The majority of electronic devices is designed on silicon wafers where the architecture of the different layers is made through lithographic techniques and resin masks. To cope with the desired increase in performance (Moore’s law), the structure size has to be reduced in order to integrate a larger number of components. Lithographic techniques then reach limits (pure top-down process). To reach a resolution of order 22 nm (the next ITRS node) new methods like e-beam etching are discussed but constraints and costs are huge. Alternative solutions can be found thanks to molecular self-assembly (bottom-up processes). Among them, block copolymer self-assembly is attractive: block copolymers are made of different A and B monomers linked by covalent bonds. At low temperatures they form ordered structures (micro-phase separation) of domains at the desired scale (sone tens of nanometers, tunable with the molecular mass of the polymer). To use this phase separation, it is requested to orient in a controlled manner the obtained structures when they are under the form of a thin film on a wafer. A new convenient method was designed for providing a first perpendicular orientation which however presents lots of defects (Fig.).
 Liu P.H., P. Thébault, P. Guenoun, J. Daillant, Macromolecules, 42, 9609 (2009)
Please contact: Patrick.Guenoun@cea.fr
a) SEM top-view image of concentric circles imprinted in a PS–PMMA BCP fi lmshowing a perpendicular organization of the BCP film with respect to the circular tracks. b) Optical microscopy image at a much larger scale of 300 μm. c) Schematic drawing showing the motion of a magnetic reading head in a magnetic storage disk.
This means to eliminate all defects which subsist after the film annealing leading to phase separation. Defectless and addressable nanostructures are a prerequisite to make a decisive step forward applications. With colleagues of LETI and IPhT), and in collaboration with Tel-Aviv University (Prof. D. Andelman), we recently used a nanostructured mold for making a heat embossing of copolymer film in order to reduce the defects at a large scale (~ 10 µm) in a self-assembled layer. This mold can be reused thousand times on different layers. By imposing a surface energy constraint and imposing a pressure, the mold induces local stresses which guide long-distance flows which orient the nanostructures, making the defects to disappear (nano-rheology). Two orientations are obtained: when the flow dynamics dominates, lamellae (the kind of nanostructures obtained for a symmetric copolymer) are oriented perpendicularly to the mold teeth while if the surface energy constraint dominates, lamellae are oriented parallel to the mold teeth. Orientation can thus be chosen (cf. Figure 18). Nanostructures without defects were obtained over sizes of several microns. As an application, the right figure suggest that using a circular mold should be very convenient for writing resolved structures for magnetic head reading with a radial symetry. Numerical simulations by self-consistent field theory did confirm the structures stability.
Developments of this study are now to build 3D oriented films for applications where the aspect ratio matters like the design of metamaterials or anisotropic structures for optics.
Maj : 29/11/2013 (1850)