COMPUTING WITH QUANTUMDOT CELLULAR AUTOMATA - Bio-Inspired and Nanoscale Integrated Computing - page 147

(contact angle 35-52 1 ), and the third a methyl-terminated surface (contact angle

105-110 ). It was found that the test molecule formed a self-limiting monolayer on

the phosphonate and hydroxyl-terminated surface, but no such layer formed on the
methyl-terminated surface; this suggests that patterned changes in the hydrophobi-
city of a SAM could be used to create patterns of molecular QCA. One of the
challenges with this approach is the resolution limits associated with using the EBL
process (around 10 nm), as well as the challenges to using such a serial patterning
approach for mass production of such devices. Line widths of 30 nm would create
QCA wires that are many cells wide; interestingly, however, wide QCA wires have
been suggested as a way of increasing the fault tolerance of QCA circuits [85].
Recent work by Hu et al. has developed the use of DNA rafts self-assembled
on an e-beam patterned PMMA surface [36]. This combination of lithography and
self-assembly appears to be very promising; however, much more work remains to
be done.
Many exciting results have been achieved toward an implementation of
molecular QCA, but many challenges to the implementation of a molecular
QCA technology still remain. The placement and patterning of individual
molecules is a very difficult challenge; in addition, the implementation of a
mechanism to measure the electronic configuration of individual molecules, to
provide a means of implementing I/O, still remains to be solved.

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4.13. CONCLUSIONS

Emerging nanotechnologies that are being explored as additions to and replace-
ments of current FET-based fabrication technologies may allow us to continue
along the Moore's Law exponential path and provide the anticipated increases in
circuit density that drive many advancements of our technological society. Several
promising emerging technologies have already been identified and others will
undoubtedly appear over the next decade or so. Whichever technologies prove to
be successful replacements, and there may be several that are finally used in a
mixed technology scenario, will need to be explored thoroughly in a sophisticated
design and simulation environment.
This chapter has focused on the promising nanotechnology of quantum cellular
automata (QCA). Our exploration has covered the basic theory of QCA including a
variety of cellular architectures, logic elements, memory cells, and processor
structures that have been designed and simulated using the QCADesigner tool.
We have covered the proposed approaches to clocking QCA cells and circuits and
provided the advantages and disadvantages of each approach. We have also
provided a round up of the most promising approaches to QCA implementation
including semiconductor, magnetic, metal-island, and molecular QCA.
QCA remains an uncommercialized technology; this is primarily due to the
challenges with realizing a scalable implementation. However, advantages such as
low power dissipation, high device density, and the lack of required connection to
each device make QCA a more suitable paradigm for computing at the molecular