Coating by optical
vacuum coater, causes of poor film strength, and combination of substrate
and film layer. Generally, this is the main
reason for the weak anti-reflection film. It is inevitable that some harmful impurities will be attached to the
surface of the substrate in the process of optical cold working and cleaning,
and there will always be some damaged layers on the surface of the substrate
due to the effect of optical cold working. Impurities (such as water vapor, oil
vapor, cleaning liquid, wiping liquid, polishing powder, etc., among which
water vapor is the main one) deeply embedded in the damaged layers are
difficult to be cleaned by general methods, especially for substrates with good
hydrophilicity and strong adsorption. When the film material molecules accumulate on these impurities, it will
affect the adhesion of the film layer, which will also affect the film strength.
In addition, if the substrate has poor hydrophilicity and
adsorption force, it will also have poor adsorption to the film layer, which
will also affect the film strength. The chemical stability of
nitrate material is poor, and the surface of the substrate has been corroded
during the circulation in the pre-processing process, forming a corrosion layer
or hydrolysis layer (perhaps local and extremely thin). If the film is plated on the corrosion layer or hydrolysis layer, its
adsorption is poor and the film firmness is poor. The surface of the substrate has dirt, oil spots, gray spots, saliva
spots, etc., and local film adhesion is poor, resulting in poor local film
firmness.
Improvement measures: (I) Strengthen
deoiling and decontamination treatment. If it is ultrasonic cleaning, it should
focus on deoiling function and ensure the effectiveness of deoiling solution; If it is hand rubbing, consider wiping with calcium carbonate powder
before cleaning.
(II) Strengthen baking before plating. If conditions permit,
the temperature of the substrate can reach above 300℃, preferably,
and keep the temperature for more than 20 minutes, so as to volatilize the
water vapor and oil vapor on the surface of the substrate as much as possible. * Note: The higher the temperature, the greater the adsorption capacity of
the substrate, and the easier it is to adsorb dust. Therefore, the cleanliness of the vacuum chamber should be improved. Otherwise, dust will adhere to the substrate before plating, which will
not only cause other defects, but also affect the film strength. (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 low film strength
when the temperature is high, and there will be spots. This has a great relationship with stress and material thermal matching.
(III) When conditions permit, the unit is equipped with a
plough cold, which can not only improve the vacuum pumping speed of the unit,
but also help to remove water vapor and oil gas from the substrate.
(IV) Improve the vacuum degree of evaporation. For coating
machines over 1m, the starting vacuum of evaporation should be higher than
3*10-3Pa. The larger the coating machine, the higher the starting vacuum of
evaporation.
(V) When conditions permit, the unit shall install ion
source, bombard before plating, clean the surface of the substrate, and assist
the coating process, which is beneficial to the compactness and firmness of the
film layer.
(VI) Dehumidification of the film materials, namely, placing
the film materials to be used in a Petri dish in a vacuum chamber for drying.
(VII) Keep the working environment dry (including lens wiping
and umbrella working area), and do not bring too much water vapor when cleaning
the working environment.
The combination of substrate and film layer is one of the important
reasons for the poor film strength of optical vacuum coating machine,
and it is also the reason that all processing manufacturers pay special
attention to.
