Microelectronic devices generally have a die (i.e., a chip) that includes integrated circuitry having a high density of very small components. In a typical process, a large number of dies are manufactured on a single wafer using many different processes that may be repeated at various stages (e.g., implanting, doping, photolithography, chemical vapor deposition, plasma vapor deposition, plating, planarizing, etching, etc.). The dies include an array of very small bond-pads electrically coupled to the integrated circuitry. The bond-pads are the external electrical contacts on the die through which the supply voltage, signals, etc., are transmitted to and from the integrated circuitry. After forming the dies, the wafer is thinned by backgrinding, and then the dies are separated from one another (i.e., singulated) by dicing the wafer. Next, the dies are typically “packaged” to connect the bond-pads to a larger array of electrical terminals that can be more easily coupled to the various power supply lines, signal lines, and ground lines.
Conventional die-level packaging processes include (a) attaching individual dies to an interposer substrate, (b) wire-bonding the bond-pads of the dies to the terminals of the interposer substrate, and (c) encapsulating the dies with a molding compound. Die-level packaging, however, has several drawbacks. First, it is time consuming and expensive to mount individual dies to interposer substrates or lead frames. Second, as the demand for higher pin counts and smaller packages increases, it becomes more difficult to form robust wire-bonds that can withstand the forces involved in molding processes. Third, the handling processes for attaching individual dies to interposer substrates or lead frames may damage bare dies.
Another process for packaging microelectronic devices is wafer-level packaging. In wafer-level packaging, a plurality of microelectronic dies are formed on a wafer, and then a redistribution layer is formed over the dies. The redistribution layer has a dielectric layer, a plurality of ball-pad arrays on the dielectric layer, and a plurality of conductive traces in the dielectric layer. Each ball-pad array is arranged over a corresponding die, and the ball-pads in each array are coupled to corresponding bond-pads of the die with the conductive traces. The conductive traces are typically constructed by laser drilling holes in the dielectric layer to expose the bond-pads on the dies, and then depositing conductive material into the holes. After forming the redistribution layer on the wafer, a highly accurate stenciling machine deposits discrete masses of solder paste onto the individual ball-pads. The solder paste is then reflowed to form small solder balls or “solder bumps” on the ball-pads. After forming the solder balls, the wafer is singulated to separate the individual microelectronic devices from one another. The individual microelectronic devices are subsequently attached to a substrate such as a printed circuit board. Microelectronic devices packaged at the wafer-level can have high pin counts in a small area, but they are not as robust as devices packaged at the die-level.
Packaged microelectronic devices can also be constructed by “build-up” packaging. For example, a sacrificial substrate can be attached to a panel that includes a plurality of microelectronic dies and an organic filler that couples the dies together. The sacrificial substrate is generally a ceramic disc that is attached to the active sides of the dies. Next, the back sides of the dies are thinned and a ceramic layer is attached to the back sides. The sacrificial substrate is then removed from the active sides of the dies and build-up layers or a redistribution layer is formed on the active sides of the dies. Packaged devices using a build-up approach on a sacrificial substrate provide high pin counts in a small area and a reasonably robust structure.
The build-up packaging process described above and conventional wafer-level packaging processes, however, have several drawbacks. First, laser drilling holes in the redistribution layer to expose the bond-pads can damage the bond-pads and/or other components on the active side of the dies. Second, because the dielectric layer covers the active side of the dies, it is difficult to properly align the laser beam with the bond-pads. Thus, laser drilling requires an expensive alignment tool to ensure that the holes are aligned with the bond-pads. Third, the bond-pads must be cleaned (e.g., desmeared) to remove residue and other debris after laser drilling the holes. Desmearing processes increase the costs of production and reduce throughput. Accordingly, there is a need to enhance the efficiency and reliability of packaging microelectronic devices.