Processing Considerations

1. Chemical Cleaning

Prior to use, silicon wafers are usually cleaned chemically to remove organic films, heavy metals and particulate.

Three common mixtures used to clean the silicon wafer are the following:

· NH₄OH + H₂O₂

· HCL + H₂O₂ ( best for removing metallic impurities)

· H₂SO₄ + H₂O₂(best to remove organic contaminants)

2. Thermal stress factors

When the wafer is removed from a high temperature furnace, the wafer edges cool rapidly by radiation to the surrounding, but the wafer centers remain relatively hot. The resultant temperature gradient creates a thermal stress S that can be estimated as

S = aEDT

Where, a is the coefficient of thermal expansion, E is Young's modulus and, DT is temperature difference across the wafer.[2]

If these stresses exceed the yield strength of the material dislocations will form. Stresses are usually kept to acceptable levels by withdrawing wafers from the furnace slowly to minimize the temperature gradient or by lowering the furnace temperature before removing the wafers, to the point where the yield strength at the removal temperature exceeds the stresses imposed. Also note that oxygen precipitates can reduce the yield strength up to five fold.

Note: The yield strength of the material is the maximum or critical shear, stress the material will accommodate without irreversible deformation.