In plasma etching technology, the core mechanisms of material removal fall into two primary categories: physical etching and chemical etching.

Physical etching, also known as sputter etching or ion milling, relies on high-energy physical particles (typically inert gas ions such as argon ions) to bombard material surfaces. When these high-energy particles collide with the material surface, they transfer their kinetic energy directly to surface atoms or molecules. If the transferred energy is sufficient to exceed the material’s binding energy and lattice energy, the surface atoms and molecules will be knocked or sputtered out of the lattice structure and detached from the surface, achieving material removal. This process is essentially a pure physical energy transfer, involving little to no chemical reactions.

Chemical etching removes materials through chemical reactions between etchant species and the material surface. Reactive chemical species in the etchant (free radicals, atoms, molecules) diffuse to the substrate surface and react with surface atoms/molecules, forming volatile byproducts that are pumped away by the vacuum system to strip materials off the surface.
Chemical etching features outstanding selectivity — formulations can be tailored to react only with target materials while yielding minimal etch rates on other layers. Additionally, it induces little physical damage to substrates due to the absence of high-energy ion bombardment.

Conventional etching technologies include four basic plasma etching mechanisms: sputtering, pure chemical etching, ion-energy-driven etching, and ion-passivation layer synergistic etching (Figure 1).

Four Fundamental Plasma Etching Processes

Sputtering describes the ejection of surface atoms under bombardment by energetic ions, as illustrated in Figure 1(a). Plasma supplies energetic ions with typical energies of several hundred volts.

Sputtering exhibits no selectivity. At fixed ion energy, sputtering yield depends on surface binding energy and the masses of target atoms and incident ions, though mass dependency is weak. The relevant parameters of different materials generally differ by a factor of only 2 to 3, so sputter rates are roughly equivalent across materials. The overall sputter rate is low, as one incident ion typically sputters out merely one atom.

The second mechanism is pure chemical etching. Plasma generates reactive species that react with the substrate surface to form volatile reaction byproducts, delivering excellent chemical selectivity.
As shown in Figure 1(b), pure chemical etching is generally isotropic. Etchant species arrive at the substrate with an evenly distributed angular flux, leading to uniform etch rates in all directions, unless crystalline reactants introduce orientation-dependent reaction speeds.
In material-processing plasmas, high flux of reactive species enables fast etching. However, the actual etch rate is rarely limited by reactant transport, but controlled by a rate-determining step among complex surface reactions.

The third type is ion-energy-driven enhanced etching (Figure 1(c)). Plasma simultaneously delivers reactive etchant species (e.g., fluorine atoms) and energetic ions. Their synergistic effect produces far superior etching performance compared to standalone chemical etching or sputtering. Similar to pure chemical etching, all reaction byproducts must be volatile.
Energetic ions travel along highly directional trajectories, yielding strong anisotropic profiles, yet the chemical selectivity is inferior to that of pure chemical etching.

The fourth process is ion-passivation layer composite etching (Figure 1(d)). This process introduces species that form protective passivation films on material surfaces. Ion bombardment suppresses or eliminates passivation layers locally, exposing the underlying substrate to chemical etching. Regions free from ion bombardment remain covered by passivation layers and are protected against etching. With precise parameter tuning, highly anisotropic vertical sidewall profiles can be achieved.