Analysis And Improvement Of Common Problems In Optical Coating

- May 21, 2019-

Analysis and improvement of common problems in optical coating


The common problems of coating products are partly caused by the coating process itself, and some are left by the former engineering. The final quality of the coating is a comprehensive reflection of the processing of the entire optical component (especially polishing and cleaning). Really find the cause, and the improvement of the countermeasures can achieve results.


First, the film strength

Film strength is an important indicator of lens coating and a common item in coating processes.

The film strength (weak film) is mainly expressed as:

1 Wipe or tear with a special tape to produce a piece of peeling off;

2 Wipe or tear with a special tape to produce a point drop;

3 boiled for 15 minutes, then pulled with a special tape to produce a point or flake off;

4 Using a special rubber head and rubbing 40 times with 1Kg force, there is a road to produce;

5 The film is wiped or not wiped with cracks in the turtle and reticular lines.

Improvement ideas: The combination of substrate and film is the primary consideration, followed by film surface hardness smoothness and film stress.

Causes and countermeasures for film strength:

1 Combination of substrate and film layer.

In general, in the anti-reflection film, this is the main reason for the weak film. Since the surface of the substrate inevitably has some harmful impurities attached to the surface during optical cold processing and cleaning, the surface of the substrate has some damage layers due to the effect of optical cold processing, and impurities deep in the damage layer (such as water vapor) , oil vapor, cleaning fluid, wiping liquid, polishing powder, etc., in which water vapor is the main), it is difficult to remove it by general methods, especially for substrates with good hydrophilicity and strong adsorption. When the film molecules accumulate on these impurities, the adhesion of the film layer is affected, which affects the film strength.

In addition, if the substrate is poor in hydrophilicity and poor in adsorption, the adsorption of the film layer is also poor, which also affects the film strength.

The chemical stability of the nitrate material is poor. During the process of the substrate during the pre-processing, the surface has been corroded, forming a corrosion layer or a hydrolysis layer (perhaps local, extremely thin). When the film is plated on the corrosion layer or the hydrolysis layer, the adsorption is poor and the film is firm.

The surface of the substrate is dirty, oil spots, gray spots, saliva points, etc., and the local film layer is attached, resulting in local film firmness.

Improve strategy:

(1) Strengthen the degreasing treatment. If it is ultrasonic cleaning, it should focus on the oil removal function and the effectiveness of the oil solution; if it is rubbed by hand, consider wiping it with calcium carbonate powder before cleaning.

(2) Strengthen the pre-plating baking, the conditions permit, the substrate temperature can reach 300 ° C or better, the temperature is more than 20 minutes, and the water vapor and oil vapor on the surface of the substrate should be volatilized as much as possible. *Note: The temperature is higher, the substrate adsorption capacity is increased, and it is easy to adsorb dust. Therefore, the cleanliness of the vacuum chamber should be improved. Otherwise, the substrate will have dust adhesion before plating, which has an effect on the film strength in addition to others. (The chemical desorption temperature of water vapor on the substrate in vacuum is above 260 °C). But not all parts need high temperature baking, and some nitrates have a high temperature, but the film strength is not high and there will be color spots. This has a large relationship with stress and material thermal matching.

(3) When there are conditions, the unit is equipped with a condensing machine (PLOYCOLD). In addition to increasing the vacuum pumping speed of the unit, it can also help the water vapor and oil and gas in the substrate to be removed.

(4) To increase the vacuum degree of vapor deposition, for a coating machine of more than 1 m, the vacuum of the vapor deposition start-up should be higher than 3*10-3Pa, the larger the coating machine, the higher the vacuum of the vapor deposition.

(5) When the condition is met, the unit is equipped with an ion source, bombarded before plating, cleans the surface of the substrate, and assists in the coating process, which is conducive to the compactness of the film layer.

(6) Dehumidification of the film material, and the film material to be used is placed in a vacuum chamber to be dried.

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

(8) For multilayer films, in the design of the film system, it is necessary to consider the matching of the film and the substrate, and consider the Al2O3 film material as much as possible, which has good adsorption force to most of the substrates. For metal films, it is also possible to consider layer-plated Cr or Cr alloys. Cr or Cr alloy also has good adsorption force on the substrate.

(9) Taking the polishing liquid (polishing liquid) to remove the corrosion layer (hydrolysis layer) on the surface of the lens

(10) Sometimes it is helpful to reduce the evaporation rate appropriately to improve the film strength, and it has positive significance for improving the smoothness of the film surface.

3 outer film surface hardness:

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

Improve strategy:

(1) When the membrane system design allows, the outer layer is coated with SiO2 layer of about 10 nm. The surface smoothness of silica is better than that of magnesium fluoride (but the surface abrasion resistance and hardness of silica are not as good as magnesium fluoride). After plating for a few minutes, the effect of the firmness will be better. (but the surface will become thicker)

(2) After the lens exits the vacuum chamber, it is placed in a relatively dry and clean place to quickly absorb moisture and reduce the surface hardness.

4 other

The reason for the film strength is that the degree of vacuum is too low (prone to occur in a manually controlled machine), the vacuum chamber is dirty, and the substrate is not heated.

When the auxiliary gas is charged, the film material is also deflated, so that the degree of vacuum is lowered, the molecular free path is reduced, and the film layer is not strong. Therefore, the filling of the auxiliary gas should consider the venting of the film material. Before the plating, the film material is fully pre-melted and fully deflated, and the vacuum degree is excessively lowered due to the venting of the film material in the vapor deposition, thereby affecting the film strength.

5 stripping

Although the release film here is also a weak film, it has some differences from the above-mentioned release film. The main features are: spot-like release, edge release, and partial release.

The main cause is the presence of dirt or contaminants in the membrane.

Improvement method: Improve the cleanliness of the substrate.

Membrane points are also a common problem in coated products. In Japan and Taiwan, the film points are called "spot holes".

As the name suggests, the film point is vapor deposition. The large particle film point is vapor deposited on the surface of the substrate along with the vapor molecules of the film. The surface of the substrate is formed with spot-like protrusions, sometimes individual points. In severe cases, it is formed into pieces. The fine spots, large particles can even damage the surface of the substrate.

The evaporation characteristics of various film materials are different, especially the melting point temperature and evaporation temperature are very different.

a material having a melting point temperature greater than the evaporation temperature, which is directly vaporized by a solid state and is a sublimation material;

A material having a melting point lower than the evaporation temperature, which is converted into a liquid state by a solid state, and then converted into a gaseous state to evaporate, which is a non-sublimation material;

A material having a melting point temperature equivalent to the evaporation temperature, evaporates from a solid state to a gaseous state, and evaporates while melting. It is a semi-sublimation material.

Among them, the non-sublimation material is easy to produce a film material point, because the liquid film material continues to heat and boils, and the bubbles in the film material in the boiling material overflow, and the possibility of splashing the film material point is increased. Sometimes there is a lot of splash when the material is pre-melted.

The film material is damp, and the water vapor escapes during pre-melting or evaporation, which also causes splashing.

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