Anti-reflective coated glass and its production method

In the field of modern optics, anti-reflective coated glass has attracted much attention for its unique optical properties. With the development of science and technology, people's requirements for glass optical performance are increasing, and ordinary glass has limited its application due to its high reflectivity in many scenarios. Anti-reflective coated glass can effectively reduce reflectivity and improve light transmittance, and is widely used in many fields such as buildings, solar energy, and electronic displays, and its production methods have also become a hot topic.

2. Overview of anti-reflective coated glass
2.1 Principle
Light is reflected and refracted as it travels at different media interfaces. Ordinary glass is unavoidable due to the difference in refractive index between the surface and the air. Anti-reflective coated glass uses the interference principle of light to reduce reflections by applying a specific film to the surface of the glass. When the thickness of the film is one-fourth of the wavelength of light in the medium, the light reflected from the upper and lower surfaces of the film is opposite and interferes with each other to cancel it out, thereby reducing the intensity of reflected light and increasing the intensity of transmitted light. For example, for light with a wavelength of λ, in a thin film with a refractive index of n, the film thickness d = λ / 4n It can achieve a better anti-reflection effect. Common coating materials such as silica (SiO₂) and titanium oxide (TiO₂) can achieve low reflection in a wider spectral range by reasonably selecting and matching materials with different refractive indices to make multi-layer films.

2.2 Features
1. Low reflectivity: The reflectivity of high-quality anti-reflective coated glass can be less than 1%, which is much lower than that of ordinary glass 4% unilateral reflectivity, greatly reducing ambient light reflection interference. If used in museum display windows, visitors can see the exhibits more clearly and avoid visual disturbances caused by reflected light.

2. High light transmittance: visible light transmittance can reach up to 99%, and the average transmittance is exceeded 95%。 In architectural lighting, it can make the interior brighter and reduce the energy consumption of artificial lighting; In solar cell modules, it can improve the absorption of sunlight and improve the efficiency of photoelectric conversion.

3. Improve visual effects: effectively weaken the whitening phenomenon caused by strong light, making the image more colorful, more contrasting, and clearer. Applied to electronic displays, it can improve the display quality and bring users a better visual experience.

4. Other characteristics: Some anti-reflective coated glass also has the characteristics of UV resistance, high temperature resistance, scratch resistance and wear resistance, acid and alkali corrosion resistance, and strong impact resistance. For example, when used in outdoor building curtain walls, it can resist ultraviolet rays and harsh environmental erosion, and maintain long-term stable performance.

2.3 Applications
1. Architectural field: used in building doors, windows, and curtain walls to reduce light reflection glare, improve lighting effects, create a comfortable indoor environment, and reduce building energy consumption. For example, some high-end office buildings use anti-reflective coated glass, which is both beautiful and energy-saving.

2. Solar energy field: It is a key material for solar cell modules, which can improve solar transmittance, reduce reflection loss, improve photoelectric conversion efficiency, and reduce power generation costs. Widely used in large solar power plants.

3. Electronic display field: used in liquid crystal displays, TVs, computer displays, touch screens, etc., to reduce reflections, enhance display clarity and color reproduction, and enhance product competitiveness.

4. Optical instruments: In optical instruments such as camera lenses, telescopes, and microscopes, anti-reflective coated glass can reduce stray light interference, improve imaging quality and observation accuracy, and help the development of scientific research, medical and other fields.

3. Production method
3.1 Physical Vapor Deposition (PVD).
1. Vacuum evaporation coating

◦Process: In a high vacuum environment, the coating material (such as SiO₂,Al₂O₃, etc.) is heated to the evaporation temperature in an evaporation source, so that its atoms or molecules evaporate into a gaseous state, and the gaseous particles fly freely in a vacuum and are deposited on the surface of the glass substrate to form a thin film.

◦Influence of process parameters: Parameters such as evaporation rate, substrate temperature, and vacuum degree have a significant impact on the quality of the coating. If the evaporation rate is too fast, the film may have problems such as coarse particles and poor adhesion. If the substrate temperature is too low, the stress in the film layer is large and it is easy to crack; If the vacuum degree is insufficient, the film will be mixed with impurities, which will affect the optical performance. For example, when coating SiO₂ films, the evaporation rate is controlled at 0.1 - 1 nm/s, the substrate temperature is 100 - 300°C, and the vacuum is controlled 10⁻⁴ - 10⁻⁶Pa for better quality coatings.

◦Advantages and disadvantages: The advantages are simple equipment, convenient operation, and high purity of the film layer; The disadvantages are that the uniformity of the film layer is poor, the coating effect on complex shape substrates is not good, and the deposition rate is relatively slow.

1. Sputtering coating

◦Process: Use a high-energy ion beam (such as argon ions) to bombard the target of the coating material, so that the atoms or molecules of the target are sputtered out and deposited on the surface of the glass substrate to form a thin film. Magnetron sputtering is a commonly used sputtering coating method, which increases the plasma density and enhances the sputtering efficiency by applying a magnetic field to confine the movement of electrons.

◦Influence of process parameters: Sputtering power, gas flow, sputtering time, substrate temperature and other parameters need to be accurately controlled. The sputtering power determines the atomic sputtering rate, which affects the deposition rate and quality of the film. The gas flow rate affects the plasma state and the composition of the film; Sputtering time controls the thickness of the film; The substrate temperature affects the stress and crystallization state of the film. For example, when preparing TiO₂ membranes, the sputtering power is 100 - 300W, the argon flow rate is 10 - 30sccm, and the sputtering time 30 - 60 min, substrate temperature 200 - 400°C, the ideal coating can be obtained.

◦Advantages and disadvantages: The advantages are good uniformity of the film layer, strong adhesion, a wide range of plating materials, and coating at lower temperatures; The disadvantages are that the equipment is complex, costly, and may introduce impurity gases.

3.2 Chemical vapor deposition (CVD).
1. Atmospheric pressure chemical vapor deposition (APCVD).

◦Process: Under atmospheric pressure, gaseous reactants containing coating elements (such as silane SiH₄, ammonia) are added NH₃, etc.) to the reaction chamber, where a chemical reaction occurs on the surface of the heated glass substrate to form a solid-state coating layer. For example, in the preparation of silicon nitride (SiNₓ) anti-reflection film, the reaction equation is: 3SiH₄ + 4NH₃ → Si₃N₄ + 12H₂.

◦Influence of process parameters: Reaction temperature, gas flow ratio, reaction time, etc. have an important impact on the quality of the film layer. The reaction temperature determines the reaction rate and film structure, generally at 700 - 900°C. The gas flow ratio affects the composition and stoichiometric ratio of the film layer. The reaction time controls the thickness of the film layer.

◦Advantages and disadvantages: The advantages are simple equipment, fast deposition rate, and large area coating; The disadvantages are high reaction temperature, high requirements for substrate, and large stress in the film layer.

1. Low-pressure chemical vapor deposition (LPCVD).

◦Process: Chemical vapor deposition at low atmospheric pressure (usually 1 - 100Pa), with APCVD is similar in principle, but the low-pressure environment reduces intermolecular collisions of gaseous reactants and improves deposition uniformity.

◦Influence of process parameters: In addition to temperature, gas flow ratio, and reaction time, pressure is also a key parameter. Lower pressure can improve the quality of the film layer, and the general pressure is controlled at 10 - 50Pa.

◦Advantages and disadvantages: The advantages are high quality of the film layer, good uniformity, and strong step coverage ability; The disadvantage is that the equipment cost is high and the deposition rate is relatively slow.

1. Plasma-enhanced chemical vapor deposition (PECVD).

◦Process: Plasma is used to enhance chemical reactivity and achieve chemical vapor deposition at lower temperatures. The reaction gas is excited to produce plasma by means such as radio frequency or microwave, so that the reaction can be carried out quickly at lower temperatures (usually 200 - 400°C).

◦Process parameter influence: RF power, gas flow, pressure, temperature and other parameters are correlated with each other. RF power affects plasma density and activity; gas flow and pressure determine the concentration and reaction rate of the reaction gas; Temperature affects the structure and properties of the layer. For example, when preparing SiO₂ films, the RF power is 100 - 200W, and the gas flow rate is SiH₄(5 - 10sccm), N₂O (50 - 100sccm), pressure 10 - 30Pa, temperature 250 - 350°C.

◦Advantages and disadvantages: The advantage is that it can deposit high-quality film layers at low temperatures, and the restrictions on substrate materials are small; The disadvantage is that the equipment is complex and the plasma parameter control is difficult.

3.3 Sol-gel method
1. Process: Mix metal alk salts (such as ethyl orthosilicate Si (OC₂H₅)₄) or inorganic salts (such as butyl titanate). Precursors such as Ti (OC₄H₉)₄) are dissolved in organic solvents (such as ethanol), and water and catalysts (such as hydrochloric acid) are added for hydrolysis and polycondensation reactions to form sols. The glass substrate is coated with a sol by impregnation, spin-coating or spraying, and after drying and heat treatment, the sol is transformed into a gel and forms a solid-state coating layer. Taking the preparation of SiO₂ membrane as an example, the hydrolysis reaction is: Si (OC₂H₅)₄ + 4H₂O → Si (OH)₄ + 4C₂H₅OH, and the polycondensation reaction is:nSi (OH)₄ → (SiO₂)ₙ + 2nH₂O。

2. Influence of process parameters: precursor concentration, catalyst dosage, reaction temperature, drying and heat treatment conditions, etc. affect the quality of the film layer. The concentration of precursors determines the thickness and structure of the film layer. the amount of catalyst controls the reaction rate; The reaction temperature affects the hydrolysis and polycondensation reaction process. Drying and heat treatment conditions affect the density and optical properties of the film layer. For example, when preparing TiO₂ membranes, the concentration of precursors is 0.1 - 0.3 mol/L, and the dosage of catalyst hydrochloric acid is 0.1 - 0.5 mol/L, the reaction temperature 25 - 60°C, drying temperature 60 - 100°C, heat treatment temperature 400 - 600°C.

3. Advantages and disadvantages: The advantages are simple process, low cost, can be operated at room temperature, can prepare a large area of uniform film layer, and can accurately adjust the composition and structure of the film layer by controlling the sol composition and process parameters; The disadvantage is that cracks are easy to occur during the drying and heat treatment of the film layer, and the adhesion between the film layer and the substrate is relatively weak.

3.4 Other production methods
1. Atomic layer deposition (ALD).: Based on self-limiting chemical reactions, the thickness and composition of the film layer are precisely controlled at the atomic scale. By alternating different precursor gases, they undergo a monolayer chemical reaction on the surface of the substrate to deposit the film layer by layer. The advantage is that the film layer has excellent uniformity and shape conformation, and is suitable for coating complex structures and nanoscale devices. The disadvantages are expensive equipment, extremely low deposition rates, and high costs, limiting large-scale applications.

2. Thermal evaporation - ion-assisted deposition (IAD).: In the process of thermal evaporative coating, ion beam assistance is introduced. The ion beam bombards the deposited atoms, improving the structure and properties of the film layer, and improving the adhesion and density of the film layer. This method combines the simplicity of thermal evaporation with the advantages of ion assistance, but the equipment is relatively complex and requires high control of ion source parameters.

4. Research progress and challenges
In recent years, the production method of anti-reflective coated glass has been continuously innovated and developed. In terms of physical vapor deposition, new sputtering technology and equipment are developed to improve the rate and quality of film deposition, and expand the application of complex shapes and high-precision optical component coatings. Chemical vapor deposition is developing in the direction of lower temperature, higher efficiency and more environmental protection, and new reactive gases and catalysts are researched to reduce energy consumption and environmental pollution. The sol-gel method solves the problem of cracking and adhesion by improving the sol formulation and process to enhance the performance of the layer. At the same time, the composite application of a variety of manufacturing methods has become a trend, such as combining physical vapor deposition and chemical vapor deposition, learning from each other's strengths and weaknesses to prepare high-performance anti-reflective coated glass.

However, there are still many challenges in the production of anti-reflective coated glass. On the one hand, how to further improve the optical performance, stability and durability of the coating while reducing costs is a key issue. On the other hand, for large-scale industrial production, it is necessary to develop efficient, stable and continuous production processes and equipment to improve production efficiency and product consistency. In addition, with the continuous expansion of application fields, the performance requirements of anti-reflective coated glass in special environments (such as high temperature, high pressure, high humidity, strong radiation, etc.) are higher, and targeted research needs to be carried out.

5. Conclusion
Anti-reflective coated glass plays an important role in many fields due to its unique optical properties. There are various methods of making them, each with its own advantages and disadvantages. Physical vapor deposition, chemical vapor deposition, sol-gel method, etc. show unique advantages in different application scenarios, but also face some problems. With the deepening of research and technological innovation, the production method of anti-reflective coated glass will continue to improve, while meeting the market demand for high-performance products and promoting the development of related industries. In the future, it is necessary to strengthen basic research, break through technical bottlenecks, develop more efficient, low-cost, and high-performance production methods, expand the application scope of anti-reflective coated glass, and inject new impetus into the development of the optical field.