Science and practice of vacuum sputtering coating: An in-depth analysis of classification, process parameters, and typical applications

1）Main classification of vacuum sputtering coating

Vacuum sputtering coating can be divided into DC sputtering, RF sputtering, magnetron sputtering and reactive sputtering according to its working mode and application characteristics. Each method has unique advantages and limitations in the process and is suitable for different target types and coating needs.

1. DC Sputtering

Operating principle

Dc sputtering is one of the earliest technologies developed in sputtering coating. By applying a DC voltage to the target, the target is set as a cathode, an inert gas (such as argon) is introduced into the vacuum chamber, and it is ionized to form a plasma. High-energy ions are accelerated and bombard the surface of the target, causing the target atoms to escape and deposit onto the substrate.

  Range of application

Dc sputtering is suitable for conductive targets and is widely used in the deposition of metal films, such as aluminum, copper and other metal materials. Because the non-conductive target accumulates positive charge during sputtering, resulting in interruption of discharge, DC sputtering is not suitable for non-conductive materials. It is mainly used in the deposition of metal interconnect layer and shield layer in electronic devices, and has the characteristics of simple process and fast deposition rate.

1. RF Sputtering

  Operating principle

  Rf sputtering provides an alternating electric field through an RF power supply (usually with a frequency of 13.56 MHz) to quickly switch the surface charge of the target and avoid the accumulation of surface charge on non-conductive materials. Ions and electrons periodically bombard the target in a radio-frequency electric field, allowing the non-conductive material to stably sputter.

Range of application

Rf sputtering is designed for non-conductive materials, suitable for deposition of oxides, nitrides, ceramics and other materials, such as SIO2, Al₂ ₃ and so on. Rf sputtering is widely used in the optical, electronic and photovoltaic fields, especially the preparation of insulating layers and anti-reflection coatings. This method has the advantages of high uniformity and stability, but the equipment and operation cost are high.

1. Magnetron Sputtering

  Operating principle

Magnetron sputtering is to apply a magnetic field parallel to the target on the surface of the target, so that the electrons produce spiral motion near the target, thereby increasing the collision frequency of electrons and enhancing the plasma density. This design improves the sputtering efficiency, resulting in faster sputtering rates and higher film quality, while reducing the working pressure of the cavity.

The enhancement effect of magnetic field on sputtering efficiency

The advantage of magnetron sputtering is to improve the production efficiency of sputtered ions and reduce the dependence on high temperature environment during deposition. Magnetron sputtering is widely used in the electronics industry and in the field of protective coatings, such as hard disk coatings, protective films and metal interconnect layers in semiconductors.

1. Reactive Sputtering

Operating principle

Reactive sputtering is the addition of reactive gases (such as oxygen and nitrogen) in the sputtering cavity, and the target atoms react chemically with the reactive gases to deposit a compound film on the substrate. For example, oxide films (e.g. TiO₂, Al₂O₃) can be deposited by introducing oxygen.

Application of reaction gases in forming compound films

Reactive sputtering can precisely control the chemical composition of the film by adjusting the flow rate of the reaction gas. It is widely used in optical thin films, wear-resistant coatings and semiconductor industries, such as anti-reflective films, insulating layers and wear-resistant coatings (such as TiN, CrN). Reactive sputtering is a coating technology with high flexibility and high control precision, which is suitable for the preparation of a variety of compound films.

• Sputtering coating process parameters and their effects

The process parameters of sputtering coating have significant influence on the quality and characteristics of the film. Common process parameters include air pressure, gas flow, target power, substrate temperature and sputtering Angle. Reasonable control of these parameters can significantly improve the uniformity, adhesion and density of the film.

1. Air pressure and gas flow

Effect of different air pressure on sputtering rate and film density

At higher pressure, the plasma density increases and the sputtering rate increases. However, too high air pressure will increase the collision of sputtered particles and reduce the energy of particles, resulting in loose film and low density. At low pressure, the kinetic energy of sputtered particles increases, which is conducive to the formation of dense films, but the reduction of gas content may affect the deposition rate. Therefore, in the actual process, medium air pressure is usually selected to balance the rate and film quality.

1. Target power and bias

Power control of sputtering rate, film composition and structure

The target power determines the energy of sputtered ions and the speed at which target atoms escape. Higher power can speed up the sputtering rate, but too high power will produce thermal stress, which may lead to cracks in the target and increase the roughness of the film. Bias controls the substrate's attraction to ions. Higher bias can increase the ion bombardment strength and improve film adhesion and compactness, but it is necessary to prevent the substrate temperature from being too high.

1. Substrate temperature and sputtering Angle

Effect of temperature and sputtering Angle on film adhesion and uniformity

Substrate temperature affects the crystal quality and stress of the film. High temperature is conducive to the crystallization and density improvement of the film, but too high temperature may cause material deformation or substrate damage. The coating uniformity of the film is affected by the Angle of sputtering, especially in large area sputtering. Optimizing the Angle of sputtering can significantly improve the uniformity and stability of the film.

• Selection and characteristics of sputtering targets

The choice of sputtering target directly determines the properties and properties of the film. The type, purity, shape and other parameters of the target material have an important effect on the sputtering effect and the quality of the film.

1. Target material

Types of target materials and different applications

Sputtering targets are mainly divided into metals, alloys and compounds. Metal targets such as aluminum and copper are often used in the preparation of conductive films. Alloy targets for films requiring composite properties (e.g. both electrical conductivity and corrosion resistance); Compound targets, such as SiO₂ and TiO₂, are widely used in optical films and insulating layers.

1. Target purity and structure

Effect of purity on coating quality

The purity of the target material directly affects the purity and structural properties of the film. High purity targets can reduce impurities and avoid deterioration of film properties, especially in the semiconductor and optical fields, high purity targets are critical. In addition, the crystallinity of the target also affects the microstructure and properties of the film, and the single crystal target is more suitable for applications requiring a high degree of uniformity.

1. Target shape and consumption

Plane target, cylindrical target and target life optimization

The choice of flat and cylindrical targets depends on the application requirements. Flat targets are more effective in small area applications, while cylindrical targets are more uniform in large area sputtering. The target is gradually consumed in the sputtering process, and in order to reduce production costs, it is usually necessary to recover and regenerate the target.

• Typical application of vacuum sputtering coating

Vacuum sputtering coating has become a key technology in many fields because of its excellent process control and flexibility.

Semiconductors and microelectronics

In the semiconductor industry, vacuum sputtering coating is used to prepare thin films such as metal interconnect layer and diffusion barrier layer. Commonly used materials include aluminum, which has high conductivity and low  cost, and copper, which has better resistance to electromigration. In addition, TiN as a barrier layer material has good high temperature resistance to prevent metal diffusion.

Optical thin film

Vacuum sputtering is widely used in the preparation of optical thin films, which is suitable for anti-reflection film, filter, etc. The thickness of the optical film needs to be extremely precise to ensure its optical properties. SiO₂ and TiO₂ are common optical thin film materials, the former has good transparency and wear resistance, and the latter has a high refractive index.

Protective coating

Vacuum sputtering coating is widely used in tools, molds, mechanical parts and other protective coatings, such as titanium nitride (TiN), chromium nitride (CrN). This kind of high-hardness coating is wear-resistant and corrosion-resistant, greatly extending the service life of tools and mechanical parts, and reducing industrial maintenance costs.

Energy materials (e.g. solar cells)

Vacuum sputtering is more and more widely used in energy field. The transparent conductive film (such as ITO), reflection film, absorption layer, etc. in solar cells need to be achieved by sputtering coating process. Vacuum sputtering can prepare uniform, transparent and good conductive films at low temperatures, so it is widely used in the photovoltaic industry.