Vacuum coating technology, abbreviated as PVD, is a technique that uses physical methods to vaporize the surface of a material source into atoms, molecules, or ions under vacuum conditions, and deposit a thin film with a certain special function on the substrate surface. The coating technology of vacuum coating equipment is mainly divided into three categories: vapor deposition, sputtering, and ion plating. There are three types of evaporation coating technology: resistance evaporation, electron beam evaporation, and induction heating evaporation.
There are three main directions for coating technology in vacuum coating equipment: evaporation coating technology, ion coating technology, and magnetron sputtering coating equipment. Each coating technology has its own advantages and disadvantages, and different substrates and targets are coated with different coating technologies.
Resistance evaporation coating technology adopts the evaporation coating technology of resistance heating evaporation source, which is generally used for evaporating low melting point materials such as aluminum, gold, silver, zinc sulfide, magnesium fluoride, chromium trioxide, etc; Heating resistors are generally made of tungsten, molybdenum, tantalum, etc. Unique advantages, simple structure, and low cost. Disadvantage: The material is prone to react with the crucible, affecting the purity of the thin film, and cannot evaporate high melting point dielectric thin films; Low evaporation rate.
Resistance evaporation plating electron beam evaporation is a technology that uses high-speed electron beam heating to vaporize and evaporate materials, and then condenses into a film on the surface of a substrate. The energy density of the electron beam heat source can reach 104-109w/cm2, and can reach over 3000 ℃. It can evaporate high melting point metals or dielectric materials such as tungsten, molybdenum, germanium, SiO2, AL2O3, etc.
The main principle of electron beam evaporation is that in a high vacuum environment, high-energy electrons emitted by an electron gun bombard the surface of a target material under the action of electric and magnetic fields, converting kinetic energy into thermal energy. The target material heats up, becomes molten, or directly evaporates, depositing a thin film on the substrate surface.
There are two types of vapor deposition sources for electron beam heating: straight gun electron guns and e-type electron guns (also circular). The electron beam is emitted from the source and focused and deflected by a magnetic field coil to bombard and heat the film material. Its advantages include the ability to evaporate any material, high purity of the film, direct action on the surface of the material, and high thermal efficiency. Disadvantages of electron guns include complex structure, high cost, easy decomposition of compounds during deposition, and chemical imbalance.
Induction heating evaporation is a technology that uses high-frequency electromagnetic field induction heating to vaporize and evaporate materials, condensing them into a film on the surface of a substrate. Its advantages include a high evaporation rate, which can be about 10 times higher than that of a resistive evaporation source. The temperature of the evaporation source is stable, making it less prone to splashing. The crucible temperature is low, and the crucible material has less membrane fouling. Its disadvantages include the need to shield the evaporation device, high cost, and complex equipment.
Although the principles of these three evaporation coating technologies for vacuum coating equipment are the same, they all use high-temperature evaporation to vaporize materials for coating. However, the environments in which they are applied are different, and the coating materials and substrates also have different requirements.
High frequency induction heating evaporation is the process of placing a crucible containing coating material at the center of a high-frequency spiral coil, causing the coating material to generate strong eddy currents and hysteresis effects under the induction of a high-frequency electromagnetic field, resulting in the heating of the film layer until it vaporizes and evaporates. The evaporation source generally consists of a water-cooled high-frequency coil and a graphite or ceramic (magnesium oxide, aluminum oxide, boron oxide, etc.) crucible. The high-frequency power supply uses a frequency of 10000 to several hundred thousand hertz, with an input power of several to several hundred kilowatts. The smaller the membrane material volume, the higher the induction frequency. Induction coil frequency is usually manufactured using water-cooled copper tubes. The disadvantage of high-frequency induction heating evaporation method is that it is not easy to fine tune the input power. It has the following advantages:
1. High evaporation rate:
2. The temperature of the evaporation source is uniform and stable, and it is not easy to produce splashing of plating droplets
3. One time loading of evaporation source, temperature control is relatively easy, and operation is simple.
The advantages of magnetron sputtering coating technology are as follows
1. High sedimentation rate. Due to the use of high-speed magnetron electrodes, a large ion current can be obtained, effectively improving the deposition rate and sputtering rate of this coating process. Compared with other sputtering coating processes, magnetron sputtering has high production capacity and output, and is widely used in various industrial productions.
2. High power efficiency. Magnetron sputtering targets generally choose voltages within the range of 200V-1000V, usually 600V, because the voltage of 600V is just within the highest effective range of power efficiency.
Low sputtering energy. The low voltage applied to the magnetron target and the magnetic field confine the plasma near the cathode, which can prevent high-energy charged particles from being incident on the substrate.
3. The substrate temperature is low. The electrons generated during anodic discharge can be utilized without the need for substrate support grounding, which can effectively reduce electron bombardment on the substrate. Therefore, the temperature of the substrate is relatively low, making it very suitable for coating some plastic substrates that are not very resistant to high temperatures.
Uneven etching on the surface of magnetron sputtering targets. Uneven surface etching of magnetron sputtering targets is caused by uneven target magnetic fields, resulting in a higher etching rate at local locations of the target and a lower effective utilization rate of the target material (only 20% -30% utilization rate). Therefore, in order to improve the utilization rate of target materials, it is necessary to change the magnetic field distribution through certain means, or use magnets to move in the cathode, which can also improve the utilization rate of target materials.
4. Composite target. Composite target coated alloy films can be produced. Currently, Ta Ti alloy, (Tb Dy) - Fe, and Gb Co alloy films have been successfully deposited using composite magnetron sputtering technology. There are four types of structures for composite targets, namely circular embedded targets, square embedded targets, small square embedded targets, and fan-shaped embedded targets. Among them, the fan-shaped embedded target structure has the best use effect.
5. Wide range of applications. The magnetron sputtering process can deposit many elements, including Ag, Au, C, Co, Cu, Fe, Ge, Mo, Nb, Ni, Os, Cr, Pd, Pt, Re, Rh, Si, Ta, Ti, Zr, SiO, AlO, GaAs, U, W, SnO, etc.
Vacuum ion coating technology
Vacuum ion plating technology (abbreviated as ion plating) was first developed by D M. Mattox was proposed and put into practice in 1963 as a coating technology that combines evaporation and sputtering. It is based on ion bombardment, which heats the coated material or workpiece to a molten state, and uses high-energy ion bombardment to deposit chemically deposited metal or semiconductor thin films onto the substrate surface, thereby obtaining thin films with specific structures and properties.
The process of ion plating is to connect the evaporation source to the anode and the workpiece to the cathode. When a high-voltage direct current of three to five thousand volts is applied, arc discharge is generated between the evaporation source and the workpiece. Due to the inert argon gas filled in the vacuum hood, some of the argon gas is ionized under the action of the discharge electric field, forming a plasma dark zone around the cathode workpiece. The positively charged argon ions are attracted by the negative high voltage of the cathode and violently bombard the surface of the workpiece, causing particles and dirt on the surface of the workpiece to be splashed and thrown out, thus allowing the surface of the workpiece to be fully cleaned by ion bombardment. Subsequently, the AC power supply of the evaporation source is connected, and the evaporated material particles melt and evaporate, entering the glow discharge zone and being ionized. The positively charged evaporated material ions, attracted by the cathode, rush towards the workpiece along with argon ions. When the amount of evaporated material ions deposited on the surface of the workpiece exceeds the amount of splashed ions, they gradually accumulate to form a firmly adhered coating on the surface of the workpiece.
The coating structure of ion plating is dense, without pinholes, bubbles, and uniform thickness. This method is very suitable for coating parts with internal holes, grooves, and narrow gaps that are difficult to coat by other methods, and does not form metal nodules. Due to its ability to repair small cracks and defects such as pitting on the surface of the workpiece, this process can effectively improve the surface quality and physical and mechanical properties of the coated parts. Fatigue tests have shown that if handled properly, the fatigue life of the workpiece can be increased by 20% to 30% compared to before plating.
Characteristics of Vacuum Ion Coating
Compared with evaporation and sputtering, ion plating has the following characteristics:
(1) Good adhesion performance of the coating
During ordinary vacuum coating, there is almost no transition layer connecting the surface of the workpiece and the coating. During ion plating, when ions bombard the workpiece at high speed, they can penetrate the surface of the workpiece and form a diffusion layer deeply implanted into the substrate. The interface diffusion depth of ion plating can reach four to five micrometers. In the early stage of coating, sputtering and deposition coexist, and a transition layer or a mixed layer of film and substrate components can be formed at the interface between the film and substrate, called a pseudo diffusion layer, which can effectively improve the adhesion performance of the film layer.
(2) Strong plating ability
During ion plating, the evaporated material particles move along the direction of the electric field in the form of charged ions. Therefore, wherever there is an electric field present, a good coating can be obtained, which is much superior to ordinary vacuum coating that can only obtain a coating in the direct direction. Therefore, this method is very suitable for areas on plated parts that are difficult to plate by other methods, such as inner holes, grooves, and narrow gaps.
(3) Good coating quality
The coating of ion plating has a dense structure, no pinholes, no bubbles, and uniform thickness. Even the edges and grooves can be uniformly coated, and parts such as threads can also be coated with high hardness, high wear resistance (low friction coefficient), good corrosion resistance, and chemical stability, resulting in a longer lifespan of the film layer; At the same time, the film layer can significantly improve the appearance and decorative performance of the workpiece.
(4) Simplify the cleaning process
Most existing coating processes require strict cleaning of the workpiece in advance, and the process is relatively responsible. During the ion plating process, a large number of high-energy particles generated by glow discharge are used to create a cathodic sputtering effect on the surface, which cleans the gas and oil adsorbed on the substrate surface by sputtering, purifying the substrate surface until the entire coating process is completed, simplifying a lot of pre plating cleaning work.
(5) Widely available plated materials
Ion plating is the process of using high-energy ions to bombard the surface of a workpiece, converting a large amount of electrical energy into thermal energy on the surface of the workpiece, thereby promoting diffusion and chemical reactions in the surface tissue, and the workpiece is not affected by high temperatures. Therefore, this coating process has a wide range of applications and is less limited. Usually, various metals, alloys, as well as certain synthetic materials, insulation materials, thermosensitive materials, and high melting point materials can be plated. Metal workpieces can be plated with non metals or metals, as well as metals or non metals, and even plastics, rubber, quartz, ceramics, etc.
Classification of Vacuum Ion Coating
There are various combinations of ionization and excitation methods for different evaporation sources and atoms, leading to the emergence of many evaporation source ion plating methods. Common methods include sputtering ion plating and evaporation ion plating based on the acquisition of membrane particles.
1. Sputtering type ion plating
By using high-energy ions to sputter the surface of the membrane material, metal particles are generated. The metal particles ionize into metal ions in the gas discharge space, and they reach the substrate under negative bias to deposit and form a film.
Evaporative ion plating
Heating the coating material through various heating methods to evaporate and produce metal vapor, which is then introduced into the gas discharge space excited in various ways to ionize into metal ions. These ions reach the substrate under negative bias and deposit into a film.
Among them, evaporative ion plating can be divided into DC two-stage ion plating, hollow cathode ion plating, hot wire arc ion plating, and cathode arc ion plating according to different discharge principles. DC secondary ion plating is a stable glow discharge; Hollow cathode ion plating and hot wire arc ion plating are both thermal arc discharges, and the reason for the generation of electrons can be simply summarized as the thermal emission of electrons outside the nucleus due to the heating of metal materials to high temperatures; The discharge type of cathodic arc ion plating is different from the previous types of ion plating, and it uses cold arc discharge.
(1) Hollow cathode ion plating (HCD)
Using hollow hot cathode discharge to generate plasma electron beam. Characteristics of hollow cathode ion plating: ① HCD hollow cathode gun is both a heat source for membrane material gasification and an ionization source for evaporated particles, and the ionization method is to use low-pressure electron beam collision; ② Using an acceleration voltage ranging from 0V to several hundred volts, ionization and ion acceleration operate independently Can perform reactive ion plating well; ④ The temperature rise of the substrate is small, and the substrate still needs to be heated during coating; ⑤ High ionization efficiency, large electron beam spot, and can be deposited on various films.
(2) Cathodic arc ion plating
Cathodic arc ion plating is the culmination of mainstream ion coating technology, which adopts cold arc discharge and has the highest particle ionization rate among many PVD coating technologies.
