What is vacuum coating technology?

The so-called vacuum coating is a process in which the material to be plated and the substrate to be plated are placed in a vacuum chamber, and the material to be plated is heated by a certain method to evaporate or sublime, and then fly and sputter to the surface of the substrate to be plated to form a film.

1. Coating method and classification

Film formation under vacuum conditions has many advantages: it can reduce the collision of atoms and molecules of the evaporated material with molecules in the process of flying to the substrate, reduce the chemical reaction (such as oxidation, etc.) between the active molecules in the gas and the evaporation source material, and Reduce the amount of gas molecules entering the film to become impurities during the film formation process, thereby improving the density, purity, deposition rate and adhesion of the film layer to the substrate. Generally, vacuum evaporation requires the pressure in the film forming chamber to be equal to or lower than 10-2Pa. For occasions where the distance between the evaporation source and the substrate is long and the film quality is required to be high, the pressure is required to be lower.

Mainly divided into the following categories:

Evaporative coating, sputter coating and ion plating.

Evaporation coating: Evaporating a substance by heating to deposit it on a solid surface, which is called evaporative coating. This method was first proposed by M. Faraday in 1857, and it has become one of the commonly used coating techniques in modern times.

Evaporation substances such as metals, compounds, etc. are placed in the crucible or hung on the hot wire as the evaporation source, and the workpiece to be plated, such as metal, ceramic, plastic and other substrates, is placed in front of the crucible. After the system is pumped to a high vacuum, the crucible is heated to evaporate its contents. The atoms or molecules of the evaporated substance are deposited on the surface of the substrate by condensation. The film thickness can vary from hundreds of angstroms to several microns. The film thickness is determined by the evaporation rate and time of the evaporation source (or by the amount of charge) and is related to the distance between the source and the substrate. For large-area coating, a rotating substrate or multiple evaporation sources are often used to ensure the uniformity of the film thickness. The distance from the evaporation source to the substrate should be less than the mean free path of the vapor molecules in the residual gas, so as to avoid chemical effects caused by the collision between the vapor molecules and the residual gas molecules. The average kinetic energy of vapor molecules is about 0.1 to 0.2 electron volts.

There are three types of evaporation sources. ①Resistance heating source: use refractory metals such as tungsten and tantalum to make boat foil or filament, and pass electric current to heat the evaporated material above it or placed in the crucible (Figure 1 [schematic diagram of evaporation coating equipment]) resistance heating The source is mainly used to evaporate Cd, Pb, Ag, Al, Cu, Cr, Au, Ni and other materials. ②High-frequency induction heating source: use high-frequency induction current to heat the crucible and the evaporated material. ③ Electron beam heating source: suitable for materials with high evaporation temperature (not lower than 2000 [618-1]), that is, bombarding materials with electron beams to make them evaporate.

Compared with other vacuum coating methods, evaporation coating has a higher deposition rate, and can coat elemental and compound films that are not easily thermally decomposed.

In order to deposit high-purity single crystal films, molecular beam epitaxy can be used. The molecular beam epitaxy device for growing the doped GaAlAs single crystal layer is shown in Figure 2 [schematic diagram of the molecular beam epitaxy device]. The jet furnace is equipped with a molecular beam source. When it is heated to a certain temperature under ultra-high vacuum, the elements in the furnace are jetted to the substrate in a beam-like molecular stream. The substrate is heated to a certain temperature, the molecules deposited on the substrate can migrate, and the crystals grow according to the lattice order of the substrate. The molecular beam epitaxy method can obtain a high-purity compound single crystal film with the required stoichiometric ratio, and the film grows the slowest The speed can be controlled at 1 single layer/second. By controlling the baffles, single crystal thin films of desired composition and structure can be made precisely. Molecular beam epitaxy is widely used to fabricate various optical integrated devices and various superlattice thin films.

Sputtering coating: When bombarding the solid surface with high-energy particles, the particles on the solid surface can obtain energy and escape from the surface to deposit on the substrate. The sputtering phenomenon began to be used in coating technology in 1870 and was gradually used in industrial production after 1930 due to increased deposition rates. Commonly used diode sputtering equipment is shown in Figure 3 [Schematic diagram of diode sputtering]. Usually the material to be deposited is made into a plate - the target, which is fixed on the cathode. The substrate is placed on the anode facing the target surface, a few centimeters away from the target. After the system is pumped to a high vacuum, it is filled with a gas (usually argon) of 10-1 Pa, and a voltage of several thousand volts is applied between the cathode and the anode, and a glow discharge is generated between the two poles. The positive ions generated by the discharge fly to the cathode under the action of the electric field, and collide with the atoms on the target surface. The target atoms that escape from the target surface by the collision are called sputtering atoms, and their energy ranges from 1 to tens of electron volts. The sputtered atoms deposit a film on the surface of the substrate. Unlike evaporation coating, sputtering coating is not limited by the melting point of the film material, and can sputter refractory substances such as W, Ta, C, Mo, WC, TiC, etc. The sputtering compound film can use the reactive sputtering method, that is, the reactive gas (O, N, HS, CH, etc.) is added to the Ar gas, and the reactive gas and its ions react with the target atoms or sputtering atoms to form compounds (such as oxides, nitrogen, etc.) compound, etc.) and deposited on the substrate. The insulating film can be deposited by high-frequency sputtering. The substrate is mounted on the grounded electrode, and the insulating target is mounted on the opposite electrode. One end of the high-frequency power supply is grounded, and the other end is connected to the electrode equipped with the insulating target through a matching network and a DC blocking capacitor. After the high-frequency power supply is turned on, the high-frequency voltage continuously changes its polarity. Electrons and positive ions in the plasma hit the insulating target in the positive and negative half cycles of the voltage, respectively. Since the electron mobility is higher than that of positive ions, the surface of the insulating target is negatively charged, and when dynamic equilibrium is reached, the target is at a negative bias potential, so that the sputtering of the target by positive ions continues. Using magnetron sputtering can increase the deposition rate by nearly an order of magnitude compared to non-magnetron sputtering.

Ion plating: The molecules of the evaporated substance are ionized by electron impact and then deposited on the solid surface as ions, which is called ion plating. This technique was proposed by D. Metaux in 1963. Ion plating is a combination of vacuum evaporation and cathode sputtering techniques. An ion plating system is shown in Figure 4 [schematic diagram of the ion plating system], the substrate stage is used as the cathode, the outer shell is used as the anode, and an inert gas (such as argon) is filled to generate a glow discharge. Ionization occurs as the molecules evaporated from the evaporation source pass through the plasma. The positive ions are accelerated to the surface of the substrate by the negative voltage of the substrate stage. Unionized neutral atoms (about 95% of the evaporated material) are also deposited on the surface of the substrate or the walls of the vacuum chamber. The acceleration effect of the electric field on the ionized vapor molecules (the ion energy is about several hundred to several thousand electron volts) and the sputtering cleaning effect of the argon ions on the substrate greatly improve the adhesion strength of the film layer. The ion plating process combines the characteristics of evaporation (high deposition rate) and sputtering (good film adhesion), and has good diffraction properties, which can coat workpieces with complex shapes.

2. Measurement of film thickness

With the advancement of technology and the application of precision instruments, there are many methods of film thickness measurement, which can be divided into two categories according to the measurement method: direct measurement and indirect measurement. Direct measurement refers to the application of measuring instruments to directly sense the thickness of the film through contact (or optical contact).

Common direct method measurements are: helical micrometric method, precision profile scanning method (step method), scanning electron microscopy (SEM);

Indirect measurement refers to converting the relevant physical quantities into the thickness of the film according to a certain corresponding physical relationship, so as to achieve the purpose of measuring the thickness of the film.

Common indirect measurement methods are: weighing method, capacitance method, resistance method, equal thickness interferometry, variable angle interferometry, and ellipsometry. According to the principle of measurement can be divided into three categories: weighing method, electrical method, optical method.

Common weighing methods are: balance method, quartz method, atomic number determination method;

Common electrical methods are: resistance method, capacitance method, eddy current method;

Common optical methods are: equal thickness interferometry, variable angle interferometry, light absorption method, and ellipsometry.

Three are briefly described below:

1. Interference Microscopy

Interference fringe spacing Δ0, fringe movement Δ, step height t=(Δ/Δ0 )*0.5λ, measure Δ0 and Δ, where λ is the wavelength of monochromatic light, such as white light, λ takes 530nm.

2. Weighing method

If the film area A, density ρ and mass m can be accurately determined, the film thickness t can be calculated:

d=m/Aρ。

3 Quartz crystal oscillator method

It is widely used in real-time measurement of thickness during film deposition, mainly for monitoring deposition speed and thickness, and in turn (combined with electronic technology) to control the rate of material evaporation or sputtering, so as to realize the accuracy of deposition process. Automatic control.

For film manufacturers, the thickness uniformity of the product is one of the most important indicators. In order to effectively control the thickness of the material, thickness testing equipment is essential, but which type of thickness measuring equipment to choose depends on the It depends on factors such as the type of flexible packaging material, the manufacturer's requirements for thickness uniformity, and the testing range of the equipment.

Three, vacuum coating machine maintenance knowledge:

1. Turn off the pump heating system, and then separate the evaporation chamber (mainly clean the dust, and the evaporation residue)

2. Turn off the power or enter the maintenance state

3. Clean the winding system (several rollers, square resistance probe, densitometer)

4. Clean the hood chamber (around the panel)

5. Open and clean the pump system after cooling (be careful not to drop debris, check the pump oil usage time and gauge to replace or add)

6. Check recooling and electrical cabinet equipment

This internship gave us an understanding of the principle and technology of coating technology, and made us understand the production of the factory. It felt very novel and gained a lot.