Approaches for Memristive Structures Using Scratching Probe Nanolithography: Towards Neuromorphic Applications

Abstract

This paper proposes two different approaches to studying resistive switching of oxide thin films using scratching probe nanolithography of atomic force microscopy (AFM). These approaches allow us to assess the effects of memristor size and top-contact thickness on resistive switching. For that purpose, we investigated scratching probe nanolithography regimes using the Taguchi method, which is known as a reliable method for improving the reliability of the result. The AFM parameters, including normal load, scratch distance, probe speed, and probe direction, are optimized on the photoresist thin film by the Taguchi method. As a result, the pinholes with diameter ranged from 25.4 ± 2.2 nm to 85.1 ± 6.3 nm, and the groove array with a depth of 40.5 ± 3.7 nm and a roughness at the bottom of less than a few nanometers was formed. Then, based on the Si/TiN/ZnO/photoresist structures, we fabricated and investigated memristors with different spot sizes and TiN top contact thickness. As a result, the HRS/LRS ratio, U_SET, and I_LRS are well controlled for a memristor size from 27 nm to 83 nm and ranged from ~8 to ~128, from 1.4 ± 0.1 V to 1.8 ± 0.2 V, and from (1.7 ± 0.2) × 10^-10 A to (4.2 ± 0.6) × 10^-9 A, respectively. Furthermore, the HRS/LRS ratio and U_SET are well controlled at a TiN top contact thickness from 8.3 ± 1.1 nm to 32.4 ± 4.2 nm and ranged from ~22 to ~188 and from 1.15 ± 0.05 V to 1.62 ± 0.06 V, respectively. The results can be used in the engineering and manufacturing of memristive structures for neuromorphic applications of brain-inspired artificial intelligence systems.

Keywords: ReRAM; Taguchi method; ZnO thin films; artificial intelligence; memristor; neuromorphic systems; pulsed laser deposition; resistive switching; scratching probe nanolithography.

Figure 1

Process flow diagram of the memristor structures to investigate the effect of memristor size (prototype 1) and top contact thickness (prototype 2) on resistive switching.

Figure 2

Study of scratching probe nanolithography on photoresist: (a)—3D-AFM image and AFM cross-section of the single indented pinhole; (b)—dependences of depth and diameter on normal load; (c)—dependences of depth and diameter on and load time; (d)—CAFM-image of the single indented pinhole and AFM cross-sections at different normal loads.

Figure 3

Study of scratching probe nanolithography on photoresist: (a)—3D-AFM image and AFM cross section of the groove; (b)—dependences of the photoresist puncture depth and structure width on the normal load; (c)—dependences of the photoresist puncture depth and structure width on the scan speed; (d)—3D-AFM image and AFM cross section of a window.

Figure 4

Study of depth and surface roughness using the Taguchi method: (a)—AFM parameters and their levels; (b)—L9 orthogonal array; (c,e)—S/N ratios for depth for each parameter; (d,f)—S/N ratios for Ra for each parameter.

Figure 5

Study of the memristor size (D) on resistive switching in the Si/TiN/ZnO/TiN structure: (a)—microphotography; (b)—current-voltage characteristics; (c)—cumulative probability; (d)—U_SET and I_LRS vs. D; (e)—retention test.

Figure 6

Study of the top contact thickness (H) on resistive switching in the Si/TiN/ZnO/TiN structure: (a)—3D-AFM image of ZnO film and TiN electrodes; (b)—AFM cross-section; (c)–current-voltage characteristics; (d)—cumulative probability; (e)—HRS/LRS ratio and U_SET vs. H; (f)—retention test.