Analysis of Common Defects of Optical Coating Products and Improvement Methods

AR coating


We often say that optical coating products include narrow-band filters, beam splitters, increasing lenses (anti-reflection mirrors), high-reflection films, neutral filters, cut-off films, etc., and the products are 100% selected and qualified for shipment. Among them, there will be a variety of different causes of poor coating. So in the process of coating, how to improve the quality? The defect of the coating product is partly caused by the coating itself, and another part is caused by the defect left over from the previous process, and the final quality of the coating.

  1. Membrane strength

Film strength is an important indicator of lens coating, and it is also the most common defective item in the coating process.

Poor membrane strength (membrane weakness) is mainly manifested in:

① Wipe or tear with special tape to cause pieces to fall off;

② Wipe or tear with special tape to cause point-like shedding;

③ After boiling in water for 15 minutes, pull and tear with special tape to produce dots or flakes off;

④ Use a special rubber head and 1Kg force to rub 40 times, and there will be scratches;

⑤ There are cracks and mesh-like fine roads on the film layer when it is wiped or not wiped.

Ideas for improvement: The combination of the substrate and the film layer is the primary consideration, followed by the hardness and smoothness of the film surface and film stress.

Causes and countermeasures of poor film strength:

① Combination of substrate and film layer.

In general, in AR coatings, this is the main cause of film weakness. Since the surface of the substrate will inevitably have some harmful impurities attached to the surface during the optical cold processing and cleaning process, and the surface of the substrate will always have some damaged layers due to the effect of optical cold processing, and impurities (such as water vapor) that penetrate into the damaged layer , oil vapor, cleaning liquid, wiping liquid, polishing powder, etc., of which water vapor is the main one), it is difficult to remove it by general methods, especially for substrates with good hydrophilicity and strong adsorption. When the film material molecules accumulate on these impurities, it affects the adhesion of the film layer, which also affects the film strength.

In addition, if the substrate has poor hydrophilicity and poor adsorption force, it will also have poor adsorption to the film layer, which will also affect the film strength.

The chemical stability of the nitrate material is poor, and the surface of the substrate has been corroded during the transfer process of the previous processing, forming a corrosion layer or a hydrolysis layer (perhaps local and extremely thin). The adsorption of the film layer on the corrosion layer or the hydrolysis layer is poor, and the film firmness is poor.

There are dirt, oil spots, gray spots, saliva spots, etc. on the surface of the substrate, and the local film layer is poorly attached, resulting in poor local film firmness.

Improve strategy:

(1) Strengthen degreasing and decontamination treatment. If it is ultrasonic cleaning, the degreasing function should be considered and the effectiveness of the degreasing solution should be considered; if it is hand rubbing, it can be considered to wipe with calcium carbonate powder first and then wipe.

(2) Strengthen the pre-plating baking, if the conditions permit, the temperature of the substrate can reach 300 ℃ or more, and keep the temperature for more than 20 minutes, so as 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, and it is also easy to absorb dust. Therefore, the cleanliness of the vacuum chamber should be improved. Otherwise, there will be dust attached to the substrate before plating, which will affect the film strength in addition to other defects. (The chemical desorption temperature of water vapor on the substrate in vacuum is above 260°C). But not all parts need to be baked at high temperature. Some nitrate materials have high temperature, but the film strength is not high and stains will appear. This has a greater relationship with stress and material thermal matching.

(3) When conditions permit, the unit is equipped with a condenser (PLOYCOLD). In addition to increasing the vacuum pumping speed of the unit, it can also help remove water vapor and oil gas from the substrate.

(iv) Increase the vacuum degree of evaporation. For a coating machine over 1 meter in length, the starting vacuum of evaporation should be higher than 3*10-3Pa. The larger the coating machine, the higher the starting vacuum of evaporation.

(5) When conditions permit, the unit installs an ion source, bombards before plating, cleans the surface of the substrate, and assists the coating process, which is conducive to the compactness and firmness of the film layer.

(vi) For the dehumidification of the film material, put the ready-to-use film material in a petri dish and dry it in a vacuum chamber.

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

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

(9) Use abrasive liquid (polishing liquid) to re-remove the corrosion layer (hydrolysis layer) on the surface of the lens

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

② Film stress:

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

The existence of stress is detrimental to the strength of the film. The light one is that the film layer cannot withstand friction, and the severe one will cause cracks or mesh fine channels in the film layer.

For the anti-reflection film, due to the small number of layers, the stress is generally not obvious. (But some glass lenses have stress problems even if they are anti-reflection films.) For high-reflection films and filter films with many layers, the stress is A common adverse factor that deserves special attention.

Improve strategy:

(1) Baking after plating. After the last layer of film is plated, do not stop baking immediately, and continue to “temper” for 10 minutes. Let the film structure tend to be stable.

(1) Prolong the cooling time properly and anneal aging. Reduce the thermal stress caused by the large temperature difference between the inside and outside of the vacuum chamber.

(2) During the evaporation process of high-reflection film and filter film, the temperature of the substrate should not be too high, and high temperature is easy to generate thermal stress. And it has a negative effect on the optical stability of titanium oxide, tantalum oxide and other film materials.

(iii) The coating process is ion-assisted to reduce stress.

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

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

(vi) Oxygenate reaction plating for all oxide film materials, and control the oxygen intake according to different film materials.

③ Surface hardness of the outer film:

Generally, MgF2 is used for the outer layer of the anti-reflection film. The section of the film layer is a relatively loose columnar structure, and the surface hardness is not high, so it is easy to wipe out holes.

Improve strategy:

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

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

④ Other

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

When the auxiliary gas is filled, the film material is also degassing, which reduces the vacuum degree, reduces the free path of molecules, and makes the film layer unstable. Therefore, the filling of the auxiliary gas should consider the degassing of the film material. The film material should be fully pre-melted and fully degassed before plating, which can also avoid the excessive decrease of the vacuum degree caused by the degassing of the film material during evaporation, which will affect the film strength.

⑤ Stripping

Although the stripping here is also a kind of weak film, it has some differences from the aforementioned stripping. The main features are: spot stripping, edge stripping, and partial stripping. The main reason is that there is dirt or pollutants in the membrane.

Improve methods:

Improve the cleanliness of the substrate.

  1. Membrane material point

Poor film material spots are also a common problem in coating products. In Japanese companies and Taiwanese companies, film material spots are called “spot holes”.

As the name implies, the film material point is the evaporation process. The large particle film material point is evaporated to the surface of the substrate along with the film material vapor molecules, forming point-like protrusions on the surface of the substrate. Sometimes it is a single point, and in severe cases it is a sheet The fine points and 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. The material whose melting point temperature is higher than the evaporation temperature is directly gasified and evaporated from the solid state, which is a sublimation material; the material whose melting point temperature is lower than the evaporation temperature first turns from a solid state into a liquid state and then evaporates into a gaseous state, which is a non-sublimation material; the melting point temperature is equivalent to the evaporation temperature The material evaporates from solid to gaseous state, and evaporates while melting. It is a semi-sublimation material.

Among them, non-sublimation materials are most likely to produce film material points, because the liquid film material will boil if it continues to be heated, and the bubbles in the film material will overflow during boiling, and the possibility of splashing film material points will increase. Sometimes there is a large splash when the material is pre-melted.

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

The following table shows the sublimation characteristics of several commonly used membrane materials

material name

Melting temperature

Evaporation temperature

splash possible

Remark

ZnS

1900

1100

extremely small

SiO2

1700

1600

very small

Al2O3

2020

2100

Small

ZrO2

2715

2700

Small

TiO2

1850

2000

big

Ta2O5

1800

2100

larger

MgF2

1266

1540

big

The film material is impure and the film material is mixed, that is, there are materials with inconsistent melting point temperature and evaporation temperature in the film material, which is also the reason for the film material point.

Improvement ideas: choose good film materials, fully pre-dissolve, and control the rate.

Improve strategy:

(1) Choose film materials with less impurities

(2) Select the film material with suitable particles for the film material that is easy to splash

(3) The film material is sieved with a mesh screen before plating

(iv) Careful pre-melting, MgF2 must melt through the film materials required for one-time evaporation, and Ta2O5 and TiO2 must be completely melted.

(v) When using one electron gun to plate several kinds of film materials, prevent the film materials from being mixed with the film materials during the rotation of the crucible and the film material slag falling from the baffle to cause film material pollution. Once the crucible membrane material is found to be contaminated, it should be replaced immediately.

(vi) Make the evaporation boat and crucible clean as much as possible. (Clean frequently).

(vii) Select the appropriate evaporation rate and the smoothness of the rate curve. Especially for non-sublimation materials, the evaporation rate should not be high.

(viii) Dehumidify the film material, put the ready-to-use film material in a petri dish and dry it in a vacuum chamber

Note: Sometimes there are some small spots in the film layer, which may not be film material spots, but dust spots. The treatment method is different from that of film material spots, and should be strictly distinguished and handled separately.

  1. Film color difference

There are two kinds of film color differences (not including stains), one is that the upper, middle and lower film colors of the whole mask are inconsistent, that is, there are differences in the spectroscopic test curves; the other is that the single film color is inconsistent.

  1. The inconsistency of the upper, middle and lower film colors is called poor uniformity of the whole umbrella (cover), also known as poor umbrella. The main reason is that there is a problem with the uniformity correction plate (correction plate).
  2. Umbrella deformed. It is also one of the reasons for the uneven color of the film, especially the umbrella that has been used for a long time. It used to be uniform, but it gradually became uneven. The deformation of the umbrella may be the main reason.
  3. Inconsistencies in the condition of the film material, especially the sublimation and semi-sublimation film materials being deflected, digging holes, etc. will also seriously affect the uniformity of the entire mask and single sheet. Especially when there is a large amount of manual pre-melting, everyone’s operation methods are different, and the obtained materials will also be different.

Improvement ideas: fully use the function of the correction board.

Improve strategy:

(1) Adjust the correction plate, and try to consider the balance and compatibility of high and low refractive index film materials. If there are two evaporation sources, use their respective correction plates independently if possible to avoid interference.

(2) If conditions permit, planetary fixtures are used.

(3) Umbrella shaping, for umbrella shaping, you can first process an R base mold (select raw materials with higher strength), and then shape the umbrella on the base mold. In order to prevent the deformation of the umbrella sheet, the thickness and raw material selection of the umbrella sheet should be appropriate when ordering the umbrella sheet.

(iv) Strengthen the management of umbrella pieces, especially when placing them, to prevent deformation due to improper placement.

(5) To improve the condition of the film material, especially when the electron gun is vapor-depositing sublimation and semi-sublimation materials, the film material cannot be collapsed or pitted.

(vi) The film material that can be automatically pre-melted should be automatically pre-melted as much as possible to reduce the influence of human factors.

▲ The correction plate is very effective in correcting the physical film thickness, but it is not enough to correct the refractive index, so it is very difficult to completely rely on the correction plate to solve the uniformity of light distribution.

▲ If one correction plate needs to correspond to two electron guns (evaporation sources) and various film materials, there will be greater difficulties.

The reasons for the uneven color of a single film:

The main reason is that the unevenness of the substrate is serious, and the curvature difference from the umbrella sheet is relatively large, and the evaporation angle formed by the normal line of the upper part or the lower part of the substrate and the evaporation source is relatively different. As a result, the conditions for receiving the film material on each part of a lens are greatly different, and the formed film thickness is greatly different.

In addition, the lens is blocked by the edge of the lens ring (disc), and the lens ring (disc) is dirty and pollutes the lens during evaporation, etc., which will also cause the problem of film color difference.

Improvement ideas: improve the evaporation angle at the edge of the lens.

Improve strategy:

(1) If conditions permit, use a planetary fixture;

(2) Choose a machine with a flat umbrella (large R);

(3) According to the hole position distribution of the umbrella sheet and the shape of the substrate, make a special zigzag correction plate.

(iv) If possible, moving the evaporation source to the middle of the vacuum chamber can also improve the color uniformity of the single film.

(v) Improve the lens ring (disc) to prevent occlusion.

(vi) Pay attention to the partial occlusion of the edge lens by the corresponding part of the rotating umbrella stand

(vii) Clean the lens ring (disc)

(viii) Improve the evaporation of film materials.

  1. Membrane dirty (also known as white gram)

As the name suggests, the membrane layer is dirty. Generally, membrane fouling occurs inside or outside the membrane. Dirty can include: dust spots, white mist, oil spots, fingerprints, saliva spots, etc. (Single row of dust spots and white mist)

Ideas for improvement: Review the process to prevent dirty pollution.

Improve strategy:

(1) The lenses sent for cleaning or wiping should not have too many bad attachments;

(2) Enhance the cleaning rate or cleaning rate of the pre-coating lens;

(3) Improve the environment for placing the lenses to be plated after the umbrella is put on, and prevent and control pollution;

(iv) Cultivate good habits of umbrella operators to prevent lens pollution (fingerprints, saliva spots and others);

(vii) Speed ​​up the replacement cycle of the vacuum chamber shield;

(viii) Clean the inflation pipeline to prevent pollution when gas is charged;

(9) Initial exhaust anti-vortex (turbulence), initial inflation anti-overshoot;

(x) Avoid oil and water vapor (edging, ultrasonic cleaning) pollution during the lens placement environment and handling process.

11 The working environment is transformed into a clean workshop.

12 Separate the operation panel of the coating machine from the main machine to reduce the harmful substances produced by the main machine from polluting the lens.

gray spot dirty

Phenomenon: There are some dots (not film material dots) on the surface or inside of the lens film, some of which can be erased, and some cannot be erased. And there will be punctate release.

cause:

  1. The vacuum chamber is dirty. The air vortex at the beginning of vacuuming will bring the dirty ash on the bottom plate and guard plate of the vacuum chamber to the lens, forming a layer of gray spots. (Inside the film, cannot be erased, and will be dotted to release the film)
  2. The lens ring or disc is dirty, with floating dust, which is attached to the lens under the action of the ion beam to form a layer of gray spots. (Inside the film, cannot be erased, and will be dotted to release the film)
  3. There are dust spots when the lens is put on the umbrella, and there is no inspection and selection when putting the umbrella on. (Inside the film, cannot be erased, and will be dotted to release the film)
  4. Environmental pollution after plating is the main cause of gray spots on the outside of the film, especially when the lens is hot, it is easier to absorb dust and it is difficult to erase. (outside the film)
  5. Dirty environment at the inflation port of the vacuum chamber, excessive inflation at the beginning, dirty inflation filter, and high temperature of the lens during inflation are also the reasons for poor dust spots on the outside of the lens film. (outside the film)
  6. Dust pollution caused by the operator (membrane inner membrane and outer membrane)
  7. Too much dust in the working environment

Improvement idea: eliminate dust source

Improve strategy:

  1. The working environment is transformed into a clean workshop, which is implemented in strict accordance with the clean workshop specifications.
  2. Clean the environment as much as possible. Make use of clean benches as much as possible.
  3. The vacuum chamber is cleaned periodically to keep it clean.

White fog outside the film

Phenomenon: After the coating is completed, there are some faint white mist on the surface. Wipe with acetone or mixed solution, and the more you wipe, the more serious the phenomenon will be. Wipe off or lighten with cerium oxide powder, (aka: wipeable acrylic)

Analysis: The cause of the white fog outside the membrane is more complicated, and the possible causes are:

① Membrane structure problem: the columnar structure of the outer membrane is loose, and the outer membrane is too rough;

② The evaporation angle is too large and the film structure is rough.

③ Temperature difference: When the lens is out of the cover, the temperature difference between the inside and outside is too large

④ Moisture; the moisture of the environment where the lens is placed after it is out of the cover

⑤ Water vapor is too heavy when POLYCOLD is thawed in the vacuum chamber

⑥ Oxygenation is not complete during evaporation, and the film structure is uneven

⑦ Stress between membranes

Ideas for improvement: There are many causes of white fog outside the membrane, but each has its own characteristics. Try to prescribe the right medicine. The main idea is to make the membrane dense and smooth so that it is not easy to adsorb, and the second is to improve the environment and reduce the objects of adsorption.

Improve strategy:

(1) Improve the film system and add silicon dioxide to the outer layer to make the surface of the film smooth and not easy to adsorb. Improve the environment when the lens comes out of the cover (dry, clean)

(2) Reduce the temperature of the lens when it is out of the cover (extend the cooling time in the vacuum chamber) to reduce the temperature difference and reduce the stress.

(iii) Improve oxygenation (increase) and improve membrane structure.

(iv) Appropriately reduce the evaporation rate and improve the columnar structure

(v) Ion-assisted coating to improve the film structure

(vi) Add a small inflation valve when the polycold is thawed (its function is to take away the water vapor in time)

(vii) Find ways to improve the evaporation angle from the evaporation source and fixture.

(viii) Improve substrate surface roughness.

(9) Pay attention to the vacuum degree when the polycold is thawed.

Intrafilm white fog

White mist is formed in the film and cannot be removed by wiping.

Possible causes:

① The substrate is dirty, residues from the pre-attachment process

② Corrosion pollution on the lens surface

③ The matching between the film material and the film material, and between the film material and the substrate.

④ Insufficient oxygenation of oxides.

⑤ The first layer of zirconia film material may produce white halo phenomenon on some substrates

⑥ The substrate is polluted by moisture before entering the hood (after cleaning)

⑦ Poor washing or wiping, washing marks, wiping marks

⑧ The vacuum chamber is dirty and the moisture is too heavy

⑨ High ambient humidity

Ideas for improvement: The problem of the substrate itself may be the main problem, and the coating is to make up for it as much as possible. The biggest problem of the coating itself may be the matching of the film material.

Improve strategy:

(1) Improve the film system, the first layer does not use zirconia.

(2) Minimize the opening time of the vacuum chamber, and clean the vacuum chamber and prepare for coating in the shortest time between hoods.

(3) After the guard plate is replaced and cleaned, it is best to vacuumize and bake the vacuum chamber with an empty cover. The replaced guard plate and other vacuum chamber components must be dry and clean.

(iv) Improve the environment

(5) Properly protect the lens on the umbrella sheet before entering the cover from pollution.

(vi) Improve cleaning and wiping effects.

(vii) Improve film matching (consider Al2O3 for the first layer)

(viii) Improved membrane oxygenation and evaporation rate (reduction)

(9) To speed up the process of the former project. The protection of the processed smooth surface is strengthened by the previous project.

(∩) The smooth surface after polishing must be cleaned immediately, and there must be no polishing powder or other impurities attached to it.

  1. Spots

Pigmentation (also known as film color pressure, ablation) refers to the local variation (generally irregular) of film color on the lens. There are two types of stains inside the membrane (including stains on the membrane layer) and stains outside the membrane. Sometimes regular local areas (such as the center or ring area of ​​concave lenses, etc.) film color variation.

Causes of spots:

Local refractive index variation: The so-called film color is a reflection of the spectral characteristics of the thin film. The design and achievement of the spectral spectral characteristics of the thin film are based on the substrate with a certain refractive index. If the local refractive index of the substrate changes, the coated film layer will Local spectroscopic properties also vary. The film color of local points will vary, forming color spots in the film.

Due to various reasons during and after the coating, the local refractive index of the film layer varies, forming the film layer or color spots outside the film.

The stains outside the film are generally shallow and can be wiped off with polishing powder or calcium carbonate powder. If it is a film stain with local refractive index variation in the middle layer or the bottom layer, it cannot be removed with polishing powder or calcium carbonate powder.

An effective means to distinguish between the inner membrane layer and the outer membrane stain is to wipe with polishing powder or calcium carbonate powder.

The treatment strategies for stains inside the film, on the film layer and outside the film are different.

Sources of intramembranous stains:

Most of the local refractive index changes on the substrate are caused by local corrosion of the substrate. The etch layer generally forms an extremely thin low refractive index layer.

  1. In the process of front processing, it is brought by fixtures and processing methods.

This part may be large, small, or mostly, so there may be stains of different sizes. This kind of stain has a characteristic, the shape of the stain is regular, the location is consistent, and the boundary is clear.

  1. Turnover, transportation, inventory generation.

Some substrates have poor chemical properties, such as H-ZK9, which has weak moisture resistance and acid resistance. During the pre-processing process, it will inevitably come into contact with water and moist air to form corrosion. If the cycle of pre-processing and coating If it is long (more than 5 hours), it is easy to produce stains in the film. If it is a reflective film or an infrared cut-off film, this stain will be very obvious. It may also be a good product just after plating, and stains will appear after a few days. and become defective.

  1. The grinding and polishing project does not clean the surface in time after the surface of the lens is completed, and the residual polishing powder and liquid dry on the surface of the lens, causing corrosion or pollution to the lens. This kind of corrosion or pollution cannot be removed by washing or wiping, and the color spots appear after coating.
  2. The pH matching of the polishing liquid used in grinding and polishing is not controlled, which affects the chemical stability of the lens between grinding and coating.

Sources of stains on the film layer and outside the film:

  1. After coating, impurities that are difficult to eliminate penetrate into the gaps of the film layer, which changes the refractive index of the local film layer, thereby forming color spots outside the film.
  2. During the coating process, the temperature of some high-refractive-index substrates is too high, causing changes in the refractive index of the local film layer (or possibly the joint between the film layer and the substrate). It can also cause stains on the film layer.
  3. In the film system matching, some films are too thin, the crystal is in an unstable state, and film stains may also occur.
  4. The poor match between the film material selection of the film system and the substrate material is also one of the reasons for the uneven color of the film layer.
  5. A small amount of oil return from the unit forms local extremely thin oil spots in the lens or film layer, which is also the cause of film stains. The film strength at such spots is generally weaker.

Countermeasures for improving spots in the film:

  1. Speed ​​up the cycle from grinding (polishing) to coating, and reduce the chance of the lens being polluted and corroded. Note: it is the entire polished surface of the lens.
  2. During the polishing process, pay attention to the protection of the other polished surface
  3. Pay attention to the tooling, fixtures and processing methods in the polishing process, so as not to cause local corrosion damage to the lens surface.
  4. The smooth surface after polishing must be cleaned immediately, and there must be no polishing powder or other impurities attached to it.
  5. Control the pH value of the grinding and polishing liquid.
  6. Before coating, use polishing powder (cerium oxide, iron oxide) or calcium carbonate powder (mixed with glycerin or water) to renew the polished surface of the lens. And clean it up as soon as possible.
  7. Strengthen the ion bombardment before plating
  8. For the anti-reflection film in the visible light area, it is designed and manufactured into a single peak shape under the premise of meeting the technical requirements, and the reflection color is light green to cover up the color spots
  9. For lenses with better chemical properties, soak them in film-removing solution or dilute acid solution to remove corrosion spots before cleaning.
  10. Selecting the appropriate film layer matching is also helpful for the improvement of stains.
  11. Increase the temperature of the substrate during plating to speed up the complete volatilization of water vapor. (However, there may be stains on the film layer, and the countermeasures should be analyzed according to the specific situation.)
  12. The first layer coated with Al2O3 film will generally have a good improvement effect.

Countermeasures for improving film layer and stains outside the film:

  1. For the anti-reflection film, when the design conditions permit, add a SiO2 layer to the outer layer, about 10nm (the general outer layer is MgF2). The outer layer tends to be smooth and dense, reducing the erosion of harmful substances. (The SiO2 layer will be rough if ions bombard it after plating.)
  2. Appropriately reduce the evaporation rate (within a certain range) to improve the smoothness of the film layer and reduce adsorption.
  3. After the lens is out of the cover, wait for it to cool down before putting it under the umbrella and wiping it.
  4. After the lens is out of the cover, place it in a clean and dry place to cool down. Reduce the possibility of pollution.
  5. Use calcium carbonate powder to gently wipe off the outer layer of attachments.
  6. Improve the humidity and temperature difference of the working environment.
  7. Improve the environment near the air filling port, so that the filled air is dry and clean.
  8. The personal hygiene of the staff (masks, clothing, gloves, finger cots, etc.) has been improved.
  9. Check the oil return status in the vacuum chamber to prevent oil return.
  10. Properly reduce the substrate temperature; (cannot affect the film strength)
  11. Improve the film system, cancel the film that is too thin, and select the appropriate film material according to the characteristics of the nitrate material.
  12. Spectral characteristics

In optical thin film products, poor spectral characteristics (poor spectroscopic) is a common problem. Poor spectral characteristics refer to spectroscopic reflection (transmission) curves that do not meet the technical requirements of parts and products. They are functional defects and must be strictly monitored during production and manufacturing.

There are many reasons for poor spectral characteristics, the main ones are:

  1. Film system design: The film thickness and refractive index tolerance during design are too small, and the spectroscopic curve during trial production is on the edge of the technical requirements. A slight deviation in manufacturing will lead to poor spectroscopic.
  2. The designed refractive index of the film material is different from the actual refractive index, or has changed.
  3. The center wavelength (film thickness) of the actual plating is different from the desired center wavelength (film thickness), or has changed. (the tooling value (also called F value) is biased)
  4. Mistakes in manufacturing: such as using the wrong film material, using the wrong program, not closing the baffle during pre-melting, etc.
  5. Changes in process conditions: vacuum degree, oxygenation amount, heating temperature, evaporation rate, substrate rotation speed, ion auxiliary conditions, etc.
  6. Material changes, such as the same optical material (substrate material and film material) produced by different manufacturers have different optical properties and chemical properties (sometimes different production batches from the same manufacturer), during the production process (especially in large quantities) Production) material sudden change (not demonstrated), it may cause poor spectroscopic.
  7. The surface characteristics of the comparison sheet used to test the spectroscopic variation vary, resulting in a poor spectroscopic test (maybe the spectroscopic lens is OK, and the color of the reflective film of the two can be compared). This may be a problem that is easily overlooked, and it is also a common problem, especially a corrosion layer is formed on the surface of the test comparison sheet with a high refractive index, which is equivalent to a layer of anti-reflection film (very thin), and the film layer is not accumulated. On the substrate, it is deposited on the corrosion layer, so the light spectrum on the comparison sheet will be inaccurate and unstable. Often the processing and storage time of comparative film is much longer than that of substrate, and the storage environment is not as good as that of lens, so the problem will be more serious.
  8. The process stability of the unit is poor. (The pumping speed is not warm, the vibration of the unit is large, the test piece or chip shakes, the rotation is not warm, the umbrella is deformed, the temperature measurement error is large, the heating power is unstable, etc.)
  9. If a crystal oscillator is used to control the film thickness, the quality of the crystal oscillator, the service life of the crystal oscillator, and the change of the activity value. For the same crystal oscillator, there are some differences in the sensitivity between the initial use and the use for a period of time (such as 6 layers of anti-reflection coating coated with 3 covers), which will also cause the overall curve to shift.
  10. The poor water cooling of the crystal control probe makes the crystal control unstable and unreliable, and the film thickness control is inaccurate.
  11. There will be differences in spectroscopic characteristics between the test just after plating and the test after a period of time.
  12. The refractive index of some film materials will change after film formation (related to film formation conditions and film layer matching), which will cause changes in spectral characteristics.
  13. The refractive index of the lens varies. Some substrate materials form an extremely thin low-refractive index layer on the surface after ultrasonic cleaning, which also affects the spectral characteristics and film color.
  14. When using light control (non-automatic control), the light value is not set properly, or the set light value is deviated due to the change of the refractive index of the film material, resulting in deviation or instability of the spectral characteristics.
  15. The monitoring film in the light control itself has a corrosion layer, which affects the value of the light control.
  16. The optical signal in the light control is unstable (voltage fluctuations, poor contact, electronic component problems, etc.) affect the accuracy and stability.
  17. In light control, some film layers are relatively thin (especially when the first layer is relatively thin) less than a peak value, which brings inaccuracy of light control.
  18. For non-automatic light control, there is an error in human judgment.

Improve strategy:

  1. Design film system:

The refractive index of the film material selected in the film system design should be consistent with the machine used for the film material;

The film system design should consider the tolerance of thickness and refractive index as far as possible (the thickness and refractive index of each layer are allowed to deviate from 1% to 2%), especially the sensitive layer, there must be a certain tolerance (1%)

●The “tooling” value of the control thickness and the actual test thickness should be accurate, and the adjustment should be confirmed frequently.

●The technical requirements of the design must be higher than those proposed in the drawings.

●The influence brought by the change of the refractive index of the variation layer of the substrate is considered in the design.

●The change of the refractive index of the film material and the matching between the film material and between the film material and the substrate are considered in the design.

  1. Once the manufacturers and models of membrane materials, chips, and nitrate materials used at the work site are confirmed, do not change them frequently, and those that must be changed should be confirmed multiple times.
  2. Eliminate and avoid the occurrence of operational errors,
  3. Strengthen the spectroscopic test and monitoring of each lens, set a warning spectroscopic curve, and adjust the film system in time.
  4. The management of test comparison sheets is strengthened to ensure that the surface of the test comparison sheets for coating in the mask is free from pollution, fresh, and the appearance meets the specified requirements. Before use, do a test on the comparison sheet to measure its reflectivity (only one wavelength point is enough). The measured value is compared with the theoretical value. Generally, the measured value is smaller than the theoretical value (corrosion layer influence), if the difference between the two values ​​is large If it is too large (for example, greater than 1%), it should be considered to renew or replace the comparison sheet.

Sixth, the spectroscopic test of the lens should be carried out after the substrate is completely cooled.

  1. Grasp the change law of the sensitivity of the crystal control chip, and correct the control data in time. The sensitivity of the crystal control chip is not exactly the same when it is new and after using several covers, and the acoustic impedance value of the chip will change slightly. Some crystal controllers (such as IC5) can be set to automatically correct, but most crystal controllers do not have the function of automatically correcting the acoustic impedance value. After mastering the law of wafer sensitivity, it can be corrected in the film thickness setting.
  2. Improve the cooling effect of the crystal-controlled probe. When the temperature of the chip is higher than 50°C, the measurement error will be large.
  3. The ion-assisted coating process can improve the stability of the film-forming spectroscopic characteristics.
  4. Review the light control applicability of the film system on the machine.
  5. Review the artificial influence in light control
  6. Check the optical path, signal, test piece, etc. of the light control frequently.
  7. Design a film system suitable for light control.
  8. Remedy for poor spectral separation (complementary color)

There are two cases of spectroscopic defect: one is that after the coating of all coatings is completed, the spectroscopic defect is tested. This type of defect is mainly dealt with by the method described in Section 6, and it is difficult to remedy the general anti-reflection coating. However, for high-reflection film, band-pass filter film, etc., it can be remedied by adding layers. The second is that the spectroscopic failure caused by interruption in the middle of plating (including the discovery of error interruption) can generally be remedied through follow-up efforts. Follow-up methods are correct, and the success rate of remediation is relatively high.

The reason for the interruption takes the form of:

① Power outage,

② Machine failure

③ Artificial interruption (interruption after finding errors and doubts)

Information after the middle paragraph:

(1) Know the number of layers to be plated, and the film thickness of each layer that has been plated;

(2) It is known which layer is plated, and the film thickness of the last layer is uncertain;

(iii) I don’t know how much was plated.

Remedial treatment:

  1. For the first case, it is easier to deal with, as long as you confirm that the previous plating is correct and the program is not wrong, you can continue the original program. It should be noted that if a part of a certain layer is plated and continues to be plated, the junction must be Reduce some film thickness (depending on the film material and evaporation rate to determine how much to reduce, generally about 0.2-1nm), if the remaining film thickness of the layer is less than 15-20 seconds, consider reducing the evaporation rate or simply not plating , is solved by adjusting the film thickness of subsequent layers.
  2. The handling of the second and third cases is more complicated,

Simulation: Input the optimized target value of the computer film system design program according to the measured spectroscopic data of the already coated lens (test comparison piece), and then input the film system information that has been mastered, and use the reverse method to optimize layer by layer to simulate the actual coating. film data.

*The test comparison sheet refers to the flat sheet coated with the lens (on the umbrella sheet, with the same refractive index as the lens), and used to test the spectroscopic curve after coating.

Optimization: Re-lock the film system data obtained through simulation, and find the best solution to achieve the goal through subsequent layer film thickness optimization.

Trial coating: according to the newly optimized follow-up coating data, try coating several lenses (1-2 pieces) or test pieces to confirm the feasibility of the complementary color coating system.

Complementary color plating: implement complementary color plating after confirming the test plating situation. Before complementary color plating, confirm whether the substrate is clean to prevent other defects.

  1. Handling of other situations:

For the use of the wrong program, wrong operation (pre-melting does not close the baffle, etc.) artificial interruption needs to be remedied; and the treatment plan for the situation that needs to be remedied after the reflective film and filter film are plated:

● Simulation: Input the measured spectroscopic data into the optimized target value of the computer film system design program. Through computer simulation (generally confirming the film thickness of the last layer), find the film system data corresponding to the test value.

● Optimization: According to the film data obtained from the simulation, input the optimization target value required by the product, and re-optimize the design of a remedial film system by adding layers and optimizing subsequent film layers.

● Trial plating: confirm and improve the effect of the remedial film system.

● Remedial plating: complete the remedial work.

Eight, broken edge, bad burst

The general coating will heat the substrate, because the substrate is mounted in a metal (aluminum, copper, stainless steel) ring or disc, and because the thermal expansion coefficient of the mirror ring or disc and the lens (substrate) is inconsistent, it will be heated during the cooling process. Cause the edge of the lens to break or burst.

Some large lenses, due to the high temperature when they come out of the cover, have a large temperature difference with the room temperature, and the thermal stress of the lens will cause the lens to burst or break the edge.

The chamfered shape of the edge of some parts is easy to cause the ring and break the edge.

Improve strategy:

① The design of the fixture (mirror ring, disc) should be reasonable in terms of size matching, and the influence of manufacturing errors should be fully considered.

② Pay attention to the deformation of the lens ring and disc, and the deformed fixture cannot be used

③ Choose suitable fixture materials (non-magnetic materials, no rust, high temperature resistance and no deformation), stainless steel is ideal (small thermal deformation coefficient), but it is difficult to process and expensive.

④ For large lenses, the temperature when coming out of the cover should be lowered to reduce the temperature difference and prevent bursting.

⑤ If it is a mirror ring, you can consider slotting the mirror ring as a buffer. (The service life of the lens ring will be reduced a lot)

Nine, scratches (film injury)

Scratches refer to scratches on the inside and outside of the film surface, the inside of the film is called scratches, and the outside of the film is called film damage. This is also a stubborn problem in the improvement of coating quality. Although the cause and improvement method are well known, it is difficult to cure it.

cause:

Scratches in the film:

① The appearance of the previous project is poor, and some scratches are not easy to find before coating, and the inspection of the previous project and the umbrella inspection before coating are not easy to find, but the scratches will appear after coating.

② Scratches on the lens caused by negligence in each operation

③ The lenses are placed too densely, causing bumps and scratches during the handling process

④ The surface of the lens is scratched by the lens placement utensils and packaging materials

⑤ Scars caused by ultrasonic cleaning

Extramembranous scars (membrane injuries)

① Film damage caused by lens placement utensils and packaging materials.

② Film damage caused by ultrasonic cleaning after plating.

③ Membrane damage caused by negligence in each operation process.

Improvement ideas: review the operation process and related equipment materials, and eliminate scratches and film damage.

Improve strategy:

(1) Strengthen the operator’s operating norms.

(2) Establish a list of operating errors to supervise operators to avoid operating errors.

(iii) Improve the lens placement interval

(iv) Improving the lens handling method

(5) Improve the placement of utensils and packaging materials.

(vi) Improve ultrasonic cleaning process parameters

(vii) Strengthen pre-engineering inspection and pre-plating inspection

It is a comprehensive reflection of the entire optical parts processing (especially polishing and cleaning), and it must be considered comprehensively when countermeasures for poor coatings. It is a comprehensive reflection of the entire optical parts processing (especially polishing and cleaning). Only in this way can we really find out the cause of the defect, and only by improving the countermeasures can we achieve results.

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