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Solution to Film Strength Defects

Time:2020/05/12 丨 source:未知 丨 visit count:

Analysis and solutions for common defects of coated product——film strength

Film strength is an important index of Lens coating, and it is also the most common defect in the coating process.
Poor film strength (film weakness) is mainly manifested as:
Wipe or tear with special tape, resulting in whole pieces film detachment;
Wipe or tear with special tape, film detached in point.
Boil for 15min, pull and tear with special tape to produce spot or pieces film detachment;
40 times with special rubber head and 1Kg force, scratches showing up.
When the film is wiped or not wiped, cracks and fine meshes appear.
Improving ideas: The bonding of substrate and film is the first consideration, and the second is the hardness and smoothness in the surface of film and film stress.
Causes and solutions of poor film strength
The bonding of substrate and film

Generally, in anti-reflection coating, bonding of substrate and film is the main cause of film weakness. Because some harmful impurities will inevitably be adhered to the surface of substrate in optical cold working and cleaning. There are damage layers on the surface of substrate due to optical cold working, in the depth of damage layer, there are impurities (such as water vapor, oil vapor, cleaning fluid, wiping fluid, polishing powder, in which water vapor is the main). It is difficult to remove these impurities, especially for substrate of strong hydrophilic and adsorption .
When molecules of film material pile up on impurities, it will affect film adhesion, which also affects film strength.
In addition, if the substrate of poor hydrophilicity, poor adsorption, the adsorption of the film is also poor, the same will affect the membrane strength.
Due to poor chemical stability of saltpeter, the surface of substrate has been corroded during pre-processing and formed a corrosion layer or hydrolytic layer (maybe local and extremely thin) . Coated on corrosion layer or hydrolysis layer, films will be poor in adsorption and fastness.
If there are dirt, oil spots, dust spots, saliva spots on the surface of substrate, local film adhesion will be poor, which result in poor fastness.
Improvement solution
Enhancing oil decontamination, if cleaning by ultrasonic, focusing on the function of oil removal, and to ensure the effectiveness of oil removal solution; if cleaning by hand wiping, calcium carbonate powder wipe fist and then wiping by hands.
Pay more attention to bakeout before coating, if possible, making substrate temperature above 300 °C for more than 20min, try to volatilize water vapor and oil vapor on the substrate.
Note: In higher temperature, adsorbability of substrate will be higher and dust is easily adsorbed. Therefore, the cleanliness of the vacuum chamber should be improved. Otherwise, there will be dust on substrate before coating, and film strength will be affected (In vacuum, the chemical desorption temperature is above 260 °C for water vapor on substrate). But not all parts need high temperature baking, for some saltpeter, film strength will not be higher in high temperature, and even there will be spots. This is related much to the stress and thermal matching of material
Installing condensers for unit, it can improve unit vacuum pumping speed and remove water vapor and oil gas on substrate.
Increasing the vacuum degree of evaporation, for more than 1 meter coater, vacuum of evaporation start-up should be higher than 3*10-3 Pa. The larger the coater you use, the higher the vacuum degree should be set.
Installing ion source for unit, Before coating, we bombard and clean the surface of substrate by the ion source. Using ion source to assist coating is conducive to strengthen film.
To dehumidify the film, put the film in a petri and dry it in vacuum chamber.
Keep working environment dry (including lens wiping and exumbrella working area). Do not bring in too much water vapor during cleaning.
For multi-layer films, in the design of the film system, it is necessary to consider the matching of the first layer of film and substrate, and it’s better to use Al2O3 film which has a good adsorption on most substrates. For metal films, the first layer can be Cr or Cr alloy films, these material has good adsorption on the substrate, too.
Adopt abrasive solution (polishing solution) to remove the corrosion layer (hydrolytic layer) on the lens surface.
Sometimes it is helpful to decrease evaporation rate to increase film strength and it’s also helpful for smoothness of the film surface.

Film Stress

The film forming process is transition of material form, and there will be stress in the film layer after film forming. For multi-layer film, there are different combinations of films, and the stress of each film layer is different, some are tensile stress, some are compressive stress and thermal stress of film and substrate.
Stress is harmful for film strength, the film can not stand friction, and even resulting in the film cracking.
For anti-reflective film, the stress is not obvious because there are not many layers(but for some nitrocellulose lenses, even anti-reflective film has stress.) For high-reflective film and filter film with more layers, stress is a common unfavorable factor, we should pay more attention.

Improvement solution

Finish films should be baked after coating, Don’t stop baking immediately after finishing coating, please keep 10minutes "tempering" to stabilize the film structure.
Cooling time need to be prolonged properly to reduce the thermal stress due to vacuum chamber temperature difference.
During evaporation of high-reflection film and filter film, the substrate temperature should not be too high, otherwise film is prone to generating thermal stress. Moreover, it will have negative effect on optical stability of film such as titanium oxide and tantalum oxide.
Adopting ion to assist coating to reduce film stress.
Please select appropriate film system matching. The first layer of film shall be matched well with substrate. (Such as five-layer anti-reflection film is coated via Al2O3 -ZrO2- Al2O3- Al2O3- ZrO2- MgF2. we can adopt SV-5 for ZrO2 film deposition, (SV-5, a kind of doping film of ZrO2 and TiO2.) ) . Or other mixed high refractive film.
Decrease evaporation rate (Al2O3 -2.5A/S; ZrO2 -3A/S; MgF2 -6A/S reference rate)
All oxide films are reactive coated with oxygen-filled, and the oxygen intake is controlled according to different films.
Surface hardness of film on outer layer 
Generally, MgF2 is used as outer layer of anti-reflection film. The profile of MgF2 film is loose columnar structure with low surface hardness and easy to be wiped with scratches.

Improvement solution

If the film design is suitable, please add about 10nm SiO2 on outer layer. The surface smoothness of silicon dioxide is better than that of magnesium fluoride (but the wear resistance and hardness of silica are not as good as magnesium fluoride). Film will get better firmness by ion bombardment for a few minutes after coating. (But film surface will become rough)
After lens being taken out of vacuum chamber, it needs to be placed in a dry and clean place to prevent rapid moisture absorption and surface hardness reduction.

Other reasons

There are some other causes for poor film, such as low vacuum degree( always occurs in manually controlled machine) , dirty vacuum chamber, substrate heating is not heated properly.
While auxiliary gas is filled, film is also degassing, which reduces the vacuum and molecular free path, as well as fastness of film layers. Therefore, when auxiliary gas is filled, the degassing of film should be considered. Before coating, film should be fully premelted and degassed to avoid the excessive decrease of vacuum degree caused by film degassing, thus affecting film strength.

Film detachment

Although it is a kind of film weakness, it is different from the former situations. it is characterized by point-like, margin and partial film detachment.
The main reason is that there are dirt or contaminants in films.
Improvement solution:
Improve the cleanliness of substrate.

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