Luis Fernando Velasquez-Garcia

Scalable, MEMS-enabled, vibrational tactile actuators for high resolution tactile displays

The design, fabrication, and characterization of a new type of tactile display for people with blindness or low vision is reported. Each tactile element comprises a piezoelectric extensional actuator that vibrates in plane, with a microfabricated scissor mechanism to convert the in-plane actuations into robust, higher-amplitude, out-of-plane (vertical) vibrations that are sensed with the finger pads. When the tactile elements are formed into a 2D array, information can be conveyed to the user by varying the pattern of vibrations in space and time. Analytical models and finite element analysis were used to design individual tactile elements, which were implemented with PZT actuators and both SU-8 and 3D-printed scissor amplifiers. The measured displacements of these 3 mm × 10 mm, MEMS-enabled tactile elements exceed 10 µm, in agreement with models, with measured forces exceeding 45 mN. The performance of the MEMS-enabled tactile elements is compared with the performance of larger, fully-macroscale tactile elements to demonstrate the scale dependence of the devices. The creation of a 28-element prototype is also reported, and the qualitative user experience with the individual tactile elements and displays is described.

A Micro-Fabricated Linear Array of Electrospray Emitters for Thruster Applications

This paper reports the design, fabrication, and experimental characterization of an internally fed linear array of electrospray emitters intended for space propulsion applications. The engine uses doped formamide as propellant and operates in the single-Taylor-cone droplet emission regime. The engine implements the concept of hydraulic and electrodynamic flow rate matching to achieve electrical control. The engine uses a set of meso-scaled silicon deflection springs to assemble the hydraulics to the electrodes, allowing to decouple the corresponding process flows. The micro-fabrication of the engine is described and novel technologies that were developed are reported. Experimental results that demonstrate cumulative uniform and steady operation are provided. Current-flowrate characteristics of the engine are in agreement with a reduced-order model. Experimental data demonstrating the low divergence of electrospray emitter arrays operated in the single Taylor Cone is in qualitative agreement with a reduced-order mode that assumes the absence of a thermalized tail in the plume

CNT-Based MEMS/NEMS Gas Ionizers for Portable Mass Spectrometry Applications

We report the fabrication and experimental characterization of a carbon nanotube (CNT)-based MEMS/NEMS electron impact gas ionizer with an integrated extractor gate for portable mass spectrometry. The ionizer achieves low-voltage ionization using sparse forests of plasma-enhanced chemical-vapor-deposited CNTs as field emitters and a proximal extractor grid with apertures aligned to the CNT forests to facilitate electron transmission. The extractor gate is integrated to the ionizer using a high-voltage MEMS packaging technology based on Si springs defined by deep reactive ion etching. The ionizer also includes a high-aspect-ratio silicon structure (¿foam) that facilitates sparse CNT growth and also enables uniform current emission. The devices were tested as field emitters in high vacuum (10-8 torr) and as electron impact ionizers using argon at pressures of up to 21 mtorr. The experimental data show that the MEMS extractor gate transmits up to 66% of the emitted current and that the ionizers are able to produce up to 0.139 mA of ion current with up to 19% ionization efficiency while consuming 0.39 W.

Chip-Scale Quadrupole Mass Filters for Portable Mass Spectrometry

We report the design, fabrication, and characterization of a new class of chip-scale quadrupole mass filter (QMF). The devices are completely batch fabricated using a wafer-scale process that integrates the quadrupole electrodes, ion optics, and housing into a single monolithic block. This process eliminates the electrode-to-housing misalignments inherent in other QMFs and enables the implementation of complex device architectures. To achieve the reported integration, 1 mm × 1 mm square electrodes of heavily doped silicon were utilized, resulting in quadrupoles with an effective aperture radius of 0.707 mm and a length of 30 mm. Mass filtering was demonstrated with this unconventional device showing a mass range of 650 amu and a resolution of ~30 at a drive frequency of 1.8 MHz. When operated in the second stability region at 2.0 MHz and a mass range of 50 amu, a peak width of 0.3 amu was achieved at mass 28, showing a resolution of ~90. This paper introduces operation in the second stability region as a reliable method for turning QMFs with less than ideal electrode geometries into high-performance devices.

Nanostructured Ultrafast Silicon-Tip Optical Field-Emitter Arrays

Femtosecond ultrabright electron sources with spatially structured emission are an enabling technology for free-electron lasers, compact coherent X-ray sources, electron diffractive imaging, and attosecond science. In this work, we report the design, modeling, fabrication, and experimental characterization of a novel ultrafast optical field emission cathode comprised of a large (>100,000 tips), dense (4.6 million tips·cm(-2)), and highly uniform (<1 nm tip radius deviation) array of nanosharp high-aspect-ratio silicon columns. Such field emitters offer an attractive alternative to UV photocathodes while providing a direct means of structuring the emitted electron beam. Detailed measurements and simulations show pC electron bunches can be generated in the multiphoton and tunneling regime within a single optical cycle, enabling significant advances in electron diffractive imaging and coherent X-ray sources on a subfemtosecond time scale, not possible before. At high charge emission yields, a slow rollover in charge is explained as a combination of the onset of tunneling emission and the formation of a virtual cathode.

A Micro Ionizer for Portable Mass Spectrometers using Double-gated Isolated Vertically Aligned Carbon Nanofiber Arrays

We report micro-fabricated double-gated vertically aligned carbon nanofiber (CNF) arrays for ionization of gasses in low power portable mass spectrometers. The devices can be operated in one of two modes - electron impact ionization (EII ) or field ionization (FI). When operated as electron impact ionizer, power dissipation was reduced from >1 W typical of thermionic emission based electron impact ionizers to <100 mW. When operated as a field ionizer, the turn-on voltage for field ionization is reduced from 5-10 kV typical of ungated ionizers to 350 V.

Uniform High-Current Cathodes Using Massive Arrays of Si Field Emitters Individually Controlled by Vertical Si Ungated FETs—Part 2: Device Fabrication and Characterization

We report the demonstration of electron sources that achieve high-current and uniform emission using dense arrays of Si field emitters (FEs) that are individually ballasted by a current source. Each FE is fabricated on top of a vertical ungated field-effect transistor (FET), a two-terminal device based on a very-high-aspect-ratio Si column. The ungated FET takes advantage of the velocity saturation of electrons in silicon, the high aspect ratio of the ungated FET, and the doping concentration to achieve current-source-like behavior to obtain reliable uniform and high-current electron emission. Emitted currents in excess of 0.48 A were demonstrated.

Precision Hand Assembly of MEMS Subsystems Using DRIE-Patterned Deflection Spring Structures: An Example of an Out-of-Plane Substrate Assembly

This paper describes a packaging concept for precise hand-assembly of microelectromechanical systems (MEMS) subsystems that uses mesoscaled deep-reactive ion etching (DRIE) patterned passive deflection spring clusters. The method is intended for applications that require decoupling of subsystem process flows to simplify device fabrication in order to attain macro three-dimensionality, or for cases where the device requires spatially referenced macro- and microfeatures with good precision. The design considerations for the deflection springs are presented, and a simple reduced-order model of the expected elastic behavior is proposed. The assembly concept is demonstrated with an electrospray array test structure. This test structure assembles perpendicularly two wafer substrates. The performance of the test structure is benchmarked using finite-element simulations and by measurements of the misalignment introduced by the assembly. A floor for the ultimate alignment accuracy of the assembly concept is proposed.

An Application of 3-D MEMS Packaging: Out-of-Plane Quadrupole Mass Filters

This paper reports the design, fabrication, and characterization of low-cost out-of-plane quadrupole mass filters that use commercially available dowel pins as electrode rods. The quadrupoles implement a 3-D MEMS packaging technology that relies on deep-reactive ion etching (DRIE)-patterned deflection springs for alignment. Quadrupoles with rod diameter ranging from 0.25 to 1.58 mm and aspect ratio of 30 to 60 were built and tested at RF frequencies of 1.44, 2.0, and 4 MHz. Assembled devices operated in the first stability region achieved a maximum mass range of 650 amu, while a minimum half-peak width of 0.4 amu at mass 28 was obtained in the second stability region. Operation in the second stability region provides a means to higher resolution, smoother peaks, and removed peak splitting at the expense of transmission. The ultimate resolution of the reported quadrupoles is also discussed.

A portable x-ray source with a nanostructured Pt-coated silicon field emission cathode for absorption imaging of low-Z materials

We report the design, fabrication, and characterization of a portable x-ray generator for imaging of low-atomic number materials such as biological soft tissue. The system uses a self-aligned, gated, Pt-coated silicon field emitter cathode with two arrays of 62 500 nano-sharp tips arranged in a square grid with 10 μm emitter pitch, and a natural convection-cooled reflection anode composed of a Cu bar coated with a thin Mo film. Characterization of the field emitter array demonstrated continuous emission of 1 mA electron current (16 mA cm − 2) with >95% current transmission at a 150 V gate-emitter bias voltage for over 20 h with no degradation. The emission of the x-ray source was characterized across a range of anode bias voltages to maximize the fraction of photons from the characteristic K-shell peaks of the Mo film to produce a quasi-monochromatic photon beam, which enables capturing high-contrast images of low-atomic number materials. The x-ray source operating at the optimum anode bias voltage, i.e. 35 kV, was used to image ex vivo and nonorganic samples in x-ray fluoroscopic mode while varying the tube current; the images resolve feature sizes as small as ~160 µm.