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approaches 0.5 in the theoretical limit of diffraction. Increasing the NA has an
adverse effect on the depth of focus (DOF), which describes the aerial image blur
through the thickness of the resist: 5
Equation 1.2
DOF = k
2
( ) 2
NA
where k2 is again a system-dependent parameter. There is a balance between
increasing NA in order to achieve better resolution, and decreasing NA in order to
achieve larger DOF. Many of the significant advances in resolution have come as a
result of using shorter exposure wavelengths.
7
λ
These resolution improvements, along with the use of different wavelengths,
require new light sources, new resist materials, new lens materials and systems, and
improvements in machine tolerances, among others. These improvements come at a
cost, as can be seen in Figure 1.4, which shows the exponentially increasing cost of
photolithography tools over time. The next generation lithography (NGL)
technologies that are currently expected to allow further resolution improvements
may in fact depart upwards from this trend, resulting in an even greater tool cost.