SiO2 thin film creation in Diffusion furnace - Process Functional Modeling
The Process Functional Modeling of the SiO2 thin film layer creation has revealed problems as follows:
- Two operations: LOADING (loading of the boat with wafers into the furnace) and UNLOADING (removing of the boat with wafers from the furnace) found as the most problematic
- It is important that the operations where wafers move within the furnace seem the most problematic!!!
Based on the concept of moving wafers, let's describe the models of the failures of bottom zone wafers' thickness sigma (thickness standard deviation)
Model 1. The loading of the boat with wafers causes overheating at the bottom zone of the furnace
During loading the wafers into the furnace (that is at 300C), the cold wafers "cool" the bottom thermocouple (TC) that resulting in strong overheating of the bottom zone.
Model 2. The Loading of the boat with wafers causes the presence of air residue in the bottom zone of the wafer.
During the loading of the wafers, the N2 flow mainly removes the air from the furnace, but some residual amount may remain and it will be collected in the bottom zone of the furnace. As a result, the wafers in the bottom zone will start oxidation earlier and will receive more O2.
Model 3. The unloading of the wafers from the furnace will result in additional uncontrolled oxidation on the bottom zone wafers
During the unloading of the boat with wafers, the bottom wafers expose to the atmosphere first. They have a higher temperature when exposed to the air and they keep the temperature longer because they are placed close to the massive bottom flange.
Possible solutions:
- Disconnect the temperature control during the loading and unloading of the boat with wafers
- Use Ar instead of N2 during the loading and unloading
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