January 20, 1997
Polymer Templates for Nanostructure Fabrication
Professor Jaeger is investigating new approaches towards fabrication of metallic structures on nanometer length scales. Conventional approaches involve drawing the desired pattern pixel by pixel by an electron beam into a polymer film, followed by further chemical and material processing. Our alternative approach is to use the natural process of self-assembly to pattern polymer films in nanometer length scales. This involves three steps: using an appropriate material that self-assembles over large areas on nanometer length scales, controlling the self-assembled pattern, and controlling the functionality of the resulting pattern.
The first of those steps is not difficult. Films of diblock copolymers annealed in an oven manifest nanometer-scale patterns called microdomains. The third step is a current area of research by us and many others. There are several strategies to achieving this goal. If one polymer is chemically removed, the patterns may be transferred to a substrate through etching or evaporation. It also is possible to use the domains as a template for decoration with metallic, magnetic, or semiconducting nanoparticles. The second step thus far has posed the major impediment to the technique: as the picture shows, the microdomains resulting from self-assembly, although spaced regularly, are arranged randomly.
The solution lies in the application of electric fields. The image at right is a picture taken using a transmission electron microscope of a roughly one square micron area of a diblock copolymer film near the corner of a simple, parallel capacitor electrode. While no field was applied in the picture above at left, an average electric field (> 3 volts per micron) caused the patterning in the image at right. As the picture shows, the microdomains follow the path of the fields, whether that path is a curve or a straight line. The required E-fields are easy to reach, requiring potentials of only a few volts for mesoscopic electrodes separated by several microns.
This technique has important ramifications. We can specify the alignment direction, even arranging different directions at different areas within a film by using multiple electrodes. Additionally, we may tailor the electrode geometry to affect the orientation of the microdomains. This technique has interesting potential applications:
- We can scale up by using interdigitated electrodes with local gaps on the order of ten microns, which should permit us to tailor various cylinder orientations over areas of several square centimeters.
- The planar electrodes used to control the orientation of the polymer patterns could serve as direct electrical contacts to the resulting structures. To provide specific examples: if liquid crystal type copolymers are used, electro-optically active devices may be realized. Even more interesting, if the patterns can be made conducting -- for example, through the method we mention in passing above -- the self-assembly of parallel quantum wires between the electrodes becomes feasible.
approved for broadcast by T. Morkved, 1/20/97
Reference:
- "Local Control of Microdomain Orientation in Diblock Copolymer Thin Films with Electric Fields" T. L. Morkved, M. Lu, A. M. Urbas, E. E. Ehrichs, H. M. Jaeger, P. Mansky, T. P. Russell, Science 273, 5277:931(1996).
The Participants:
- Terry Morkved: senior grad student on the project
- Mei Lu: grad student
- Augustine Urbas: undergrad (did Senior Thesis on project)
- Ed Ehrichs: (former) postdoc, now at AMD
- Paul Mansky and Tom Russell: collaborators from IBM Almaden (now both at UMass/Amherst)