Measures to improve film adhesion strength
The film layer’s adhesion strength refers to the film layer’s firmness on the substrate surface.
Film stress is an essential factor affecting the adhesion strength of the film layer. Film stress is divided into thermal and internal stress, and internal stress is divided into deposition internal stress and additional internal stress.
Since metal vapor molecules are combined with the substrate at high temperatures, thermal stress will inevitably be generated if the thermal expansion coefficients of the evaporation material and the substrate are different. Deposition internal stress is generated during the film formation process due to the large amount of heat released by gas phase atoms during the condensation process and the accumulation and contraction of grains when deposited on the substrate. Additional internal stress is generated when the film is exposed to the atmosphere after it is made or the atmosphere is introduced into the vacuum chamber, causing the film to oxidize.
The thermal stress, deposition internal stress, and additional internal stress of the film will generate a shear stress between the interface of the film layer and the substrate. When the shear stress is greater than the adhesion between the interface of the film layer and the substrate, the film layer will crack, warp, or fall off. Therefore, the reasonable matching of film materials and substrates can reduce the thermal stress of the film. The correct formulation of the film deposition process can minimize the internal stress or make the stresses compensate for each other. This is a crucial measure to improve the film adhesion strength.
In addition, it is necessary to avoid the influence of humid gas, oil pollution, and dust on the film adhesion strength. In the case of moisture or oil pollution, the surface of the substrate will adsorb a layer of water molecules or oil molecules. In this way, it is difficult for the evaporated metal vapor molecules to directly produce physical adsorption with the substrate and even more challenging to produce chemical adsorption. Instead, they can only adhere to the water molecule or oil molecule layer adsorbed by the substrate, weakening the adhesion strength between the film layer and the substrate interface. The dust attached to the substrate hinders the adsorption of the evaporated metal vapor molecules to the substrate. Sand holes (or pinholes) without the film layer are formed where the dust is—the more dust attached to the substrate, the more serious the sand holes.
The following measures are used to improve the film adhesion strength.
Film stress is an essential factor affecting the adhesion strength of the film layer. Film stress is divided into thermal and internal stress, and internal stress is divided into deposition internal stress and additional internal stress.
Since metal vapor molecules are combined with the substrate at high temperatures, thermal stress will inevitably be generated if the thermal expansion coefficients of the evaporation material and the substrate are different. Deposition internal stress is generated during the film formation process due to the large amount of heat released by gas phase atoms during the condensation process and the accumulation and contraction of grains when deposited on the substrate. Additional internal stress is generated when the film is exposed to the atmosphere after it is made or the atmosphere is introduced into the vacuum chamber, causing the film to oxidize.
The thermal stress, deposition internal stress, and additional internal stress of the film will generate a shear stress between the interface of the film layer and the substrate. When the shear stress is greater than the adhesion between the interface of the film layer and the substrate, the film layer will crack, warp, or fall off. Therefore, the reasonable matching of film materials and substrates can reduce the thermal stress of the film. The correct formulation of the film deposition process can minimize the internal stress or make the stresses compensate for each other. This is a crucial measure to improve the film adhesion strength.
In addition, it is necessary to avoid the influence of humid gas, oil pollution, and dust on the film adhesion strength. In the case of moisture or oil pollution, the surface of the substrate will adsorb a layer of water molecules or oil molecules. In this way, it is difficult for the evaporated metal vapor molecules to directly produce physical adsorption with the substrate and even more challenging to produce chemical adsorption. Instead, they can only adhere to the water molecule or oil molecule layer adsorbed by the substrate, weakening the adhesion strength between the film layer and the substrate interface. The dust attached to the substrate hinders the adsorption of the evaporated metal vapor molecules to the substrate. Sand holes (or pinholes) without the film layer are formed where the dust is—the more dust attached to the substrate, the more serious the sand holes.
The following measures are used to improve the film adhesion strength.
Increase substrate temperature
Increasing the substrate temperature is beneficial for removing residual gas molecules from the substrate surface. At the same time, high temperature will promote the transformation of physical adsorption to chemical adsorption and increase the adhesion between the substrate and the deposited material. For example, for aluminum-plated mirrors, a baking chamber is set up near the vacuum chamber, and the glass is baked first and then placed in the vacuum chamber. The baking temperature can be above 50°C. When preparing metal oxide films, the baking temperature is preferably around 150°C.
The baking temperature of the substrate or workpiece cannot be the same as the working temperature of the evaporation source. Otherwise, the vapor molecules of the evaporating material will evaporate again after reaching the substrate or workpiece, resulting in the undesirable effects of difficult film formation and coarse grains.
The baking temperature of the substrate or workpiece cannot be the same as the working temperature of the evaporation source. Otherwise, the vapor molecules of the evaporating material will evaporate again after reaching the substrate or workpiece, resulting in the undesirable effects of difficult film formation and coarse grains.
Choose the right working pressure
When coating aluminum films, it is recommended to maintain a vacuum chamber pressure of approximately 10-2 PaPa to ensure optimal film quality.
Because a few microparticles always adhere to the surface of the substrate or workpiece, when the vapor molecules of the evaporating material are obliquely projected, the particles block them, and an enlarged sand hole is formed behind the particles. The angle between the evaporation source and the substrate is generally not more than 60°. Otherwise, the mirror diffuse reflection is severe, the adhesion is reduced, and the film layer is uneven.
The substrate needs to be carefully treated before coating
Before the substrate enters the coating room, the surface dust, oil, moisture, and various compounds must be removed, especially the oxide layer with spots, so that the substrate surface is clean; different substrate materials require different treatment methods. Glass substrates are often polluted to varying degrees during production, transportation, and storage, and are even corroded by chemical substances. To improve the quality of the mirror film layer, the glass film layer must be carefully cleaned, dried, and stored.
During production, try to avoid stopping the machine midway and filling the air multiple times to prevent the airflow with impurities from rushing to the substrate surface, causing the film layer to lose adhesion.
During production, try to avoid stopping the machine midway and filling the air multiple times to prevent the airflow with impurities from rushing to the substrate surface, causing the film layer to lose adhesion.
Specific requirements for substrate
The substrate used for evaporation coating must meet the following requirements: the surface has high flatness and small optical deformation; good chemical stability and no reaction with the film material; certain thermal stability and thermal shock resistance to withstand the baking heating during the process; certain mechanical strength; the thermal expansion coefficient should be close to the film expansion coefficient to prevent the film from generating stress and peeling off.
Choose the appropriate film thickness
The shear force generated by stress at the interface between the film layer and the substrate is proportional to the film thickness. Therefore, if the film is too thick, the shear force may be greater than the adhesion force, which may cause the film to fall off. Generally, the film thickness is preferably 40 to 100 nm.
Before film formation, the substrate is bombarded and heated with electrons or ions.
The cleaned substrate is placed in a plasma zone to be bombarded and heated with electrons or ions before film formation so that the water, oil molecules, and dirt adsorbed on the substrate are further removed, and the substrate temperature is appropriately increased to reduce internal stress and improve film adhesion. For glass mirrors, as long as the glass is baked at 50-100°C, the requirements can be met.
Contact Us
Your feedback fuels our growth, and your questions drive our solutions.
We value your feedback, inquiries, and suggestions. Please feel free to get in touch with us
General inquiries
Please contact us via sales@bo-glass.com, and we will reply to you as soon as possible.
Interested to work with us
Drop your resume at info@bo-glass.com
and we will get back to you shortly.
We uses the contact information you provide to us to contact you about our relevent content, products, and services.