Electrostatic MEMS Actuators using Gray-scale Technology

The majority of fabrication techniques used in micro-electro-mechanical systems (MEMS) are planar technologies, which severely limits the structures available during device design. In contrast, the emerging gray-scale technology is an attractive option for batch fabricating 3-D structures in silicon using a single lithography and etching step. While gray-scale technology is extremely versatile, limited research has been done regarding the integration of this technology with other MEMS processes and devices. This work begins with the development of a fundamental empirical model for predicting and designing complex 3-D photoresist structures using a pixilated gray-scale technique. A characterization of the subsequent transfer of such 3-D structures into silicon using deep reactive ion etching (DRIE) is also provided. Two advanced gray-scale techniques are then introduced: First, a double exposure technique was developed to exponentially increase the number of available gray-levels; improving the vertical resolution in photoresist. Second, a design method dubbed compensated aspect ratio dependent etching (CARDE) was created to anticipate feature dependent etch rates observed during gray-scale pattern transfer using deep reactive ion etching (DRIE). The developed gray-scale techniques were used to integrate variable-height components into the actuation mechanism of electrostatic MEMS devices for the first time. In static comb-drives, devices with 3-D comb-fingers were able to demonstrate >34% improvement in displacement resolution by tailoring their force-engagement characteristics. Lower driving voltages were achieved by reducing suspension heights to decrease spring constants (from 7.7N/m to 2.3N/m) without effecting comb-drive force. Variable-height comb-fingers also enabled the development of compact, voltage-controlled electrostatic springs for tuning MEMS resonators. Devices in the low-kHz range demonstrated resonant frequency tuning >17.1% and electrostatic spring constants up to 1.19 N/m (@70V). This experience of integrating 3-D structures within electrostatic actuators culminated in the development of a novel 2-axis optical fiber alignment system using 3-D actuators. Coupled in-plane motion of electrostatic actuators with integrated 3-D wedges was used to deflect an optical fiber both horizontally and vertically. Devices demonstrated switching speeds <1ms, actuation ranges >35μm (in both directions), and alignment resolution <1.25μm. Auto-alignment to fixed indium-phosphide waveguides with <1.6μm resolution in <10 seconds was achieved by optimizing search algorithms

Enhancement of an indium phosphide resonator sensor microsystem through the development of an adaptive feedback circuit and electrospray deposition

Cantilever resonator sensor enhancement through the development of an adaptive feedback circuit and the use of electrospray deposition is presented. The feedback system adapts to a wide range of resonators by implementing a hill climbing algorithm, locking onto the cantilever's resonance condition. Eight different cantilever-based sensors (Length=40-75μm), resonating at 201.0kHz to 592.1kHz, with a minimum standard deviation of 11.8Hz, corresponding to a mass resolution limit of 123fg for the device, have been dynamically detected using a single circuit. Electrospray deposition of thin-films on multiple substrate materials and released microstructures has been performed. An average deposition rate of 9.5±5nm/min was achieved with an average surface roughness of 4.5nm on a 197nm thick film. This technology will enable a post-processing method for depositing absorbing layers for sensing applications. With the development of these two technologies, the practical functionality of a chip-scale sensor microsystem will be more readily realized

An Optical Microsensor Utilizing Genetically Programmed Bioreceptor Layers for Selective Sensing

Protein engineering is a rich technology that holds the potential to revolutionize sensors through the creation of highly selective peptides that encode unique recognition affinities. Their robust integration with sensor platforms is very challenging. The goal of this research project is to combine expertise in micro-electro-mechanical systems (MEMS) and biological/protein engineering to develop a selective sensor platform. The key enabling technology in this work is the use of biological molecules, the Tobacco mosaic virus (TMV) and its derivative, Virus-Like-Particle (VLP), as nanoreceptor layers, in conjunction with a highly sensitive microfabricated optical disk resonator. This work will present a novel method for the integration of biological molecules assembly on MEMS devices for chemical and biological sensing applications. Particularly in this research, TMV1Cys-TNT and TMV1Cys-VLP-FLAG bioreceptor layers have been genetically engineered to bind to an ultra-low vapor pressure explosive, Trinitrotoluene (TNT), and to a widely used FLAG antibody, respectively. TNT vapor was introduce to TMV1Cys-TNT coated resonator and induced a 12 Hz resonant frequency shift, corresponding to a mass increase of 76.9 ng, a 300% larger shift compared to resonators without receptor layer coating. Subsequently, a microfabricated optical disk resonator decorated with TMV1Cys-VLP-FLAG was used to conduct enzyme-linked immunosorbent assay and label-free immunoassays on-a-chip and demonstrated a resonant wavelength shift of 5.95 nm and 0.79 nm, respectively. The significance of these developments lies in demonstrating the capability to use genetically programmable viruses and VLPs as platforms for the display and integration of receptor peptides within microsystems. The work outlined here constitutes an interdisciplinary investigation on the integration capabilities of the bio-nanostructure materials with traditional microfabrication architectures. While previous works have focused on individual components of the system, this work addresses multi-component integration, including biological molecule surface assembly and fabrication utilizing both top-down and bottom-up approaches. Integrating biologically programmable material into traditional MEMS transducers enhances selectivity, sensitivity, and simplifies fabrication and testing methodologies. This research provides a new avenue for enhancing sensor platforms through the integration of biological species as the key to remedying challenges faced by conventional systems that utilize a wide range of polymers or metals for nonspecific bindings

Indium Phosphide Based Optical Waveguide MEMS for Communications and Sensing

Indium phosphide (InP) is extensively used for integrated waveguide and photonic devices due to its suitability as a substrate for direct bandgap materials (e.g. In1-XGaXAsYP1-Y) operating at the lambda=1550 nm communications wavelength. However, little work has been reported on InP optical waveguide micro-electro-mechanical systems (MEMS). In this work, InP cantilever and doubly-clamped beams were micromachined on an In0.53Ga0.47As "sacrificial layer" on (100) InP substrates. Young's modulus was measured using nanoindentation and microbeam-bending. Intrinsic stress and material uniformity (stress gradient) were obtained by measuring the profile of doubly-clamped and cantilever beams using confocal microscopy. The study resulted in a Young's modulus of 80.4-106.5 GPa (crystal orientation-dependent). Although InP was grown lattice-matched to the substrate, arsenic from the underlying In0.53Ga0.47As sacrificial layer resulted in intrinsic compressive stress. Adding trace amounts of gallium to the InP layer during epitaxial growth induced tensile stress to offset the effect of arsenic. The materials characterization was extended to develop optical waveguide switches and sensors. In the first device, two parallel waveguides were actuated to vary the spacing between them. By modulating the gap using electrostatic pull-in actuation, the optical coupling strength was controlled via the evanescent field. Low voltage switching (<10 V), high speed (4 us), low crosstalk (-47 dB), and low-loss (<10 %) were achieved. Variable coupling over a 17.4 dB dynamic range was also demonstrated. The second device utilized a single movable input waveguide, which was actuated via electrostatic comb-drives to end-couple with one of several output waveguides. Low voltage switching (<7 V), 140 us switching speed (2 ms settling time), low crosstalk (-26 dB), and low-loss (<3.2 dB) were demonstrated. Sensing techniques based on mass-loading were developed using end-coupled cantilever waveguides. Here, the mechanical resonance frequency was measured by actuating the cantilever and measuring the end-coupled optical power at the output waveguide. A proof-of-concept experiment utilized a focused-ion-beam to mill the cantilever tip and resulted in a measurable resonance shift with mass-sensitivity delta_m/delta_f=5.1 fg/Hz. The cantilever waveguide devices and measurement techniques enable accurate resonance detection in mass-based cantilever sensors and also enable single-chip sensors with on-chip optical detection to be realized

Reliable Packaging and Development of Photodiode Module for Operation at 4 K

AC voltage standards based on pulse-driven Josephson junction arrays, operated at 4 K, enable quantum-accurate generation of arbitrary voltage waveforms. These devices are operated by biasing the arrays with high-frequency current pulses. Utilizing an optically controlled current source, consisting of a modulated laser source and optical fibercoupled photodiodes, operated at 4 K, rather than an electrical link, is expected to have several advantages in terms of performance. The PhD-project has focused on the development of an optoelectronic module for realizing this concept. Packaging techniques and characterization of the module was investigated. In particular, designs and materials for reducing thermal stress at 4 K were considered. A packaging technique for assembling multiple optical fiber-coupled high-speed photodiodes on silicon substrates was demonstrated. The assembly was shown to have sufficient precision and to be robust against thermal stress. Finite element simulations of the thermomechanical stresses were performed in order to validate the robustness claim. The frequency response of commercial InGaAs/InP photodiodes up to 14 GHz, as well as DC-response, was measured at room temperature and at 4 K. It was shown that the effect of low temperature did not negatively affect the frequency response. The thesis also includes a review of devices, packaging techniques and materials for cryogenic optoelectronics. In addition, initial work on semiconductor simulations of photodiodes at low temperature is discussed

NASA Tech Briefs, May 1997

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Near-field characterization of plasmonic waveguides

This PhD thesis presents investigation of plasmonic waveguides and waveguiding components by means of scanning near-field optical microscopy characterizations, far-field optical observations, and numerical simulations. The plasmonic waveguiding attracts huge interest due to several reasons: 1) it is believed to bridge naturally optical and electronic circuits; 2) it looks natural and most efficient for active applications due to the presence of the metal inside the core of the plasmonic mode; 3) the mode size and correspondent field confinement of plasmonic waveguides can be tuned in a vast range simply by changing geometric parameters of the waveguide, keeping in mind the trade-off between confinement and propagation losses. A broad variety of plasmonic waveguides and waveguide components, including antennas for coupling the light in/out of the waveguide, requires correspondent characterization capabilities, especially on experimental side. The most straight-forward and powerful technique for such purpose is scanning near-field optical microscopy, which allows to probe and map near-field distribution and therefore becomes the main tool in this project. The detailed description of the used setups and their imaging techniques is included additionally to the main research of plasmonic waveguides (channel plasmon polariton, long-range dielectric-loaded surface plasmon polariton, and plasmonic slot waveguides) and waveguide components (antennas, S-bends, and directional couplers) included as a reprint of papers

Near-field characterization of plasmonic waveguides

This PhD thesis presents investigation of plasmonic waveguides and waveguiding components by means of scanning near-field optical microscopy characterizations, far-field optical observations, and numerical simulations. The plasmonic waveguiding attracts huge interest due to several reasons: 1) it is believed to bridge naturally optical and electronic circuits; 2) it looks natural and most efficient for active applications due to the presence of the metal inside the core of the plasmonic mode; 3) the mode size and correspondent field confinement of plasmonic waveguides can be tuned in a vast range simply by changing geometric parameters of the waveguide, keeping in mind the trade-off between confinement and propagation losses. A broad variety of plasmonic waveguides and waveguide components, including antennas for coupling the light in/out of the waveguide, requires correspondent characterization capabilities, especially on experimental side. The most straight-forward and powerful technique for such purpose is scanning near-field optical microscopy, which allows to probe and map near-field distribution and therefore becomes the main tool in this project. The detailed description of the used setups and their imaging techniques is included additionally to the main research of plasmonic waveguides (channel plasmon polariton, long-range dielectric-loaded surface plasmon polariton, and plasmonic slot waveguides) and waveguide components (antennas, S-bends, and directional couplers) included as a reprint of papers