A More Detailed Look at Immersion Lithography
If the air between the lens of an exposure tool and the wafer is replaced with a fluid, such as water, it is possible to increase resolution while simultaneously maintaining practical depths-of-focus.
Effectively, the wavelength of light in the fluid is the wavelength of the light in vacuum divided by the index of refraction n of the fluid.
Since resolution is proportional to the wavelength of light, the use of a dense fluid enables an increase of resolution. For example, the index of refraction of water at a vacuum wavelength of 193 nm is 1.435. Hence, the use of water as an immersion fluid has the potential to improve resolution over 40% compared to imaging in air. The resolution is also related to the maximum angle of incidence of the light at the wafer plane, illustrated in the following figure.
The depth-of-focus (DOF) is given by the expression
where NA = nsin. Large values ofÂ increase the resolution but limit the depth-of-focus. For small values ofÂ one finds that
Thus, the use of the immersion fluid increases the depth-of-focus by a factor of n at smaller values ofÂ while continuing to provide increase at larger values.
Because of the resolution and depth-of-focus benefits, GLOBALFOUNDRIES uses immersion lithography for our leading-edge processes.