What are the requirements for the coating process of ultraviolet quartz lenses

UV quartz lenses are widely used in precision optical fields such as photolithography, UV spectroscopy analysis, laser processing, etc. due to their efficient transmission of ultraviolet light, high temperature resistance, and strong chemical stability. To further enhance its optical performance (such as anti reflection, reflection, filtering, etc.), the coating process is crucial. The coating process of ultraviolet quartz lenses is not simply thin film deposition, but requires careful consideration of material selection, process parameter control, and film structure design based on the characteristics of the ultraviolet band, quartz substrate, and application scenarios. The following analyzes the core requirements of UV quartz lens coating process from multiple dimensions.

1、 Matching selection of coating materials

The photon energy in the ultraviolet band (usually referring to 10-400nm) is high, and the requirements for the optical stability, chemical inertness, and radiation resistance of materials are much higher than those in the visible or infrared bands. The selection of coating materials must meet three core requirements:

High transmittance/reflectance characteristics in the ultraviolet band

For anti reflective films, it is necessary to choose materials with high transmittance in the target ultraviolet band (such as deep ultraviolet 193nm, vacuum ultraviolet 254nm), such as magnesium fluoride (MgF ₂), lithium fluoride (LiF), silicon dioxide (SiO ₂), etc. Among them, MgF ₂ has excellent transparency in the 120-7000nm wavelength range and a moderate difference in refractive index compared to quartz substrates (quartz refractive index is about 1.46, MgF ₂ is about 1.38), making it a commonly used material for ultraviolet anti reflective films.

For reflective or filter films, high reflectivity materials such as aluminum (Al), silver (Ag), and other metal films should be selected. However, metals are prone to oxidation in the deep ultraviolet band and need to be packaged with protective dielectric films (such as SiO ₂). For example, aluminum film has a reflectivity of over 80% in the ultraviolet band, but it needs to be treated with oxidation or covered with a dielectric layer to prevent corrosion.

Adhesion and compatibility with quartz substrate

Quartz (SiO ₂) has a smooth surface and strong chemical stability, and the coating material needs to form strong chemical bonds or physical adsorption with the substrate. If the difference in thermal expansion coefficient between the material and quartz is too large, stress will be generated during temperature changes, leading to cracking or detachment of the film layer. For example, the thermal expansion coefficient of calcium fluoride (CaF ₂) is similar to that of quartz (quartz is about 0.5 × 10 ⁻⁶/℃, CaF ₂ is about 18 × 10 ⁻⁶/℃), but it has better transparency in the deep ultraviolet band and is suitable for high-precision scenes; The adhesion between the metal film and quartz is poor, and a transition layer (such as chromium Cr) needs to be plated first to enhance the bonding strength.

Environmental and radiation resistance performance

UV lenses often work in high temperature, strong radiation, or corrosive environments (such as laser equipment, lithography systems), and coating materials need to withstand long-term UV irradiation without aging, discoloration, or structural damage. For example, SiO ₂ film layer has strong stability under UV irradiation, while organic materials (such as certain polymers) may experience performance failure due to UV degradation, and therefore are prohibited from being used for UV coating.

2、 Accurate control of coating process parameters

The coating process of ultraviolet quartz lenses is mainly based on physical vapor deposition (PVD), including vacuum evaporation coating, magnetron sputtering coating, ion beam assisted deposition (IAD), etc. The parameter control of different processes directly affects the quality of the film layer:

Vacuum degree and environmental cleanliness

Coating should be carried out in a high vacuum environment (usually ≤ 1 × 10 ⁻⁴ Pa) to avoid impurities such as oxygen, water vapor, and dust from the air from entering the film layer. For example, water vapor can cause the formation of hydroxyl groups (- OH) in the membrane layer, resulting in absorption peaks in the ultraviolet band and reducing transmittance; Dust particles can cause defects in the film layer, leading to light scattering. Therefore, the vacuum chamber needs to be strictly baked and degassed (temperature 80-150 ℃), and a molecular pump or ion pump should be used to maintain high vacuum.

Sedimentation rate and substrate temperature

Excessive sedimentation rate can lead to membrane looseness, high porosity, and easy adsorption of impurities; If it is too slow, it will increase production time, and the film layer is easily affected by residual gases in the vacuum environment. For ultraviolet outer film, the rate is usually controlled between 0.1-1nm/s, and the thickness is precisely controlled through real-time monitoring (such as crystal oscillating plates).

The substrate temperature needs to be adjusted according to the material: the temperature during metal film deposition should be low (room temperature to 100 ℃) to avoid metal oxidation; The dielectric film (such as SiO ₂) needs to be heated to 200-300 ℃ to promote molecular diffusion and form a dense structure. However, excessive temperature can cause deformation of the quartz substrate (quartz melting point is about 1713 ℃, but long-term high temperature may affect optical accuracy), and it is necessary to optimize the heating scheme through thermal simulation.

Ion assisted and membrane densification

Ion beam assisted deposition (IAD) is a key technology for improving the quality of ultraviolet outer film layers: during the deposition process, high-energy ion beams (such as Ar ⁺) are used to bombard the surface of the film layer, which can reduce the porosity of the film layer, enhance atomic level binding force, and reduce internal stress. For example, introducing ion assistance during MgF ₂ film deposition can increase the density of the film layer by more than 30% and improve the transmittance in the deep ultraviolet band by 5% -10%.

3、 Fine design of membrane structure

The film structure of ultraviolet quartz lenses needs to be designed according to optical performance objectives (such as anti reflection, splitting, filtering), while also considering mechanical strength and stability:

Selection of single-layer and multi-layer films

Single layer anti reflective film is suitable for low precision scenarios, such as MgF ₂ film, which can reduce the reflectivity of quartz lenses in the ultraviolet band from about 8% to 1% -2%; But for wideband or high-precision requirements (such as photolithography lenses covering 193nm-365nm), it is necessary to design multi-layer film systems (such as alternating stacking of "high refractive index materials low refractive index materials") to cancel reflections through interference between film layers. For example, a multilayer film composed of SiO ₂ (low refractive index) and HfO ₂ (high refractive index) can achieve a reflectivity of ≤ 0.1% in the 200-400nm wavelength range.

Uniformity and accuracy of film thickness

The wavelength of the ultraviolet band is extremely short (such as 193nm deep ultraviolet), and a small error in the thickness of the film layer (± 1nm) can significantly deviate the optical performance from the design value. Therefore, it is necessary to control the thickness in real-time through high-precision monitoring systems such as ellipsometers and cavity ring down spectrometers, and ensure that the thickness deviation of each point on the lens surface is ≤ λ/40 (where λ is the target wavelength). For example, for a 193nm lens, the thickness deviation of the film layer needs to be controlled within ± 4.8nm.

Stress matching and anti damage design

The stress of different materials in multilayer films will be superimposed on each other. If the total stress is too high, it will cause lens warping or film peeling. By selecting the combination of "compressive stress material+tensile stress material" (such as SiO ₂ as compressive stress and TiO ₂ as tensile stress), stress balance can be achieved. In addition, in ultraviolet laser scenarios (such as excimer lasers), the film layer needs to withstand high energy densities (up to 1J/cm ²), and it is necessary to reduce the laser-induced damage threshold (LIDT) by increasing the film layer density or introducing gradient refractive index structures.

4、 Substrate pretreatment and post-treatment processes

The surface condition of quartz lenses directly affects the quality of coating, and pre-treatment and post-treatment are important steps that cannot be ignored

Thoroughly clean the substrate

Oil stains, scratches, and adsorbed water on the surface of quartz can cause a decrease in adhesion or defects in the film layer. The cleaning process usually includes: ultrasonic cleaning (using anhydrous ethanol and acetone to remove organic matter) → plasma etching (using O ₂ plasma to remove residual carbon pollution while activating the surface) → pure water rinsing (removing ion impurities) → vacuum drying (avoiding secondary pollution). The cleaned surface should meet the standard of "continuous water film" (i.e. water evenly spreads on the surface without rupture), and the surface roughness Ra should be ≤ 0.5nm.

Stability enhancement of post-processing

After coating is completed, annealing treatment (heating to 150-300 ℃ in vacuum or inert gas, holding for 2-4 hours) is required to eliminate internal stress in the film layer and improve stability; For metal films or easily oxidizable materials, a protective film (such as SiO ₂) should be coated and subjected to densification treatment (such as ion beam bombardment); In addition, low-temperature plasma treatment (such as N ₂ plasma) can be used to improve the surface energy of the film layer and enhance its anti pollution ability.

5、 Quality inspection and performance verification

The detection of UV quartz lens coating should cover optical performance, mechanical performance, and environmental adaptability:

Optical performance testing

Measure the transmittance/reflectance using a UV spectrophotometer and ellipsometer to ensure that it meets the design values in the target wavelength band; By using a laser interferometer to detect the wavefront distortion of the coated lens, the optical accuracy is ensured to be unaffected.

Mechanical and Environmental Testing

Conduct adhesion testing (such as tape peeling test, scratch test), requiring no peeling of the film layer; Verify the ability to resist temperature changes through high and low temperature cycling tests (-40 ℃ to 80 ℃); Salt spray test (for outdoor applications) to test corrosion resistance.

Long term stability monitoring

Simulate long-term irradiation (cumulative dose up to 1000 hours) in a UV aging chamber, monitor whether the film layer changes color, cracks, or reduces light transmittance, and ensure that the performance degradation is ≤ 5% within the service life (usually 5-10 years).

summary

The coating process of ultraviolet quartz lenses is a combination of material science, optical design, and precision manufacturing, with the core emphasis on selecting matching coating materials based on the characteristics of the ultraviolet band; Realize the uniformity and density of the film layer through high-precision process control; Design a reasonable film structure to balance optical performance and mechanical stability; And through strict preprocessing, post-processing, and testing, ensure that the final product meets the stringent requirements of the application scenario. With the development of technologies such as deep ultraviolet lithography and extreme ultraviolet (EUV) detection, the coating process of ultraviolet quartz lenses will evolve towards higher precision (nanometer level thickness control), higher stability (strong radiation resistance, laser damage resistance), and wider frequency bands (extending to vacuum ultraviolet below 100nm), providing core support for high-end optical equipment.