Vacuum coating technology mainly involves the use of different vacuum coating equipment and process methods to generate coated materials on the surface of specific substrates in order to prepare a variety of thin film materials with specific functions. Application areas for vacuum coating technology include flat-panel displays, semiconductors, solar cells, magnetic and optical recording medias, optical components, energy saving glass, LEDs, tool modifications, high-end decorative items, etc. Thin film materials grow on top of substrate materials (such as screen glass, optical glass, etc.) and are generally formed by metal, non-metal, alloy or compound materials (collectively referred to as coating materials) after coating, and have the functions of increased transmission, absorption, cut-off, spectroscopy, reflection, light filtering, interference, protection, water and dirt repellency, anti-static, electrical conductivity, magnetic conductivity, insulation, abrasion resistance, high temperature resistance, corrosion resistance, oxidation resistance, radiation protection, decoration and composite and other functions. Thin film materials can improve product quality, environmental protection, energy saving, extend the life of the product and improve the original performance, etc. As the thin film material is formed after the transfer of the coating material to the substrate, the quality of the film is directly related to the quality of the coating material. Currently, thin film material preparation technologies mainly include: physical vapour deposition (PVD) and chemical vapour deposition (CVD) technologies. Among them, physical vapour deposition (PVD) technology mainly includes vacuum sputtering coating, vacuum evaporation coating and vacuum ion coating.
Vacuum sputter coating. It refers to the technique of using ions generated by an ion source, which are accelerated and gathered in a vacuum to form a high ion beams, to bombard the surface of the target (coating material), where kinetic energy is exchanged between the ions and the atoms on the surface of the target, causing the atoms on the surface of the target to leave and be deposited on the surface of the substrate material. The object bombarded with ions is the raw material for the deposition of thin film materials by the vacuum sputtering method and is called a sputtering target.
Generally speaking, sputtering targets are mainly composed of target blanks, back plates (or back tubes) and other parts, of which, target blanks are the target material bombarded by high-speed ion beams, belonging to the core part of sputtering targets. In the sputtering coating process, target blanks are hit by ions and its surface atoms are sputtered out and deposited on the substrate, making thin film materials. As the sputtering target needs to be installed in special equipment to complete the sputtering process, the equipment has a high voltage, high vacuum inner working environment and most of the target blank material is soft or highly brittle, not suitable for direct installation in the equipment, therefore, it need to be bound with the back plate (or back tube) which mainly plays a role as the fixator of sputtering target and has good electrical and thermal conductivity.
Characteristics of vacuum sputtered films:
- Good controllability and repeatability of film thickness. The thickness of the film is controlled at a predetermined value, known as controllability of film thickness. The required film thickness can occur repeatedly, called film thickness repeatability. In vacuum sputter coating, the film thickness can be controlled by controlling the target current.
- Strong adhesion of the film to the substrate. The energy of the sputtered atoms is 1-2 orders of magnitude higher than the energy of the evaporated atoms, and the energy conversion of the high-energy sputtered atoms deposited on the substrate is much higher than that of the evaporated atoms, generating higher energy and enhancing the adhesion of the sputtered atoms to the substrate.
- In the preparation of alloy and compound films, the components of the target material are very close to that of the film material deposited onto the substrate, avoiding variation and inconsistency in the components and structure of the coating material as it is transferred to produce the film material.
- New material films can also be prepared which are different from the target material. If a reactive gas is passed through the sputtering so that it reacts chemically with the target, a new material film which is completely different from the target can be obtained.
- High purity of the film layer. The purity of the film is high. The sputtering method does not have the crucible component of the evaporation method, so the sputtering coating does not mix with the crucible heater material and has a higher purity. The disadvantages of the sputtering method are that the film formation speed is lower than that of evaporation, the substrate temperature is higher, it is more susceptible to impurity gases and the device structure is more complex.
Vacuum sputtering has become one of the mainstream technologies for the preparation of thin film materials due to the reproducibility and controllable thickness of the sputtering process, the ability to obtain uniform thickness films on large areas of substrate materials, the high purity of the films prepared, the good density and the strong bonding with the substrate materials. Various types of sputtering targets have been widely used, so the demand for sputtering targets, a functional material with high added value, is increasing at a high rate year by year, and sputtering targets have become the largest market application of PVD coating materials.
Vacuum evaporation coating. It is a deposition technique that uses the thermal energy of a film heating device (called an evaporation source) under vacuum conditions to deposit a substance on the surface of a substrate material by heating it to evaporate. When the mean free path of the evaporated molecules is larger than the dimension line between the evaporation source and the substrate, the evaporated particles escape from the surface of the evaporation source and are rarely hindered by the collision of other particles (mainly residual gas molecules) in the process of flying towards the surface of the substrate, and arrive directly at the surface of the substrate, condensing and producing a thin film. The evaporated material is the raw material for the deposition of thin film materials by vacuum evaporation coating and is called vapour deposition material.
Vacuum evaporation coating system generally consists of three components: the vacuum chamber, the evaporation source or evaporation heating unit, and the device for placing and heating the substrate. In order to evaporate the material to be deposited in a vacuum, a vessel is required to support or contain the evaporate and to provide evaporation heat to bring the evaporate to a high enough temperature to generate the required vapour pressure.
Features of vacuum evaporation coating technology: simple equipment, easy operation, high purity and quality of the film made, accurate controllability of the thickness, fast filming rate , high efficiency, relatively simple growth mechanism of the film, etc.; the disadvantage are that it is not easy to obtain the film with crystalline structure; the adhesion of the film formed on the substrate is small; the repeatability of the process is not good enough, etc.
Vacuum ion coating. This refers to a process in which the film is evaporated or sputtered in a vacuum atmosphere, using an evaporation source or sputtering target, and part of the evaporated or sputtered particles are ionized into metal ions in the gas discharge space, and these particles are deposited onto the substrate under the action of an electric field to produce a thin film. The principle is shown in followed Figure. Firstly, the pressure of the coating chamber is evacuated to below 10-3pa, then the working gas is passed in to increase the pressure to 10^0~10^-1pa and the high pressure is accessed. As the cathode of the evaporation source grounded, the substrate is connected to the adjustable negative bias voltage, then the power supply can establish a low temperature plasma area with low pressure gas discharge between the evaporation source and the substrate, after the evaporation source of resistance heating type is energised and heats the film material, part of the neutral atoms escape from the surface of the film material and ionise into positive ions due to collision with electrons when passing through the plasma in the process of migration to the substrate; another part of the neutral atoms ions can also be generated by colliding with ions in the working gas and exchanging charges. These ions are then accelerated by the electric field and shot towards the substrate where they are connected to the negative potential, resulting in a thin film.
Characteristics of vacuum ion coating:
- film/substrate adhesion is strong and the film layer is not easy to fall off;
- ion plating has good diffraction, thus improving the coverage of the film layer;
- high quality of the coating;
- high deposition rate, film speed. 30 micron thick film can be prepared;
- coating can be applied to a wide range of substrate materials and film materials.
Chemical Vapour Deposition (CVD). It is a filming technique that uses heating, plasma enhancement and light assist to make a solid film on the surface of a substrate by chemical reaction of gaseous substances under atmospheric or low pressure conditions.
Chemical vapour deposition CVD technology has the following characteristics:
- the process and operation of the equipment is relatively simple and flexible, enabling the preparation of single or composite film layers and co-layers in various ratios;
- the chemical vapour deposition CVD method is widely applicable;
- the deposition rate can be as high as a few microns to hundreds of microns per minute, resulting in high production efficiency;
- compared to the PVD method (vapour deposition, sputtering), it has good diffraction properties and is suitable for coating substrates with complex shapes;
- good denseness of coating;
- low damage after exposure to radiation and integration with the MOS integrated circuit (an integrated circuit consisting of metal-oxide-semiconductor field-effect transistors as the main components) process.
Summary: Vacuum coating technology mainly includes physical vapour deposition (PVD) technology and chemical vapour deposition (CVD) technology. The above mentioned vapour deposition, sputtering and ion deposition are all physical vapour deposition PVD, the basic principle of which can be summarised as follows: vapourisation of the plating material → migration of the atoms, molecules or ions in plating materials → deposition of atoms, molecules or ions in the plating materials on the substrate to recreate a thin film. Chemical vapour deposition CVD can be summarised as: formation of volatile substances → transfer of the substances to the deposition area → chemical reaction on the solid and production of a solid film substance.