What are the countermeasures for improving the adverse reactions of vacuum coating?

Improve strategy:

Strengthen the degreasing and decontamination treatment. If it is ultrasonic cleaning, the degreasing function should be focused on and the effectiveness of the degreasing solution should be ensured; if it is hand wiped, consider wiping with calcium carbonate powder and then wiping.

Strengthen the baking before plating. If conditions permit, it is better that the substrate temperature can reach 300℃ or more, and the constant temperature should be more than 20 minutes to make the water vapor and oil vapor on the surface of the substrate volatilize as much as possible. *Note: The higher the temperature, the greater the adsorption capacity of the substrate, it is also easy to adsorb dust. Therefore, the cleanliness of the vacuum chamber should be improved. Otherwise, dust will adhere to the substrate before plating, which will affect the strength of the film in addition to other defects. (The chemical desorption temperature of water vapor on the substrate in vacuum is above 260℃). However, not all parts need to be baked at high temperature, and some nitrate materials have high temperature, but the film strength is not high, and stains may occur. This has a greater relationship with stress and thermal matching of materials.

When conditions permit, the unit is equipped with a condenser (PLOYCOLD), which can not only increase the vacuum pumping speed of the unit, but also help remove water vapor and oil and gas from the substrate.

Increase the vacuum degree of evaporation. For the coating machine above 1 meter, the starting vacuum of the evaporation should be higher than 3*10-3Pa. The larger the coating machine, the higher the starting vacuum of the evaporation.

When possible, the unit is equipped with an ion source, bombarded before plating, cleans the surface of the substrate, and assists the plating process, which is conducive to the compactness and firmness of the film.

To remove moisture from the membrane material, place the membrane material to be used in a petri dish in a vacuum chamber for drying.

Keep the working environment dry (including lens wiping, umbrella working area), and do not bring in too much water vapor when cleaning the working environment.

For multi-layer films, when designing the film system, it is necessary to consider the matching of the film and the substrate, and consider using Al2O3 film material as much as possible, which has a good adsorption force for substrates. For metal films, the layer of Cr or Cr alloy plating can also be considered. Cr or Cr alloys also have better adsorption power to the substrate.

Use polishing liquid (polishing liquid) to rehabilitate and remove the corroded layer (hydrolyzed layer) on the surface of the lens

Sometimes a proper reduction of the evaporation rate is helpful to increase the film strength and has a positive meaning in improving the smoothness of the film surface.

The film formation process of a thin film is a transformation process of material form. It is inevitable that there will be stress in the film layer after film formation. For multilayer films, there are combinations of different film materials, and the stresses reflected by each film layer There are differences, some are tensile stress, some are compressive stress, and there are thermal stresses of the film and the substrate.

The existence of stress is detrimental to the strength of the film. The lighter one means that the film cannot withstand friction, and the heavy one causes cracks or thin nets in the film.

For anti-reflection films, the stress is generally not obvious due to the small number of layers. (However, some nitrate lenses have stress problems even if they are anti-reflection films.) For high-reflection films and filter films with more layers, the stress is A common undesirable factor should be paid special attention to.

Bake after plating. After the last layer of film is plated, do not stop baking immediately, and continue "tempering" for 10 minutes. Make the film structure more stable.

The cooling time is appropriately extended, and the annealing is aging. Reduce the thermal stress caused by the excessive temperature difference between the inside and outside of the vacuum chamber.

For high-reflection films, filter films, etc., during the evaporation process, the substrate temperature should not be too high, as high temperatures are likely to cause thermal stress. And it has a negative effect on the optical stability of film materials such as titanium oxide and tantalum oxide.

Ion assist in the coating process to reduce stress.

Choose the appropriate film system to match, the layer of film material and the substrate match. (For example, the five-layer anti-reflection film uses Al2O3-ZrO2-Al2O3-Al2O3-ZrO2-MgF2; ZrO2 can also use SV-5 (a ZrO2 TiO2 mixed film material) or other mixed high refractive index film materials.

Appropriately reduce the evaporation rate (Al2O3-2.5A/S; ZrO2-3A/S; MgF2-6A/S reference rate)

Oxygen-filling reaction plating is performed on all oxide film materials, and the oxygen gas intake rate is controlled according to different film materials.

The anti-reflection film generally uses MgF2 as the outer layer. The cross-section of the film is a relatively loose columnar structure, and the surface hardness is not high, so it is easy to wipe the road.

Improve strategy:

When the film system design permits, add about 10nm SiO2 layer to the outer layer, and the surface smoothness of silicon dioxide is better than that of magnesium fluoride (but the wear resistance and hardness of silicon dioxide are not as good as magnesium fluoride). After a few minutes of ion bombardment after plating, the effect of firmness will be better. (But the surface will become thicker)

After the lens leaves the vacuum chamber, place it in a dry and clean place to prevent rapid moisture absorption and reduce surface hardness.

The reasons for the poor film strength include low vacuum (prone to occur in manually controlled machines), dirty vacuum chamber, and insufficient substrate heating.

When the auxiliary gas is filled in, the membrane material is also outgassing, which reduces the vacuum degree, reduces the molecular free path, and the membrane layer is not strong. Therefore, the filling of the auxiliary gas should consider the degassing of the film material, and the film material is fully pre-melted and fully degassed before plating, which can also avoid the excessive decrease of the vacuum degree due to the degassing of the film material during the evaporation, thereby affecting the film strength.

Although the film release here is also a kind of weak film, it has some differences from the previous film release. The main features are: point release, edge release, and partial release.

The main reason is that there are dirt or pollutants in the membrane.

Improvement method: improve the cleanliness of the substrate.