Package structure

A package structure, comprising:

• a first device region over a first substrate;
• a second device region below a second substrate;
• a bonding structure between the first device region and the second device region, wherein the bonding structure comprises a first intermetallic compound (IMC) and a second intermetallic compound (IMC);
• and
• a first barrier layer on the second IMC, wherein a sidewall of the first barrier layer is exposed to an air gap.

A package structure, comprising:

• a first die comprising a first device region;
• a second die comprising a second device region;
• a bonding structure between the first die and the second die, wherein the bonding structure comprises a first intermetallic compound (IMC) and a second intermetallic compound (IMC);
• a first barrier layer interfacing a bottom surface and a sidewall of the first IMC;
• and
• a second barrier layer on the second IMC, wherein the second IMC interfaces a bottom surface and a sidewall of the second barrier layer.