Mechanical, structural, and optical properties of PEALD metallic oxides for optical applications

Abstract

Structural, optical, and mechanical properties of Al₂O₃, SiO₂, and HfO₂ materials prepared by plasma-enhanced atomic layer deposition (PEALD) were investigated. Residual stress poses significant challenges for optical coatings since it may lead to mechanical failure, but in-depth understanding of these properties is still missing for PEALD coatings. The tensile stress of PEALD alumina films decreases with increasing deposition temperature and is approximately 100 MPa lower than the stress in thermally grown films. It was associated with incorporation of -OH groups in the film as measured by infrared spectroscopy. The tensile stress of hafnia PEALD layers increases with deposition temperature and was related to crystallization of the film. HfO₂ nanocrystallites were observed even at 100°C deposition temperature with transmission electron microscopy. Stress in hafnia films can be reduced from approximately 650 MPA to approximately 450 MPa by incorporating ultrathin Al₂O₃ layers. PEALD silica layers have shown moderate stress values and stress relaxation with the storage time, which was correlated to water adsorption. A complex interference coating system for a dichroic mirror (DCM) at 355 nm wavelength was realized with a total coating thickness of approximately 2 μm. Severe cracking of the DCM coating was observed, and it propagates even into the substrate material, showing a good adhesion of the ALD films. The reflectance peak is above 99.6% despite the mechanical failure, and further optimization on the material properties should be carried out for demanding optical applications.

Review—Investigation and Review of the Thermal, Mechanical, Electrical, Optical, and Structural Properties of Atomic Layer Deposited High-kDielectrics: Beryllium Oxide, Aluminum Oxide, Hafnium Oxide, and Aluminum Nitride

“…60,180,181 For the latter, we note that in some cases the degree of crystallinity in ALD HfO 2 has been observed to increase with thickness/number of growth cycles, 18,30,56 and that other tetragonal and orthorhombic crystalline phases have been reported. 180,181 Post-deposition annealing at 500-900…”

“…22 Due to exceptional thickness control and uniformity, atomic layer deposition (ALD) has become the preferred method for depositing most high-k dielectric materials in micro-/nano-electronic applications. 23 The low deposition temperature, 23 excellent surface topography coverage, [23][24][25] low pinhole/defect density, 26,27 high mass/atomic density, 28 and thermodynamic stability 29 of ALD high-k materials have further enabled these materials to serve additional roles in complex interference coatings 30 as well as in moisture, 31 oxygen, 32 and metal 33 diffusion barriers in hermetic packaging, 34 organic light emitting diode, 35 and metal interconnect 36 applications.…”

Antireflection Coatings for Strongly Curved Glass Lenses by Atomic Layer Deposition

“…Al 2 O 3 has been applied in ALD antireflection coatings in combination with TiO 2 [17,20] or Ta 2 O 5 [28]. Next to this, SiO 2 is a very important low-index material that we recently applied in ALD optical coatings [19,21,28,29]. The properties of the single-layer films resulting from the ALD processes used in this work are summarized in Table 2.…”

“…Note that no in situ control of the film thicknesses has been applied during the ALD process. In situ monitoring might be necessary for more complex AR coatings or interference coatings such as narrow bandpass filters or dichroic mirrors [20,21]. Table 2.…”

“…We have previously shown a broadband AR coating on flat high refractive index glasses using SiO2/HfO2 multilayers [19]. Atomic layer deposition is also being considered for more complex interference coatings such as dichroic mirrors and narrow bandpass filters [16,20,21]. Atomic layer deposition is a modified form of chemical vapor deposition, where the precursors are sequentially exposed to the surface until saturation is reached [22].…”