Templated Self-Assembly of Block Copolymers for Nanolithography

Project Staff:

C.A. Ross, F. Ilievski, V. Chuang, Y. S. Jung, in collaboration with H. I. Smith, E. L. Thomas, G.J. Vancso, A. Mayes

Sponsors:

National Science Foundation, MIT Center for Materials Science and Engineering, Semiconductor Research Corporation, Singapore-MIT Alliance

Fabrication of large-area periodic nanoscale structures using self-organizing systems is of great interest because of the simplicity and low cost of the process. Block copolymers consist of polymer chains made from two chemically distinct polymer materials. These can self-assemble to form small-scale domains whose size and geometry depend on the molecular weights of the two types of polymer and their interaction. The domains have a very uniform distribution of sizes and shapes. We have been using block copolymers as templates for the formation of structures such as magnetic particles, by selectively removing one type of domain and using the resulting template to pattern a nanostructured magnetic film. For example, we have applied this technique to the patterning of magnetic CoCrPt films with perpendicular magnetic anisotropy. Each ‘dot’ of CoCrPt can be magnetized ‘up’ or ‘down’. Measurements show that the magnetic switching volume is very close to the physical volume of the particles, indicating that these ‘dots’ reverse independently.

These self-organizing systems can be used to create fine-scale periodic patterns, with good short-range order. However, the long-range order of such patterns is typically poor, limiting their usefulness in nanoscale structures or devices. We are also developing methods to induce long-range order in block copolymer systems by regularly patterning the substrate. This approach is called ‘templated self-assembly’ and may be used in future integrated circuit manufacture. Patterning is carried out by topographically or chemically modulating a substrate using interference lithography, or using other lithography methods such as electron-beam lithography. Alternatively, nanoimprinting has been used to confine the polymer. The overall goal of the project is to develop methods by which controlled nanoscale patterns or structures can be created using a combination of ‘conventional’ lithography and self-assembly. Of particular interest is how the quality of the templating is affected by the relative length-scales of the template and the natural period of the self-assembled system.

Well-ordered block copolymer sphere arrays form in shallow grooves with all the close-packed rows aligned within the grooves, provided the groove width is comparable to the ‘grain size’ of the block copolymers. We have found that the number of rows within the groove, the spacing of the rows, and the deliberate introduction of defects such as vacancies and dislocations, can be controlled by adjusting the groove dimensions. The distribution of the domains can be modeled, allowing templates to be designed to produce particular domain arrangements. Further, we have explored the effects of the edge-roughness of the template on the packing of the domains, and the registration of the domains using two-dimensional templates. Recently we have studied the ordering of single rows of spheres, and we have explored the three-dimensional packing of spheres within a V-groove.

Figure 1. PS-PFS block copolymer in a 1D template forms an ordered array of PFS spheres. Although this appears to be a perfect array, the positions of the spheres deviate from perfect close-packing due to the roughness of the template. The difference between the grid and actual positions of the polymer domains are represented by vectors, which are magnified 2 times with respect to the axes.

Self-assembled block copolymer in a v-shaped groove. For imaging, one of the blocks was removed by oxygen plasma etching to reveal the structure of the other block. The number of rows of spheres increases in a quantized manner from the bottom up (one row of spheres, then two, etc.)