Functional Modeling of Dry Etch at Semiconductor Manufacturing

Defect reduction should focus on moisture control, fast SiF₄ evacuation, reduced sputtering, and proper coordination of source and bias generators. The source generator should drive the chemical etch, while the bias generator should provide only the minimum ion energy needed for profile control. The process should be strong enough to etch SiO₂, but not so aggressive that it creates the particles it is trying to avoid.

The Functional Model shows that the main problem in the dry etch process is not simply insufficient etching, but the formation of SiO₂-based particles and residues during the etch itself.

Two defect-generation mechanisms were identified:

1. Chemical particle formation
2. Fluorine plasma reacts with SiO₂ and forms volatile SiF₄, which should be removed by the vacuum system. However, if water or moisture-containing by-products are present, SiF₄ can react and form SiO₂ or silicon-oxyfluoride particles. These particles may redeposit on the wafer, block vias or trenches, and cause local underetch.
3. Mechanical particle formation
4. Ion bombardment is required to make the etch directional and remove material from the feature bottom. However, excessive ion energy causes sputtering of SiO₂, photoresist, SLAM, chamber coatings, or focus-ring material. These sputtered fragments can redeposit as particles, micromasks, or residues.

Therefore, the dry etch process is controlled by two coupled mechanisms:

Chemical etch: formation of volatile SiF₄

Mechanical / sputter etch: ion-assisted directional removal

The key conclusion is that increasing plasma power or bias power alone is not a reliable solution. If the chemical part is too strong, too much SiF₄ and reactive by-products are generated. If the mechanical part is too strong, sputtering and redeposition increase. In both cases, the process may create more defects while trying to etch faster.

The winning direction is to balance the two generators:

1. The source generator should provide enough plasma density and fluorine radicals for efficient chemical conversion of SiO₂ into volatile SiF₄.
2. The bias generator should provide only the necessary ion energy for anisotropic etching and bottom clearing, without excessive sputtering.

A practical improvement strategy is:

Use chemical etching to remove SiO₂ efficiently, and use ion bombardment only as much as needed to control profile and directionality.

This can be achieved by optimizing source power, bias power, pressure, gas composition, wafer temperature, pumping efficiency, and overetch time. In many cases, a multi-step recipe may be preferable: stronger chemical etch during the main etch, controlled bias for profile formation, purge or evacuation of volatile products, and a lower-bias final overetch to reduce sputtering and particle generation.

The main principle is:

A clean dry etch is not the most aggressive etch. A clean dry etch is the process where chemical reaction products leave the chamber before they become particles, and ion bombardment is kept below the sputtering-defect threshold.

Thus, defect reduction requires not more power, but proper coordination of chemical and mechanical etching mechanisms.