Chapter 3
Micro- and Nanopatterning Polymers Downloaded from pubs.acs.org by AUBURN UNIV on 04/25/19. For personal use only.
Synthesis of Stereoregular Polymers as Precursors to Highly Conducting Carbon for Use in Applications in Micro- and Nanolithography C. B. Gorman, R. W. Vest, J. L. Snover, T. L. Utz, and S. A. Serron Department of Chemistry, North Carolina State University, Box 8204, Raleigh, NC 27695-8204
Organic conductors have received virtually no attention in lithographic processes and may be uniquely suited in these applications. In particular, it should be possible to design polymers that can be converted from electrical insulators into electrical conductors in one, dry step. This process is anticipated to save time and minimize the use of organic solvents in processing. Moreover, very small ( ≤100 nm) features of organic conductors can be covalently linked thoughout and should not sufferfrommigration processes (particularly electromigration) found in metallic structures with small size features. Up to the present, it does not appear that polymers capable of facile direct conversion from an insulating phase to a conducting phase are available. Potential routes to lithographically defined, graphitic carbon from precursor polymers based on stereoregular poly(acrylonitrile) and poly(cyanoacetylene) are illustrated. As stereoregular versions of these polymers are not available, routes for their preparation are presented and discussed.
There are a number o f fundamental challenges in improving lithographic methods. The process is time consuming, requiring a number o f steps just to fabricate electrical interconnects between circuit elements. Omission o f some coating and developing steps w i l l lower the time required to fabricate circuitry and reduce the waste generated by the process. A t smaller length scales, a number o f process issues arise, most notably the need for new methods to write information rapidly and with good contrast at sub-micron sizes. In addition, materials issues may limit the reliability o f such nano-circuitry. Metals used i n device fabrication, such as copper, migrate, particularly under the application o f an electric field (electromigration) (1,2).
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© 1 9 9 8 American Chemical Society
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For metals such as copper, this process inevitably results i n the loss o f electrical continuity for wires o f < 100 nm width. Organic conductors have a real potential to overcome some o f the processing problems as well as some of the materials problems mentioned above. It is envisioned that they may be uniquely suited for certain nanolithography applications. Graphitic organic polymers can display high electrical conductivities (> 1000 Q - ^ c n r ) . To date, several routes have been explored for their fabrication in bulk structures such as films and fibers (3,4). Comparatively little is known about how to fashion structures on small length scales. Moreover, most routes to intrinsically conductive organics involve high temperature (500 - 900 °C) carbonization processes, and few routes based on processes such as U V or electron beam lithography have been described. 1
Electron Beam Generator
Figure 1. Schematic for the fabrication o f a conductive organic circuit pattern. P A N refers to any i n a series o f nitrile-based polymers discussed below. Unconverted P A N may be an acceptable insulating phase. Alternatively, brief acid or base hydrolysis might convert it into the insulating poly (amide). The use o f a precursor polymer offers several potential advantages: • Precursor polymers may be applicable i n direct writing applications (5). That is, they w i l l be convertable, either by U V , X-ray or electron beam radiation from an insulating phase to a conducting phase i n one step. Reduction i n processing steps is of interest i n any lithographic process. • The processing could be performed without organic solvents. Although organic vapors w i l l not be completely excluded, these conditions should ameliorate disposal costs and concerns. • A n ideal process could be designed so that the conversion from an insulating to a conducting polymer proceeds efficiently under irradiation but does not proceed i n the absence o f a threshold flux. These considerations w i l l permit
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the simultaneous fabrication o f highly electrically conductive structures with large contrast between insulating and conducting regions. • Graphitic polymers would be linked by strong bonds between the atoms. Unlike i n metals, where metal-metal bonding is weak, these covalent structures should not be susceptible to electromigration or other processes that could change the shape o f a nanometer scale fabricated structure under the influence o f an electrical or magnetic field. Here, we describe potential routes to new precursor polymers for nanolithography applications. Specifically, two known polymers, poly(acrylonitrile) and poly(cyanoacetylene), have some promise in this area. It is argued, however, that their current lack o f applicability as acceptable graphitic precursors stems from a lack o f stereoregularity along the polymer backbone. Correct alignment o f reactive groups in a precursor polymer i n the solid state is critical for graphitization. Incorrect alignment o f reactive groups can completely prevent a desired solid-state reaction. A s the stereoregularity o f the polymer chain w i l l determine this alignment, this parameter is the focus o f the work described below.
Poly(acrylonitrile) as a Graphitic Precursor Polymer Polyacrylonitrile ( P A N , Figure 2, top left) can be subjected to thermal treatment to produce a conjugated, conducting pyrolysed structure (6-9). A n idealized depiction o f the chemical changes that result upon heating P A N is shown in Figure 2. Particularly the second step o f this process in which the rings formed in the first step are aromatized to form the completely unsaturated structure does not occur without a number o f side reactions, particularly in the presence o f oxygen (7). Conjugated ladder structures such as pyrolysed P A N have nevertheless commanded a great deal o f attention because of their rigid-rod nature and their electronic, electrochemical and nonlinear optical properties. Thermal conversion in bulk has resulted in fibers, sheets and foams (5) with modest electrical conductivities (4). M a x i m u m conductivites, however, are reached after very high temperature conversion required to carbonize the structure. Because o f its potential to be converted from an insulator to a conductor, P A N and its derivatives are an attractive starting point for use in direct writing schemes. Reports i n the literature, however, unambiguously illustrate that the scheme shown in Figure 2 is extremely idealized and shows only part o f what actually occurs upon heating the polymer. It is unclear that a conjugated ladder structure must be extensively crosslinked (graphitized) in order to become highly electrically conducting. Rather, defects produced during such high temperature operations may serve to limit conductivity, requiring higher temperatures to further carbonize the structure. Conjugated, acene-type polymers have potential to act as intrinsic electrical conductors i f their structure is regular enough (10-12). A t minimum, they should be easily oxidized extrinsic conductors using photooxidants or treatment with chemical oxidants.
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