Mechanical and optical properties of as-grown and thermally annealed titanium dioxide from titanium tetrachloride and water by atomic layer deposition

Oili M.E Ylivaara,Andreas Langner,Xuwen Liu,Dieter Schneider,Jaakko Julin,Kai Arstila,Sakari Sintonen,Saima Ali,Harri Lipsanen,Timo Sajavaara,Simo-Pekka Hannula,Riikka L Puurunen

doi:10.1016/j.tsf.2021.138758

Abstract

The use of thin-films made by atomic layer deposition (ALD) is increasing in the field of optical sensing. ALD TiO2 has been widely characterized for its physical and optical properties, but systematic information about the influence of thermal history to optical and mechanical properties of the film is lacking. Optical applications require planar surface and tunability of the refractive index and residual stress. In addition, mechanical properties such as elastic modulus and film hardness influence the performance of the layer, especially, when optics is integrated with microelectromechanical systems. In this work, optical properties, density, elemental analysis, residual stress, elastic modulus and hardness of as-grown ALD TiO2 thin films on silicon were studied at temperature range from 80 to 350 °C and influence of post-ALD thermal annealing was studied on films annealed up to 900 °C. ALD TiO2 films were under tensile stress in the scale of hundreds of MPa. The stress depended both on the ALD temperature and film thickness in a complex way, and onset of crystallization increased the residual stress. Films grown at 110 and 300 °C were able to withstand post-ALD annealing at 420 °C without major change in residual stress, refractive index or extinction coefficient. Elastic modulus and hardness increased upon crystallization with increasing ALD temperature. The results presented here help to improve the design of the optical devices by choosing films with desired optical properties, and further help to design the post-ALD thermal budget so that films maintain their desired features.

Highlights

TiO2 is a polymorphic material [1,2], and at temperature range used in thermal atomic layer deposition (ALD), it exists at least in amorphous, anatase and rutile forms [3,4,5,6]
Optical properties, density, elemental analysis, residual stress, elastic modulus and hardness of as-grown ALD TiO2 thin films on silicon were studied at temperature range from 80 to 350 ◦C and influence of post-ALD thermal annealing was studied on films annealed up to 900 ◦C
The results presented here help to improve the design of the optical devices by choosing films with desired optical properties, and further help to design the post-ALD thermal budget so that films maintain their desired features

Summary

Introduction

TiO2 is a polymorphic material [1,2], and at temperature range used in thermal atomic layer deposition (ALD), it exists at least in amorphous, anatase and rutile forms [3,4,5,6]. Considering the use of ALD TiO2 in optical applications requires precise understanding, of the desired optical properties of the grown films, and of the material properties and their dependence on process conditions. For ALD TiO2, both tensile and compressive residual stress have been measured at temperature range from 80 to 300 ◦C depending on the used substrate material [13,43,44,48]. As post-ALD thermal treatments are known to create, for example adhesion problems [51], it is important to do sys tematical characterization of the relation of processing conditions to optical and mechanical properties of the material. The goal of this work was to gain better understanding of the residual stress, elastic modulus, and hardness in ALD TiO2 films related to their use in optical applications. This article continues the article series on mechanical characterization of ALD thin films previously published on Al2O3 [49,52,53], Al2O3-TiO2 nanolaminates[49,54,55, 56] and TiO2[48,49,53]

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Conclusion