One of the main factors driving improvements in the cost and complexity of integ

Question

One of the main factors driving improvements in the cost and complexity of integrated circuits (ICs) is improvements in photolithography and the resulting ability to print ever smaller features. Modern circuits are made using a variety of complicated lithography techniques, with the goal to make electronic traces as small and as close to each other as possible (to reduce the overall size, and thus increase the speed). In the end though, all optical techniques are limited by diffraction.
Assume we have a scannable laser that draws a line on a circuit board (the light exposes a line of photoresist, which then becomes impervious to a subsequent chemical etch, leaving only the narrow metal line under the exposed photoresist). Assume the laser wavelength is 248.0 nm (Krypton Fluoride excimer laser), the initial beam diameter is 1.0 cm, and the focusing lens (diameter = 1.3 cm) is extremely 'fast', with a focal length of only 0.625 cm.

1. One proposal to further increase the resolution without changing the laser source is to operate the system in a fluid such as water. Microscopists have known for a very long time that the resolution of a microscope can be improved by using an oil- or water-immersion lens, with index of refraction n > 1.
Going back to the lithography system with a laser that produces light (in air) at ? = 248.0 nm, what minimum line separation can be expected in this case (i.e., what is the new answer to b), if the space between the lens and the photoresist is filled with water [n = 1.33 for water])? Hint: What effect does the index have on the wavelength of the light?

Explanation / Answer

a. ?=1.22?/D(laser) d=2?f=2.44?f/D(laser) (2.44)(248 nm)(6250000 nm)/(10000000 nm)=378.2 nm b. 378.2/2=189.1 nm c. Same as parts A and B ?=1.22?/D(laser) d=2?f=2.44?f/D(laser) (2.44)(157 nm)(6250000 nm)/(10000000 nm)=239.425 239.425/2=119.7125 nm d. Use the answers to parts B and C as radii of circles and calculate the areas. Then divide them. Answer=2.495

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