Yongha Hwang

A stacked memory device on logic 3D technology for ultra-high-density data storage

We have demonstrated, for the first time, a novel three-dimensional (3D) memory chip architecture of stacked-memory-devices-on-logic (SMOL) achieving up to 95% of cell-area efficiency by directly building up memory devices on top of front-end CMOS devices. In order to realize the SMOL, a unique 3D Flash memory device and vertical integration structure have been successfully developed. The SMOL architecture has great potential to achieve tera-bit level memory density by stacking memory devices vertically and maximizing cell-area efficiency. Furthermore, various emerging devices could replace the 3D memory device to develop new 3D chip architectures.

Suspended few-layer graphene beam electromechanical switch with abrupt on-off characteristics and minimal leakage current

Suspended few-layer graphene beam electro-mechanical switches (SGSs) with 0.15 μm air-gap are fabricated and electrically characterized. The SGS shows an abrupt on/off current characteristics with minimal off current. In conjunction with the narrow air-gap, the outstanding mechanical properties of graphene enable the mechanical switch to operate at a very low pull-in voltage (VPI) of 1.85 V, which is compatible with conventional complimentary metal-oxide-semiconductor (CMOS) circuit requirements. In addition, we show that the pull-in voltage exhibits an inverse dependence on the beam length.

Dielectrophoresis-Assembled Zeolitic Imidazolate Framework Nanoparticle-Coupled Resonators for Highly Sensitive and Selective Gas Detection

This work reports on zeolitic imidazolate framework (ZIF)-coupled microscale resonators for highly sensitive and selective gas detection. The combination of microscale resonators and nanoscale materials simultaneously permits the benefit of larger capture area for adsorption from the resonator and enhanced surface adsorption capacity from the nanoscale ZIF structure. Dielectrophoresis (DEP) was demonstrated as a novel method for directly assembling concentrated ZIF nanoparticles on targeted regions of silicon resonant sensors. As part of the dielectrophoretic assembly process, the first ever measurements of the Clausius-Mossotti factor for ZIFs were conducted to determine optimal conditions for DEP assembly. The first ever real-time adsorption measurements of ZIFs were also performed to investigate the possibility of inherent gas selectivity. The ZIF-coupled resonators demonstrated sensitivity improvement up to 150 times over a bare silicon resonator with identical dimensions, and real-time adsorption measurements of ZIFs revealed different adsorption time constants for IPA and CO2.

Porous Silicon Resonators for Improved Vapor Detection

This paper presents a microscale resonant sensor that has been fabricated with nanoscale pores for enhanced sensitivity to chemical vapors. By building resonators that are made of porous silicon, we take advantage of the increased area for molecular binding and improve the sensitivity of the resonators to the vapor concentration of interest. We present results for resonators whose surfaces are entirely porous silicon. We also examine the use of targeted regions of porosity to keep critical parts of the beam nonporous and mechanically stable while still maximizing surface area. Surface micromachining processes were used to fabricate the silicon resonator mass sensor, allowing nanostructured devices to be fabricated using only standard top-down processing techniques. We have demonstrated an improvement up to 261% and 165% in resonator sensitivity to isopropyl alcohol forfully porous resonators and partially porous resonators, respectively, as compared to nonporous silicon resonators. Combining this increased sensitivity with resonator quality factor suggests an improvement in minimum detectable resolution over the nonporous resonators by 41% and 56% for the fully porous and partially porous resonators, respectively.

Capillary Flow in PDMS Cylindrical Microfluidic Channel Using 3-D Printed Mold

This letter investigates the capillary filling in polydimethylsiloxane (PDMS) microchannels using 3-D printed molds to produce channels with circular cross sections. The circular cross sections are prevalent in biology and anatomy, yet they cannot readily be mimicked with existing soft-lithography techniques. The molds are printed directly from computer-aided design files, making rapid prototyping of microfluidic devices possible in hours, demonstrating microscale features in PDMS channels. The PDMS channels with variable channel diameters ranging from 200 to 1000 μm in a single device that are obtained from four different 3-D printers are compared in terms of capillary flow. Technology limits, including surface roughness and resolution, are also characterized, and estimated as an equivalent contact angle which is a fit parameter dependent on the 3-D printer.

Receptor-coated porous silicon resonators for enhanced sensitivity of vapor detection

We present microscale silicon resonators with nanoscale pores for increased surface area, which are capable of providing enhanced sensitivity in vapor sensors. Increased mechanical stability and detection performance are also achieved by keeping parts of the resonating device nonporous and adding a receptor coating. We demonstrate improvements up to 165% and 654% in resonator sensitivity to isopropyl alcohol (IPA) for partially-porous silicon resonators and receptor-coated partially-porous silicon resonators, respectively, as compared to nonporous silicon resonators.

Fabrication Process for Integrating Nanoparticles With Released Structures Using Photoresist Replacement of Sublimated p-Dichlorobenzene for Temporary Support

This letter introduces a fabrication technique that enables deposition of a wide variety of materials on released microelectromechanical systems devices by providing a temporary support layer of photoresist (PR). The technique is particularly useful for materials that cannot survive aggressive wet etchants or high-temperature processes. The novel step of the process involves continuous replacement of solid p-dichlorobenzene (p-DCB) by liquid PR as the p-DCB sublimates from a released structure, without ever allowing the structure to dry. The PR serves as both a temporary support for the device and a mold for targeted deposition of the material. As a demonstration of the process, fully released silicon microresonators were coupled with nanoparticles that would not survive common sacrificial oxide etch processes. The process is shown to produce nanoparticle-functionalized resonators that operate as functional resonant gas sensors.

Fabrication of truly 3D microfluidic channel using 3D-printed soluble mold

The field of complex microfluidic channels is rapidly expanding toward channels with variable cross-sections (i.e., beyond simple rounded channels with a constant diameter), as well as channels whose trajectory can be outside of a single plane. This paper introduces the use of three-dimensional (3D) printed soluble wax as cast molds for rapid fabrication of truly arbitrary microfluidic polydimethylsiloxane (PDMS) channels that are not achieved through typical soft lithography. The molds are printed directly from computer-aided design files, followed by simple dissolution using a solvent after molding PDMS, making rapid prototyping of microfluidic devices possible in hours. As part of the fabrication method, the solubility of several build materials in solvents and their effect on PDMS were investigated to remove the 3D-printed molds from inside the replicated PDMS microfluidic channels without damage. Technology limits, including surface roughness and resolution by comparing the designed channels with fabricated cylindrical channels with various diameters, are also characterized. We reproduced a 3D image of an actual human cerebral artery as cerebral artery-shaped PDMS channels with a diameter of 240 μm to prove the developed fabrication technique. It was confirmed that the fabricated vascular channels were free from any leakage by observing the fluorescence fluid fill.

A Field-Portable Cell Analyzer without a Microscope and Reagents

This paper demonstrates a commercial-level field-portable lens-free cell analyzer called the NaviCell (No-stain and Automated Versatile Innovative cell analyzer) capable of automatically analyzing cell count and viability without employing an optical microscope and reagents. Based on the lens-free shadow imaging technique, the NaviCell (162 × 135 × 138 mm3 and 1.02 kg) has the advantage of providing analysis results with improved standard deviation between measurement results, owing to its large field of view. Importantly, the cell counting and viability testing can be analyzed without the use of any reagent, thereby simplifying the measurement procedure and reducing potential errors during sample preparation. In this study, the performance of the NaviCell for cell counting and viability testing was demonstrated using 13 and six cell lines, respectively. Based on the results of the hemocytometer (de facto standard), the error rate (ER) and coefficient of variation (CV) of the NaviCell are approximately 3.27 and 2.16 times better than the commercial cell counter, respectively. The cell viability testing of the NaviCell also showed an ER and CV performance improvement of 5.09 and 1.8 times, respectively, demonstrating sufficient potential in the field of cell analysis.

High-throughput and real-time microalgae monitoring platform using lens-free shadow imaging system (LSIS)

Floc size analysis is one of the major processes in bio-flocculant efficiency determination, which is critical for microalgae harvesting. Till now the flocculation analysis has been performed by using photospectrometry. In this conventional method, optical density of a sample is measured in a time interval by collecting the sample from a fixed point of the container, up to ~5 hrs of observation period. This time consuming process may not guarantee the viability of the algae, which limits the efficient harvesting. To address this issue, we introduce a real time flocculation monitoring system based on the lens-free shadow imaging technique (LSIT). This simple, fast, and cost-effective system automatically analyzes the thousands of single microalgae all in parallel and monitors their flocculation by using a custom developed algorithm. We evaluate the performance of this approach by comparing the results with the standard method, showing a good agreement between two modalities.