Specific Process Knowledge/Bonding/Eutectic bonding
Eutectic Bonding
Eutectic bonding depends on a formation of an eutecticum in the bond interphase. This makes bonding of two substates by use of eg. a AuSi eutecticum possible at temperatures much lower than the actual melting points of either of the two (in this case bonding can be done at around 390oC). The bonding strecth is quite good, but far from as good as anodic bonding. Different eutecticums have a different eutectic (bonding) temperature.
| Eutecticum | Eutectic temperature | Bonding temperature in EVG NIL |
|---|---|---|
| AuSi | 365oC | 390oC |
| AuSn | <300oC | 300oC |
| AuSnNi | . | . |
Anodic bonding can only occur between a Silicon wafer and a Pyrex (Borofloat) glass, not a quartz (fused silica). It is a strong chemical bond made by an E-field depleting the surface of the Pyrex wafer from Na+ ions. Hence it is very important to stack the Si wafers and the Pyrex wafer in the right order. Otherwise the Pyrex wafer could bond to the chuck or electrode, depending on the direction of the E-Field. The EVG NIL only operates with an negative voltage meaning that one should always place the Si wafer closest to the chuck, and the Pyrex wafer closest to the electrode. Almost no force is needed to push the wafers together normally 1000N. For a normal bond the temperature used is 400C and the voltage is 600V. It is possible to decrease the temperature if one increases the voltage, eg. T=320C and V=1200V.
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.
Try and put this as one of the last steps in you process sequence, since you cannot do high temperature steps in DANCHIPS cleanroom with Pyrex wafer. This also includes the plasma systems.
Bonding procedure
- If the wafers are to be alinged remeber to put the Si wafers in first (Si wafer closest to the chuck).
- If the wafers are just stacket on top of each other remeber to put the Si bellow (Si wafer closest to the chuck).
- 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.