2 Ω cm, which is close to the result reported by Xu et al. [17]. In addition, TiO2 has a high melting point (approximately 2116 K) and will be thermally stable under high temperature (approximately 900 K) during the reset operation. Generally speaking, with the suitable electrical C646 resistivity, thermal conductivity and thermal stability, a crystalline TiO2 layer should hopefully serve as the bottom heating layer in PCM cells

to improve the thermal efficiency and, therefore, reduce the power requirement during phase transitions. In this study, the atomic layer deposition (ALD) TiO2 was used as a buffer layer which was expected to improve the thermal efficiency and reduce the reset voltage of PCM. Methods The PCM cells in this study are fabricated Nutlin3a using 0.18 μm CMOS technology. Figure 1a shows a cross-section transmission electron microscopy (TEM) image of the LY2835219 price fabricated cell without TiO2 buffer layer. The diameter and height of the columnar W electrode are 260 and 700 nm, respectively. Figure 1b shows a schematic diagram of the cross-section structure of the fabricated cell with TiO2 buffer layer. The thin TiO2 layer was interposed between the phase change layer (PCL) and W plug. A 2-, 4-, and 8-nm thick TiO2 buffer layer was deposited by ALD at 400°C using Beneq TFS 500 ALD system (Beneq, Vantaa, Finland).

One deposition cycle was composed of Ti precursor (TiCl4) pulse (250 ms), 200 sccm N2 purge (2 s), water (H2O) pulse (250 ms), and 200 sccm N2 purge (s2 s). The deposition rate is 0.5 A/cycle. The as-deposited films were crystallized science with rutile structure measured by X-ray diffraction. Then, 100-nm thick AST PCL was deposited by magnetron sputtering. The background pressure and Ar gas pressure were 2.0 × 10-4 and 0.18 Pa, respectively. The stoichiometry of the deposited films was confirmed by electron dispersive spectroscopy.

The Al/Sb/Te ratio was 1:3:1. Then, 20 nm TiN and 200 nm Al were deposited by sputtering as top electrode. For comparison, sputter-deposited AST film without the interposed TiO2 layer was also fabricated with the same structure. The electric property tests of PCM were carried out by a Tektronix AWG5012b arbitrary waveform generator (Tektronix, Inc., Shanghai, China) and a Keithley 2602A parameter analyzer (Keithley Instruments, Inc., OH, USA). Figure 1 Cross-sectional structures of PCM cells. (a) Cross-sectional structure of PCM cell without TiO2 buffer layer and (b) schematic diagram of the cross-section structure of the fabricated cell with TiO2 buffer layer. Results and discussion Figure 2a shows the sheet resistance change of AST films as a function of temperature. The sample with a thickness of 100 nm was prepared on the SiO2/Si(100) by sputtering at room temperature. Upon heating, the sheet resistance of AST films decreased with a rapid drop at the crystallization temperature (T c).