Impact of Surface Roughness and Material Properties of Inert Electrodes on the Threshold Voltages and Their Distributions of ReRAM Memory Cells

Abstract

We explored the impact of the surface roughness (SR) and other material properties of metal electrodes on the statistical distributions of the switching threshold voltages, Vform, Vset, and Vreset, of a resistive memory (ReRAM) cell. The surface roughness of Pt, Ru, Co, and Cu in Cu/TaOx/Pt, Cu/TaOx/Ru, and Cu/TaOx/Co devices is extensively characterized and related to switching characteristics of the devices. We find that SF has both impact on the mean and on the standard deviation of the Vform , Vset, and partly also on the Vreset distributions. The surface roughness of free metal surfaces of Pt, Ru, and Co when annealed at 600 and 900°C is found to increase significantly, but this increase is substantially suppressed when passivated by the thin switching layer of the solid electrolyte, such as TaOx or SiO2. The increase of SR of those metals with increasing temperature correlates well with their melting temperature. Overall, we find that the differences in surface roughness along with material properties such as the thermal conductivity and work function explain well the differences between the distributions of the threshold voltages of the ReRAM devices with Pt, Ru, and Co serving as the inert electrode.

Effect of Electrochemically Active Top Electrode Materials on Nanoionic Conductive Bridge Y2O3 Random-Access Memory

“…The work function difference between Ag and ITO is −0.44 eV (4.26–4.7 eV), and that between Cu and ITO is −0.05 eV (4.65–4.7 eV) [ 36 , 41 ]. The built-in field of the device was determined using the following expression [ 40 ]: where E bi represents the built-in field, Δ Φ is the difference between the work function of the TE and BE, q is the charge amount, and t ox is the oxide thickness (55 nm). The E bi obtained for the Ag device was 0.8 × 10 5 V·cm −1 , while that for the Cu device was 0.09 × 10 5 V·cm −1 .…”

Inertness and Other Properties of Thin Ruthenium Electrodes in ReRAM Applications

“…For micro size asperities with c = 16 μm and b = 0.7 μm one obtains an enhancement factor of β = 187. Countless studies have confirmed since then the impact of the electric field enhancement factor on various physical systems [34].…”

“…For a more detailed discussion of how the field enhancement factor correlates with SR, see ref. [34]. The differences in SR explain that in order to reach a critical field required for the formation of a filament a lower voltage is needed in case of a rougher surface of Co than for smoother interfaces of Pt or Ru.…”

“…The impact of SR of metal electrodes on local field enhancement, dielectric breakdown, including ReRAM performance is well known and has recently attracted a lot of interest. A review of the impact of surface roughness on numerous devices and phenomena, including gas breakdown, dielectric breakdown in RF MEMS devices, in triboluminescence, MIM structures, capacitors, and ReRAM memory cells can be found in [34]. As early as 1928, Eyring et al [39] found that logarithms of currents of extracted electrons from asperities display linear relation with the reciprocal of electric field strength at the tip of an asperity.…”

“…denotes a free metal surface annealed at different temperatures. SiO 2 denotes the cases in which the metal was covered by a 30 nm SiO 2 during the anneal, and the SiO 2 was removed by a soft oxide etch after the anneal[34] © The Electrochemical Society reproduced by permission of IOP Publishing. Chakraborty A, Al-Mamun M, Orlowski M. Impact of surface roughness and material properties of inert electrodes on the threshold voltages and their distributions of ReRAM memory cells" ECS J. Sol.…”