NIRT: Architectures and Devices for Quantum-dot Cellular Automata

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CCR-0210153Kogge, PeterThis proposal was received in response to the Nanoscale Science and Engineering initiative, NSF 01-157, category NIRT. Quantum-dot cellular automata (QCA) is a revolutionary computing paradigm that is well suited to nanoelectronic implementation and scaling to molecular dimensions. The central feature of QCA is that binary information is encoded in the position of single electrons among a group of dots forming a cell. This represents a significant break with the transistor-based paradigm in which information is encoded by the state of the transistor current switch. In QCA, electrons switch between quantum dots within a cell, but no current flows between cells. This leads to extremely low power dissipation, avoiding the problem of heat generation that will ultimately limit the integration density of transistor circuits. Clocking of QCA circuits has proven to be extremely important from the standpoint of both architectures and devices. It allows arrays of QCA cells to be broken into sub-arrays for pipelined processing, and it enables cells to produce signal power gain to replace signal energy lost to the environment. Functioning QCA devices have already been demonstrated in an aluminum/oxide tunnel junction scheme, confirming the operation of QCA cells, shift registers, logic gates, and memory elements. Power gain in a QCA shift register has also been achieved. This project will advance the architectural development of QCA, investigate questions of switching speed in nanoelectronic devices, and develop advanced fabrication techniques to implement the architectural and circuit theory concepts. Since QCA represents a dramatic break from conventional devices, significant changes in architecture are needed to fully exploit the capabilities of QCA. In QCA layout, timing, and architecture are intimately related, requiring a unified design approach. This is analogous to the approach begun by Mead and Conway which revolutionized VLSI design by making a connection between architecture and layout and building on that connection to enable designers to quickly synthesize large and complex functional blocks. Likewise, QCA system designers will be able to exploit timing in addition to layout to produce highdensity functional designs. In particular we will investigate the development of simple, yet complete, QCA based "Field Programmable Gate Arrays", where 2D arrays of identical cells are tiled together, with programmable interconnect and function. Timing plays a pivotal role in QCA designs, so it is vital to achieve a complete understanding of switching and switching dynamics in arrays of coupled electrons. Some recent theoretical results indicate that electron switching speeds would be orders of magnitude lower than that expected from the capacitances and resistances of the dots and tunnel junctions, contrary to theoretical work done at Notre Dame. To resolve this issue we will apply high frequency measurement techniques to the study of switching in QCA cells and in arrays of cells.At present, experimental demonstrations of QCA devices are limited to a small number of cells by the large capacitances produced by the aluminum tunnel junctions. To support and experimentally confirm the advances made in architecture and circuit theory, we will employ advanced fabrication techniques based on AFM lithography to produce QCA with greatly enhanced operating characteristics. This will allow us to fabricate and measure arrays of cells with significant extent and complexity. QCA presents a unique opportunity for a broad impact on the educational experience of students, and on research in the field of electronic devices. We will develop instructional modules based on QCA simulation tools to teach the concepts of QCA architecture to undergraduate and graduate students. These modules will benefit students by introducing them to alternative architectural concepts. In addition, by broadening their horizons, it will strengthen their understanding of conventional architectures by emphasizing the foundational concepts of architectural concepts.