Thin film deposition refers to the deposition of specific materials on a substrate to form a thin film with optical, electrical, mechanical, and other related properties. According to the principle of sedimentation, the methods of thin film deposition can be divided into physical vapor deposition, chemical vapor deposition, and atomic layer deposition.

There are many factors that affect the adhesion of thin films. The deposition temperature of thin films will affect their adhesion by influencing the interface morphology. At lower deposition temperatures, there is a lack of diffusion and chemical bonding of interface atoms, and the adhesion of the film mainly comes from mechanical meshing and van der Waals attraction. The adhesion is relatively small, and the work of peeling off the film is about 0.1 eV/atom. Moreover, the adhesion will rapidly decrease with the increase of interfacial atomic spacing; At higher deposition temperatures, significant diffusion occurs at the interface, leading to chemical bonding and a significant increase in interfacial adhesion, which can reach the level of 1 to 10 eV/atom. Using surface activators or cleaning agents to activate the substrate surface, as well as bombarding the substrate with ions of certain energy, can reduce surface pollution, promote the diffusion of surface atoms, and facilitate the formation of effective interface mechanical meshing and chemical bonding, thereby improving interface adhesion. The properties of thin films and substrate materials also have a significant impact on adhesion. Substances with significant differences in bonding types and poor wettability are difficult to form strong bonds, resulting in poor adhesion. For example, Au has poor adhesion on SiO2 substrates; When there are the same or similar chemical bonding types, or when there are differences in bonding types but high chemical affinity between each other, it can effectively reduce the interfacial energy between the film and the substrate, and improve the adhesion of the film. For example, good adhesion can be formed between Au and Cu, Cu and Zn, but thicker and more brittle interfacial compounds can also deteriorate the interfacial adhesion performance.

There are two basic conditions for the tight bonding between the film layer and the substrate. Firstly, the gap between the film layer and the substrate should be as small as possible. Secondly, there should be as many contact surfaces between the film layer and the substrate as possible, and increasing the contact area and improving surface roughness is an extremely effective means. To achieve the above objectives, it is necessary to carry out necessary surface pretreatment on the substrate. There are many methods of surface pretreatment, but these methods must all achieve the following two goals: first, to completely remove impurities from the substrate surface as much as possible and expose the fresh surface of the substrate as thoroughly as possible; Secondly, creating a certain degree of roughness on the surface, expanding the actual surface area of the substrate, and enhancing the adhesion of the film layer per unit area of the substrate surface.

Here are some methods to improve film adhesion

Control sedimentation temperature: Increase sedimentation temperature appropriately to promote interface diffusion and chemical bonding formation, thereby enhancing adhesion.

Surface pretreatment: necessary cleaning, activation, and other pretreatment of the substrate to remove impurities, increase roughness, and expose fresh surfaces.

Using ion bombardment: such as plasma cleaning, activating the surface atoms of the substrate, providing energy conditions, reducing surface pollution, and forming a rough surface for bonding.

Choose the appropriate material combination: Ensure that the film matches the substrate material in terms of bonding type, chemical affinity, etc., to improve adhesion.

Optimize process parameters: Carefully adjust various parameters during the deposition process to achieve a state conducive to improving adhesion.

Adding an intermediate layer or coating: In some cases, adding a suitable intermediate layer or coating between the film and substrate can improve adhesion.

The purpose of plasma cleaning before thin film deposition

Plasma cleaning refers to the process of removing pollutants such as grease, oxides, and gases from the surface of a substrate by bombarding it with high-energy particles. The purpose is to provide environmental conditions for good bonding between the film layer and the substrate. Ion bombardment has an activating effect on the surface atoms of the substrate, thereby providing certain energy conditions for the bonding between the film layer atoms and the substrate.

Plasma cleaning before film deposition can remove or reduce substrate surface contamination and factors such as oxides that are not conducive to the bonding strength of the film layer. It can also form a micro rough surface, increase the bonding area between the film layer and the substrate, and help improve the bonding strength.

Advantages of plasma cleaning for thin film deposition

Plasma cleaning has the following advantages: firstly, the workpiece will be very dry after plasma cleaning and can be directly sent to the deposition equipment for the next deposition; Secondly, this method is an environmentally friendly green cleaning method that does not produce harmful pollutants after cleaning; Thirdly, plasma can deeply clean the micro pores and depressions of objects, which is beneficial for the next deposition; Fourthly, the cleaning efficiency is high, and the entire cleaning process can be completed within a few minutes; Fifthly, it can process various substrates, including metals, semiconductors, oxides, and polymer materials, all of which can be treated with plasma. This method is very suitable for use in flexible deposition manufacturing; Sixth, while completing the cleaning, it can change the surface properties of the material, such as enhancing the wetting ability of the surface, improving the adhesion of the film, etc., which is beneficial for the bonding between layers during the next deposition.