Heterogeneous bonding structure and method forming same

A method comprising:

• forming a first package component comprising:
• depositing a first dielectric layer;
• forming a first metal pad in the first dielectric layer, wherein the first metal pad comprises a first metallic material;
• and
• after the first metal pad is formed, depositing a fill-in metal layer extending into a recess of the first metal pad, wherein the fill-in metal layer comprises a second metallic material different from the first metallic material;
• bonding a second package component to the first package component, wherein the fill-in metal layer forms an alloy with an additional fill-in metal in the second package component, and wherein the second package component comprises:
• a second dielectric layer bonded to the first dielectric layer to form a first interface;
• and
• a second metal pad in the second dielectric layer, wherein the second metal pad is in contact with the first metal pad to form a second interface parallel to the first interface, and wherein the alloy separates the second interface from a respective closest edge of the first metal pad.

A method comprising:

• forming a first package component comprising:
• forming a first dielectric layer;
• forming a first metal pad in the first dielectric layer, wherein the first metal pad comprises a first metallic material;
• and
• depositing a first fill-in metal layer on the first metal pad, wherein an entirety of the first fill-in metal layer is formed of a homogeneous material;
• forming a second package component comprising:
• forming a second dielectric layer;
• forming a second metal pad in the second dielectric layer, wherein the second metal pad comprises a second metallic material;
• and
• depositing a second fill-in metal layer on the second metal pad;
• and
• bonding the first package component to the second package component, wherein a first portion of the first fill-in metal layer forms an alloy layer with the second fill-in metal layer, and a second portion of the first fill-in metal layer separates the alloy layer from the first metal pad.