xx Nano-Lithography
Improvements were first made by decreasing the exposure wavelength λ. This
was why, in the 1980s, the first tools started using different radiations from a
mercury lamp (λ = 436 nm, called g-line radiation; 405 nm, or h-line; and then
365 nm, or i-line), usually using reduction projection optics based on quartz. Each
wavelength change was accompanied by process changes that were major at the
time, but which, in retrospect, could now be qualified as minor.
The first transition came in the 1990s with the use of deep ultraviolet excimer
lasers, first with 248 nm (with a KrF laser) and then 193 nm (with an ArF laser), and
allowed feature size resolution below the 0.1 µm limit to be reached. However, this
evolution required major changes in either projection optics (use of CaF
2
in addition
to quartz) or in the choice of the transparent matrix of the photosensitive resist.
The normal evolution would have moved towards F
2
lasers (λ = 157 nm) which
need projection optics made mainly out of CaF
2
, a material whose bi-refringence has
proven to be a major obstacle: in the decade after 2000, after many years of
development, industry finally concluded that it was illusory to continue down this
path for mass production.
Reducing the k
1
parameter then appeared very promising. This is achieved by
first improving the resist process, for example by increasing its contrast with
nonlinear phenomena or by controlling the diffusion of the photosensitive
compound. By optimizing illumination techniques (annular, quadripolar, etc.), it is
also possible to gain resolution and process control but often by promoting certain
shapes or pattern orientations.
It has been, above all, by mastering diffraction phenomena, and thus influencing
the exposure light phases, that progress has been the most spectacular: it has been
acknowledged that it is now possible to go beyond the Rayleigh criterion and print
patterns even smaller than the exposure wavelength. From laboratory curiosities,
these techniques have now become the workhorse of the microelectronics industry
and are now known under the name “Resolution Enhancement Techniques”.
In a very schematic manner, and for a certain illumination and resist process, we
will try to calculate what the patterns and phase-differentiated areas on a mask
should be in order to achieve an image on a wafer which matches an image initially
conceived by circuit designers. The reverse calculations are extremely complex and
demand very powerful computers in order to obtain the result (in some cases taking
up to several days, which affects the cycle time of prototypes of new circuits). In the
end, the goal is to take proximity effects between close patterns (thus a
combinational explosion of the calculation time) into account, by in turn taking into
account the most precise possible optical models (and, as the technologies improve,
it is important to not only take into account intensity and phase but also light
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