Stress in Optical Coatings
Optical coatings are integral to the performance and durability of modern laser systems, yet they present unique challenges due to the stresses that can arise during their fabrication and use. Understanding these stresses — whether mechanical, thermal, or chemical — is crucial for ensuring the reliability and longevity of coated optics.
This article delves into the complexities of stress in optical coatings, offering a detailed analysis of its origins, effects, and mitigation strategies. By exploring both the fundamental principles and practical considerations, the paper aims to provide valuable insights for scientists, engineers, and professionals involved in the design and application of high-performance optical coatings.
Definition of Stress
The presence of stress within thin films presents a very real challenge in optical coatings as it can adversely affect coating adhesion, cause bending in the substrate (introducing unwanted aberrations), or possibly even lead to a catastrophic failure of the coating. In severe cases, stresses can be of such magnitude that they are of the order of GPa. Knowing is half the battle however, and a keen awareness of the mechanisms that cause stress and stress-induced failures is the first step in combating it.
The total mechanical stress within a thin film σ is composed of three components:
σ = σext+ σtherm + σint
The term σext represents stress from external sources, σtherm is the thermal stress caused by the difference between the thermal expansion coefficients of the film, σint represents the intrinsic stress within the material.
The thermal stress is represented by the equation:
σtherm = Yf(af – as)(Tp – Tm)
Yf is Young’s modulus of the film. af and as are the thermal expansion coefficients of the film and substrate, respectively. Tp and Tm are the temperatures taken during measurement and during the coating process, respectively.
The most complex and important component of the stress is the intrinsic stress (σint) arising from the film microstructure and growth. It exists in two principal forms, tensile stress due to the film contracting parallel to the substrate and compressive stresses due to the film expanding parallel to the substrate. Tensile stress has a tendency to induce cracks in the film that in some cases may even transfer to the substrate, whereas compressive stress causes buckling and delamination of the film. Due to the complexity of intrinsic stress and its dependency on the deposition process there exist many models which provide useful but incomplete pictures of stress description.
Intrinsic stress arises from a variety of sources, such as lattice defects, variations in interatomic, contamination, phase transformations, and interactions at grain boundaries or voids to name a few. Packing density and the resultant microstructure play a big role. A film with pronounced columnar growth or polycrystalline structure and low packing density will experience strong tensile stresses due to forces between grain boundaries/columns. Conversely films with a closely packed micro-structure tend to be under compressive stress. Additionally, even amorphous or polycrystalline films with a higher packing density experience tensile stress due to the attractive forces present within voids, cavities/ and grain boundaries. In some cases, moisture filling the voids can reduce tensile stress or even convert it into compressive stress!
Origins of Thin Film Stress
Analyzing the evolution of stress during film growth also gives insight into the nature and origins of thin film stress. As an example we can consider the Volmer Weber (V-W) growth mode. Consider an initially bare substrate with a flux of evaporated material incident upon it. Initially the material will form lone islands, a process known as “nucleation.” Over time these islands grow and eventually intersect and coalesce signifying the formation of grain boundaries. This process continues until the film becomes relatively flat and uniform, at which point it begins to grow in thickness. The behavior of stress throughout these three processes follows a compressive-tensile-compressive (CTC) pattern typical for high mobility films. The islands cause compressive stress initially until they begin to coalesce. The stress is tensile until atoms are driven into grain boundaries, returning the film to a compressive stress state.
The stress is also dependent on the material utilized in conjunction with the process, for example pure metal films are generally under tensile stress though the exact value depends on the method of production. Metal oxides are capable of showing either stress state though they tend to be compressive under energetic processes. Thick SiO2 films produced via electron-beam evaporation show compressive stresses of around 160 MPa, though sub-100-nm thick films can show extremely high stresses of up to 1.1 GPa.
Countering Tensile Stress
The primary method of countering tensile stress is of course the utilization of energetic or sputtering methods (such as Electron Beam PVD, Magnetron Sputtering, Ion-Assisted Deposition) as this results in a higher packing density and a less columnar micro-structure, though in some cases this pushes the film stress from tensile to compressive! Encouraging the growth of larger grains also results in a reduced tensile stress. Generally speaking, thicker films tend to be under lower intrinsic stress.
About Manx Precision Optics
Manx Precision Optics (MPO) manufactures optical components and precision optics. The company offers customized products as well as a wide range of off-the-shelf components at highly competitive prices. The main categories of MPO’s standard products are: coated optics (including components for ultrafast lasers), uncoated optics, and polarization optics.
MPO works with customers to determine the most suitable optical coatings for their specific applications. With years of experience and an intimate knowledge of thin film coatings design MPO can produce customized, quality coatings according to unique specifications.
To request more information or a quotation for Manx Precision Optics products, contact IL Photonics.