How thick is the water film in 193i?

[Tanj]

I'm curious how thick the water is. I was assuming a few microns but then I came across a 2008 document taking about a millimeter. I was thinking the lens "flies" low almost like an HDD head.

 

[nghanayem]

That would be information of the utmost secrecy for Nikon, ASML, and their customers. I wouldn’t be shocked if different tools and maybe even applications nesecitated different values, but I am not a litho guy so this is pure speculation on my part.

 

[Fred Chen]

Micro-Objectives of Immersion Lithography

The optical lithography community is gaining confidence that immersion lithography can further reduce critical dimensions based on theoretical models.

www.azooptics.com

 

[Tanj]

Beautiful find, Fred. Thanks!

That instrument design is somewhat different to a full stepper, but the data there is quite useful.

 

[SPQR54]

Seen only now. There are multiple considerations for choosing thickness of water in an immersion stepper and not all of them purely due to the lenses. Just to name two of them, you need the flow of the water to be laminar otherwise you have distortions due to turbulence and you need the water temperature in the optical path to stay constant to avoid changes in index of refraction (dn/dT=10e-4 hence a variation of 0.01C will give a variation of n of 10e-6 so a deflection of 1nm for a water thickness of 1mm...).

 

[Tanj]

Seen only now. There are multiple considerations for choosing thickness of water in an immersion stepper and not all of them purely due to the lenses. Just to name two of them, you need the flow of the water to be laminar otherwise you have distortions due to turbulence and you need the water temperature in the optical path to stay constant to avoid changes in index of refraction (dn/dT=10e-4 hence a variation of 0.01C will give a variation of n of 10e-6 so a deflection of 1nm for a water thickness of 1mm...).

Thanks! It has become clear that the depth is on the order of a mm, and as you say that is thick enough to add fun variations. I am looking at a proximity scanning system with much slower movement (< 1cm/sec) but also much thinner (< 10 microns) so I will clearly be working in a different regime with different problems.