Thick Photoresist processing for Gold bump and Copper pillar processing
In the formation of both gold bumps and copper pillars it is required to contain the “plated up” structure within a “mold” of polymer material that is most effectively created using photolithographic techniques. As the bumps and pillars need to be relatively high above the surface of the chip the thickness of the Photoresist needs to be relatively thick, the same as the height of the pillar. If the pillar needs to be 50 microns high then the polymer needs to be 50 microns high or perhaps a bit higher, to ensure that the pillar or bump does not “spread” at its top.
See Figure 1.
Film thickness of 50 microns is a very thick film for spun on films, especially in one applications dry resists of 10-50 microns thick are common, however the material cost of dry film resist is substantially higher than for the simpler common Photoresist. It is desirable therefore to use such resists if the overall process costs can be made lower by their use. There are however, challenges to thick resist processing, they are:
- To achieve thick films normally requires either very viscous resists or multiple applications of resist or both. The pumps for very viscous resists are very expensive and for the very viscous materials the costs also tend to rise. Alternatively more expensive dry film resist can be used
- The Bake of such thick films must be done in such a way to ensure that the film does not “film over” … dry out on the upper most layer before evolving solvent from lower levels of the resist, thereby creating bubbles and blisters in the resist film. The rate of heating of the substrate needs to be controlled and should not be too fast so that film defects can be avoided.
- As the resist film are very thick there is a lot of solvent that must evolve from the cast film in the hot plate chamber. If the hot plate chamber is not properly designed and configured the evolving solvent will condense in the hot plate chamber and reflux (drip) on the wafer(s) being processed resulting in the need to at least rework or suffer reduced yield. Such poor design can only be mitigated by frequent cleaning of the hot plate chamber. A properly designed and configured hot plate chamber/assembly requires no such cleaning.
- As the resists used are normally very viscous it becomes very difficult to drain and maintain clean the spin cups, by appropriate design and facilitization the spin cups can be made self cleaning
- To minimize costs a uniform very thick resist film can be achieved using S-Cubed’s tools and patented technology.