining a precise range of micron/sub-micron fibers onto a desired, programmed location via automated process control. Here, the discussed self-assembled micron/sub-micron scale 3D structures have been employed to fabricate suspended structures to create micron/sub-micron fluidic devices and bioengineered scaffolds.

Direct Write Fabrication of Polymer Fibers for Microscale Applications

A new direct write technique has been developed for processing viscous polymer solutions into suspended, three-dimensionally-oriented polymer fibers. This process has been successfully implemented to produce fibers from multiple materials including poly(methyl methacrylate) (PMMA), multi-wall carbon nanotube (MWNT)-doped PMMA, and biodegradable poly(L-lactic acid) (PLLA). Process characterization performed with PMMA suggests that fiber diameter is controllable through modulation of process geometry and/or polymer solution properties. Furthermore, suspended microchannels have been produced by employing directly written fibers as sacrificial structures.

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

Direct-Write Fabrication of Polymer Nanocomposite Fibers

The unique properties of carbon-nanotube (CNT)-doped polymers have generated several promising applications including gas sensors, high-strength/light-weight materials, and electromagnetic interference shielding. The ability to process CNT-doped materials into complex architectures may enable further advancement of these devices. We have developed a direct-write technique for processing CNT-doped poly(methyl methacrylate) (PMMA) into 3D arrays of precisely-positioned fibers with micro- and sub-microscale diameters. In this method, a programmable micromanipulator-controlled syringe was loaded with solvated CNT/PMMA and utilized to draw an array of freely-suspended solution filaments on a substrate in a “connect-the-dots” fashion. As the filaments are drawn, they are thinned by surface tension-driven necking as they dry and form solid fibers. The degree of thinning can be controlled by varying the viscosity of the solution, which acts to resist the necking while the volatile solvent evaporates and solidification occurs. Multiple fibers were drawn to investigate the effects of several factors on fiber diameter and process yield. These variables included fiber length (4, 8, and 18 mm), fiber drawing velocity (5 and 20 mm/s), polymer concentration in solution (22 and 24% by wt.), and CNT concentration in solution (0, 0.5, 1, and 1.5% by wt.), with the latter two of these variables strongly influencing solution viscosity. Measurement of the fibers via scanning electron microscopy (SEM) revealed several trends: Fiber diameter was not influenced by CNT concentration, but increased with increasing PMMA concentration (P 100 μm. Furthermore, fiber yield exceeded 75% for all tested solutions except for the lowest viscosity CNT-doped solution (24% PMMA/0.5% CNT, η=50.1 Pa*s), which experienced capillary breakup prior to solidification. The conductivities of direct-write PMMA/CNT fibers ranged from -7 to 0.15 S/m, with shorter fibers having higher conductivities (P

Out-of-plane micro-needle arrays using silicon micromachining

An array of out-of-plane silicon processed micro-needles has been successfully fabricated using relatively simple fabrication techniques. This abstract describes the methods explored to produce 100-element needle arrays (10times10) on a single die. Applications of the micro-needles include transdermal drug delivery and tissue delivery (during surgery). Two types of needles were fabricated: solid electrode and hollow through-hole. The solid-core micro-needles can be used for painless perforation of the skin layer to increase absorption of drugs through patches. These types are fabricated using only oxidation, dicing and isotropic etching. The hollow versions have more applications for direct delivery of drugs through the skin and require additional steps of lithography and deep reactive ion etch (DRIE).

Micromachined tacks for retinal implant applications

To fasten 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) ultra-high-precision micromilling. Metrology was performed on each microtack design following fabrication, measuring all critical and overall dimensions. Force measurement experiments were performed to determine the loads required to insert and remove each microtack design in a synthetic material. This was carried out to verify the validity of the new geometric features added to the microtack. Each experiment consisted of three programmed zones: Insertion, Hold, and Removal. The required displacement and velocity were determined for each zone dependent upon microtack design.