why are different metals used for different interconnect levels

Different interconnect levels use different metals because of cost, performance, and what's practical. In the typical flow: TUNGSTEN (W) is used for the lowest-level CONTACTS and vias (good fill, but higher resistivity, so used where geometry is forgiving). ALUMINUM (Al) was the old main line metal. COPPER (Cu) replaced Al for the main signal lines because of its low resistivity (less RC delay) — but Cu needs a BARRIER (Ta) and liner because it diffuses into silicon/dielectric (Lesson 57). At the smallest dimensions, you see COBALT (Co) and RUTHENIUM (Ru) as liners/seeds or local interconnect because they scale better than Cu at very tight pitches. The one-line frame: 'I use the cheapest metal that meets the job — tungsten for contacts, copper for the wires because it's low-resistance, and cobalt/ruthenium as the ultra-thin liners where copper stops scaling.' It shows real depth and ties to the ALD/liner lessons.