Jung-Sub Wi

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.

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.

Fabrication of silicon nanopillar teradot arrays by electron-beam patterning for nanoimprint molds.

The formation of high-density, nanometer-scale dot (nanodot) arrays is a challenging task. Such arrays are considered important not only for scientific study of the fundamental quantum-mechanical behavior of materials, but also for achieving a practical goal of ultrahigh-density data storage and manipulation devices. [1‐3] In particular, methods for producing isolated high-density magnetic nanodot arrays with a pitch of 25nm or less have been extensively studied with the aim of fabricating the next generation of patterned magnetic media with a recording density of up to 1 terabit inch � 2 . [4‐6] With this application in mind, various fabrication attempts have been made, such as those based on a block copolymer, anodized aluminum oxide, colloid lithography, or laser interference lithography with extreme UV light. [7‐10] How

Sensitivity Characteristics of Positive and Negative Resists at 200 kV Electron-Beam Lithography

The contrast curve of positive and negative electron-beam resists such as polymethylmethacrylate (PMMA), ZEP520A, and hydrogen silsesquioxane (HSQ) at 200 kV electron-beam was estimated by using continuous slow down approximation (CSDA) model with both non-relativistic and relativistic Bethe stopping power. Experimental results show that simple CSDA model well explains the overall response of these various electron-beam resists to high energy electron-beam only if we use the relativistic Bethe stopping power. The difference between non-relativistic and relativistic Bethe stopping power is discussed.

Electron-beam lithography of Co∕Pd multilayer with hydrogen silsesquioxane and amorphous Si intermediate layer

We propose a patterning method to form nanostructures of a Co∕Pd multilayer by using electron-beam lithography with an amorphous silicon (a-Si) layer and two-step etching process. On the Co∕Pd multilayer, a-Si is sputter deposited and hydrogen silsesquioxane (HSQ), the electron-beam resist, is spin coated sequentially. We found that an a-Si intermediate layer between the Co∕Pd underlayer and HSQ overlayer improves adhesion of HSQ on the metallic underlayer after electron-beam dosing and chemical development; it also increases etch selectivity between the Co∕Pd multilayer and its overlayers. We demonstrate that a Co∕Pd multilayer can be patterned successfully as a nanowire array using the suggested process.

Electron-Beam Lithography Patterning of Ge2Sb2Te5 Nanostructures Using Hydrogen Silsesquioxane and Amorphous Si Intermediate Layer

We report the successful electron-beam patterning of Ge 2 Sb 2 Te 5 nanostructures, in the form of both lines and dots, using hydrogen silsesquioxane (HSQ) resist. Although HSQ has proven to be a good resolution electron-beam resist, the adhesion between Ge 2 Sb 2 Te 5 and the HSQ resist is problematic when trying to form fine patterns. To promote their adhesion, we introduced an amorphous Si (a-Si) layer between the Ge 2 Sb 2 Te 5 and HSQ layers, and the layers were then sequentially removed using Cl 2 reactive ion etching and Ar etching with good selectivity of each layer. The selectivities for HSQ:a-Si etched by Cl 2 reactive gas and a-Si:Ge 2 Sb 2 Te 5 etched by Ar gas were 1:2.7 and 1:17, respectively. On the basis of this multilayer structure and a two-step dry etching process, various Ge 2 Sb 2 Te 5 nanostructures as small as 40 nm were successfully fabricated.

The fabrication scheme of a high resolution and high aspect ratio UV-nanoimprint mold

We propose a new scheme of fabricating molds for UV-nanoimprint lithography (UV-NIL) that is both high resolution and has a high aspect ratio. The scheme involves the utilization of a hydrogen silsesquioxane (HSQ) electron beam resist for high resolution patterning and the sputter-deposited alpha-Si layer that defines the high-aspect-ratio mold pattern obtained from the high etch selectivity between the HSQ and the alpha-Si. We obtained high resolution line patterns and dot patterns with feature sizes of 40 nm and 25 nm, respectively. The aspect ratio of the patterns was about 3.5 for line patterns and about 5 for dot patterns. These molds also demonstrate successful UV-nanoimprint patterning.

Method of improving the quality of nanopatterning in atomic image projection electron-beam lithography

The authors demonstrated successful nanopatterning using a high resolution lattice image obtained by transmission electron microscopy (TEM) as a mask signal [H. S. Lee et al., Adv. Mater. (Weinheim, Ger.) 19, 4198 (2007)]. In this process, the quality of the patterning result is critically dependent on the lattice image, which, in turn, is critically dependent on the quality of the mask. It is often noted that the quality of the mask prepared by the conventional TEM sample preparation technique is far from perfect, which critically limits the quality of patterning. In this work, they first discuss the various origins of the noise signal generated by the mask and then introduce a special type of objective aperture (noise reduction aperture) to remove the noise signal. The effect of the noise reduction aperture, designed for the Si [110] zone axis, is experimentally demonstrated.

High Speed Phase Change Random Access Memory with (Ge1Sb2Te4)0.9(Sn1Bi2Te4)0.1 Complete Solid Solution

We investigated structures and phase transformation kinetics of (Ge1Sb2Te4)0.9(Sn1Bi2Te4)0.1 alloy mixture and its application for the phase change random access memory device. As-sputtered (Ge1Sb2Te4)0.9(Sn1Bi2Te4)0.1 thin film forms crystalline fcc phase. Meanwhile, we could obtain amorphous RESET state and crystalline SET state reproducibly by using appropriate voltage pulse conditions in device structure. We demonstrate that the minimum time for SET operation of phase change random access memory device with (Ge1Sb2Te4)0.9(Sn1Bi2Te4)0.1 goes down to 20 ns, which is much smaller than 100 ns for device with Ge1Sb2Te4. The accelerated SET operation of the device with (Ge1Sb2Te4)0.9(Sn1Bi2Te4)0.1 is interpreted to originate from reduced bond strength in comparison to pure Ge1Sb2Te4.