S.M. Bishop

Ion implantation synthesized copper oxide-based resistive memory devices

Copper oxide resistive memory layers have been synthesized by ion implantation. Devices fabricated from off-stoichiometric Cu2O exhibited unipolar switching in forward/reverse bias without a forming voltage. The on-state conduction of these devices is likely dominated by a metallic filament, which ruptures via Joule heating to transition the device to the high resistance off-state. Technology scaling was achieved by oxygen implanting copper filled vias. The resulting via-based memory devices exhibited unipolar resistive switching down to 48 nm in diameter.

Influence of the SET current on the resistive switching properties of tantalum oxide created by oxygen implantation

Resistive memory devices fabricated from oxygen implanted tantalum exhibited bipolar switching without a forming voltage. The influence of the current limit during SET on the switching properties has been studied using endurance measurements. The SET/RESET voltages did not change with the current limit. The RESET current increased proportionally to the SET current, while the on-state resistance varied inversely to the SET current. These results are consistent with a RESET process that is directly linked to the peak power during SET. The trade-off between the switching endurance and memory window that results from the SET process is also shown.

Effect of crystallinity on endurance and switching behavior of HfO x -based resistive memory devices

This paper compares the resistive switching properties of crystalline and amorphous HfOx thin-film resistive memory devices (RMDs), which were fabricated by physical vapor deposition films using two different O2 partial pressures. The crystallinity of the two HfOx samples was verified by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Ni/HfOx/Cu devices fabricated from both 50 nm thick amorphous and crystalline HfOx films exhibited consistent bipolar switching. Average electroforming voltage for the crystalline and amorphous weare <;20 V and <;11 V, respectively. Both devices showed similar average set (Vset) and reset (Vreset) voltages of -2.25 V and 0.35 V, respectively, independent of electrode size and current compliance. Preliminary endurance data shows that the amorphous device shows the better endurance (14,300 cycles) compared to that of the crystalline device (102,000 cycles), which is at about an order of magnitude higher than the endurance of the crystalline device. Switching uniformity for both devices showeds similar trends with dispersions (standard deviation/mean ratio) of about 30% for Vset and Vreset.

Ion implantation synthesis and conduction of tantalum oxide resistive memory layers

In this paper, the ion implantation synthesis of tantalum oxide resistive memory material is introduced, the salient switching properties are described, and the results from an analysis of the off- and on-state conduction are presented. The tantalum oxide layers were synthesized by oxygen ion implanting (5 × 1016/cm2 O+ ions at 30 keV) tantalum metal. From composition-depth profiling, the oxygen implant profile is estimated to peak at ∼20 at. %. The properties of memory devices fabricated from the implantation-synthesized oxide were investigated through endurance testing. A stable 2× memory window was obtained for >103 switching cycles with low SET and RESET voltages 2 V, Frenkel-Poole emission current was identified; thus, trap states that may be induced by the implantation process impact high voltage conduction in this memory state.In this paper, the ion implantation synthesis of tantalum oxide resistive memory material is introduced, the salient switching properties are described, and the results from an analysis of the off- and on-state conduction are presented. The tantalum oxide layers were synthesized by oxygen ion implanting (5 × 1016/cm2 O+ ions at 30 keV) tantalum metal. From composition-depth profiling, the oxygen implant profile is estimated to peak at ∼20 at. %. The properties of memory devices fabricated from the implantation-synthesized oxide were investigated through endurance testing. A stable 2× memory window was obtained for >103 switching cycles with low SET and RESET voltages <|1|. Both the on- and off-state resistance decreased inversely with the current used during programming. Analyses of the current–voltage data show that the platinum-tantalum oxide Schottky barrier factors largely into the resistance difference between memory states. Lastly, defect-related conduction dominated the current of the off-state res...

Analysis of nonpolar resistive switching exhibited by Al/Cu x O/Cu memristive devices created via room temperature plasma oxidation

Resistive memory devices based on plasma oxidized copper have been shown previously; however, the influence of the reactive ion etch (RIE) power on device operating parameters has not been established. To investigate this relationship, CuxO was produced on blanket ECD copper substrates by room temperature plasma oxidation under varying RIE powers levels. The RIE power was varied from 100–300 W, yielding copper oxide thicknesses ranging from 40–625 nm, as determined by SIMS analysis. This analysis also indicated that Cu:O atomic ratios of the resulting films increased from 1:1–3:2 with increasing RIE power. Following copper oxide formation, 400 nm thick aluminum top contacts (100 um diameter) were patterned with a shadow mask, resulting in Al/CuxO/Cu resistive memory devices. Devices from each sample exhibited both stable bipolar and unipolar switching behaviors over repeated set/reset measurements; however, device stability decreased with oxide thickness such that the RIE 100 W sample (∼40nm oxide) possessed the best switching characteristics. This nonpolar behavior was operable with set and reset voltages of ± 2–3 V and ± 0.5V, respectively, with an average maximum reset current less than 8 mA (Fig. 1A & 1B). ROFF/RON ratios of up to 4,000 were observed.

Experimentally demonstrated filament-based switching mechanism for Al/Cu x O/Cu memristive devices

Al/CuxO/Cu memristive devices created via a plasma oxidation step have previously demonstrated complete nonpolar switching behavior [1]. An additional material contamination control measure resulted in improved uniformity of I-V curve behavior but necessitated an initial forming step in most devices. The operation voltages were irrespective of switching style and top electrode (TE) size. The high resistance state resistance increased with decreasing TE size; the low resistance state resistance remained invariant. Lateral switching of memristive device pairs unambiguously indicated filament-based device switching. A voltage-driven CuxO filament composition modulation switching mechanism is suggested instead of the popular Joule heating RESET mechanism.

Properties and challenges of scaled resistive memory

Transition metal oxide resistive memory devices (RMDs) are a promising replacement for transistor-based non-volatile memory. Because of their vertical metal-insulator-metal design, resistive memory devices have the potential for smaller footprints and higher densities than their transistor counterparts. To fully realize the spatial and performance advantages of these devices, new integration pathways must be developed that are compatible with state-of-the-art CMOS (complementary metal-oxide semiconductor) processing. Because there is a significant lack of information available in the open literature on the fabrication of nanoscale resistive memory devices, the objective of this work was to explore multiple process routes for fabricating these devices in a via-based platform that is transferable to current CMOS technology nodes.