Article Cite This: Chem. Mater. 2017, 29, 8690-8703
pubs.acs.org/cm
Reaction and Growth Mechanisms in Al2O3 deposited via Atomic Layer Deposition: Elucidating the Hydrogen Source Carlos Guerra-Nuñez,*,† Max Döbeli,‡ Johann Michler,† and Ivo Utke*,† †
Laboratory for Mechanics of Materials and Nanostructure, EMPA, Swiss Federal Laboratories for Materials Science and Technology, Feuerwerkerstrasse 39, CH-3602 Thun, Switzerland ‡ Ion Beam Physics, ETH Zürich, Otto-Stern-Weg 5, CH-8093 Zürich, Switzerland S Supporting Information *
ABSTRACT: In this work, we have quantitatively elucidated the source of the hydrogen content in the atomic layer deposition of Al2O3 at different temperatures (80−220 °C), by replacing the H2O precursor with heavy water (D2O) to use as a tracer and discern between the H coming from the unreacted metal precursor ligands and that from the unreacted −OD (hydroxyl) groups coming from the (heavy) water. The main source of impurities arises from the unreacted hydroxyl groups (−OD), reaching ∼18 atom % of deuterium at a deposition temperature of 80 °C. Reconsidering carefully our own and literature experimental data, we concluded that the generally accepted mechanism of steric hindering by monodentate Al(CH3)2 adsorbates (dimethylaluminum) cannot be solely responsible for the retention of hydroxyls during atomic layer deposition (ALD). On this regard, we propose two additional mechanisms that can lead to sterically hinder hydroxyl groups which will then remain unreacted in the film: surface rehydroxylation resulting in the reconfiguration of bidentate or tridentate adsorbates into monodentate adsorbates and hindered subsurface hydroxyl groups during the (heavy) water pulse and the hydroxylation of sterically hindered dissociated methyl chemisorbed species. Based on these three steric hindrance mechanisms, we constructed a growth model that consists of the initial chemisorption configurations of trimethyl-aluminum molecules with the alumina surface and the subsequent reconfiguration of the resulting adsorbates into a monodentate configuration that consequently leads to sterically hindered hydroxyl groups. The fraction of AlOx adsorbates arranged in monodentate and bidentate configurations entails a specific number of O/Al atoms and unreacted hydroxyl groups inside the film. This model was able to explain the deuterium content, the O/Al ratio, and the density obtained from Rutherford back-scattering and heavy ion elastic recoil detection analysis measurements. Furthermore, this model was able to predict more accurately the growth per cycle to what has been reported to be the ALD window of alumina. Our findings will spur further detailed investigations of the reaction and growth modes in ALD films.
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have been broadly researched.3 However, the deposition of Al2O3 is known to contain significant amounts of hydrogen remaining in the bulk, and the exact origin of it remains unclear. These hydrogen residuals may arise either from the unreacted metalorganic precursor ligands (i.e., AlCH3)3) or unreacted −OH groups which remained in the film during growth. It is wellknown that ALD Al2O3 films are almost carbon free, opening the question whether the hydrogen is in the form of unreacted hydroxyl groups and how they remain unreacted. Quantifying these impurities and their origins can help us to understand the strengths and limitations of the ALD processes and shed light into the reaction and growth mechanisms of the ALD films. Previous studies have shown that the total hydrogen
INTRODUCTION Atomic layer deposition (ALD) is now a standard deposition technique of thin films of various materials and an essential tool for a wide range of applications in nanotechnology.1,2 The selflimiting behavior of the precursors involved is the key for the subnanometer control of the thickness and conformality of the films. Each precursor is sequentially dosed into the chamber, in which the gas precursor molecules react with the active surface, resulting in self-saturating chemisorbed gas molecules or adsorbates. These adsorbates form a new reactive surface for the subsequent gas precursor to react with, thus completing one ALD cycle. Most of the physicochemical properties of the deposited film depend on these two processes, reaction and growth. Therefore, in order to develop further this technology it is important to understand these processes. On this regard, aluminum oxide may be the most extensively studied film deposited by ALD, and the reaction and growth mechanisms © 2017 American Chemical Society
Received: July 3, 2017 Revised: September 18, 2017 Published: September 18, 2017 8690
DOI: 10.1021/acs.chemmater.7b02759 Chem. Mater. 2017, 29, 8690−8703
Article
Chemistry of Materials
a range foil and time-of-flight heavy ion ERDA (13 MeV 127I) were used to quantify the concentration depth profiles of H and D. The detection limits of heavy ion ERDA are
