Dry etching technology is the process of using plasma to achieve etching. Once gases are introduced into the chamber, they will be ionized and transformed into a plasma state. In the plasma state, it exhibits two significant characteristics: firstly, the gas in the plasma exhibits higher chemical activity compared to the normal state. This means that we can select appropriate etching gases (i.e. reactants) based on the characteristics of the material to be etched, thereby accelerating the chemical reaction with the material and achieving the etching goal. Secondly, plasma can be guided and accelerated by an electric field, carrying a certain amount of energy. When these high-energy plasmas collide with the etched surface at acceleration, etching is completed through the mechanism of physical energy transfer. Therefore, dry etching is actually the result of both physical and chemical interactions. The unique feature of plasma dry etching technology is that it can not only perform anisotropic etching, but also maintain precise pattern transfer with extremely small feature critical dimensions.

In the process of dry etching, the glow discharge principle is first used to generate a gas containing neutral radicals, ions, and free electrons in a low-pressure environment. This gas is in an ionized or partially ionized state and is collectively referred to as plasma. Subsequently, by applying a voltage to the plasma, electrons are accelerated and collide with neutral gas molecules, producing ions and more neutral free radicals. As shown in Figure 1-1, due to the relatively small mass of electrons, they can move faster than ions when in contact with plasma, resulting in a rapid negative charge on the surface; Therefore, compared to plasma, electrons exhibit a negative potential. So, under the action of electric field force, ions accelerate and repel electrons, ensuring that the net charge current on the material surface remains at zero. Moreover, due to the high energy carried by ions, they can work together with active neutral substances to achieve the goal of removing impurities or unwanted components on the material surface. When etching is carried out through chemical reactions involving only active neutral substances, the isotropic properties exhibited by these neutral substances, including their uniform angular distribution and low adhesion probability, result in an isotropic profile on the etched surface. Therefore, this chemical etching, also known as isotropic etching, has a relatively high selectivity advantage. Conversely, if there are no neutral species and the surface is subjected to physical etching or high-energy ion impact, the etched surface will exhibit anisotropic characteristics due to the different directions of ion impact on the surface. When ions bombard the surface of materials, the etching effect is achieved by accelerating the deposition of ions on surface atoms and physical interactions; However, the selectivity of physical bombardment is relatively low. Therefore, in today's rapidly developing integrated circuit field, etching is usually carried out by combining chemical reactions and physical bombardment, and this type of etching process is called reactive ion etching (RIE).

Figure 1-1 Plasma Etching Process Diagram

Inductively Coupled Plasma Reactive Ion Etching Technology (ICP-RIE)

ICP-RIE is a more optimized RIE etching technique that achieves high-density, low-energy plasma by controlling the excitation and etching of the plasma separately through a separate RF source. It has the advantages of low damage and maintaining a high etching rate under low pressure.

Figure 2-2 Schematic diagram of inductively coupled plasma reactive ion etching system

Figure 2-2 shows a schematic diagram of the chamber structure of ICP-RIE (Inductively Coupled Plasma Reactive Ion Etching). From the figure, it can be seen that the system couples additional RF energy from the outside through induction coils, keeping the plasma in the chamber at a high density (>1011/cm ³). Under low-pressure conditions, plasma can remain stable, which helps to accurately control the etching morphology. The generation and distribution of plasma pass through the plasma generation chamber and are separated from the actual etching area. High power RF signals drive free electrons to undergo high-speed swirling motion within the plasma cavity, significantly increasing the ionization probability of electrons and increasing the concentration of plasma. In addition to the main RF source, the second RF source (usually referred to as an RF source or RIE source) connected to the sample stage substrate can independently adjust the power, thereby achieving precise control of the self bias voltage to optimize the etching effect. Overall, the ICP-RIE etching process is mainly divided into the following three stages:

1. Adsorption of Active Neutral Particles - Active particles in the etching gas adhere to the surface of the material.

2. Chemical reaction - the reaction gas interacts with the etched material to generate volatile reaction products.

3. Physical and chemical bombardment - Plasma applies energy to surface reaction products, causing them to detach from the surface and ultimately achieve etching.

ICP-RIE has been optimized compared to traditional Reactive Ion Etching, achieving more efficient plasma excitation by adding an Inductively Coupled Plasma source. The system uses two independent RF sources for plasma generation and etching rate control, significantly improving machining accuracy and process controllability. Compared with traditional RIE technology, ICP-RIE has advantages such as high etching rate, high selectivity, and low damage. High etching rate helps improve processing efficiency, high selectivity ensures precise etching between different materials, and low damage characteristics reduce the impact on the sample, making it particularly suitable for fine processing of nanoscale structures. In addition, this technology can also operate in lower pressure environments, further improving etching uniformity and making etching depth easier to control. In summary, ICP-RIE has been widely used in the field of micro nano manufacturing due to its excellent etching performance and high controllability, especially for the processing of semiconductor devices, optical metasurfaces, and nanoscale electronic components.