The big thing about introducing immersion lithography into production has been defects, since the wafer surface is now directly in contact with water. And, obviously, the water is dynamically moving along the wafer surface during exposure.

The two major immersion vendors, ASML and Nikon, have drastically different ways to address dispensing of water on the surface. ASML traps the water within a hood, using blowing air on all sides to prevent it from leaking out. Nikon continually fills the gap between the lens and the wafer with water and then sucks it out, thereby always replenishing the water.

Because the water dispensing is so different, troubleshooting the defects is also different, as the defect sources and mechanisms would be different, depending on whether use one vendor or another.

The way things have evolved, vendor-dependent yield has become a characteristic of immersion lithography.

As I indicated here, water limits contaminant diffusion better than air, because there are so many water molecules blocking the contaminants per unit volume, so in terms of optics contamination, immersion lithography is safest. But it seems better to continually supply fresh water than to have the same drop over the whole wafer.