Plasma etching is a process for the selective removal of materials by means of plasma or ion beams. Featuring high precision and excellent reproducibility, it is widely adopted in the fabrication of semiconductors, MEMS and optical devices.
Within semiconductor manufacturing workflows, plasma etching serves for pattern transfer. To put it simply, patterns are transferred downwards by stripping the underlying material unprotected by photoresist or other masking substances.
Plasma etching can perform patterned etching on a wide range of materials including metals, silicon dioxide, silicon nitride, polysilicon, silicon carbide and gallium nitride. Based on the differing etching effects of plasma, the classification and characteristics of plasma‑etching mechanisms are listed in Table 1‑1.

Table 1‑1 Plasma‑Etching Mechanisms

Three mechanisms co‑exist during plasma etching: physical sputtering, pure chemical etching and ion‑assisted chemical etching. Most practical plasma‑etching applications are dominated by ion‑assisted chemical etching.
Ion bombardment carries out two key functions in this procedure. First, it breaks atomic bonds on the material surface and generates chemically‑active dangling bonds to accelerate chemical‑reaction rates. Second, it desorbs reaction by‑products accumulated on the reaction interface, enabling reactive species to keep contacting the material surface.
When reaction deposits on the sidewalls of etched features cannot be efficiently eliminated by directional ion bombardment, the ratio of vertical‑to‑lateral etch rate rises and anisotropic etching is achieved.

The surface‑reaction principle of plasma etching is illustrated in Figure 1‑1.

Figure 1‑1 Schematic diagram for the surface‑reaction mechanism of plasma etching

The complete etching cycle inside the vacuum chamber consists of five primary stages, as simplified in the workflow diagram Figure 1‑2.

Figure 1‑2 Simplified flow chart of the plasma‑etching procedure

(1) Equipment preparation
The apparatus conducts temperature regulation and chamber pumping according to preset temperature and chamber‑pressure parameters. Etching commences only after the chamber environment stabilises at target setpoints.

(2) Process‑gas inlet into the etching chamber
Gas inflow is regulated by the Mass Flow Controller (MFC) at configured flow‑rate values. Chamber pressure fluctuates after gas admission, so the vacuum system readjusts the internal pressure back to the preset value.

(3) Plasma generation
The radio‑frequency (RF) source is activated with impedance matching to minimise energy loss, which produces glow discharge maintained throughout the whole reaction. Each process gas yields a characteristic glow‑discharge colour; abnormal luminescence generally indicates compromised gas purity.
The injected gas then undergoes ionisation, decomposition, excitation and relaxation, whose typical reaction formulas are given from Equation (2.1) to (2.4).

Here A and B denote general‑purpose atoms; A* stands for an excited‑state atom, h represents Planck’s constant and ν is the light frequency.

(4) Surface‑based reactions
After plasma generation, reactive free radicals and positive ions travel toward the wafer surface driven by the bias‑voltage electric field and trigger etching. The three etching modes shown in Figure 1‑2 take place, accompanied by particle adsorption‑desorption, as well as by‑product desorption and redeposition.

(5) By‑product exhaust
The vacuum system maintains stable chamber pressure all through processing. Most reaction by‑products are evacuated out of the reaction chamber by the pumping subsystem, while a fraction may adhere to the inner‑chamber wall and require routine cleaning.