5.1 Atomic layer deposition

This module was developed as part of the Rice University course CHEM-496: Chemistry of Electronic Materials . This module was prepared with the assistance of Julie A. Francis.

Introduction

The growth of thin films has had dramatic impact on technological progress. Because of the various properties and functions of these films, their applications are limitless especially in microelectronics. These layers can act as superconductors, semiconductors, conductors, insulators, dielectric, or ferroelectrics. In semiconductor devices, these layers can act as active layers and dielectric, conducting, or ion barrier layers. Depending on the type of film material and its applications, various deposition techniques may be employed. For gas-phase deposition, these include vacuum evaporation, reactive sputtering, chemical vapor deposition (CVD), especially metal organic CVD (MOCVD), and molecular beam epitaxy (MBE). Atomic layer deposition (ALD), originally called atomic layer epitaxy (ALE), was first reported by Suntola et al. in 1980 for the growth of zinc sulfide thin films to fabricate electroluminescent flat panel displays.

ALD refers to the method whereby film growth occurs by exposing the substrate to its starting materials alternately. It should be noted that ALE is actually a sub-set of ALD, in which the grown film is epitaxial to the substrate; however, the terms are often used interchangeably. Although both ALD and CVD use chemical (molecular) precursors, the difference between the techniques is that the former uses self limiting chemical reactions to control in a very accurate way the thickness and composition of the film deposited. In this regard ALD can be considered as taking the best of CVD (the use of molecular precursors and atmospheric or low pressure) and MBE (atom-by-atom growth and a high control over film thickness) and combining them in single method. A selection of materials deposited by ALD is given in [link] .

How ald works

The premise behind the ALD process is a simple one. The substrate (amorphous or crystalline) is exposed to the first gaseous precursor molecule (elemental vapor or volatile compound of the element) in excess and the temperature and gas flow is adjusted so that only one monolayer of the reactant is chemisorbed onto the surface ( [link] a). The excess of the reactant, which is in the gas phase or physisorbed on the surface, is then purged out of the chamber with an inert gas pulse before exposing the substrate to the other reactant ( [link] b). The second reactant then chemisorbs and undergoes an exchange reaction with the first reactant on the substrate surface ( [link] c). This results in the formation of a solid molecular film and a gaseous side product that may then be removed with an inert gas pulse ( [link] d).