Recent Progress of Atomic Layer Technology in Spintronics: Mechanism, Materials and Prospects

Abstract

The atomic layer technique is generating a lot of excitement and study due to its profound physics and enormous potential in device fabrication. This article reviews current developments in atomic layer technology for spintronics, including atomic layer deposition (ALD) and atomic layer etching (ALE). To begin, we introduce the main atomic layer deposition techniques. Then, in a brief review, we discuss ALE technology for insulators, semiconductors, metals, and newly created two-dimensional van der Waals materials. Additionally, we compare the critical factors learned from ALD to constructing ALE technology. Finally, we discuss the future prospects and challenges of atomic layer technology in the field of spinronics.

Keywords: atomic layer deposition; atomic layer etching; atomic layer technology; atomic scale; reaction mechanism; self-limiting; spintronics.

Figure 1

TEM images of a single-crystal MTJ with the Fe (001)/ MgO (001) (1.8 nm)/Fe (001) structure fabricated by MBE. Reprint with permission form Ref. [5]. Copyright 2004 Springer Nature.

Figure 2

TEM images of a single-crystal MTJ with a highly oriented (100) MgO tunnel barrier fabricated by MS. High-resolution images along the [110] zone axes showing atomically resolved lattice planes with (100) planes perpendicular to the growth direction. Reprint with permission from Ref. [39]. Copyright 2004 Springer Nature.

Figure 3

Schematic illustrations of one typical ALD cycle. First, the precursor A is introduced to the chamber to be chemically adsorbed on the substrate, and then the by-products and excess precursors are removed by the inert gas. Last, the precursor B is introduced to react with the adsorbed molecules to form the monolayer of the desired material before cleaning again by the inert gas.

Figure 4

Schematic illustrations of the ozone-based atomic layer deposition process on graphene-coated Ni electrodes. Ni electrodes exposed to air. Through the ALD cycle, by pulsing of ozone and trimethylaluminum (TMA), the electron-transparent ${\mathrm{Al}}_2{\mathrm{O}}_3$ tunnel barrier is finally formed. Reprint from Ref. [83].

Figure 5

Timeline for the development in the use of ALD for MTJ [83,92,93,94,95,96,98,100].

Figure 6

Illustrations of a typical cycle of plasma ALE in the form of a diagram. In the 1st reaction, the substrate is affected by the modified gas and subsequently cleaned by the inert gas. The high-energy particles are utilized in the 2nd reaction to etch the modification, and the cycle is completed following the inert gas cleaning steps.

Figure 7

Illustrations of a typical thermal ALE cycle in the form of a schematic diagram. In the 1st reaction, the substrate is affected by the modified gas and subsequently cleaned by the inert gas. The 2nd reaction employs an active reactive gas to react with the modification, after which the volatile products are released, and the cycle is completed after the inert gas cleaning steps.

Figure 8

A schematic of the oxidation–conversion–fluorination method for W and ${\mathrm{WO}}_3$. First, the W is oxidated by the ${\mathrm{O}}_2$/${\mathrm{O}}_3$. Second, the ${\mathrm{WO}}_3$ is conversed to ${\mathrm{B}}_2{\mathrm{O}}_3$ by ${\mathrm{BCl}}_3$ to form ${\mathrm{WO}}_{\mathrm{x}}{\mathrm{Cl}}_{\mathrm{y}}$ products. Last, the ${\mathrm{B}}_2{\mathrm{O}}_3$ is fluorinated by HF to form the volatile ${\mathrm{BF}}_3$ and ${\mathrm{H}}_2\mathrm{O}$.

Figure 9

A schematic of etching reaction of TiN. First, the TiN is oxidated by the ${\mathrm{O}}_3$ to the ${\mathrm{TiO}}_2$. Last, the ${\mathrm{TiO}}_2$ is fluorinated by HF to form the volatile ${\mathrm{TiF}}_4$ and ${\mathrm{H}}_2\mathrm{O}$.

Figure 10

A schematic of the etching reaction of Si. First, the Si is modified by ${\mathrm{Cl}}_2$ to form the ${\mathrm{SiCl}}_{\mathrm{x}}$. Last, the ${\mathrm{SiCl}}_{\mathrm{x}}$ surface is removed by the high-energy Ar ion.

Figure 11

Schematic illustrations of one typical Fe ALD cycle. In the first step, Fe is chlorinated with chlorine to form ${\mathrm{FeCl}}_{\mathrm{x}}$. In the second step, the ${\mathrm{FeCl}}_{\mathrm{x}}$ is reacted with acetylacetone (acac) to form volatile metal complexes. The gases A and B are a halogen precursor ${\mathrm{Cl}}_2$ and an organic precursor acetylacetone (acac). Reprint with permission from Ref. [146]. Copyright 2018 American Vacuum Society.

Figure 12

The proposed general approach and viable reactions during the CoFeB alloy atomic layer etching cycle using sequential exposure of chlorine and acacH. Reprint with permission from Ref. [164]. Copyright 2022 Elsevier.

Figure 13

A schematic of spatial ALD. Differently from temporal ALD, the substrate can swing between different areas.