Principle and characteristics of thin film preparation by sputtering method

In a low voltage DC glow discharge, the process is divided into five zones: the cathode zone, the negative glow zone, the Faraday dark zone, the positive column zone and the anode zone. One of these zones, the anomalous glow discharge, is the one actually used for sputtering the deposited film. In sputtering the substrate acts as the anode, usually placed in the negative glow region, and the target acts as the cathode. Since the energy and velocity of electrons are much higher than those of ions, a plasma sheath layer is formed during the glow discharge and, due to the presence of the sheath potential, a large portion of the applied voltage at the electrode is taken up by the cathode sheath potential. The establishment of the plasma sheath potential causes the ions reaching the cathode to be accelerated accordingly and gain energy. Therefore, the ions have high energy when they reach the surface of the cathode through the plasma region of the glow discharge, and produce a bombardment effect on the surface of the cathode, so that the molecules and atoms of the cathode material are sputtered out and the sputtering phenomenon occurs. These molecules and atoms have a certain amount of kinetic energy and are shot in a certain direction toward the surface of the workpiece to form a thin film.

Sputtering is a complex process in which sputtering is accompanied by various phenomena of ion bombardment. The basic condition for thin film deposition is that most of the particles ejected from the solid surface under the high-speed collision of incident particles are neutral atoms or molecules. In addition to this, the radiated secondary electrons are the elementary particles that maintain the glow discharge in sputtering, and their energy is equal to the potential of the target.

Diode sputtering is the earliest sputtering method to realize technical application. The sputtering device of diode sputtering consists of two electrodes, the cathode and the anode, so it is also called cathodic sputtering or direct current (DC) sputtering.

Due to the existence of dipolar sputtering gas ionization rate is low (in 0.3 % ~ 0.5%), the deposition rate is slow, the target material utilization rate is low, the electrons make the substrate temperature rise and other shortcomings, so the sputtering technology was not widely used, in order to make up for these shortcomings, in the last century, 1970's put forward the magnetron sputtering technology. Magnetron sputtering technology attenuates the damage to the film caused by the heat of secondary electrons hitting the substrate and improves the sputtering rate of dipolar sputtering. Therefore, magnetron sputtering technology has gained rapid development and wide application as soon as it was introduced, and now it has become one of the mainstream technologies for material surface coating.

The magnet is placed at the back of the target, so that the surface of the magnetic line of force that penetrates out of the target forms a structure that is perpendicular to the electric field and eventually returns to the target surface. The binding effect of the magnetic field on the secondary electrons on the target surface significantly increases the plasma concentration near the target surface, thus effectively solving the problem of low deposition rate of ordinary dipole sputtering. Electrons in the electric and magnetic fields, the trajectory is bent, in the process of flying to the substrate electrons and gas atoms collision, so that the gas molecules ionized cations and electrons, electrons to the anode, and cations will be accelerated under the action of the electric field to the target, and high energy bombardment of the target surface, so that the target material sputtering, sputtering out of the neutral target atoms (or molecules) in the substrate deposition of the film formation.