Traditional fabrication methods for the integrated circuit (IC) and the microelectromechanical systems (MEMS) industries have been developed primarily for two-dimensional fabrication on planar surfaces. More recently, commercial electronics are expeditiously emerging with non-planar displays and rapid prototype machines can be purchased for the price of a modern laptop. While electrospinning (ES) has been in existence for over 100 years, this fabrication method has not been adequately developed for commercial fabrication of electronics or the rapid prototyping industries. ES provides many benefits as a fabrication method including tunability of fiber size and affordable hardware. To realize the full potential of ES as a commonplace fabrication method for modern devices, precise control, real-time fiber morphology monitoring, and the creation of a comprehensive databank of accurate models for prediction is essential. The aim of this research is to accomplish these goals through several avenues. To improve fiber deposition control, both passive and active methods are employed to modify electric field lines during the ES process. COMSOL models have been developed to meticulously mimic experimental results for predictive planning, and an in situ laser diagnostic tool was developed to measure real-time fiber morphology during electrospinning. Further, post-processing data was generated through the use of two-dimensional fast Fourier transform (2D-FFT) to monitor alignment, and four-point conductivity measurements were taken via four independently-positioned micromanipulator probes. This article describes the devices developed to date, the a priori modeling approach taken, and resultant capabilities which complement ES as an attractive fabrication method for the electronic and photonic industry.
Large arrays of periodic nanostructures are widely used for plasmonic applications, including ultrasensitive
particle sensing, optical nanoantennas, and optical computing; however, current fabrication
processes (e.g., e-beam lithography and nanoimprint lithography) remain time consuming and expensive.
Previously, researchers have utilized double casting methods to effectively fabricate large-scale arrays of
microscale features. Despite significant progress, employing such techniques at the nanoscale has
remained a challenge due to cracking and incomplete transfer of the nanofeatures. To overcome these
issues, here we present a double casting methodology for fabricating large-scale arrays of nanostructures.
We demonstrate this technique by creating large (0.5 cm × 1 cm) arrays of 150 nm nanoholes and 150 nm
nanopillars from one silicon master template with nanopillars. To preclude cracking and incomplete
transfer problems, a hard-PDMS/soft-PDMS (h-PDMS/s-PDMS) composite stamp was used to replicate the
features from: (i) the silicon template, and (ii) the resulting PDMS template. Our double casting technique
can be employed repeatedly to create positive and negative copies of the original silicon template as
desired. By drastically reducing the cost, time, and labor associated with creating separate silicon
templates for large arrays of different nanostructures, this methodology will enable rapid prototyping for
diverse applications in nanotechnological fields.
Electrothermal actuation has been used in microelectromechanical systems where low actuation voltage and high contact force are required. Power consumption to operate electrothermal actuators has typically been higher than with electrostatic actuation. A method of designing and processing electrothermal actuators is presented that leads to an order of magnitude reduction in required power while maintaining the low voltage, high force advantages. The substrate was removed beneath the actuator beams, thereby discarding the predominant power loss mechanism and reducing the required actuation power by an order of magnitude. Measured data and theoretical results from electrothermally actuated switches are presented to confirm the method.
Optical filters based on resonant gratings have spectral characteristics that are lithographically defined. Nanoimprint lithography is a relatively new method for producing large area gratings with sub-micron features. Computational modeling using rigorous coupled-wave analysis allows gratings to be designed to yield sharp reflectance maxima and minima. Combining these gratings with microfluidic channels and micromechanical actuators produced using micro electromechanical systems (MEMS) technology forms the basis for producing tunable filters and other wavelength selective elements. These devices achieve tunable optical characteristics by varying the index of refraction on the surface of the grating. Coating the grating surface with water creates a 33% change in the resonant wavelength whereas bringing a grating into contact with a quartz surface shifts the resonant wavelength from 558 nm to 879 nm, a fractional change of 58%. The reflectivity at a single wavelength can be varied by approximately a factor of three. Future applications of these devices may include tunable filters or optical modulators.
Large voltage differences between closely spaced MEMS structures can cause electrical breakdown and destruction of devices <sup>1-2</sup>. In this study, a variety of planar thin film electrode configurations were tested to characterize breakdown response. All devices were fabricated using standard surface micromachining methods and materials, therefore our test results provide guidelines directly applicable to thin film structures used in MEMS devices. We observed that planar polysilicon structures exhibit breakdown responses similar to published results for larger metal electrode configurations <sup>3-6</sup>. Our tests were performed in air at atmospheric pressure, with air gaps ranging from 0.5 μm to 10 μm. Our results show a sharp rise in breakdown level following increases in gap width up to about 3 μm, a plateau region between 3 μm and 7 μm, and breakdown in gaps over 7 μm following the Paschen curve. This profile indicates an avalanche breakdown process in large gaps, with a transition region to small gaps in which electrode vaporization due to field emission current is the dominant breakdown process. This study also provides information on using multiple-gap configurations, with electrically floating regions located near the energized electrodes, and the added benefit this method may provide for switching high voltage with MEMS devices. In multiple-gap configurations, we noted a transition between direct tip to tip breakdown across electrode gaps of 40 μm, and a preferential breakdown path through the electrically floating contact head region for electrode gaps over 100 μm.
Metal films perforated with periodically-spaced subwavelength diameter holes have been shown to transmit light with greater efficiency than predicted by classical models for evanescent propagation. This transmission mechanism is caused either by the coupling of light to surface plasmon polariton modes on the surfaces of the metal film or by diffraction patterns in the lateral evanescent modes of electromagnetic field. Regardless of the root cause, this characteristic performance leads to electric field enhancement at the apertures in the metal, an effect that holds promise for nanoscale optical sensors. In particular, the propagation of these modes is very sensitive to changes in the index of refraction on either surface of the metal. This paper will describe our work on patterned metal films which are interrogated using infrared (IR) radiation. These metal gratings are fabricated using a surface micromachining process, allowing MEMS actuators to be integrated alongside the optically-active surface. The integration of MEMS structures with subwavelength optical structures can be used to create structures whose optical properties are modulated by changes in the position of a MEMS element, resulting in mechanical sensors and tunable optical filters. We will describe structures in which small changes in the separation between the metal film and a dielectric substrate result in large changes in the optical transmission and reflection spectra.