This is because starting the process from the beginning will almost always require new photomasks for all layers, and each of the 20 or so masks in a modern semiconductor fabrication process is quite expensive.
这是因为从头开始流程几乎总是需要为所有层新造光罩，并且现代半导体器件制造工艺中每20个或更多的掩膜中每一个都相当昂贵。