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Analysis and Solutions to Film Strength
更新时间: 2021-10-28 16:39:29 访问次数: 0

Analysis and solutions for 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:
膜强度的不良(膜弱) 主要表现为:
Wipe or tear with special tape, resulting in whole pieces film detachment;
① 擦拭或用专用胶带拉撕,产生成片脱落;
Wipe or tear with special tape, film detached in point.
② 擦拭或用专用胶带拉撕,产生点状脱落;
Boil for 15min, pull and tear with special tape to produce spot or pieces film detachment;
③ 水煮15分钟后用专用胶带拉撕产生点状或片状脱落;
40 times with special rubber head and 1Kg force, scratches showing up.
④ 用专用橡皮头、1Kg力摩擦40次,有道子产生;
When the film is wiped or not wiped, cracks and fine meshes appear.
⑤ 膜层擦拭或未擦拭出现龟裂纹、网状细道子。
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
① 基片与膜层的结合。
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.
一般情况, 在减反膜中, 这是膜弱的主要原因。 由于基片表面在光学冷加工及清洗过程中不可避免地会有一些有害杂质附着在表面上, 而基片的表面由于光学冷加工的作用,总有一些破坏层, 深入在破坏层的杂质(如水汽、 油汽、 清洗液、 擦拭液、 抛光粉等,其中水汽为主要), 很难以用一般的方法去除干净, 特别对于亲水性好,吸附力强的基片尤其如此。 当膜料分子堆积在这些杂质上时, 就影响了膜层的附着, 也就影响了膜强度。
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.
硝材化学稳定性差, 基片在前加工过程中流转过程中,表面已经受到腐蚀,形成了腐蚀层或水解层(也许是局部的、极薄的)。膜层镀在腐蚀层或水解层上其吸附就差,膜牢固度不良。
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 methods
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.
㈠ 加强去油去污处理,如果是超声波清洗, 应重点考虑去油功能, 并保证去油溶液的有效性; 如若是手擦,可考虑先用碳酸钙粉擦拭后再清擦。
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.
㈡ 加强镀前烘烤,条件许可, 基片温度能达到 300℃以上更好,恒温 20 分钟以上, 尽可能使基片表面的水汽、油汽挥发。
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
*注意:温度较高,基片吸附能力加大,也容易吸附灰尘。所以,真空室的洁净度要提高。 否则基片在镀前就有灰尘附着,除产生其它不良外, 对膜强度也有影响。(真空中基片上水汽的化学解吸温度在 260℃以上)。但不是所有的零件都需要高温烘烤,有的硝材温度高了反而膜强度不高还会有色斑产生。这与应力以及材料热匹配有较大的关系。
Installing condensers for unit, it can improve unit vacuum pumping speed and remove water vapor and oil gas on substrate.
㈢ 有条件时, 机组安装冷凝机(PLOYCOLD),除提高机组真空抽速外, 还可以帮助基片水汽、 油气去除。
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.
㈣ 提高蒸镀真空度,对于1米以上的镀膜机,蒸镀启动真空应高于 3*10-3 Pa,镀膜机越大,蒸镀启动真空更高。
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.
㈤ 有条件时, 机组安装离子源, 镀前轰击, 清洁基片表面, 镀膜过程辅助, 有利于膜层的密实牢固。
To dehumidify the film, put the film in a petri and dry it in vacuum chamber.
㈥ 膜料的去潮, 将待用膜料用培养皿盛放在真空室干燥。
Keep working environment dry (including lens wiping and exumbrella working area). Do not bring in too much water vapor during cleaning.
㈦ 保持工作环境的干燥(包括镜片擦拭、 上伞工作区),清洁工作环境时不能带入过多的水汽。
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.
㈧ 对于多层膜, 在膜系设计时, 就要考虑第一层膜与基片的匹配,尽可能考虑用 Al2O3 膜料, 该膜料对大部分基片有较好的吸附力。 对于金属膜, 也可考虑第一层镀Cr或Cr合金。Cr或Cr合金对基片也有较好的吸附力。
Adopt abrasive solution (polishing solution) to remove the corrosion layer (hydrolytic layer) on the lens surface.
㈨ 采取研磨液(抛光液) 复新去除镜片表面的腐蚀层(水解层)
Sometimes it is helpful to decrease evaporation rate to increase film strength and it’s also helpful for smoothness of the film surface.
㈩ 有时候适当降低蒸发速率对膜强度的提高有帮助,对提高膜表面的光滑度有积极意义。
Film Stress
② 膜层应力:
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.
薄膜的成膜过程,是一个物质形态的转变过程,不可避免地在成膜后的膜层中会有应力存在,对于多层膜来说有不同膜料的组合,各膜层体现出的应力是有所不同的,有的是张应力、 有的是压应力,还有膜层及基片的热应力。
Stress is harmful for film strength, the film can not stand friction, and even resulting in the film cracking.
应力的存在对膜强度是有害的, 轻者是膜层耐不住摩擦,重者,造成膜层的龟裂或网状细道子。
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.
对于减反膜, 由于层数不多, 应力一般体现不明显,(但有些硝材的镜片即便是减反膜也有应力问题存在)而层数较多的高反膜、滤光膜,应力是一个常见的不良因素,应特别注意。
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.
㈠ 镀后烘烤,最后一层膜镀完后,烘烤不要马上停止,延续10分钟“回火”。让膜层结构趋于稳定。
Cooling time need to be prolonged properly to reduce the thermal stress due to vacuum chamber temperature difference.
㈡ 降温时间适当延长, 退火时效。 减少由于真空室内外温差过大带
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.
㈢ 对高反膜、滤光膜等在蒸镀过程中,基片温度不宜过高, 高温易产生热应力。并且对氧化钛、氧化钽等膜料的光学稳定性有负面作用。
Adopting ion to assist coating to reduce film stress.
㈣ 镀膜过程离子辅助,减少应力。
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.
㈤ 选择合适的膜系匹配。第一层膜料与基片的匹配。(如五层减反膜采用Al2O3-ZrO2-Al2O3-Al2O3-ZrO2-MgF2;ZrO2也可以采用SV-5(一种ZrO2 、TiO2混和膜料)或其他混合高折射率膜料。
Decrease evaporation rate (Al2O3 -2.5A/S; ZrO2 -3A/S; MgF2 -6A/S reference rate)
㈥ 适当减小蒸发速率(Al2O3 -2.5A/S; ZrO2 -3A/S; MgF2 -6A/S参考速率)
All oxide films are reactive coated with oxygen-filled, and the oxygen intake is controlled according to different films.
(七) 对氧化物膜料全部充氧反应镀,根据不同膜料控制氧进气量。
Surface hardness of film on outer layer 
③ 外层膜表面硬度:
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)
㈠ 膜系设计允许时,外层加 10nm左右的SiO2层,二氧化硅的表面光滑度优于氟化镁(但二氧化硅表面耐磨度、硬度不如氟化镁)。镀后离子轰击几分钟, 牢固度效果会更好。(但表面会变粗)
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.
㈡ 镜片出真空室后,放置在较干燥洁净的地方,防治快速吸潮,表面硬度降低。
Other reasons
④ 其它
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.
辅助气体充入时, 膜料也在放气致使真空度降低,使分子自由程减少,膜层不牢。所以辅助气体的充入要考虑膜料的放气,镀前对膜料充分预熔充分放气,也可以避免由于蒸镀中膜料放气造成真空度过度下降,从而影响膜强度。
Film detachment
⑤ 脱膜
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.
改善方法: 提高基片的洁净度。

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