A Study on the cleaning efficiency of chip-let Micro-structure

Recently the AI and big data application is booming and the chip-let technology is reaching another step height. The chip 3-D integration is more and more complex and the critical dimension of the chip interconnection is getting smaller and smaller. Taking the chip-let pre under-fill process as an example, when the bump pitch on the IC chip is less than 40 micron, the flux cleaning before the under filling process, neither traditional face up cleaning method nor the high pressure spray cleaning method works, in other words, the flux around the bump can not be washed away by the above two methods. Because when the bump height reaches less than 40µm, the liquid tension becomes a major effect for the liquid flow. The surface tension will blocking the cleaning liquid goes into the inner side of the chip, a lot of residual is found after cleaning, especially at chip center. A novel cleaning method is introduced in this paper, that is to clean the chip with the bump height under 40µm under a vacuum environment, for example the vacuum pressure is less than 150torr. The cleaning procedure is as follow: first the wafer or/chip-let is put into a vacuum chamber, then the chamber is pumping down to a pre-set vacuum, then the cleaning liquid in injected onto the wafer surface. Because the surface tension has a dramatic change in vacuum environment, there is almost no air in and between the chip-let structure, the cleaning liquid can go inside and out freely. The cleaning efficiency is increased a lot, and the test results showing that all the flux on both the top die and the bottom die is flushed away, there is no residual. The vacuum cleaning method is a novel cleaning technology, which is called as 3rd generation of the flux cleaning method. It will be used more and more as the chip-let technology moving forward.