Specific Process Knowledge/Bonding/Fusion bonding
Fusion Bonding
Fusion bonding is a weaker bond between e.g. two clean Si wafers. It is absolutely necessary for the wafers to be very clean and to be annealed at 1000C afterwards to avoid and minimize defects. Fusion bonding can be made as a Si to Si direct bonding or with SiO2 layers in between. It is also possible to use nitride in between but it should be close to 100% particle free. We have good experience with Sintef but unfortunetly not with the old DANCHIP nitride furnace.
Please be advised that it is notoriously difficult to use the EVG NIL bond aligner, due to its manual nature it is strongly advised to book extra time to do alignment. However alignment of +-2 microns is possible by very exprienced users. The alignment marks (on the masks) are to be positioned at y=0 and x=+-40mm for 4", for optimal result. It is possible to use both backside alignment and IR alignment however IR alignmant is more accurate, since only one aligning is used. However it is very important to use dobbelt polished wafers to have enough IR light to see the patterns.
Try and put a fusion bonding as early in a process sequence as possible, since during annealing is not possible to have any kind of metals on your wafer. Furthermore the thermal budget could drive dopants further into the wafer than wanted.
Bonding procedure
- Remember to use double side polished wafers for IR alingment.
- Clean the wafers with IMEC clean.
- Transport the wafers to the bonder in a clean box.
- Clean the chuck with ethanol.
- Clean the glass top with ethanol.
- CLean the graphite electrode with ethanol.
- Clean the bond glass with Ethanol.
- Place the wafers in the alinger with a clean tweezer.
- Align the wafers
- Place the chuck and graphite electrode in the chamber.
- Close the chamber.
- Turn the µm screw on top of the chamber to set the desired stack height.