An embodiment of the invention provides a device package. The device package includes a semiconductor die, a molding compound extending along sidewalls of the semiconductor die, and a planarizing polymer layer over the molding compound and extending along the sidewalls of the semiconductor die. The molding compound includes first fillers, and the planarizing polymer layer includes second fillers smaller than the first fillers. The device package further includes one or more fan-out redistribution layers (RDLs) electrically connected to the semiconductor die, wherein the one or more fan-out RDLs extend past edges of the semiconductor die onto a top surface of the planarizing polymer layer. The invention also provides a method of forming the device package.