Functional Modeling of Wet Etch at Semiconductor Manufacturing
The product of the wet etch system is the wafer. The system processes the wafer by removing selected material, residue, particles, or contamination from its surface and prepares it for the next manufacturing step.
The wafer is delivered into the wet etch chamber by a robot blade and placed on lift pins. The chuck then clamps the wafer at the edge or bevel, the lift pins move down, and the wafer starts rotating according to the recipe.
A movable nozzle dispenses the required chemistry, typically near the wafer center. Wafer rotation spreads the chemistry from the center to the edge. The chemistry removes material by dissolving it or by loosening and carrying away residues and particles with the liquid flow. If needed, several independent nozzles can dispense different chemistries or DI water according to the process sequence.
After the chemical step, DI water rinses the wafer and removes remaining chemistry, dissolved material, and movable particles. Used chemistry and rinse water flow into the process bowl, then to the drain and waste-collection system. Filtered clean air or nitrogen is supplied to the chamber to reduce airborne contamination and remove vapors.
At the end of the process, the wafer is dried by high-speed rotation, sometimes assisted by nitrogen flow. The desired output is a clean, etched, rinsed, and dry wafer with minimal residues, particles, watermarks, and surface damage.
Operational Effectiveness – OE
Effective
Ineffective
OE 0.17
Operational Perfectness - OP
Basic functions
Components
Supersystems
Chuck | 8 |
Chamber | 5 |
Chemistry 2 | 4 14 |
Chemistry 1 | 4 27 |
Air flow in the chamber | 4 35 |
DI water | 4 27 |
Nozzle for chemistry 1 | 2 |
Nozzle for chemistry 2 | 2 |
Nozzle for DI water | 2 |
Fan | 4 |
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.
- Control air flow inside the chamber
- 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.
- Reduce evaporation of chemistry and DI water
- 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.
- Improve wafer wetting at the beginning of the process
- 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.
- Prevent bubble formation and air trapping
- 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.
- Consider alternative chamber/process architecture
- 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:
Similar projects
The process is related to microelectronics - microchip manufacturing. The purpose of the process is to create a SiO2 layer on the surface of a Si wafer. Equipment: Vertical furnace to heat the wafers in the Q2 atmosphere and perform oxidation on the wafer surface. Process: The oxidation occurs on the front side and on the back side of the wafer Requirements: Create a SiO2 thin layer with a certain thickness and low sigma - low standard deviation of the thickness between the wafers and within the wafer Failure: Wafers from the lower zone have higher thickness and significantly higher within wafer sigma (standard deviation of the thickness within the wafer)
Wet cleaning is widely used in microchip manufacturing. Single wafer equipment is working as follows. A wafer rotates, and chemistry is poured from a movable nozzle. Water rinsing is performed at the end of the process. Loading of a new batch of the chemistry resulted in excursion - a strongly increased amount of defects was observed on the wafer after the processing. The project is dedicated to the failure analysis and creation of innovative solutions.
This project investigates particle-defect formation during SiO₂ dry etch. Functional modeling revealed that defects can be generated by two coupled mechanisms: chemical formation of SiO₂/SiOFₓ particles from SiF₄ interaction with moisture, and mechanical generation of particles by excessive ion bombardment and sputtering. The proposed improvement direction is to balance chemical etching and ion-assisted etching by properly adjusting the source and bias generators, reducing moisture, improving by-product evacuation, minimizing sputtering, and using pulsed or multi-step process recipes. The goal is to reduce particles and micromasking while maintaining etch rate, profile control, and process stability.