Sung-Wook Nam

Ionic Field Effect Transistors with Sub-10 nm Multiple Nanopores

We report a new method to fabricate electrode-embedded multiple nanopore structures with sub-10 nm diameter, which is designed for electrofluidic applications such as ionic field effect transistors. Our method involves patterning pore structures on membranes using e-beam lithography and shrinking the pore diameter by a self-limiting atomic layer deposition process. We demonstrate that 70-80 nm diameter pores can be shrunk down to sub-10 nm diameter and that the ionic transport of KCl electrolyte can be efficiently manipulated by the embedded electrode within the membrane.

Direct evidence of phase separation in Ge2Sb2Te5 in phase change memory devices

Phase changematerials in phase change random access memory (PRAM) undergo very high electrical stress as well as thermal stress during operation. These can cause intrinsic problems in stability of the PRAM devices. We investigate the stability of Ge 2 Sb 2 Te 5 , the most common phase changematerial for PRAM, under electrical stress. After multiple programming cycles, cross-sectional transmission electron microscopy and energy dispersive analysis show that electromigration as well as incongruent melting causes phase separation by mass movement by which the peak position of Sb and Ge lies shifted toward the cathode and that of Te toward the anode.

Sub-10-nm nanochannels by self-sealing and self-limiting atomic layer deposition.

We report on a novel fabrication method of a nanochannel ionic field effect transistor (IFET) structure with sub-10-nm dimensions. A self-sealing and self-limiting atomic layer deposition (ALD) facilitates the fabrication of lateral type nanochannels smaller than the e-beam or optical lithographic limits. Using highly conformal ALD film structures, including TiO(2), TiO(2)/TiN, and Al(2)O(3)/Ru, we have fabricated lateral sub-10-nm nanochannels with good control over channel diameter. Nanochannels surrounded by core/shell (high-k dielectric/metal) layers give rise to all-around-gating IFETs, an important functional element in an electrofluidic-based circuit system.

High-Resolution Transmission Electron Microscopy Study of Electrically-Driven Reversible Phase Change in Ge2Sb2Te5 Nanowires

By combining high-resolution transmission electron microscopy (HRTEM) characterization and electrical measurements on a unique device platform, we study the reversible electrically-driven phase-change characteristics of self-assembled Ge(2)Sb(2)Te(5) nanowires. Detailed HRTEM analyses are used to correlate and understand the effect of full and intermediate structural transformations on the measured electrical properties of the nanowire devices. The study demonstrates that our unique approach has the potential to provide new information regarding the dynamic structural and electrical states of phase-change materials at the nanoscale, which will aid the design of future phase-change memory devices.

Phase separation behavior of Ge2Sb2Te5 line structure during electrical stress biasing

We report the breakdown behavior of a patterned Ge2Sb2Te5 multiline structure during the voltage-driven electric stress biasing. Scanning Auger microscope analysis shows that the breakdown process accompanies with a phase separation of Ge2Sb2Te5 into an Sb, Te-rich phase and a Ge-rich phase. The phase separation is explained by the incongruent melting of Ge2Sb2Te5 based on the pseudobinary phase diagram between Sb2Te3 and GeTe. It is claimed that this phase separation behavior by incongruent melting provides one of the plausible mechanisms of the device failure in a phase change memory.

Electric-Field-Induced Mass Movement of Ge2Sb2Te5 in Bottleneck Geometry Line Structures

We report an electric-field-induced directional mass movement of Ge 2 Sb 2 Te 5 in bottleneck geometry. Under high-electric-stress circumstances (>10 6 A cm -2 ), a mass of Ge 2 Sb 2 Te 5 tends to move toward the cathode (-) by the remaining mass depletion at the anode (+). The high electric stress induces an asymmetric compositional separation such that Sb is distributed toward the cathode (-) whereas Te is distributed toward the anode (+). Ionicity in Ge 2 Sb 2 Te 5 at high temperature and high electric stress can be one of the origins of the asymmetric behavior. The electric-field-induced mass movement may provide insight on the device reliability of phase-change random access memory.

Analysis of the electric field induced elemental separation of Ge2Sb2Te5 by transmission electron microscopy

The chemical instability of line patterned Ge2Sb2Te5 was studied by transmission electron microscopy after electrically inducing melt and solidification. Compositional analysis showed elemental separation of Te to the anode side, while Ge and Sb mutually separated at the cathode side. Such elemental separation of Ge2Sb2Te5 is explained by the electric field effects and thermodynamic driving forces.

Contrast enhancement behavior of hydrogen silsesquioxane in a salty developer

The authors investigated a contrast enhancement behavior of hydrogen silsesquioxane (HSQ) in a salty development system (NaOH∕NaCl). Time-resolved analysis of contrast curves and line-grating patterns were carried out to investigate the unique properties of a salty development process. In NaOH developer without salt, the development process was saturated beyond a certain development time. On the other hand, the addition of salt enabled a continuous development, which was not observed in the pure NaOH development. The continuous thinning process enhances the contrast of HSQ in the salty developer, which allows a fast collapsing behavior in HSQ line-grating patterns. During development process, salt seems to have the role of modifying HSQ by breaking network bonds preferentially, leading to a continuous development rate.

Formation of Ge2Sb2Te5 – TiO x Nanostructures for Phase Change Random Access Memory Applications

Amorphous Ge 2 Sb 2 Te 5 clusters with a size of 20 nm, self-enclosed by a thin layer of TiO x , were obtained by cosputtering Ge 2 Sb 2 Te 5 and TiO 2 targets at room temperature with the aim of reducing the reset current for phase change random access memory applications. Eutectic decomposition during the deposition caused a phase separation of Ge 2 Sb 2 Te 5 and TiO x . The temperature-dependent resistance change results showed that the activation energy for crystallization increased from 2.44 ± 0.76 to 3.84 ± 1.43 eV in the Ge 2 Sb 2 Te 5 film. The set resistance can be tuned within an acceptable range, and the reliability of this microstructure during repetitive laser melt-quenching cycles was tested.

Two-step resist-development process of hydrogen silsesquioxane for high-density electron-beam nanopatterning

It is identified that the development of hydrogen-silsesquioxane resist after electron-beam exposure, by using a 25% tetramethylammonium-hydroxide (TMAH) developer, almost stops after 1min of development time and it severely limits the delineation of high-density nanometer-scale patterns. By using x-ray photoelectron spectroscopy, the authors identified that this development-stopping phenomenon is due to the formation of a siloxane-type bond structure which is insoluble to the TMAH developer. Here, the authors propose a two-step development method that involves the removal process of siloxane layer using a dilute hydrofluoric acid between development processes. This method successfully eliminates the insoluble layer, thus generating isolated high-density dot patterns with 25nm pitch.