本揭露之实施例提供封装结构及其形成方法，此方法包含提供第一集成电路晶粒并形成重布线结构于第一集成电路晶粒上方，形成基底层于重布线结构上方，基底层具有复数个第一开口和复数个第二开口，且第一开口宽于第二开口，形成复数个第一凸块于重布线结构上方，第一凸块具有下部填入第一开口中。此外，此方法包含透过具有下部填入第二开口中的复数个第二凸块将第二集成电路晶粒接合至重布线结构，在第二集成电路晶粒与基底层之间有一空间。此方法也包含形成模塑化合物层于基底层上方，模塑化合物层填入此空间中并围绕第一凸块和第二凸块。

Package structures and methods for forming the same are provided. The method includes providing a first integrated circuit die and forming a redistribution structure over the first integrated circuit die. The method also includes forming a base layer over the redistribution structure. The base layer has first and second openings. The first openings are wider than the second openings. The method further includes forming first bumps over the redistribution structure. The first bumps have a lower portion filling the first openings. In addition, the method includes bonding a second integrated circuit die to the redistribution structure through second bumps having a lower portion filling the second openings. There is a space between the second integrated circuit die and the base layer. The method also includes forming a molding compound layer over the base layer. The molding compound layer fills the space and surrounds the first and second bumps.