Analog Resistive Switching in BEOL, Ferroelectric Synaptic Weights

Abstract

A Ferroelectric, two-terminals, analog memristive device is fabricated with a Back-End-Of-Line, CMOS compatible process. A bilayer composed of a ferroelectric material, HfZrO4 (HZO) and a semiconducting oxide, WOx layer is comprised between two TiN electrodes. The devices demonstrate reversible and remanent resistive switching, with a record endurance (>10 10 switching cycles) and ON/OFF ratio up to 10. The analog resistive switching is obtained, with a cycle-to-cycle reproducibility of 90%. The synaptic behavior is explored with pulses of varying sign, length, or amplitude. For the scheme with constant pulse width the linearity coefficients for the potentiation and depression are -1.4 and 4. The dynamics of the resistive switching is shown to be governed by the ferroelectric domains switching. Finally, temperature dependent transport measurements indicate Ohmic conduction at low bias and Modified Schottky Emission at larger bias. They provide insights on the role of defects and oxygen vacancies in the transport and resistive switching mechanisms.

Effect of cycling on ultra-thin HfZrO4, ferroelectric synaptic weights

“…when carriers are not accumulated, but depleted in the later. This was already observed for WO x (3 nm)/HZO (5 nm) devices fabricated in similar conditions [18], where we showed that the resistive switching occurred within the HZO layer, the WO x layer being considered as a metal. In this work, the electric field is assumed to drop mainly across the ferroelectric layer (i.e., the thickness t is assumed to be the thickness of HZO, 2.7 nm).…”

“…The as-grown HZO layer was amorphous [21]. Although 400 • C was sufficient to crystallize 5 nm thick films [18], in this work films flash-annealed at 450 • C did not show any sign of crystallization. The increased temperature required as the thickness decreases was observed by Tahara et al [17] on TiN/HZO/TiN stacks.…”

“…In order to provide HZO with asymmetric interfaces while ensuring a maximal current density, the combination of an ultra-thin metal oxide interlayer and a metallic bottom electrode was preferred to a thick metal oxide electrode. TiO x was first studied as a metal oxide interlayer: for 5 nm thick HZO synaptic weights, replacing TiO x [19] by WO x [18] resulted in a higher On/Off ratio and suppressed the need for a wake-up sequence, indicating that the HZO/metal oxide interface is active on the device characteristics such as On/Off ratio, rather than the bulk of the metal oxide. The crystallization was performed using the millisecond flash lamp annealing technique [21]: the sample was preheated to 500 • C, then a 20 ms long energy pulse of 90 J cm −2 was applied.…”

“…At the cost of a reduced On/Off ratio, the 2.7 nm thick HZO layer enables the devices to operate at low bias (<100 mV), in a linear regime, with a large current density of 0.01 A cm −2 at 100 mV. Such a device operating in the quasiohmic regime was already demonstrated using 5 nm thick HZO films with WO x [18] or TiO x [19] interlayers: they have the advantage of back-end-of-line compatibility, but the current density was only 1 μA cm −2 at a read voltage of 100 mV. This work further establishes the depression/potentiation capability of ferroelectric, two-terminals synaptic weights.…”

Scaled, Ferroelectric Memristive Synapse for Back‐End‐of‐Line Integration with Neuromorphic Hardware

“…The role of the WO x interlayer, as discussed in ref. [33], is to induce an asymmetry in the energy profile of the memristor, leading to a resistive switching effect driven by the ferroelectric domain switching. TiN was chosen as a capping layer to promote the crystallization of HZO in the ferroelectric phase by a mechanical constraint during the annealing.…”