In Situ Transmission Electron Microscopy with Biasing and Fabrication of Asymmetric Crossbars Based on Mixed-Phased a-VOx

Summary

Presented here is a protocol for analyzing nanostructural changes during in situ biasing with transmission electron microscopy (TEM) for a stacked metal-insulator-metal structure. It has significant applications in resistive switching crossbars for the next generation of programmable logic circuits and neuromimicking hardware, to reveal their underlying operation mechanisms and practical applicability.

Abstract

Resistive switching crossbar architecture is highly desired in the field of digital memories due to low cost and high-density benefits. Different materials show variability in resistive switching properties due to the intrinsic nature of the material used, leading to discrepancies in the field because of underlying operation mechanisms. This highlights a need for a reliable technique to understand mechanisms using nanostructural observations. This protocol explains a detailed process and methodology of in situ nanostructural analysis as a result of electrical biasing using transmission electron microscopy (TEM). It provides visual and reliable evidence of underlying nanostructural changes in real time memory operations. Also included is the methodology of fabrication and electrical characterizations for asymmetric crossbar structures incorporating amorphous vanadium oxide. The protocol explained here for vanadium oxide films can be easily extended to any other materials in a metal-dielectric-metal sandwiched structure. Resistive switching crossbars are predicted to serve the programmable logic and neuromorphic circuits for next-generation memory devices, given the understanding of the operation mechanisms. This protocol reveals the switching mechanism in a reliable, timely, and cost-effective way in any type of resistive switching materials, and thereby predicts the device's applicability.

Introduction

Resistance change oxide memories are increasingly used as the building block for novel memory and logic architectures due to their compatible switching speed, smaller cell structure, and the ability to be designed in high capacity three-dimensional (3D) crossbar arrays¹. To date, multiple switching types have been reported for resistive switching devices^2,3. Common switching behaviors for metal oxides are unipolar, bipolar, complementary resistive switching, and volatile threshold switching. Adding on to the complexity, single cell has been reported to show multifunctional resistive swi....

Protocol

1. Fabrication process and electrical characterization

1. Use standard image reversal photolithography⁹ to pattern bottom electrode (BE layer 1) with photoresist of the devices using the following parameters:
   1. Spin coat the photoresist at 3,000 rpm, soft bake it at 90 °C for 60 s, expose with 25 mJ/cm² with a 405 nm laser, bake at 120 °C for 120 s, perform flood exposure with 21 mW/cm² and a 400 nm laser, develop using developer, and rinse with deionized water.
2. Deposit 5 nm titanium (Ti) for adhesion and 15 nm of platinum (Pt....

Results

The results achieved using this protocol for the a-VO_x cross-point devices are explained in Figure 8. Figure 8A shows the TEM micrograph of the intact lamella. Here the diffraction patterns (inset) indicate the amorphous nature of the oxide film. For the in situ TEM measurements, controlled voltages were applied starting from 25 mV to 8 V in 20 mV steps with the bottom electrode (BE) positively biased and top.......

Discussion

This paper explains the protocol for in situ biasing with transmission electron microscopy including the fabrication process for the device, gridbar designing for biasing chip mounting, lamella preparation and mounting on the biasing chip, and TEM with in situ biasing.

The fabrication methodology of cross-point devices, which can be easily scaled up to crossbar structures, is explained. The Ti capping of vanadium oxide is essential to incorporate amorphous vanadium oxide, because it .......

Materials

Name	Company	Catalog Number	Comments
Resist processing system	EV group	EVG 101
Acetone	Chem-Supply	AA008
Biasing Chip - E-chip	Protochips	E-FEF01-A4
Developer	MMRC	AZ 400K
Electron beam evaporator - PVD 75	Kurt J Leskar	PRO Line - eKLipse
Focused Ion beam system	Thermo Fisher - FEI	Scios DualBeamTM system
Hot plates	Brewer Science Inc.	1300X
Magnetron Sputterer	Kurt J Leskar	PRO Line
Mask aligner	Karl Suss	MA6
Maskless Aligner	Heildberg instruments	MLA150
Methanol	Fisher scientific	M/4056
Phototresist	MMRC	AZ 5412E
Pt source for e-beam evaporator	Unicore
The Fusion E-chip holder	Protochips	Fusion 350
Ti source for e-beam evaporator	Unicore
Transmission Electron Microscope	JEOL	JEM 2100F