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

Two-terminal ferroelectric synaptic weights are fabricated on silicon. The active layers consist of a 2 nm thick WOx film and a 2.7 nm thick HfZrO4 (HZO) film grown by atomic layer deposition. The ultra-thin HZO layer is crystallized in the ferroelectric phase using a millisecond flash at a temperature of only 500 °C, evidenced by x-rays diffraction and electron microscopy. The current density is increased by four orders of magnitude compared to weights based on a 5 nm thick HZO film. Potentiation and depression (analog resistive switching) is demonstrated using either pulses of constant duration (as short as 20 nanoseconds) and increasing amplitude, or pulses of constant amplitude (+/−1 V) and increasing duration. The cycle-to-cycle variation is below 1%. Temperature dependent electrical characterisation is performed on a series of device cycled up to 108 times: they reveal that HZO possess semiconducting properties. The fatigue leads to a decrease, in the high resistive state only, of the conductivity and of the activation energy.ISSN:2634-438

Ferroelectric memristors for neuromorphic applications: design, fabrication, and integration

An artificial synaptic element consisting of a three terminal Ferroelectric Field-Effect Transistor (FeFET) with an oxide channel is presented in this thesis. Bio-inspired computing emerged as the forefront technology to harness the growing amount of data generated in an increasingly connected society. Dedicated hardware solutions are required to leverage its full potential, especially regarding power consumption and parallelism by co-locating memory and computing. A common denominator among most proposed neuromorphic computing architectures is a neural network consisting of neurons and synapses. In the analog domain, the state of a synapse is emulated by a programmable and persistent electrical conductance, for which multiple physical effects can be exploited. Among them, the ferroelectric effect promises a low power operation and high endurance due to the electrostatic nature of the polarisation switching. In the first part of this thesis, the process development for the materials is reviewed. A Back-End-Of-Line (BEOL) compatible crystallisation of HfZrO4 (HZO), a CMOS friendly and scalable material, in the metastable ferroelectric phase is demonstrated by a millisecond flash lamp anneal. Also, the effect of the electrodes and film thickness is studied. It is found that TiN and WOx electrodes both support the stabilisation of the metastable ferroelectric phase and that the ferroelectricity vanishes for very thin HZO. Furthermore, the development of a semiconducting WOx channel is presented, including the effect of the deposition method and processing conditions on its electrical properties. In the second part of the manuscript, the developed materials are combined in a FeFET device: a simple gate-first device layout is designed and then used to establish a direct link between the ferroelectric polarisation and the channel conductance. The fine-grained domain structure of HZO is used to demonstrate a programmable and persistent multi-state conductance. Moreover, the FeFETs display a good linearity and symmetry, a low cycle-to-cycle noise, fast programming speed, and low write energy. The device area, dynamic range, endurance, and large device-to-device variability call for additional improvement. In the last part, the process is further developed with the objective of decreasing the device area, reducing the device-to-device variability, and increasing the dynamic range and endurance. An important change to allow for such improvements is the growth of WOx by atomic layer deposition instead of sputtering. In addition, a more complex design enables the integration in cross bar arrays. The result is a sub-μm size artificial synaptic element with a quasi-continuous resistance tuning and a fine-grained weight update. Moreover, the change of conductance appears over two timescales. It is found that a fast, saturating ferroelectric effect and a slow, less saturating ionic drift and diffusion process are responsible for the multi time scale behaviour. The FeFET exhibits an excellent endurance and ferroelectric retention thanks to the good interface between the ferroelectric and the oxide channel. Its reduced footprint is an important step towards dense integration. Also, it is found that as a consequence of the two physical effects leading to different timescales, the symmetry and linearity of the device deteriorate. Taking all these characteristics into account, the performance of the FeFET is assessed by simulating the classification of the MNIST dataset, resulting in an excellent accuracy of 88 % accuracy, making it well suited for neuromorphic and cognitive computing

Physical modeling of HZO-based ferroelectric field-effect transistors with a WOx channel

The quasistatic and transient transfer characteristics of Hf0.57Zr0.43O2 (HZO)-based ferroelectric field-effect transistors (FeFETs) with a WOx channel are investigated using a 2-D time-dependent Ginzburg-Landau model as implemented in a state-of-the-art technology computer aided design tool. Starting from an existing FeFET configuration, the influence of different design parameters and geometries is analyzed before providing guidelines for next-generation devices with an increased “high (RH) to low (RL)” resistance ratio, i.e., RH/RL. The suitability of FeFETs as solid-state synapses in memristive crossbar arrays depends on this parameter. Simulations predict that a 13 times larger RH/RL ratio can be achieved in a double-gate FeFET, as compared to a back-gated one with the same channel geometry and ferroelectric layer. The observed improvement can be attributed to the enhanced electrostatic control over the semiconducting channel thanks to the addition of a second gate. A similar effect is obtained by thinning either the HZO dielectric or the WOx channel. These findings could pave the way for FeFETs with enhanced synaptic-like properties that play a key role in future neuromorphic computing applications.ISSN:2673-301

Ferroelectric, Analog Resistive Switching in Back‐End‐of‐Line Compatible TiN/HfZrO4/TiOx Junctions

Due to their compatibility with complementary metal–oxide–semiconductor technologies, hafnium‐based ferroelectric devices receive increasing interest for the fabrication of neuromorphic hardware. Herein, an analog resistive memory device is fabricated with a process developed for back‐end‐of‐line integration. A 4.5 nm‐thick HfZrO4 (HZO) layer is crystallized into the ferroelectric phase, a thickness thin enough to allow electrical conduction through the layer. A TiOx interlayer is used to create an asymmetric junction as required for transferring a polarization state change into a modification of the conductivity. Memristive functionality is obtained, both in the pristine state and after ferroelectric wake‐up, involving redistribution of oxygen vacancies in the ferroelectric layer. The resistive switching is shown to originate directly from the ferroelectric properties of the HZO layer.ISSN:1862-6270ISSN:1862-625

Stabilization of ferroelectric HfxZr1-xO2 films using a millisecond flash lamp annealing technique

We report on the stabilization of ferroelectric HfxZr1-xO2 (HZO) films crystallized using a low thermal budget millisecond flash lamp annealing technique. Utilizing a 120 s 375 degrees C preheat step combined with millisecond flash lamp pulses, ferroelectric characteristics can be obtained which are comparable to that achieved using a 300 s 650 degrees C rapid thermal anneal. X-ray diffraction, capacitance voltage, and polarization hysteresis analysis consistently point to the formation of the ferroelectric phase of HZO. A remanent polarization (P-r) of similar to 21 mu C/cm(2) and a coercive field (E-c) of similar to 1.1 MV/cm are achieved in 10 nm thick HZO layers. Such a technique promises a new alternative solution for low thermal budget formation of ferroelectric HZO films. (C) 2018 Author(s)

Stabilization of ferroelectric HfxZr1−xO2 films using a millisecond flash lamp annealing technique

We report on the stabilization of ferroelectric HfxZr1−xO2 (HZO) films crystallized using a low thermal budget millisecond flash lamp annealing technique. Utilizing a 120 s 375 °C preheat step combined with millisecond flash lamp pulses, ferroelectric characteristics can be obtained which are comparable to that achieved using a 300 s 650 °C rapid thermal anneal. X-ray diffraction, capacitance voltage, and polarization hysteresis analysis consistently point to the formation of the ferroelectric phase of HZO. A remanent polarization (Pr) of ∼21 μC/cm2 and a coercive field (Ec) of ∼1.1 MV/cm are achieved in 10 nm thick HZO layers. Such a technique promises a new alternative solution for low thermal budget formation of ferroelectric HZO films

High-Conductance, Ohmic-like HfZrO4 Ferroelectric Memristor

The persistent and switchable polarization of ferroelectric materials based on HfO2-based ferroelectric compounds, compatible with large-scale integration, are attractive synaptic elements for neuromorphic computing. To achieve a record current density of 0.01 A/cm(2) (at a read voltage of 80 mV) as well as ideal memristive behavior (linear current-voltage relation and analog resistive switching), devices based on an ultra-thin (2.7 nm thick), polycrystalline HfZrO4 ferroelectric layer are fabricated by Atomic Layer Deposition. The use of a semiconducting oxide interlayer (WOx<3) at one of the interfaces, induces an asymmetric energy profile upon ferroelectric polarization reversal and thus the long-term potentiation / depression (conductance increase / decrease) of interest. Moreover, it favors the stable retention of both the low and the high resistive states. Thanks to the low operating voltage (<3.5 V), programming requires less than 10(-12) J for 20 ns long pulses. Remarkably, the memristors show no wake-up or fatigue effect

A multi-timescale synaptic weight based on ferroelectric hafnium zirconium oxide

Brain-inspired computing emerged as a forefront technology to harness the growing amount of data generated in an increasingly connected society. The complex dynamics involving short- and long-term memory are key to the undisputed performance of biological neural networks. Here, we report on sub-µm-sized artificial synaptic weights exploiting a combination of a ferroelectric space charge effect and oxidation state modulation in the oxide channel of a ferroelectric field effect transistor. They lead to a quasi-continuous resistance tuning of the synapse by a factor of 60 and a fine-grained weight update of more than 200 resistance values. We leverage a fast, saturating ferroelectric effect and a slow, ionic drift and diffusion process to engineer a multi-timescale artificial synapse. Our device demonstrates an endurance of more than 10 10 cycles, a ferroelectric retention of more than 10 years, and various types of volatility behavior on distinct timescales, making it well suited for neuromorphic and cognitive computing.ISSN:2662-444