Mon-Shu Ho

RRAM-based 7T1R nonvolatile SRAM with 2x reduction in store energy and 94x reduction in restore energy for frequent-off instant-on applications

This study proposes a 7T1R nonvolatile SRAM (nvSRAM) to 1) reduce store energy by using a single NVM device, 2) suppress DC-short current during restore operations through the use of a pulsed-overwrite (POW) scheme, and 3) achieves high restore yield by using a differentially supplied initialization (DSI) scheme. This initialization-and-overwrite (IOW) 7T1R nvSRAM improves breakeven-time (BET) by 6+x, compared to previous nvSRAMs. We fabricated a 16Kb IOW-7T1R nvSRAM using HfOx RRAM and a 90nm process. This represents the first ever silicon verified single-NVM nvSRAM macro. Measurements obtained in test-mode confirm that the proposed nvSRAM reduces store energy by 2x and restore energy by 94x, compared to 2R-based nvSRAMs.

Atomic force microscopic observation of surface-supported human erythrocytes

The nanomechanical characteristics of the membrane cytoskeleton of human erythrocytes were studied using atomic force microscopy (AFM). The self-assembly, fine structure, cell diameter, thickness, and reticulate cytoskeleton of erythrocytes on the mica surface were investigated. The adhesive forces that correspond to the membrane elasticity of various parts of the erythrocyte membrane surface were measured directly by AFM to be 0.64±0.14nN for cell indentation, 4.2±0.7nN for cell hump, and 11.5nN for side waist, respectively. The deformation of erythrocytes was discussed. Standing waves on the membrane that were set up by increased AFM amplitude were observed. The propagating velocity on the erythrocyte membrane was estimated to be ∼2.02×10−2m∕s. Liquid physiological conditions were considered throughout.

Thermally tunable polymer microlenses

Polymer microlenses capable of using heat to control its focal length are presented. The microlenses are created by exposing droplets of the polymer SU-8 to UV light. By altering the temperature of the microlenses via on-chip heating, their curvature and focal length are actively controlled without mechanical movements. By directly and indirectly measuring temperature-dependent changes of the focal length, we test the ability of the microlenses as a tunable imaging component. The microlenses have potential use in applications such as laser systems, functional biomimetics, and endoscopy.

Scanning tunneling microscope study of dynamic phenomena on clean Si(111) surfaces: Si magic clusters and their role

Abstract On Si(111) surfaces, we observed a special type of Si magic clusters with a variable-temperature scanning tunneling microscope (STM). At temperatures above 400°C, these clusters migrate on Si(111)-(7×7) surfaces as a whole. They can hop within a half-cell of Si(111)-(7×7), but sometimes they hop away from their original halves, leaving the 7×7 structure intact. When this happens, the magic cluster usually reappears at a site a few hundred A away. We characterize its structure and derive path-specific hopping parameters using Arrhenius analysis. In the long hops, interestingly, magic clusters exhibit a strong bias for moving in the direction of the heating current. Effects of the directed motion in electromigration and those in thermal migration are determined separately and quantitatively. We also observed fluctuations of step edges through detachment and attachment of magic clusters. The filling of two-dimensional (2-D) craters and the decay of 2-D islands are also found to occur preferentially at the cathode side. These observations provide important clues for understanding the atomic processes in epitaxial growth and in electromigration on Si(111) surfaces. Based on our observations, the phase transition of 7×7↔1×1 is also discussed.

Electron and atom dynamics at solid surfaces and relation to epitaxy

Abstract At the surface, the three dimensional symmetry of a solid is broken. Electrons and atoms near the surface may rearrange to lower the free energy of the system. Scattering by defects and confinement by boundaries of electrons may produce long-range charge density oscillations. Adatoms interact with each other via mutual perturbation of the surface, known as indirect electronic and elastic interactions. These interactions are very weak and are also oscillatory. For some systems, formation of adsorption layer superstructures can be directly correlated to adatom–adatom interactions. When the temperature is raised, adatoms and admolecules can start to diffuse, interact, or react. They may aggregate into clusters and islands, and grown into a thin film. The stability of clusters may exhibit magic numbers in size and thickness. When the temperature is changed, island shape transitions may occur. The growth of islands and ultra-thin films can also be influenced by electronic effects as well as by the addition of a surfactant layer. All these growth behaviors in epitaxy can be understood from the mechanisms and energetics of elementary surface atomic processes, and atom and electron dynamics. They, in turn, can be studied in details using atomic resolution microscopy.

A 260mV L-shaped 7T SRAM with bit-line (BL) Swing expansion schemes based on boosted BL, asymmetric-V TH read-port, and offset cell VDD biasing techniques

This work proposes bit-line (BL) swing expansion schemes (BL-EXPD), which minimize the product (A*VDDmin) of SRAM cell area (A) and the minimum operation voltage (VDDmin) to the best of our knowledge. The key-enablers to minimize A*VDDmin are: L-shaped 7T cell (L7T) and BL-EXPD. The L7T features: (1) an area efficient cell layout, (2) a read-disturb free decoupled 1T read port (RP), and (3) a half-select disturb free write back scheme[1]. The BL-EXPD enables a 9× larger read-BL (RBL) swing at the 6σ point than that in our previously proposed Z8T[2] and allows single BL sensing to reduce cell area. A fabricated 65nm 256-row BL 32Kb L7T SRAM achieved a 260mV VDDmin. As a result, its A*VDDmin is ~50% lower than for Z8T and conventional 8T SRAM cells [3,4].

Dynamical Study of Single Silver Atoms on Si(111)-7 ×7 Surfaces

The dynamics and interactions of silver atoms on Si(111)-7 ×7 surfaces are investigated using variable-temperature scanning tunneling microscopy (STM). The activation energies and pre-exponential factors upon the room temperature are estimated to be 0.81 and 0.9 eV, and 109.65 and 1010.68 for the hopping out of the faulted halves and for hopping out of unfaulted halves, respectively. The behavior exhibited by two silver atoms that jump into a single half-unit cell is also addressed. Details of the tracking of single silver atoms that hop within a 7 ×7 half-unit cell are reexamined at ~80 K using low-temperature STM. The analytical data revealed that silver atoms diffuse within a 7 ×7 unit cells via a corner adatom–rest atom–center adatom–rest atom–corner adatom diffusion pathway in a faulted half under the stress associated with 7 ×7 reconstruction.

Read circuits for resistive memory (ReRAM) and memristor-based nonvolatile Logics

Resistive memory device (Memristor) is one of the candidates for energy-efficient nonvolatile memory and nonvolatile logics (nvLogics) in the applications of wearable devices, Internet of Things (IoT), cloud computing, and big-data processing. However, resistive RAM (ReRAM) and memristor-based nvLogics suffer limited performance and low yield due to process variations in transistors and resistance of memristor. This presentation discusses the design challenges in read circuits for high-speed, area-efficient, and low-voltage ReRAM and nvLogics. Memristor-based nvLogics, such as nonvolatile-SRAM (nvSRAM), nonvolatile flip-flops (nvFF), and nonvolatile TCAM (nvTCAM) are included in this presentation. Several silicon-verified solutions on read scheme and sense amplifiers are also discussed in this presentation.

A low-power subthreshold-to-superthreshold level-shifter for sub-0.5V embedded resistive RAM (ReRAM) macro in ultra low-voltage chips

Many mobile chips with low supply voltage (VDD) require low-voltage embedded nonvolatile memory (eNVM) to enable low-power read operations and zero-standby-current power-off storage. ReRAM has a lower write-voltage (VW), smaller write-current (Iw), and larger resistance-ratio than other NVMs, making it a good candidate for low-VDD eNVM, as long as the following two challenges can be overcome: (1) the failure of level-shifters (LSs) to operate with a low input voltage (VDDL) during write operations, particularly under high converting voltages (VDDH); (2) the consumption of large DC current by LS resulting in degradation in VDDmin for the on-chip charge-pump, particularly in NVMs with a large number of rows. This study proposes a pseudo-diode-mirrored (PDM) LS to achieve low VDDL, while maintaining a small DC current. PDM LSs were fabricated in a 65nm 4Mb embedded ReRAM macro, which achieved 0.1V minimum-VDDL at VDDH=2V.

Controlling the mechanics and nanotopography of biocompatible scaffolds through dielectrophoresis with carbon nanotubes

Creating a biocompatible carbon‐nanotube polymer scaffold is an area that has a number of potential applications. Herein, a dielectrophoretic approach was pursued to integrate carbon nanotubes into a polymeric material for fabricating a nanoscale composite scaffold with increased and controllable mechanical strength. The adhesion force, which combines the surface energy of the sample and the interfacial energy between the tip and sample, was estimated to be 55.39 ± 6.72 nN away from the center of the protrusions at a distance of 0.5 μm while being 24.01 ± 4.45 nN at the center. The adhesion force for the center of the cavities was 42.47 ± 6.91 and 88.21 ± 15.05 nN at 0.5 μm away from the center. NIH 3T3 fibroblast cells were then utilized to test the cellular biocompatibility of this multiwalled carbon nanotubes (MWCNTs) film. Cells were cultured on the surface and then their attachment, spreading, and proliferation behaviors were observed. This nanotube‐polymer scaffolding approach has a wide range of potential applications including in complex device fabrication as well as in developing biomimetic and tissue engineering scaffolds, and artificial organs.