Plasma state is the fourth state of matter other than solid, liquid, and gas states. When the temperature of a substance changes from low to high, it will transition from a solid state to a liquid and gas state. As the temperature of a gaseous object further increases, atoms and molecules in the gas ionize, resulting in a mixed system composed of positive ions, negative ions, and neutral molecules. This mixed system contains a large number of active ions or molecules, but the overall state is electrically neutral. Objects in this special system state are called plasmas. In the field of semiconductor processing and manufacturing, radio frequency ion sources are commonly used as power sources, which apply a radio frequency voltage between two electrode plates to generate plasma. As shown in Figure 1.1. Under the action of an electric field, free electrons gain energy, and the accelerated free electrons collide with the gas in the reaction chamber. The collision of atoms or molecules in a gas produces an ion and a free electron. The generation of ions is formed by the collision with free electrons and the loss of one free electron. Reaction collision is a continuous process, and the entire reaction chamber will be filled with electrons and ions in a very short time. This system state is called plasma.

Figure 1.1 Schematic diagram of plasma formation process

At present, etching techniques are divided into dry etching and wet etching, but dry etching belongs to plasma etching technology. As shown in Figure 1.2. Among them, dry etching is the selective removal and retention of certain areas of the material through physical and chemical methods. Chemical wet etching is the process of removing the exposed material on the surface of a silicon wafer using chemical reagents. In the semiconductor manufacturing process, dry etching is currently the mainstream etching technology, with plasma dry etching being the main method. It has advantages such as high directionality, controllable etching profile, high resolution, uniformity, and material selectivity. However, wet etching cannot control the line width and cannot process precise circuit diagrams. Therefore, the use of scenarios is also decreasing.

Figure 1.2 (a) Dry etching, (b) Wet etching

Principle of plasma etching

Dry etching technology is one of the common plasma etching methods. As shown in Figure 1.3, based on the morphology of etching, it can be divided into: (1) uniform etching; (2) Uneven etching; (3) Grain etching (note: green box in Figure 1.3 (d)); (4) Grain boundary etching (note: red box in Figure 1.3 (d)).

Figure 1.3 (a) Unetched surface, (b) Uniform etching, (c) Uneven etching, (d) Grain and grain boundary etching

Figure 1.4 Schematic diagram of plasma etching

From the principle of etching, it can be divided into: (1) physical etching; (2) Chemical etching; (3) Reactive etching. The schematic diagram of plasma etching is shown in Figure 1.4. Physical etching relies on physical energy transfer to remove surface materials, using high-energy inert gas ions (such as Ar, Xe) to bombard the material surface, knocking out atoms and molecules on the material surface. Its main core is energy transfer. Physical etching processing depends on the energy of incident ions, bombardment power, bombardment time, angle, energy density, and the physical properties of the substrate (including atomic binding energy, relative atomic mass, material density, etc.). Physical etching has the characteristics of high heterogeneity and non selectivity. Chemical etching is the process of removing surface materials through chemical reactions, utilizing the chemical reactions between active molecules in plasma and the material being etched to produce etching. Chemical etching is related to the activity, density, and physical properties of reaction products of molecules. Reactive etching is the process of accelerating charged ion bombardment to etch the surface of a substrate, resulting in a series of physical and chemical reactions on the surface to obtain an anisotropic etching profile, while exhibiting high selectivity and etching rate during the etching process.