Investigation of the Effect of Mechanical Pressure on Memristor Device Characteristics

Abstract

In recent years, neuromorphic computing has been introduced as a new computing paradigm to continue the exponential acceleration of computing power observed over more than half a century. The impending End of Moore’s Law and the von Neumann bottleneck seen in the implementation of modern computers have motivated the development of neural networks, which represent one of the numerous approaches to implement neuromorphic computers. The research community is directing its focus on the benefits brought by dedicated hardware integrations of neural networks over pure software solutions, which cannot be optimally run on a classical computing hardware. The realisation of neuromorphic hardware asks for the development of artificial synapses, to which memristors are one of the possible answers. In this PhD thesis, the effect of mechanical pressure as an external stimuli on memristors is investigated. For a long time, it has been wellknown that a mechanical force can influence the physical properties of materials. The benefits from the change of the electrical characteristics is of special interest, as shown by the recent development of strained-Si channel transistors in which the increase of carrier mobility due to strain in the material lead to a performance enhancement. The objective of this work is to discover if similar improvements are possible in memristor devices. Furthermore, the investigation focuses on the possibility to use mechanical force as a way to fine-tune the resistive state of memristors, a concept which allows to introduce an additional input to the devices. In this work, three technologies are developed and evaluated in order to find the most suitable candidate for the study of the effect of pressure. Filamentary devices HfO2/Ti Resistive Random Access Memory (RRAM), TaOx/HfO2 bilayer RRAM, and Hafnium Zirconium Oxide (HZO) Ferroelectric Random Access Memory (FeRAM) are fabricated and measured with a characterisation setup created for this purpose. A new probestation is built from scratch and automated measurements are performed using a custom control software. The resulting data are then analysed using a newly written program to run statistical analysis on the memristor measurement data. To further understand the underlying switching mechanisms of these memristors, an impedance spectroscopy analysis method is refined and used on the bilayer RRAM and FeRAM technologies. After setting up and validating the method, a novel fitting procedure is utilised to model the behaviour of the devices. Following the Direct Current (DC) characterisation and impedance measurements, the ferroelectric memristor technology is chosen as the main candidate for the pressure experiments because amongst the three technologies considered it exhibits the highest reproducibility of memristive states in operation. The development of an indenter tool is presented and used to precisely apply forces up 2.75 N on integrated devices with a flat surface sapphire tip of ∼75 μm diameter. A repeatable, consistent, and reversible effect due to pressure is observed and quantified. An increase up to 25% of the resistance of the FeRAM is shown in this work for a pressure of 0.54 GPa. The role of the piezoresistivity of the layer stack and of the pressure-induced ferroelectric domain switching are presented as the source of the resistance change. As another means of applying stress to a device, the co-integration of a memristor and of a nano-actuator is targeted. Out of the many developed processing techniques, two main process modules are presented: the vertical sidewall Ion Beam Etching (IBE) etch of Lead Zirconate Titanate (PZT) and the advanced planarisation method of complex device topographies. Building on top of this, an integrated nano-actuator platform is developed and fabricated devices are characterised using a optimised Double Beam Laser Interferometer (DBLI) system with a 2 μm diameter measurement beam. An unparalleled piezoelectric response of ∼200 pm/V is measured for Chemical Solution Deposition (CSD) PZT on platinised Si actuators. The role of layer unclamping is investigated as the reason for the large piezoelectric response. As an outlook, the results of the development and fabrication of the fully co-integrated device are presented. Further future improvements to the different topics of this thesis are suggested as well.