Plated Layers Need Constant Density ...Two of the masks we are preparing involve plating -- the RDL mask and the UBM mask. In both cases the mask defines areas where metal will be deposited and plated up. The very first thin layer of metal is applied by sputtering -- but the sputtering process builds up the metal very slowly so it is only used to put a very thin layer down. Then more metal is applied by plating over the sputtered layer. You could say that plating is both a chemical and a physical process. It turns out the the rate of plating of any particular location depends on the density of the plating chemicals and the density of the electrical current. These are both affected by the local density of the metal being plated. If you have abrupt changes in metal density, you will get non-uniform plating and this can adversely affect the yield of your circuits. The bottom line is that one has to maintain the metal density over the "good" cells or you may lose them due to poor plating. |
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If we were to step out our RDL circuit only to the limit of the good cells on the wafer we would have metal density discontinuity along the periphery which will cause non-uniform plating. To get better density along the wafer periphery we should instead use a smooth round clipping window as shown in the next illustration. |
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What we've done is to a) extend the array past the desired good die and b) clip smoothly to the "edge" of the wafer. The metal on the partial die acts as a buffer between the edge of the wafer so that the good die are uniformly plated. How do we achieve this effect? Well, we could use a GDSII polygon editor to manually explode and edit each cell along the outer edge of the array but you are talking about editing several hundred cells. But we'll get the job done in seconds ... not in hours ... using the HExtract program. |
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