Just remebered, when i was processing 5" Si wafer on 1996, we were using Al-1%Si-0.5%Cu. But on the street of wafer, sometimes, there were Si residue. When we cut the crosssection on street ( before Metal etch), Al-Si-Cu/Si wafer, comparing 1% Si and 100% Si, 99% Al and 0 % Al, 0.5% Cu and 0% Cu, here comes "diffusion" for Al, Cu and Cu at high temperature. When we use chemical metal etch, the etch solution can not etch away Si, then, Si residues remains on the wafer. Those are very interesting things happen on the wafer.
However, when we think about our blood, our blood also have several doping materials, we can not just easily treat our blood as " Pass"or " Scrap". If our blood has residue, what can we do? flush away? use chemical to clean? or just leave or ignore these residues there. At least, we need a "clean blood" in our body.