Negative capacitance effect in HfO2-based imprinted antiferroelectric / dielectric bilayer for capacitance boosting applications

Abstract

Using computing-in-memory (CIM) with a synaptic array architecture, brain-inspired neuromorphic systems have been investigated extensively as potential solutions to the von Neumann bottleneck. Charge trap flash (CTF) memory is an attractive option for usage in synaptic devices. Using its stable wide memory window, high on/off ratio, remarkable retention properties, and mature device technology, the utility of CTF memory as a synapse has been demonstrated previously. To accomplish CIM for artificial neural network inference, which entails recurring multiply-accumulate (MAC) operations at the edge, CTF memory must undergo more study concerning device features and array designs. Because the charge tunneling process of CTF memory demands long-duration and high-amplitude voltage pulses, it presently requires a high operating voltage of 20 V and a slow speed of 10-3 s. In addition, incremental step pulse programming (ISPP) is a general programming method that can be used to generate multilevel cell (MLC) CTF memory with tight threshold voltage (VTH) distributions. Idealistically, the ISPP slope, which represents program efficiency, approaches 1 in a linear fashion. Nevertheless, the slope of the ISPP is normally nonlinear and less than 1, which is a fatal characteristic for high-performance MLC programming and reliability. Various high-performance CTF memory techniques with a functional blocking layer (BL) have been developed to address the limitations of CTF memories. BLs with a larger k value are required to improve the device characteristics of MLC CTF memory and to maintain scalability. Lately, both ferroelectric (FE)/dielectric (DE) and antiferroelectric (AFE)/DE BLs based on HfO$_2$ have been presented. The band structure of HfO$_2$-based (A)FE is almost identical to that of HfO$_2$ DE, and the capacitance rises with FE polarization switching. On the other hand, the existence of remanent polarization (Pr) is unfavorable to the reliability and performance of CTF memory. Furthermore, given that the capacitance improvement of the (A)FE/DE BL owing to (A)FE polarization switching is always less than that of the component DE layer, a BL with stronger capacitance enhancement characteristics is needed to further improve memory performance. The introduction of the negative capacitance (NC) effect to CTF memory is an undeveloped category of CTF memory. The NC effect can result in differential amplification of the local potential and an increase in capacitance for the overall system, relative to its component layers. This aspect of the NC effect has the ability to overcome the physical limitations of the performance of electronic devices. The NC state for FE materials corresponds to an energetically unstable condition in comparison to the polar state, according to the Landau-Ginzburg-Devonshire (LGD) model. In light of the fact that FE domain formation and a corresponding multi-domain system are very probable under most cases, sustaining the NC state is exceedingly difficult. It has been suggested that the NC state of the FE layer in the FE/DE bilayer system can be stabilized by suppression of spontaneous polarization in the FE layer, which is related to the electrostatic interaction between the FE and DE layers. Nevertheless, the majority of the NC effect has only been reported in perovskite-structured epitaxial-FE/DE bilayers. These materials are incompatible with CMOS technology, even when the FE/DE bilayer is exploited as a supercapacitor layer. The (A)FE materials must be grown on a polycrystalline or amorphous TiN/Si substrate in order to be CMOS-compatible. To stabilize and utilize the NC effect, we formed a Hf0.25Zr0.75O2 (HZO) film of homogeneously aligned ferroelectric phase, such as a single-domain, with reversible domain switching (reversible single-domain ferroelectric, RSFE) in this study. We gave considerable consideration to the process of domain formation, a basic property of the HfO$_2$ FE material. Without processes or extra layers, CMOS-compatible RSFE-HZO films were grown on TiN/Si substrates by applying forming gas high-pressure post-deposition annealing (FG-HPPDA) to the HZO film. FG-HPPDA produces a homogeneously aligned domain phase and reversible domain switching by means of a strain gradient-induced internal field and chemically-induced surface polarization pinning. In addition, we developed for the first-time negative capacitance-charge trap flash (NC-CTF) memory by integrating an RSFE and DE heterostructure layer in which the NC effect is stabilized as the BL. In the NC-CTF memory, an ISPP slope of 1.1, which is greater than the physical limit of the CTF memory, was obtained with an 8 V memory window even at a high-speed ISPP operation of 100 ns. This is possible because the NC effect in NC-CTF memory may amplify the internal field delivered to the tunneling layer (TL) (voltage amplification). In addition, we present a high-density CIM architecture that is noise-immune and energy-efficient by resolving the limitations of conventional CTF memory-based CIMs and leveraging the benefits of NC-CTF memory. CIM computing arrays based on NC-CTF are implemented by source-follower read, which multiplies neural network inputs and weights (VTH), and charge-sharing, which accumulates multiplied values. Image recognition has been successfully achieved utilizing simulations based on experimentation with the fabrication of actual devices.