Conclusion: Functional Modeling of Wet Etch

Functional Modeling shows that the wet etch system has two key groups of components: components that provide useful process functions and components that create the main harmful effects.

The most functional component is the chuck, because it holds the wafer and rotates it. Wafer rotation is essential for spreading chemistry, controlling liquid flow, removing spent solution, supporting rinsing, and drying the wafer.

The most problematic component is air — both the air initially present on the wafer surface and the air flowing inside the chamber.

Main challenge 1: Air on the wafer surface

Before chemistry or DI water reaches the wafer, the wafer surface is covered by air. For effective wet etch or cleaning, the liquid must replace this air and fully wet the wafer surface.

If air is not completely displaced, it may remain on the wafer as a thin air film or small bubbles. These air pockets block contact between the solution and the material that must be removed. As a result, the chemistry cannot properly dissolve residues or etch the target material, and local defects or incomplete cleaning may remain.

Therefore, one important mechanism of wet etch failure is:

Air prevents solution from contacting the wafer surface.

Main challenge 2: Air flow inside the chamber

Air flow inside the chamber can also create harmful effects. Although clean air or exhaust flow is used to control vapors and particles, excessive or poorly directed air flow can intensify evaporation of chemistry or DI water from the wafer surface.

When liquid evaporates too quickly, dissolved material, chemicals, or particles can remain on the wafer. This can lead to residues, watermarks, particle redeposition, and local contamination. Air flow can also move particles or aerosols inside the chamber and increase the risk of redeposition on the wafer.

Therefore, the second important mechanism of wet etch failure is:

Air flow accelerates evaporation and can leave defects on the wafer.

Recommended improvement directions

The key direction is to improve wetting and reduce uncontrolled evaporation.

1. Control air flow inside the chamber
2. Air flow should not directly disturb the liquid film on the wafer. Instead of simply increasing airflow, it should be minimized, redirected, or isolated from the wafer surface while still maintaining safe vapor removal.
3. Reduce evaporation of chemistry and DI water
4. Evaporation can be reduced by controlling chamber atmosphere, humidity, pressure, temperature, exhaust rate, and process timing. The goal is to prevent the liquid film from drying too early and leaving residues or particles behind.
5. Improve wafer wetting at the beginning of the process
6. The process should ensure complete replacement of air by liquid. Possible actions include pre-wet steps, lower initial rotation speed, temporary stop of wafer rotation during first dispense, optimized nozzle position, improved dispense profile, and chemistry additives that improve wetting.
7. Prevent bubble formation and air trapping
8. The dispense sequence should avoid trapping air between the liquid and wafer surface. Liquid should contact the wafer smoothly and continuously, especially in patterned areas and high-aspect-ratio features.
9. Consider alternative chamber/process architecture
10. A more radical solution is to redesign the process so the wafer is processed face down toward a shallow liquid bath. This could improve wetting, reduce air trapping, and reduce evaporation. A shallow bath may also allow fast replacement of chemistry and rinse water while keeping the wafer continuously covered by liquid.

The main conclusion is:

Wet etch defects are not caused only by chemistry. They are also caused by the interaction between liquid, wafer rotation, air on the wafer, and air flow inside the chamber. To improve the process, the system must ensure full wetting of the wafer and prevent uncontrolled evaporation before residues and particles are removed.

Face-down concept: