Why does dry etching require fluorine-based plasma?

I. Etching Fundamentals: Free Radicals and Plasma Control

A. The Role of the Free Radical

The chemical activity in plasma etching is driven by free radicals, which are atoms or molecules possessing one or more unpaired electrons. Due to the presence of these unpaired electrons, radicals are highly unstable and reactive, rapidly reacting with other molecules to form new chemical bonds.

The F radical, derived from fluorine-containing gases, is particularly corrosive due to fluorine's extremely high electronegativity, which drives it to form strong chemical bonds with exposed atoms (e.g., silicon) on the wafer or chamber surface. This high reactivity allows F radicals to rapidly and efficiently etch (corrode) materials, forming volatile byproducts (e.g., SiF4). In a plasma environment, the reaction rate between these radicals and materials is extremely high.

B. Etching Mechanisms: F/C Ratio

In dry etching, control over the Fluorine-to-Carbon (F/C) ratio is essential for achieving the required profile and selectivity.

High F/C Ratio (e.g., SF6): Favors chemical etching, generating abundant F radicals for rapid material removal.

Low F/C Ratio (e.g., C4F8): Favors the deposition of fluorocarbon (FC) polymer fragments, which serve as a protective film for sidewall passivation, crucial for creating anisotropic (vertical) etch profiles.

II. Wafer Etching and Profile Control Gases

These gases are used directly on the wafer surface to create precise patterns and structures by etching silicon (Si), silicon dioxide (SiO2), and silicon nitride (Si3N4).

1. Tetrafluoromethane (CF4)

CF4 is a widely used general etchant for SiO2, Si3N4, and Si. Its etching effect, primarily driven by F radicals, is often optimized by blending with additives, such as oxygen (O2), which helps regulate the F/C ratio. O2 acts as a carbon scavenger, consuming polymerizing carbon species and thereby increasing the F radical concentration to enhance the etch rate.

2. Sulfur Hexafluoride (SF6)

SF6 is an extremely efficient Si etchant that produces a high flux of F radicals.

Application: It is ideal for high-rate etching of silicon, used in blanket (non-patterned) area cleaning and as the aggressive etching step (E-step) in Deep Reactive Ion Etching (DRIE).

3. Octafluorocyclobutane (c - C4F8)

c - C4F8 is a strongly polymerizing gas with a low F/C ratio.

Application: It is specialized for sidewall passivation in Deep Silicon Etching (DRIE) / Bosch Process. In the passivation step (P-step), it deposits a protective fluorocarbon (FC) film on the sidewalls to block lateral etching by F radicals

4. Trifluoromethane (CHF3)

CHF3 is used for etching SiO2 and Si3N4. The hydrogen content in CHF3 helps to moderate the plasma, promoting a slight degree of polymerization.

Application: It is suitable for Shallow Trench Isolation (STI) processes, where the gentle polymer formation provides necessary profile control and high selectivity for etching dielectrics over the Si substrate.

5. Trifluoroiodomethane (CF3I)

CF3I is an alternative gas that offers low-damage etching. It provides stable F radicals at low temperatures.

Application: It is being adopted as a replacement for CF4 in advanced etching processes, such as High-Aspect-Ratio (HAR) oxide etching, due to its high selectivity and lower material damage.

III. CVD Chamber Cleaning Gases

These gases are essential for in-situ cleaning of Chemical Vapor Deposition (CVD) chambers by removing unwanted silicon-based residues and precoat materials from the chamber walls.

6. Nitrogen Trifluoride (NF3)

NF3 is the mainstream cleaning gas for CVD equipment.

Mechanism: It requires plasma activation (e.g., RF power density > 1.4W/cm₂) 12 to decompose and generate highly active F radicals, which react with the silicon deposits to form volatile SiF4.

7. Chlorine Trifluoride (ClF3)

ClF3 is a highly potent interhalogen compound known for its extreme cleaning and etching capabilities.

Mechanism: It is used for thermal (plasmaless) in-situ cleaning of CVD equipment, relying on the chamber's operating temperature to drive the chemical reaction, eliminating the need for plasma activation.

8. Chlorine Pentafluoride (ClF5)

ClF5 is an oxidizing gas with even stronger cleaning capabilities than ClF3.

Application: Like ClF3, it is used for high-efficiency thermal in-situ cleaning of CVD chambers, though its chemical activity and associated handling risks are proportionally higher.

IV. Plasma-Resistant Materials

To mitigate the corrosive effects of highly reactive plasma species, especially F radicals, critical components (such as chamber linings and showerheads) are constructed from plasma-resistant materials, Cersol can provide semiconductor parts made from the following ceramic materials: Click here to learn more!

V. Summary of Gas Applications