Fabrication of Bumping Mask for Flip-Chip Process on Stainless Steel Using through Mask Electrochemical Micro Machining(TMEMM)

As semiconductor devices become smaller as well as better in performance with the improvement done in device level, packaging technology has also attracted much attention. Especially, flip-chip packaging technology which connects between chips by making micro-bumps on the land position has more advantages than the wire bonding packaging technology which connects chip and substrate using wire. In order to make micro-bumps, bumping process which deposits solder ball or solder paste through the holes in bumping mask is used. Therefore hole size uniformity of bumping mask is the key technology for high accurate bumping process. Usually bumping mask is fabricated by several methods such as electroforming, laser machining and chemical wet etching. But they have several limitations, respectively. Electroforming is an expensive process due to high cost of Ni which is a main component. Laser machining process also has a cost and process time issues when it comes to high density holes in large area because it makes hole on the substrate one by one directly with laser beam. Lastly although chemical wet etching process has good metal removal rate, the surface roughness after etching is high and hard to control etching process.

In order to improve the limitations of conventional machining methods, we introduced Through-Mask Electrochemical Micro Machining (TMEMM) process. TMEMM process is originated from Electrochemical Machining (ECM) which uses electrochemical reaction at the interface between metal and electrolyte that makes it possible to fabricate smooth surface. TMEMM process uses electrochemical reaction as same with ECM, but resist patterning process is added to make resist pattern on substrate. By using resist pattern as a mask layer, only exposed area to electrolyte is dissolved selectively. Through this process, many patterns in large area can be etched at the same time.

In this study, we optimized TMEMM process to fabricate bumping mask by controlling the process parameters. During optimization we had introduced a new TMEMM process design. In this process, electrolyte is circulated by pump system. The temperature of the electrolyte was kept constant with the help of refrigerated circulating bath. To make hole shaped resist pattern on the stainless steel substrate in large area, dry-film resist (DFR, HW440, Hitachi, Japan) was used. 400 x 400 mm DFR patterned stainless steel substrate was used as an anode, and Cu plate was used as a cathode. Pulse current was used with different current value, pulse on/off time and duty cycle. Phosphoric acid (H₃PO₄, 86 Vol%) was used as an electrolyte. The effect of temperature of electrolyte, distance and flow rate of electrolyte between electrodes, cathode material and insulation shield to prevent edge effect are investigated in this study. After TMEMM process, resist pattern on the stainless steel substrate was removed by dipping in 3M NaOH solution for 3 min. To analyze the uniformity of fabricated holes and surface quality of the wall of hole, Optical Microscope (L-150A, Nikon, Japan) and Field Emission Scanning Electron Microscope (FE-SEM, MIRA3, TESCAN, Czech) were used and we used coefficient of variation (CV) value as an indicator of uniformity. As a result, we fabricated the bumping mask which has 250.6 µm of average hole diameter with 2.8% of CV value and smooth surface.