Wafer cleaning issues at the wet process
To analyze why the defects remain on the wafer while cleaning with chemistry.
The process is as follows:
- a wafer is kept in the special chuck in 4 points
- chemistry pores from the nozzle on the wafer
- the wafer rotates allows removing the chemistry from the wafer together with the defects
- parameters of the process - chemistry flow, temperature, process time, wafer rotation rate and so on can be adjusted
It seems like the process conditions are not optimal
Linear (5+ whys) analysis is needed to search for a root cause of insufficient defects removal with the chemistry and with water at rinsing
The most important point is that chemistry does not perform the process properly - improve the process is needed
Defects remain on the wafer surface after the wet cleaning process step
Try to eliminate the washing. Do we really need this?
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Because the wet process does not remove the defects
To increase the density of the chemistry by temperature reduction.
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Defects remain on the wafer till the very end of the process.
Ensure that the chemistry remains on the wafer as long as possible (reduce rotation of the wafer)
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The chemistry is removed from the wafer before it removes the defects
Reduce evaporation rate of the chemistry, reduce rotation of the wafer
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Because the chemistry contaminates the wafer due to impurities or other properties
Replace the chemistry
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If | The wafer will be washed as much as needed. |
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Then | A big amount of chemistry or water will remain on the wafer during the cleaning process. |
But | The process cost will be significantly increased. |
Defects remain on the wafer surface. The wet cleaning process suddenly becomes ineffective.
Let's apply the Effective Brainstorming (EBS) tool to generate ideas and prioritize them.
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The purpose of the wet etch process is to clean the wafer, dissolve and remove all soluble material, and remove insoluble material as small particles carried by the solution flow.
Defects remain on the wafer surface after the wet cleaning process step.
Single wafer wet cleaning system.
The chemistry is poured on the rotated wafer to remove the stuff and not leave residue or particles. The wafer rotates, and the solution with the dissolved and movable particles is removed from the wafer by centrifugal forces.
Pure air operates as an important component of the system (not a supesystem component). Pure air is filtered by an air filter, but it might also move air particles within the chamber, and can stimulate the evaporation of the solution from the wafer
The sketch of the chamber is shown below:
Operational Effectiveness – OE
Effective
Ineffective
OE 0.22
Operational Perfectness - OP
Basic functions
Components
Supersystems
OP 0.02
Frontside Chemistry | 17 17 |
Wafer | 12 |
Residue | 5 5 |
Stuff to remove | 5 |
Backside chemistry | 5 |
Chuck | 4 |
Chemistry particles | 3 |
Frontside nozzle | 3 |
Pump | 3 |
Chemistry Filter | 2 |
The most problematic components are Air and Front side Chemistry:
Do we really need an airflow? Maybe it would be better just filter without a fan. The exhaust will compensate for the pressure variations.
Front side chemistry is also the most functional component. It means the improvement can be achieved by variation of the chemistry parameters
All components of low functionality can be trimmed.
The functionality of the Exhaust can be significantly improved by relocation within the chamber
The general conclusion is to eliminate the air supply and exhaust system.
Let air enter the chamber and convert exhaust to drain with very little under-pressure.
This will stabilize the performance, reduce the evaporation rate of the chemistry, reduce the cost and simplify the system.
Defects remain on the wafer surface after the wet cleaning process step.
Single wafer wet cleaning system.
The chemistry is pored on the rotated wafer to remove the stuff and not leave residue or particles.
The sketch of the chamber is shown below:
Operational Effectiveness – OE
Effective
Ineffective
OE 0.39
Operational Perfectness - OP
Basic functions
Components
Supersystems
OP 0.09
Chuck | 12 |
Front side chemistry | 8 20 |
Pump | 4 |
Residue | 4 4 |
Tank with chemistry | 4 |
Back side chemistry | 4 |
Air | 3 77 |
Air particles | 2 2 |
Heater | 2 |
Chemistry filter | 2 6 |
The air flow is the most harmful component - prevent air flow to avoid drying and evaporation of the chemistry that results in the leaving of the residue on the wafer surface
The system is built for wet cleaning of silicon wafers. The wafer rotates and the solution is delivered through a nozzle to the center of the wafer. The solution purpose is to dissolve the defects and remove the defects both chemically and mechanically from the wafer. The problem is that the process is not very effective and some defects remain on the wafer.
Pure air passes into the chamber and since the air can keep particles the filter is installed in the top of the chamber. Nevertheless, filter does not stop all the particles and the particles fall down to the wafer and additionally contaminate the wafer. The air, even pure air, also increases the evaporation of the solution, which reduces the time of the interaction of the solution with the defects and with the residue that remains on the wafer.
Please avoid using supesystem here - all the parts of the system should be taken and treated as components of the system.
Also we need to take into account that the chemistry wets the wafer, moves the particles and residue on the wafer
Definition of defects: defects are particles that remain on the wafer.
We suspect that the solution is flying away from the wafer too fast
The scheme of the wet cleaning system is shown in the picture below:
Operational Effectiveness – OE
Effective
Ineffective
OE Ideal operation
Operational Perfectness - OP
Basic functions
Components
Supersystems
OP 0.47
Chuck | 8 |
Chamber | 4 |
Back Side Chemistry Nozzle | 4 |
Front Side Chemistry Nozzle | 4 |
Filter And Fan | 2 |
Exhaust | 2 |
Tank With Chemistry | No impact |
Pump | No impact |
Heater | No impact |
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