3D packaging has a bright future. It simply makes a lot of sense. So, being an equipment supplier, the next question is: what opportunities does 3D packaging offer for the equipment industry?
In fact, the third dimension not only increases package density but also adds a diversity of new package designs. New, or at least modified, process techniques will be required. This certainly offers new opportunities for equipment suppliers. But diversity and opportunities alone do not generate good business, in the end, standards are needed to drive the economy of scale.
Through silicon vias, (TSVs) have become the Holy Grail of ultimate 3-D interconnect. TSVs offer opportunities for DRIE, laser drill, and plating equipment makers to optimize their products and drive TSV yield up and cost down. But what about photo-lithography? Does current equipment satisfy the requirements for 3D packaging? There is more involved than just TSVs.
Image sensors are interesting products to consider. Wafer-level packaging (WLP) of image sensors includes several common characteristics of 3D packaging such as TSVs, redistribution layers (RDL) on the back side of the wafer, and wafer-level assembly. Additionally, they are early adopters of 3D packaging processes.
Specific requirements for photo-lithography equipment include the need for back-side-lithography – an equipment option previously not required for WLP – and conformal coating of TSVs with photo resist and organic dielectrics. Equipment manufacturers are familiar with these requirements from the MEMS industry, and have developed bottom-side-alignment systems and spray-coating equipment. Now these equipment options are needed on 300-mm equipment.
Image sensor stacks and optics are assembled at the wafer level using wafer bonding. Economics of this technology is attractive, especially for small die. Moreover, with a wafer-level lens replication technique, the cost of manufacturing image sensors can be further reduced.
3D packaging requires processes on the back side of very thin wafers. Currently, for wire-bonded 3D packages, back-side processes are more or less limited to stress-relief and dicing. Many of the more advanced 3D packages will require processes such as sputtering, DRIE, electroplating, and photolithography on very thin wafers. That’s why thin wafer handing has become so critical. In this respect, image sensor packaging is different: although image sensor wafers are thinned to less than 100-