This works on modern processes because modern chips are made using CMP (chemical-mechanical polishing) to planarize the layers. On older processes the layers we

This works on modern processes because modern chips are made using CMP (chemical-mechanical polishing) to planarize the layers. On older processes the layers were just stacked on each other without being flat in the depth dimension, so you couldn't just cut cleanly in one plane like you can now. Then again, on older chips you can usually just take a top layer shot and tell apart most of the layers anyway.