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| Fig.
1: Dispersion characteristics for a material with an
active (amplifying) resonance. The real index (n') is
enhanced on resonance. |
Currently optical lithography used to pattern
microchips in the semiconductor electronics industry is running
up against some serious challenges. The features made are
already smaller than the wavelength of the light used to define
them, requiring complex optical techniques be applied. The
next serious step in reducing feature dimensions is "immersion
lithography". In this technique, the final lens and device
substrate are immersed in a fluid with as high a refractive
index as possible. The effective wavelength of the light is
then reduced, and it becomes possible to make even smaller
features (with the result that computers will continue to
get faster in the future).
But
there is a limit to how high an index can be achieved with
conventional materials. Most materials have indexes of refraction
of less than 2. Materials with higher indexes of refraction
tend to be highly absorbing because of a fundamental relationship
between absorption and index of refraction. If the index of
refraction of materials cannot exceed 2 without reasonable
optical transmission, the continued scaling of computer power
cannot be assured.
We
have conceived of a solution to this problem. We propose using
an active optical material as the immersion fluid, so the
index is enhanced (to as high as 6) but there is no loss.
Figure 1 shows the spectral dependence of the effect--it is
quite narrow, and in fact in this simple form the concept
has other problems. So we have proposed a similar material
but now have included a homogeneous mixture of absorbing material.
The result is a broader spectral component and much more robust
characteristics.
With
continued work and improvements, careful tuning of the optical
properties of materials could yield a high index of refraction
material with ideal characteristics for application to optical
lithography.
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