Dae-Hwan Kang

An experimental investigation on the switching reliability of a phase change memory device with an oxidized TiN electrode

Fluctuations (or drifts) in switching voltages such as programming set/reset voltages and threshold voltage pose serious obstacles to the reliable operation of electrical phase change memory devices. Using a phase change memory device having a GeSb2Te4 phase change material and TiN electrode, these fluctuations are demonstrated to result from device resistances varying with programming cycles. Fluctuating resistances appear to stem primarily from large contact resistances at the interface between the phase change material and the TiN electrode and from inhomogeneous phase distribution across the GeSb2Te4 layer due to unsuccessful heat confinement near the interface with TiN. Oxidation of a TiN electrode surface (via thermal annealing at 350°C under an atmospheric gas mixture of 97.9vol% N2 and 2.1vol% O2) is very effective in the reduction of fluctuations in device resistances and switching voltages hence the resulting increase in the programming cycles by two orders of magnitude. From a high resolution t...

Lower voltage operation of a phase change memory device with a highly resistive TiON layer

The electrical switching behaviors of an offset-type phase change memory device with a highly resistive TiON layer were investigated, where the TiON layer (7 nm thick) was formed at a 70 nm wide contact between Ge1Sb2Te4 and TiN layers. Reversible transitions between crystalline (set) and amorphous (reset) phases were found to occur at relatively lower reset and set voltages, as compared with a device having no TiON layer. These results hold a high promise for a low-power operation of a phase change memory device.

Multilayer diffusion barrier for copper metallization using a thin interlayer metal (M=Ru, Cr, and Zr) between two TiN films

A multilayer structure [TiN(5 nm)/interlayer metal(2 nm)/TiN(5 nm)] has been evaluated as a diffusion barrier between copper and silicon. Three metals (M=Ru, Cr, and Zr) were tested as interlayer materials for the multilayer diffusion barrier scheme, expecting that they will stuff the TiN grain boundaries by forming metal oxides therein. The diffusion barrier performance was found to be the best when the Zr was used as an interlayer material. X-ray diffractometry showed that the Zr interlayer diffusion barrier failed only after annealing at 800 °C for 30 min due to the formation of copper silicide while the reference sample without a metal interlayer failed when annealed at 600 °C. The insertion of Cr also was also found to enhance the diffusion barrier performance, whereas, Ru has little effect on the improvement of the barrier performance. High-resolution transmission electron microscopy and energy dispersive spectroscopy showed that Zr significantly diffused into TiN grain boundaries while Ru did not d...

A nonvolatile memory based on reversible phase changes between fcc and hcp

A nonvolatile memory technology utilizing reversible changes between fcc and hcp crystalline phases is proposed. In this new type of phase-change memory, data are stored in different forms of crystalline phases of (Ge/sub 1/Sb/sub 2/Te/sub 4/)/sub 0.8/(Sn/sub 1/Bi/sub 2/Te/sub 4/)/sub 0.2/ chalcogenide alloy. RESET operation produces the less conductive metastable fcc phase via melt-quenching from the more conductive stable hcp phase and SET operation involves a phase change from fcc directly to hcp. Both RESET and SET operations can be completed as fast as 70 ns with large changes in cell resistance.

Evaluation of the ion bombardment energy for growing diamondlike carbon films in an electron cyclotron resonance plasma enhanced chemical vapor deposition

The amorphous hydrogenated carbon (a-C:H) films were deposited on (100) silicon substrates using a divergent Ar+CH4 electron cyclotron resonance plasma. During the deposition sequence, the rf bias was applied to the substrate to increase the ion bombardment energy. The results of the microhardness test and the Raman spectroscopy revealed that these films showed abrupt changes in the mechanical properties and in the bonding characteristics from polymerlike to diamondlike when the rf substrate bias (Vsub) was increased to −40 V. Such changes were attributed to the structural and compositional modifications of the growing film by the bombarding ions. In addition, the results of the plasma diagnostics and calculation showed that the required ion energy for growing a hard diamondlike carbon film was about 65 eV or higher. The bombardment by high energetic ions caused the a-C:H film to have diamondlike properties with a strong three-dimensional carbon network by dislodging hydrogen atoms from CH3 radicals on th...

Effect of the substrate state on the formation of diamond film in a low temperature microwave‐plasma‐enhanced chemical vapor deposition system

Diamond films were deposited on substrates using a low temperature microwave‐plasma‐enhanced chemical vapor deposition system. Low temperature (below 500 °C) deposition can be achieved only by making the substrate position remote from the plasma under the normal deposition condition. The substrate was untreated or was pre‐treated by 30 μm diamond powder. Among various substrates (untreated silicon, pre‐treated silicon, pre‐treated quartz, untreated glass, and pre‐treated glass), pre‐treated quartz and pre‐treated glass enhance the diamond formation, while untreated glass gives only graphite formation even under the same deposition conditions. We understand that the better formation of diamond on pre‐treated glass and pre‐treated quartz was due to the existence of a diamond particle on the substrates. We also deposited diamond films on pre‐treated glass at various CH4 concentrations. The largest grain size can be acquired at 3% of CH4 concentration and the growth rate of the film increases with increasing ...

Growth kinetics of MgB2 layer and interfacial MgO layer during ex situ annealing of amorphous boron film

This paper describes the growth kinetics of an interfacial MgO layer as well as those of an MgB 2 layer during ex situ annealing of the evaporated amorphous boron ( a -B) film under Mg vapor overpressure. A thin MgO layer is formed at the interface between a -B and Al 2 O 3 substrate before the formation of crystalline MgB 2 layer and the interfacial layer is epitaxially related with Al 2 O 3 substrate (MgO (111)[1 1 0] // Al 2 O 3 (0001)[1 1 00]). The interfacial MgO layer continues to grow during the annealing, and its apparent growth rate is about 0.1 nm/min. The analysis of MgB 2 layer growth kinetics using cross-sectional transmission electron microscopy reveals that there exist two distinct growth fronts at both sides of an MgB 2 layer. The growth kinetics of the lower MgB 2 layer obeys the parabolic rate law during the entire annealing time. The growth of the upper MgB 2 layer is controlled by the surface reaction between out-diffused boron and Mg vapor up to 10 min, resulting in a rough surface morphology of MgB 2 layer. By considering the mass balance of Mg and boron during ex situ annealing, we obtained the diffusivities of Mg and boron in MgB 2 layer which were in the same order range of approximately 10 −12 cm 2 /s.

Understanding on the current-induced crystallization process and faster set write operation thereof in non-volatile phase change memory

We experimentally demonstrate that the crystallization process of Ge-Sb-Te crystallites during the set operation in non-volatile phase change memory commences after threshold switching event. It is also shown that the nucleation and growth rates have opposite behaviors with the increase of set operation power: the incubation time in nucleation stage can be minimized at higher power, whereas the percolation time in growth stage is smaller at lower power. Based on these results, we introduce a two-step set pulse of high-power nucleation and low-power growth making the set write operation much faster than conventional simple rectangular or slow-quenched form.

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.

Interface-controlled thermal transport properties in nano-clustered phase change materials

We measured the thermal conductivity of nano-clustered Ge2Sb2Te5(GST)–TiOx films in situ upon annealing from room temperature to 200 °C by the time-domain thermoreflectance method. The nano-clustered structure was found to significantly reduce the thermal conductivity of the crystallized GST–TiOx films. The reduction is attributed to the thermal resistance provided by the TiOx boundaries, of which the impact is identified by estimating the apparent interfacial thermal conductance of the embedded GST/TiOx interfaces. We suggest how to deal with the electronic contribution to thermal transport for this procedure. The apparent interfacial thermal conductance of the embedded GST/TiOx interfaces was found to tune closer to the intrinsic value 30 MW/m2 K as the microstructure of the films evolved into a distinctly clustered structure.