Dirk Wouters

10×10nm 2 Hf/HfO x crossbar resistive RAM with excellent performance, reliability and low-energy operation

We report on worlds smallest HfO2-based Resistive RAM (RRAM) cell to date, featuring a novel Hf/HfOx resistive element stack, with an area of less than 10×10 nm2, fast ns-range on/off switching times at low-voltages and with a switching energy per bit of <0.1pJ. With excellent endurance of more than 5.107cycles, large on/off verified-window (>50), no closure of the on/off window after 30hrs/200C and failure-free device operation after 30hrs/250C thermal stress, the major device-level nonvolatile memory requirements are met. Furthermore, we clarify the impact of film crystallinity on cell operation from a scalability viewpoint, the role of the cap layer and bring insight into the switching mechanisms.

Evidences of oxygen-mediated resistive-switching mechanism in TiN\HfO2\Pt cells

In this letter, we study the influence of the Pt top-electrode thickness and of the chamber atmosphere during cell operation on the resistive switching of TiN\HfO2\Pt cells. The oxygen permeability of the Pt electrode directly in contact with the atmosphere significantly affects the resistive switching and the resistance states of the cell. The results provide strong experimental indications that the electroforming operation leads to oxygen-vacancy formation and that the subsequent reset operation relies on the available oxygen species in the filament neighborhood. Significant implications with respect to endurance and retention assessment of resistive-switching memory devices are discussed.

Coexistence of the bipolar and unipolar resistive-switching modes in NiO cells made by thermal oxidation of Ni layers

In this paper, we show the coexistence of the bipolar and unipolar resistive-switching modes in NiO cells realized using an optimized oxidation process of a Ni blanket layer used as the bottom electrode. The two switching modes can be activated independent of the cell switching history provided the appropriate programming conditions are applied. The bipolar and unipolar switching modes are discussed as driven by electrochemical- and thermal-based mechanisms, respectively. The switching versatility between these two modes is demonstrated both for large oxidized Ni films and for Ni films oxidized at the bottom of small dimension contact holes. The perspective of selecting the desired switching mode in a scaled device made in a small diameter single hole is highly attractive because the specific advantages of the two modes broaden the application scope of the cell and enable larger flexibility in terms of memory architecture.

Subthreshold slope in thin-film SOI MOSFETs

The subthreshold conduction regime in thick- and thin-film SOI MOSFETs is studied. Using the depletion approximation, a one-dimensional analytical expression for the subthreshold slope is derived, and equivalence with a simple capacitive network is proven. The model accounts for the influence of the back interface properties on the subthreshold swing in the thin-film regime. The coupling between front and back surface potential and the influence of the backside conduction on the front interface characteristics are accounted for. The case of double gate control is studied in more detail. Experimental verification of the model with measured subthreshold slopes in thin-form MOSFET devices is given. >

Endurance/Retention Trade-off on

The endurance/retention performance of HfO2/ Metal cap RRAM devices in a 1T1R configuration shows metal cap dependence. For Hf and Ti caps, owning strong thermodynamic ability of extracting oxygen from HfO2, long pulse endurance (>1010 cycles) could be achieved. For Ta cap, owning lower thermodynamic ability of extracting oxygen from HfO2, better retention can be achieved. Therefore, an endurance/retention performance tradeoff is identified on the 40 nm × 40 nm HfO2/Metal cap bipolar RRAM devices. The tradeoff of endurance/retention performance can be explained by a different filament constriction shape depending on metal cap layer as derived from fitting I-V curves in the quantum point contact model. This difference in filament constriction shape is attributed to the thermodynamics difference of metal cap: Hf and Ti have a stronger thermodynamical ability to extract oxygen from HfO2 than Ta. The possibility of tuning the intrinsic reliability performance by changing the cap materials paves a way for optimizing the operation of RRAM devices into the desired specifics.

\hbox{HfO}_{2}/\hbox{Metal}

It is shown that the charge-pumping technique can be successfully applied to SOI structures, directly providing important and reliable information about the quality of both front- and back-gate interfaces. The possibility of performing measurements on a transistor level makes direct correlation with other MOS characteristics and material parameters possible. In particular, the ability of this technique to perform measurements on thin-film transistors and to separate the information from front and back gates makes it indispensable for characterization of advanced SOI CMOS structures. Although the technique was demonstrated only on 5- mu m channel length devices, it has sufficient sensitivity to be applicable to transistors of micrometer and submicrometer dimensions. Charge-pumping measurements on laser-recrystallized SOI MOSFETs showed that the front interface is only slightly deteriorated compared to that of bulk MOSFET devices, while the back interface is of a substantially lower quality, with about 10 times higher interface trap densities. >

Cap 1T1R Bipolar RRAM

By tuning the SET/RESET pulse amplitude conditions, the pulse endurance of our 40-nm HfO2/Hf 1T1R resistive-random-access-memory devices demonstrates varying failure behaviors after 106 cycles. For unbalanced SET/RESET pulse amplitude conditions, both low-resistance state (LRS) and high-resistance state (HRS) failures may occur, while varying the pulsewidths influences the LRS/HRS window and the stability of the LRS/HRS states. The failure of the HRS or LRS state during cycling is ascribed to the depletion or excess of oxygen vacancies at the switching interface. Through a dc SET/RESET recovery operation, LRS/HRS states can be recovered after failure, indicating that the distribution of oxygen vacancies can be restored. By optimally balancing the SET/RESET pulse conditions, more than 1010 pulse endurance cycles is achieved.

Characterization of front and back Si-SiO/sub 2/ interfaces in thick- and thin-film silicon-on-insulator MOS structures by the charge-pumping technique

This article reports the bipolar and unipolar resistive-switching modes coexisting in TiN\HfO 2 \Pt systems. The unipolar switching characteristics exhibit progressive forming as well as gradual reset traces, while the set switching trace is abrupt. Based on our results, we propose a simple model based on the generation/recovery of oxygen-vacancy defects at the Pt interface during switching, in accordance with the gradual character of the resistance change. We show that the programming of the intermediate resistance levels, corresponding to only a partial recovery of the vacancy defects, leads to weaker retention properties.

Balancing SET/RESET Pulse for

In this brief, we integrate oxygen-deficient NiO cells in 80-nm-wide contact holes using complementary metal-oxide-semiconductor-compatible Ni electrodes. Ni/NiO/Ni memory-cell arrays are forming free, and can be operated using very low reset current (< 50 muA) and switching voltage (< 1 V). In contrast to metallic-type filaments formed at high-power switching, low-power switching involves high-resistance semiconducting filaments, probably consisting of oxygen-vacancy-rich paths. Retention tests carried out at 150degC indicated excellent stability of both the high- and low-power set states. Drastic reduction of reset current is also demonstrated for single-contact cells with TiN top electrodes.

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Voltage shift and deformation in the hysteresis loop of Pb(Zr,Ti)O3 (PZT) capacitors have been studied by varying the annealing temperature after patterning the top sputter‐deposited Pt electrode using reactive ion etch (RIE) with Ar gas. It was observed that the hysteresis loop of the film was seriously deformed by both sputtering and RIE induced defects. Voltage shift and polarization suppression can be explained by the charge trapping at electrode interfaces and at defect levels in the film, respectively. Space charges trapped at defect levels in the film suppress polarization parallel to poling direction, however, enhance polarization opposite to the poling direction.