To illustrate the unusual exposure dependence of PROVE initiators, we fabricated patterns with a constant velocity in the x direction (hori-
zontal in these images) and a sinusoidal velocity in the y direction (vertical in these images). A photoresist that has a photoinitiator with
a conventional exposure dependence (left) gives a larger feature size at the turning points, where the stage moves more slowly and the
resist receives more exposure. A photoresist with a PROVE initiator (middle) instead exhibits a smaller feature size at the turning points. A
photoresist with an appropriate mixture of the two types of initiator (right) gives a feature size that is independent of fabrication velocity.
deactivation. Surprisingly, several of these molecules are in the
rhodamine family and can be used for MAP (and RAPID)
despite having high fluorescence quantum yields.
Another application of PROVE initiators is in the creation
of photoresists for which the feature size is independent of
velocity over a wide range. To grasp the importance of this
concept, imagine a photographic film composition for which
the exposure is always perfect. Such velocity-dependent
photoresists can be prepared using an appropriate mixture of
a photoinitiator with a conventional velocity dependence and
one with a PROVE dependence.
Moving forward RAPIDly
One ultimate dream for visible-light nanolithography is to be
able to drastically reduce the manufacturing costs of integrated
circuits by obviating the need for short-wavelength radiation.
As promising as the results with RAPID lithography have
been, much work remains to be done before we can assess its
potential in industrial lithographic processes.
Although we have demonstrated RAPID on a point-by-point basis using MAP, this strategy is not viable for creating
structures on the wafer scale. Thus, a major goal is to be able
to use RAPID to pattern large areas simultaneously. Achieving
this objective will require further advances—both in materials
and in optics. For large-scale patterning, we must be able to
accomplish deactivation at relatively low light intensities.
Two strategies that could decrease the intensity needed for
deactivation are finding molecules with even higher deactivation probabilities and finding methods of further increasing
the lag time between excitation and initiation of polymerization. These attributes would also be useful in enabling the use
of single-photon excitation for exposure of RAPID resists,
which is another essential step for reaching the wafer scale.
There is also an important distinction between creating
small features and developing tightly packed small features.
Feature pitch is the area in which RAPID runs headlong into
the Rayleigh criterion, which determines how close together
two dark regions of a deactivation field can be. Large-scale
RAPID lithography will therefore require the use of multiple
exposures with different masks in order to attain a suitable
feature pitch. To minimize the number of exposures required,
it will also be desirable to identify initiators that can be deactivated with shorter wavelengths of light.
Finally, it will be important to address any compatibility issues that might exist between RAPID photoresists and
current processing technologies. For instance, we have yet to
perform detailed tests of the etch resistance of RAPID photoresists. It may be necessary to develop new formulations that
optimize etch resistance while maintaining high resolution.
It is difficult to predict the ultimate range of applications of
RAPID lithography until these challenges are addressed. As we
develop a better understanding of the photochemistry involved
in RAPID, we should be able to design next-generation materials that are highly optimized for this technique. Overall, we
believe that the future looks bright for RAPID lithography. t
John T. Fourkas ( firstname.lastname@example.org) is in the department of chemistry
and biochemistry and the Institute for Physical Science and Tech-
nology at the University of Maryland, College Park, Md., U.S.A. Member
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