1 Principles of optical films
In this article, we will introduce the principles of optical thin films, commonly used design software and coating technology.
The basic principle of why optical films can achieve unique functions such as anti-reflection, high reflection or light splitting is the thin-film interference of light. Thin films are usually composed of one or more groups of high refractive index material layers and low refractive index material layers alternately superimposed. These film layer materials are generally oxides, metals or fluorides. By setting the number, thickness and different film layers of the film, The difference in refractive index between layers can regulate the interference of light beams between film layers to obtain the required functions.
Let's take a common anti-reflection coating as an example to illustrate this phenomenon. In order to maximize or reduce interference, the optical thickness of the coating layer is usually 1/4 (QWOT) or 1/2 ( HWOT). In the figure below, the refractive index of the incident medium is n0, and the refractive index of the substrate is ns. Therefore, a picture of the refractive index of the film material that can produce interference cancellation conditions can be calculated. The light beam reflected by the upper surface of the film layer is R1, The light beam reflected by the lower surface of the film is R2. When the optical thickness of the film is 1/4 wavelength, the optical path difference between R1 and R2 is 1/2 wavelength, and the interference conditions are met, thus producing interference destructive interference. Phenomenon.
In this way, the intensity of the reflected beam becomes very small, thereby achieving the purpose of anti-reflection.
2 Optical thin film design software
In order to facilitate technicians to design film systems that meet various specific functions, thin film design software has been developed. The design software integrates commonly used coating materials and their parameters, film layer simulation and optimization algorithms and analysis functions, making it easier for technicians to develop and analyze. Various film systems. Commonly used film design software are as follows:
A.TFCalc
TFCalc is a universal tool for optical thin film design and analysis. It can be used to design various types of anti-reflection, high-reflection, bandpass, spectroscopic, phase and other film systems. TFCalc can design a double-sided film system on a substrate, with up to 5,000 film layers on a single surface. It supports the input of film stack formulas and can simulate various types of lighting: such as cone beams, random radiation beams, etc. Secondly, the software has certain optimization functions, and can use methods such as extreme value and variational methods to optimize the reflectivity, transmittance, absorbance, phase, ellipsometry parameters and other targets of the film system. The software integrates various analysis functions, such as reflectivity, transmittance, absorbance, ellipsometry parameter analysis, electric field intensity distribution curve, film system reflection and transmission color analysis, crystal control curve calculation, film layer tolerance and sensitivity analysis , Yield analysis, etc. The operation interface of TFCalc is as follows:
In the operation interface shown above, by inputting parameters and boundary conditions and optimizing, you can get a film system that meets your needs. The operation is relatively simple and easy to use.
B. Essential Macleod
Essential Macleod is a complete optical film analysis and design software package with a true multi-document operation interface. It can meet various requirements in optical coating design, from simple single-layer films to strict spectroscopic films. , it can also evaluate wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) filters. It can design from scratch or optimize existing designs, and can survey errors in the design. It is rich in functions and powerful.
The design interface of the software is shown in the figure below:
C. OptiLayer
OptiLayer software supports the entire process of optical thin films: parameters - design - production - inversion analysis. It includes three parts: OptiLayer, OptiChar, and OptiRE. There is also an OptiReOpt dynamic link library (DLL) that can enhance the functions of the software.
OptiLayer examines the evaluation function from design to target, achieves the design target through optimization, and performs pre-production error analysis. OptiChar examines the difference function between the layer material spectral characteristics and its measured spectral characteristics under various important factors in thin film theory, and obtains a better and realistic layer material model and the influence of each factor on the current design, pointing out the use What factors need to be considered when designing this layer of materials? OptiRE examines the spectral characteristics of the design model and the spectral characteristics of the model measured experimentally after production. Through engineering inversion, we obtain some errors generated during production and feed them back to the production process to guide production. The above modules can be linked through the dynamic link library function, thereby realizing functions such as design, modification and real-time monitoring in a series of processes from film design to production.
3 Coating technology
According to different plating methods, it can be divided into two categories: chemical coating technology and physical coating technology. Chemical coating technology is mainly divided into immersion plating and spray plating. This technology is more polluting and has poor film performance. It is gradually being replaced by a new generation of physical coating technology. Physical coating is carried out by vacuum evaporation, ion plating, etc. Vacuum coating is a method of evaporating (or sputtering) metals, compounds and other film materials in a vacuum to deposit them on the substrate to be coated. In a vacuum environment, coating equipment has fewer impurities, which can prevent oxidation of the material surface and help ensure the spectral uniformity and thickness consistency of the film, so it is widely used.
Under normal circumstances, 1 atmospheric pressure is about 10 to the power of 5 Pa, and the air pressure required for vacuum coating is generally 10 to the power of 3 Pa and above, which belongs to high vacuum coating. In vacuum coating, the surface of optical components needs to be very clean, so the vacuum chamber during processing also needs to be very clean. Currently, the way to obtain a clean vacuum environment is generally to use vacuuming. Oil diffusion pumps, A molecular pump or condensation pump is used to extract vacuum and obtain a high vacuum environment. Oil diffusion pumps require cooling water and a backing pump. They are large in size and consume high energy, which will cause pollution to the coating process. Molecular pumps usually require a backing pump to assist in their work and are expensive. In contrast, condensation pumps do not cause pollution. , does not require a backing pump, has high efficiency and good reliability, so it is most suitable for optical vacuum coating. The internal chamber of a common vacuum coating machine is shown in the figure below:
In vacuum coating, the film material needs to be heated to a gaseous state and then deposited onto the surface of the substrate to form a film layer. According to the different plating methods, it can be divided into three types: thermal evaporation heating, sputtering heating and ion plating.
Thermal evaporation heating usually uses resistance wire or high-frequency induction to heat the crucible, so that the film material in the crucible is heated and vaporized to form a coating.
Sputtering heating is divided into two types: ion beam sputtering heating and magnetron sputtering heating. Ion beam sputtering heating uses an ion gun to emit an ion beam. The ion beam bombards the target at a certain incident angle and sputters out its surface layer. atoms, which deposit onto the surface of the substrate to form a thin film. The main disadvantage of ion beam sputtering is that the area bombarded on the target surface is too small and the deposition rate is generally low. Magnetron sputtering heating means that electrons accelerate towards the substrate under the action of an electric field. During this process, electrons collide with argon gas atoms, ionizing a large number of argon ions and electrons. The electrons fly towards the substrate, and the argon ions are heated by the electric field. The target is accelerated and bombarded under the action of the target, and the neutral target atoms in the target are deposited on the substrate to form a film. Magnetron sputtering is characterized by high film formation rate, low substrate temperature, good film adhesion, and can achieve large-area coating.
Ion plating refers to a method that uses gas discharge to partially ionize gas or evaporated substances, and deposits evaporated substances on a substrate under the bombardment of gas ions or evaporated substance ions. Ion plating is a combination of vacuum evaporation and sputtering technology. It combines the advantages of evaporation and sputtering processes and can coat workpieces with complex film systems.
4 Conclusion
In this article, we first introduce the basic principles of optical films. By setting the number and thickness of the film and the difference in refractive index between different film layers, we can achieve the interference of light beams between the film layers, thereby obtaining the required Film layer function. This article then introduces commonly used film design software to give everyone a preliminary understanding of film design. In the third part of the article, we give a detailed introduction to coating technology, focusing on the vacuum coating technology that is widely used in practice. I believe that through reading this article, everyone will have a better understanding of optical coating. In the next article, we will share the coating testing method of the coated components, so stay tuned.
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Post time: Apr-10-2024
