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possible, to position individual nanostructure, especially in a massive number. On
the other hand, photolithography has been a successful and evolving technology
for microscale patterning.
On the other hand, most nanoscale fabrication methods, including self-
assembly, nanoimprinting, and the LB process, are bottom-up in nature, unable
to accurately position a nanostructure effectively. 2 Although small logic and
memory units have been demonstrated, it is expensive, if at all possible, to use the
same methods to fabricate large-scale integrated systems. The bottom-up fabrica-
tion is only effective with making repetitive patterns, or homogeneous structures.
It is extremely difficult to directly connect nanoscale units thus fabricated with
each other, or even with microscale structures, as highlighted above, with
certainty.
Combining the bottom-up and top-down approaches, or meet-in-between
paradigm, is likely to provide solutions to such a dilemma. In this paradigm,
instead of positioning each individual nanoscale element, traditional photolitho-
graphy is employed to pattern microscale units and interconnects between them,
which are potentially programmable. Each unit is then filled with nanoscale
elements through bottom-up nanoscale fabrication methods. Finally, each unit is
individually programmed through the microscale interconnection in order to
produce functional circuits. Figure 11.7 illustrates this three-step process. The
paradigm leverages the ever increasing computing power to relieve the limitations
of fabrication. The meet-in-between paradigm is complementary to existing
proposals to combine the strength of lithographically fabricated MOSFETs and
nanoscale elements, including molecular switches [70, 77, 78]. In most of these
proposals, MOSFETs are employed for logic and signal regeneration while
nanoscale elements are employed for highly regular structures, such as memory
units.
The meet-in-between paradigm is based on a trade off between design and
fabrication capabilities. Instead of fabricating a prefixed design, it fabricates a
prefixed but programmable microscale structure. It leverages the programmability
of the nanoscale elements and existing computing power to work around the
nanoscale uncertainty inside of the microscale units. Critical to the meet-in-
between paradigm is a microscale unit made of nanoscale switching elements that
are rapidly programmable and feature-rich. The two compelling candidates are
nanowire crossbars and molecular nanocells [79]. As we have already addressed
nanowire crossbar arrays, we describe the nanocell design and fabrication next.
11.4.1. Nanocell
We have developed a nanocell architecture that fits into the meet-in-between
paradigm and takes advantage of the smallness in size of the molecules via
lithographic tools [38, 79-81]. The nanocell architecture also offers enormous
2 While nanoimprinting and the SNAP method can more or less accurately control the separation
between nanowires, they are less capable of absolutely positioning a single nanowire.
 
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