Package and forming method thereof

In an embodiment, a package includes an integrated circuit device attached to a substrate; an encapsulant disposed over the substrate and laterally around the integrated circuit device, wherein a top surface of the encapsulant is coplanar with the top surface of the integrated circuit device; and a heat dissipation structure disposed over the integrated circuit device and the encapsulant, wherein the heat dissipation structure includes a spreading layer disposed over the encapsulant and the integrated circuit device, wherein the spreading layer includes a plurality of islands, wherein at least a portion of the islands are arranged as lines extending in a first direction in a plan view; a plurality of pillars disposed over the islands of the spreading layer; and nanostructures disposed over the pillars.