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Question:
How can I tell if my wafers have been properly treated with HMDS (Hexamethyldisilazane)?
Answer:
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By:
Ken Sautter |
The easiest way to tell is using water contact angle. There are tools called, goniometers, which can give precise measurements of the contact angle, but one can eyeball it as well.
Before priming, take an eyedropper or syringe of deionized (DI) water. Create as small a drop as possible and place gently on the surface of the wafer. Note how much it spreads out. If the wafer is very clean or has been properly plasma treated, the drop will spread completely (hydrophilic). If it has been sitting around for a while, the angle will be about 40°.
In order to properly prepare a wafer for a coating of photoresist, it must be completely dehydrated-not only surface moisture, but chemically bound water molecules, as well. To achieve this, Yield Engineering Systems (YES) developed a process combining high temperature with low pressure. YES-TA vacuum bake/ vapor prime ovens remove the moisture that is chemically bound to the surface so the silane/substrate bond isn't impaired.
Then, when chemical is applied, a superior bond is formed that is stable even after exposure to atmospheric moisture. Wafers properly treated will last for weeks with no change to surface adhesion.
After treatment, repeat the eyedropper test. The water should now bead up on the surface. A properly primed surface will form a contact angle of 70-75°. If there is too much HMDS, the contact angle may reach 85-90°. If the wafers are overprimed, photoresist will roll off the wafer and the wafer will appear to have grease spots. If the wafer is underprimed, the resist will simply not have great adhesion.
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