SK Hynix & SNU demonstrate 28nm RRAM cell with ultra thin ALD Ta/Ta2O5 stack

Here is a impressive report by SK Hynix & Prof. Hwang and co-workers SNU on a RRAM device with Ta/Ta2O5 stacked RS layers with ultra-thin Ta2O5 thicknesses (0.5–2.0 nm) deposited by ALD. Woah that´s thin  - like the same order of thickness like native oxide, which makes me wonder if teh extra couple of ALD cycles was needed --> I need to study the paper more carefully!

Thickness effect of ultra-thin Ta₂O₅ resistance switching layer in 28 nm-diameter memory cell 

C.S. Hwang et al

Scientific Reports 5, Article number: 15965 (2015)
doi:10.1038/srep15965

(a) Schematic diagram and (b) TEM image of the TiN/Ta₂O₅/Ta/TaN device. Ta₂O₅ (0.5 nm) device, and (d) Ta₂O₅ (1.5 nm) device. Insets show the linear I-V plot. Scanning transmission electron microscopy (STEM) high angle annular dark field (HAADF) images of (c) 0.5 nm-thick device and (d) 2.0nm-thick device. 

Resistance switching (RS) devices with ultra-thin Ta₂O₅ switching layer (0.5–2.0 nm) with a cell diameter of 28 nm were fabricated. The performance of the devices was tested by voltage-driven current—voltage (I-V) sweep and closed-loop pulse switching (CLPS) tests. A Ta layer was placed beneath the Ta₂O₅ switching layer to act as an oxygen vacancy reservoir. The device with the smallest Ta₂O₅ thickness (0.5 nm) showed normal switching properties with gradual change in resistance in I-V sweep or CLPS and high reliability. By contrast, other devices with higher Ta₂O₅ thickness (1.0–2.0 nm) showed abrupt switching with several abnormal behaviours, degraded resistance distribution, especially in high resistance state, and much lower reliability performance. A single conical or hour-glass shaped double conical conducting filament shape was conceived to explain these behavioural differences that depended on the Ta₂O₅ switching layer thickness. Loss of oxygen via lateral diffusion to the encapsulating Si₃N₄/SiO₂ layer was suggested as the main degradation mechanism for reliability, and a method to improve reliability was also proposed.