SMTA International Conference Proceedings


Authors: Shu-Ching Ho, Hsiang-Ming Huang, Yi-Chang Lee, Yeong-Jyh Lin, and An-Hong Liu
Company: ChipMOS Technologies Inc.
Date Published: 10/11/2007   Conference: SMTA International

Abstract: Redundant cells are added to a memory chip to replace defective cells during IC fabrication to increase overall wafer yields. Compared to other repairing methods, only a small chip area is needed to design laser fuses, moreover, just a few redundant rows or columns can significantly enhance the yields of memory wafers. The laser fuses are hence the most commonly implemented method to increase yields in both DRAM and SRAM IDMs.

In conventional memory packages, either TSOP II or FBGA, the memory chips are probed and repaired first, then packaged, burn-in, and final test. Therefore, there is no issue in covering the laser windows on a memory chip after laser repairs since the laser windows are all encapsulated by molding compound. In flip chip technologies or wafer-level packaging, memory ICs are packaging in wafer forms instead of individual dies. Moreover, memory wafers are normally probed and repaired before packaging, therefore, the laser windows are also encapsulated by underfill materials or by molding compound. However, if the conventional chip probing, burn-in, and laser repair can be done after flip chip bumping or wafer-level packaging, the testing steps can be simplified, the testing cost can be greatly reduced, and more wafer yield can be gained.

This study primarily focuses on the laser repair after flip chip bumping to explore the feasibility of laser repair through BCB passivation on top of laser fuses and to develop new processes to protect laser fuses during flip chip bumping. Three different conditions are studied, original laser windows with PI passivation opened, laser windows with BCB coated on the top of PI passivation, and laser windows with BCB coating without PI passivation. The laser windows with PI opened, the laser fuses can easily be “burned” with 1.0 µJ of laser energy and the die can successfully be repaired since a fuse is sublimated by the laser beam with its vapor exploded and broken through the covered dielectrics. There are two different thicknesses of coated BCB in laser windows, one is laser windows with BCB coated on the top of PI passivation and the other is laser windows with BCB coating without PI passivation where the thicknesses are 10.69 µm and 7.32 µm, respectively. The results of laser repair are almost identical. Even with the laser repair energy up to 1.8 µJ, the fuse can not be “burned” and its vapor can not explode and break through the dielectrics and the BCB on top, all the more, its vapor will go sidewards and cause cracks along the interfaces of BCB and the dielectrics, in the worst case, causing shorts between the adjacent fuses.

From the results of the laser repair, it can easily be concluded that the fuse vapor cannot easily break through the dielectrics with a thick BCB on top, therefore, no further study on the parameters of laser repair through BCB is necessary. The laser repair should be performed without any BCB passivation nor any bumping residual inside the laser windows. Then, how to protect laser fuses inside the laser windows during flip chip bumping processes becomes very crucial. The protection of laser fuses can be done by using appropriate UBM materials during flip chip bumping processes, then after solder ball reflow, the UBM was removed by etching. It has been proven that the dielectrics on top of laser fuses will not be damaged during flip chip bumping processes.

Key words: laser fuses, flip chip bumping processes, laser windows, BCB passivation

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