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 390^oC). 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
	For bonding two substrates by use of an interphase that makes an eutecticum.	For bonding two identical materials.	For bonding Si and Glass.
Bonding temperature	Depending on the eutecticum 310$^{o}$C to 400$^{o}$C.	Depending on defects 50$^{o}$C to 400$^{o}$C.	Depending on the voltage 300$^{o}$C to 500$^{o}$C Standard is 400$^{o}$C.
Annnealing temperature	No annealing	1000$^{o}$C in the bond furnace C3	No annealing
Materials possible to bond	Bonding of substrates is done by use of the eutectica Au/Si, Au/Sn and Ni/Si	Si/Si, SiO$_{2}$/SiO$_{2}$	Si/Pyrex (glass)
Substrate size	Up to 6" (aligning only possible for 4" and 6")	Up to 6" (aligning only possible for 4" and 6")	Up to 6" (aligning only possible for 4" and 6")
Cleaning	Cleaning by N2.	Wet chemical cleaning, IMEC.	Cleaning by N2.
IR alignment	Double side polished wafers.	Double side polished wafers.	Not relevant.

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 400${\rm {^{o}}}$C and the voltage is 600V. It is possible to decrease the temperature if one increases the voltage, eg. T=320${\rm {^{o}}}$C 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.