Dae-Hwan Kang

A one-dimensional heat conduction model is developed for a phase change random access memory device with an 8F2 memory cell structure (F=0.15 μm). The required current level for a reset operation, which corresponds to the phase switching from a crystalline (“1” state) to an amorphous phase (“0” state) of Ge2Sb2Te5, was investigated by calculating one-dimensional temperature profiles for the memory cell structure. It is revealed that a reset operation is not achieved at the current level (2 mA) reported for existing devices with a subquarter micron plug size when only TiN is used as a resistive heater. However, it is possible when an additional heating layer of 5 nm thickness is inserted between the TiN and Ge2Sb2Te5 layers, for which the electrical resistivity ρelec is higher than 105 μΩ cm, and the thermal conductivity κ and specific heat c are as low as those of Ge2Sb2Te5. In addition, it is shown that a reset operation at a low current level of 1 mA can be realized in this memory cell when amorphous carbon (κ=0.2 W/m K and ρelec=106 μΩ cm) is used as an additional heating layer. It is believed that this relatively simple one-dimensional heat conduction model is a useful tool for analyzing the device operation of phase change random access memory devices and for selecting the proper conditions for an additional heating layer allowing for low-current operation.

A cross-point-type phase-change random access memory (PRAM) cell without an access transistor is successfully fabricated with the In/sub 2/Se/sub 3/ resistor, which has much higher electrical resistivity than conventionally used Ge/sub 2/Sb/sub 2/Te/sub 5/ and of which electrical resistivity can be varied by the factor of 10/sup 5/ related with the degree of crystallization. Thus, the switching power can be delivered more effectively due to its higher electrical resistivity and device failure related to phase decomposition can be avoided since In/sub 2/Se/sub 3/ is single phase binary compound. The static-mode switching (dc test) is tested for the 5 /spl mu/m In/sub 2/Se/sub 3/ PRAM device. In the first sweep, the as-grown amorphous In/sub 2/Se/sub 3/ resistor showed the high resistance state at low voltage region. However, when it reached the threshold voltage, the electrical resistance of the device was drastically reduced through the formation of an electrically conducting path. The pulsed-mode switching of the 5 /spl mu/m In/sub 2/Se/sub 3/ PRAM device shows that the reset (crystalline /spl rarr/ amorphous) of the device was done with a 70 ns 3.1 V pulse and the set (amorphous /spl rarr/ crystalline) of the device was done with a 10 /spl mu/s 1.2 V pulse. Reading was accomplished by measuring the device resistance at 0.2 V and as high as 100 of switching dynamic range (ratio of R/sub high/ to R/sub low/) was observed.

Ruthenium (Ru) thin films were grown on thermally-grown SiO2 substrates by thermal atomic layer deposition (ALD) using a sequential supply of a zero metal valence precursor, isopropyl-methylbenzene-cyclohexadiene Ru(0) (IMBCHRu, C16H22Ru) and molecular oxygen (O-2) at substrate temperatures ranging from 185 to 310 degrees C. The growth rate at 185 degrees C was approximately 0.059 nm/cycle but its resistivity was > 3000 mu Omega cm. When the deposition was done at 200 degrees C, the resistivity was decreased drastically to similar to 100 mu Omega cm, and the film growth rate increased to 0.075 nm/cycle. An ALD temperature window from 225 to 270 degrees C was observed. A high growth rate of 0.086-0.089 nm/cycle was obtained at this ALD temperature window. The film deposited at 270 degrees C showed a minimum resistivity of similar to 30 mu Omega cm and a high density of 11.7 g/cm(3) and with no impurities in the film, such as oxygen and carbon. At 310 degrees C, the growth rate increased to 0.136 nm/cycle due to a partial decomposition of the precursor. In addition, the film resistivity increased slightly to similar to 40 mu Omega cm with the incorporation of carbon and the formation of a less-dense film. The step coverage of the ALD-Ru film was dependent on the dimensions of the contact and deposition temperature. At the contact with an aspect ratio of similar to 4.6 (top opening diameter: 80 nm), the step coverage was excellent irrespective of the deposition temperature. However, at the contact with an aspect ratio of similar to 25, the step coverage of the film deposited at 310 degrees C (or above ALD temperature window) was degraded, even though those prepared within ALD temperature window were similar to 100%. Finally, ALD-Ru film was used successfully as a seed layer for Cu electroplating. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3575163] All rights reserved.

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 transmission electron microscopy, the oxidized surface was shown to consist of a titanium oxide layer primarily with Ti2O3 crystallites which is presumed to yield enhanced stability of the device by the following two effects. Firstly, Ge, Sb, and Te atoms would have stronger bonds to oxygen atoms than to nitrogen atoms by about 0.5eV, thereby producing more robust interface. Accordingly, the magnitude of contact resistance and its variation are reduced significantly so as to have little influence on the device resistances and their fluctuations. Secondly, thermally and electrically more resistive nature of the oxide layer would tend to yield, by enhanced generation and confinement of Joule heat, more uniform temperature distribution across the phase change material layer, rendering possibly a more homogeneous single phase material hence steadier sheet resistances with programming cycles.

The properties of WNxCy films deposited by atomic layer deposition (ALD) using WF6, NH3, and triethyl boron were characterized as diffusion barriers for copper metallization. The films deposited at 313degreesC showed resistivity of about 350 muOhm cm with a density of 15.4 g/cm(3). The film composition measured by Rutherford backscattering spectrometry showed W, C, and N of approximately 48, 32, and 20 atom %, respectively. Transmission electron microscopy analyses showed that the as-deposited film was composed of a face-centered-cubic phase with a lattice parameter similar to both beta-WC1-x and beta-W2N with an equiaxed microstructure. The film kept its nanocrystalline microstructure until annealing at 700degreesC, although some amount of simple hexagonal alpha-WC was identified to be formed and the beta-W2N phase disappeared. As the annealing temperature increased to 800degreesC, relatively larger grains of body-centered-cubic W were newly formed with smaller grains of hexagonal-close-packed alpha-W2C or alpha-WC. All the phenomena are related to nitrogen release after annealing at 700 and 800degreesC. The results of diffusion barrier performance between Cu and Si analyzed by X-ray diffractometry showed that ALD-WNxCy film (12 nm) failed only after annealing at 700degreesC for 30 min by the formation of copper silicide, while the sputter-deposited Ta (12 nm) and ALD-TiN (20 nm) films failed at 650 and 600degreesC annealing, respectively. It is thought that the superior diffusion barrier performance of ALD-WNxCy film is the consequence of both the formation of equiaxed microstructure and the high-density nature of the film. (C) 2004 The Electrochemical Society.