Radio frequency discharge plasma generally has two different excitation methods, namely inductively coupled plasma and capacitively coupled plasma, which can generate high-density plasma at low pressure. The electron energy in radio frequency plasma can usually reach several eV or even higher, which can promote the reaction through the high activity of electrons without causing damage to the material background. Therefore, it is widely used in the field of material surface modification. For RF plasma sources, common plasma sources include capacitive coupled plasma sources and inductive coupled plasma sources.

Capacitive coupled plasma source

The basic discharge structure of Radio frequency Capacitive Coupled Plasma (RF-CCP) source is shown in Figure 1.1. The RF power is applied to two parallel plate electrodes to form a discharge device similar to a capacitive structure. The structure of the discharge zone between two electrodes is: sheath/plasma/sheath, similar to a sandwich structure. The controllable parameters of CCP sources usually include discharge power, working pressure, parallel plate electrode area, electrode spacing, etc. By adjusting the above discharge parameters, the discharge properties can be improved to meet practical process requirements. In order to achieve anisotropic etching process, a lower discharge pressure of 10-100mTorr, radio frequency of 1-100MHz, and electrode spacing of 1-5cm are usually selected. When the size of the two electrodes is exactly the same, it is a symmetrical CCP, as shown in Figure 1.1 (a), otherwise it is an asymmetrical CCP, as shown in Figure 1.1 (b). In asymmetric CCP, the RF power is applied to the smaller driving electrode, while the area of the ground electrode is much larger than that of the driving electrode. Therefore, due to the asymmetry of the two electrodes, the voltage drop in the sheath layers on both sides will not be equal, that is, the sheath layer near the driving electrode is thick and the potential drop is large; The sheath layer near the grounding electrode is thin and the potential drop is small. In order to maintain current balance, a large DC self bias is formed on the driving electrode side. Generally, the processed workpiece is placed on a smaller electrode to effectively utilize the self bias of the sheath layer and improve the efficiency of the surface treatment process. In radio frequency discharge, the frequency of a typical radio frequency power supply is 13.56MHz, and the electron density is related to the radio frequency frequency. Its discharge density is relatively low, generally between 1015-1017m-3. But in some special cases, the discharge frequency will be selected in the frequency band greater than 13.56MHz, or even higher (known as very high frequency CCP).

Figure 1.1 Schematic diagram of RF capacitive coupled plasma discharge structure: (a) Symmetric electrode structure; (b) Asymmetric electrode structure

Inductively coupled plasma source

The ICP source is a radio frequency coil placed at the top or side wall, which is driven by a radio frequency power source. Then, an alternating radio frequency current is input into the coil to generate an alternating magnetic field in the reactor. The alternating magnetic field induces an alternating electric field, which ionizes the background gas in the reactor to produce plasma. Usually, ICP uses a radio frequency power supply with a frequency of 13.56MHz. Due to the fact that the radio frequency electric field moves along the circumferential direction, the electrons accelerated by the electric field mainly move along the circumferential direction, and there are more opportunities for collision with neutral particles. Therefore, uniform high-density plasma can be obtained in a large range under low pressure.

According to the different shapes and positions of the discharge coils, ICP sources in applications are mainly divided into the following two types: one is to wrap the RF coil around the side of the discharge chamber to form a spiral cylindrical coil, as shown in Figure 1.2. Generally, quartz dielectric windows are used to separate the discharge chamber from the coil. Due to the fact that the transmitting antenna of the RF source is wrapped outside the insulated vacuum chamber at this time, when RF power is applied to the antenna through a matching network, RF current will pass through the antenna and generate RF magnetic flux. The RF electric field will be induced along the angular direction of the cylindrical descriptor inside the vacuum container, thereby breaking down the discharge gas and generating plasma. Another type of ICP source places the coil at the top of the chamber, called a planar coil, as shown in Figure 1.2 (b). The substrate is placed below, with a vacuum window and a distance of about 5-10cm between the substrate. The electron acceleration mechanism is the same as that of a spiral cylindrical coil, and the uniformity of the plasma can be controlled by the structure of the antenna.

Figure 1.2 Schematic diagram of inductively coupled plasma source structure: (a) columnar coil, (b) planar coil

In ICP discharge, the RF current (angular direction) in the coil will generate an alternating magnetic field (two components along the radial and axial directions), and this changing magnetic field will generate a changing RF electric field (angular direction), so this discharge is called an electromagnetic mode (Hmode), which is inductive discharge. The voltage drop across the coil can generate an electrostatic field similar to that in CCP discharge in the discharge chamber, hence the discharge is called capacitive or electrostatic mode (Emode). ICP has the advantages of low discharge pressure (<50mTorr), simple device structure, and no need for external magnetic field, so it is widely used in the etching of polycrystalline silicon and metals in the semiconductor industry.