Selection of underfill material in Cu hybrid bonding and its effect on the transistor keep-out-zone

Hybrid bonding of Cu with PECVD oxide (PE-TEOS), benzocyclobutane (BCB), and polyimide (PI) is gaining importance in third dimensional IC fabrication. Thermal mechanical simulation is performed on Cu hybrid bonding structure as the gap-filling materials are not thermally matched to the back-end-of-line (BEOL) materials. Two bonded Si layer connected by TSV with different underfill materials are compared with metal bonding without underfill. Stress profile in the Si substrate for back-to-face and face-to-face bonding orientations is extracted. By using the piezoresistive equation, keep-out-zone (KOZ) for MOS transistor is estimated. TEOS is found to be able to minimize the mobility change for n-MOSFET but similar effect is not observed in p-MOSFET. Mobility change when organic polymer (BCB or PI) underfill is used shows dependency on the via diameter and channel orientation. This study provides a guideline for transistor placement in hybrid bonding to minimize mobility change.