Plasma Etching Mechanisms: From Physical Sputtering to Atomic-Level Directional Control

In modern semiconductor manufacturing, the ability of dry etching to achieve high aspect ratio structures and nanoscale pattern transfer is not simply a matter of "etching using plasma," but more importantly, the precise control of different etching mechanisms within the plasma. The plasma environment itself is extremely complex, containing both high-energy ion bombardment and a large number of highly chemically reactive free radicals and neutral particles. Therefore, the etching process is often not dominated by a single factor, but rather a result of the coupling of physical effects and chemical reactions. In different process systems, the proportions of these two factors vary, ultimately resulting in completely different etching morphologies, directions, and material selectivity.

Physical etching (sputter etching)

The most basic etching mechanism is physical etching, also known as sputtering etching. This process is essentially a momentum transfer process under high-energy ion bombardment. Under high negative bias conditions, positive ions in the plasma are accelerated by the electric field of the sheath and bombard the material surface with high energy perpendicularly. When the ion energy is high enough, surface atoms are directly "knocked out" of the lattice, thereby achieving material removal. Since the ion bombardment direction is basically perpendicular to the substrate, this mechanism can form very excellent anisotropic structures, which is of great significance in early micro and nano fabrication.

However, the drawbacks of pure physical sputtering are also quite obvious. Since material removal relies primarily on mechanical momentum rather than chemical reactions, both the substrate and mask materials can be simultaneously removed when bombarded by high-energy ions. This means that the mechanism typically lacks sufficient material selectivity, easily leading to rapid mask wear. Furthermore, high-energy ions can cause significant lattice damage, inducing surface defects, dislocations, and interface amorphization. Therefore, in advanced device fabrication, pure physical etching is rarely used alone, and is more often used as an auxiliary mechanism in composite plasma etching processes.

Pure chemical etching

In contrast, there is the pure chemical etching mechanism. In this case, the main role of the plasma is not to provide ion bombardment, but to generate highly reactive free radicals. For example, in CF₄ plasma, electrons colliding with CF₄ molecules dissociate into active fluorine atoms. These fluorine free radicals can rapidly react with silicon to generate volatile SiF₄ products, which are then extracted from the reaction chamber. Because chemical free radicals can diffuse uniformly across all exposed surfaces, this type of etching typically exhibits high selectivity but poor directionality, making it more prone to forming isotropic etching morphologies.

Essentially, pure chemical etching and wet etching share some similarities; both rely on chemical reactions to remove material, except the reaction environment changes from a liquid phase to a gas-phase plasma. Due to the lack of ion directionality constraint, both the sidewalls and the bottom surface react simultaneously, making lateral undercutting a common phenomenon. While this mechanism is unsuitable for machining high aspect ratio structures, it still has applications in certain process scenarios requiring low damage, low stress, and high selectivity, such as partial sacrificial layer removal and surface cleaning processes.

Plasma etching

The establishment of the ion-enhanced etching mechanism truly drove the development of modern plasma etching. This mechanism is considered the core foundation of modern reactive ion etching (RIE) and high-density plasma etching. Its essence is not simply the superposition of physical bombardment and chemical corrosion, but rather a synergistic coupling effect between the two. In this mechanism, high-energy ions first bombard the substrate surface, breaking local atomic bonds, increasing surface activity, or removing the original passivation layer, thereby bringing the material surface, which was originally difficult to react, into a highly reactive state. Subsequently, chemical free radicals in the plasma react rapidly with the activated surface, generating volatile products and achieving material removal.

The greatest advantage of this mechanism lies in its simultaneous balance of directionality and selectivity. Since ion bombardment is primarily concentrated on the horizontal surface, the reactivity in the bottom region is significantly enhanced, while the sidewalls, which experience almost no ion bombardment, exhibit a markedly lower chemical reaction rate, resulting in an excellent anisotropic structure. Simultaneously, because material removal relies mainly on chemical reactions rather than purely mechanical bombardment, mask loss is relatively low, and selectivity is significantly improved. Modern silicon etching, dielectric layer etching, and polysilicon gate fabrication are largely based on ion-enhanced etching mechanisms.

For example, in chlorine plasma, undoped silicon itself does not undergo a significant spontaneous reaction with chlorine. However, under ion bombardment, the activation energy of the silicon surface decreases, and chlorine radicals can rapidly form volatile SiCl₄ with silicon, thereby achieving efficient etching. This "ion activation-chemical removal" mode is actually the most common working mechanism in modern advanced etching processes.

Advanced directional etching mechanism

In higher-precision directional control, ion-suppressed etching mechanisms have been further developed into sidewall protection technology. This mechanism typically introduces two types of gases simultaneously: an etchant and a polymeric inhibitor. The etchant reacts with the material to generate volatile products, while the inhibitor deposits a polymer film across the entire surface. Without ion bombardment, this polymer layer would cover the entire surface and prevent further etching; however, because vertical ions continuously bombard the bottom region, the bottom polymer is gradually removed, allowing etching to continue downwards. Meanwhile, the polymer layer remains in the sidewall regions due to the lack of ion bombardment, effectively suppressing lateral corrosion.

This mechanism forms the important theoretical basis for deep reactive ion etching (DRIE) and Bosch processes. By periodically alternating the "etch-passivation" process, extremely high aspect ratio structures can be obtained, achieving near-vertical sidewall morphologies. Currently, this type of process has become one of the core manufacturing technologies in MEMS, TSV through-silicon vias, and 3D NAND deep trench fabrication.

In recent years, with the continuous development of FinFET, GAAFET, and two-dimensional semiconductor devices, the research focus of plasma etching mechanisms is also changing. Past process development focused more on etching rate and directionality, while current advanced nodes place greater emphasis on atomic-level interface damage control, plasma-induced defects, and sub-nanometer-scale line-edge roughness. Therefore, low-energy ion etching, pulsed plasma, and atomic layer etching (ALE) have gradually become research hotspots. In particular, ALE, through its self-confined surface reaction and layer-by-layer atomic removal mechanism, can achieve material processing with near-single-atom-layer precision and is considered an important development direction for the future fabrication of advanced transistor structures.

In general, the development of plasma etching mechanisms is essentially a process of semiconductor manufacturing evolving from "coarse-scale material removal" to "atomic-level precise control." From the initial physical sputtering to chemical free radical reactions, and then to the synergistic mechanism of ion enhancement and sidewall suppression, modern etching technology has been able to achieve highly selective, highly anisotropic, and low-damage processing in complex three-dimensional structures. This is also an important foundation for the continuous miniaturization of advanced semiconductor manufacturing.