ABSTRACT
Since the miniaturization and the performance increase of electronic components nowadays are ongoing parallel trends in the microelectronics industry, the introduction of nanometer (nm)-scale materials emerges even in the field of packaging, and interconnections within the back-end-of-line (BEoL) processes are also subjects of these processes [1]. Their potential as thermally, mechanically, or electrically enhancing additive to established packaging materials reduces their potential to a passive role. The use of carbon nanotubes (CNTs) inside Cu-based through-silicon vias (TSVs) [2,3] or packaging polymers [4,5] are prominent examples. On the other hand, the use of one-dimensionally elongated nanoparticles featuring a high aspect ratio (AR, the ratio of particle length to diameter), so-called nanowires (NWs) gains more and more attention when it comes to an active, that is, interconnecting incorporation of nanoparticles into packaging [6–8]. Their mostly bottom-up-based fabrication has already been realized by versatile approaches [9]. In general, one-dimensional growth can be achieved either by energetically favored growth based on high crystal structure anisotropy of the growing material or by spatial confinement during growth achieved by a template with parallel pores of a defined geometry. In the latter case, this geometry determines the later geometry of the NWs, that is, their diameter, length, and order. Diameters in the range of a few nanometers up to several hundred nanometers have already been achieved using different template materials. Among mesoporous silica [10], track-etched polycarbonate [11], or self-organizing block copolymers [12], 642anodically oxidized alumina (AAO) [13–15] enables beneficial features for template-assisted NW synthesis. AAO offers a wide range of variable pore geometries (pore diameters from 25 to 500 nm, interpore distances from 50 to 400 nm, pore depths from a few nanometers to micrometers, depending on the thickness of the Al used) while enabling once tremendous pore densities up to 1011 pores/cm2, through-going and nearly defect-free pores, and a very high degree of hexagonal pore order. Pores of AAO templates have been successfully filled with metals [16–18], metallic alloys [19], or (conductive) polymers [20], demonstrating the high material versatility for AAO-assisted NW synthesis. Different architectures, such as core–shell structures [21], multisegmented NWs [22], or nanoporous NWs [23], have also been obtained by filling the pores of AAO templates.
