Contacts and Interconnects for Germanium-based Monolithic 3D Integrated Circuits

  • Date:
  • Location: Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala
  • Doctoral student: Jablonka, Lukas
  • About the dissertation
  • Organiser: Fasta tillståndets elektronik
  • Contact person: Jablonka, Lukas
  • Disputation

Disputation

Three-dimensional integrated circuits have great potential for further increasing the number of transistors per area by stacking several device tiers on top of each other and without the need to continue the evermore complicated and expensive down-scaling of transistor dimensions. Among the different approaches towards the realization of such circuits, the monolithic approach, i.e. the tier-by-tier fabrication on a single substrate, is the most promising one in terms of integration density. Germanium is chosen as a substrate material instead of silicon in order to take advantage of its low fabrication temperatures as well as its high carrier mobilities. In this thesis, the work on two key components for the realization of such germanium-based three-dimensional integrated circuits is presented:the source/drain contacts to germanium the interconnects.

As a potential source/drain contact material, nickel germanide is investigated.In particular, the process temperature windows for the fabrication of morphologically stable nickel germanide layers formed from initial nickel layers below 10 nm are identified and the reaction between nickel and germanium is further studied by means of in-situ x-ray diffraction. The agglomeration temperature of nickel germanide is increased by 100 °C by the addition of tantalum and tungsten interlayers and capping layers. In an effort to more thoroughly characterize the contacts, a method to reliably extract the specific contact resistivity is implemented on germanium.

As a potential interconnect material cobalt is investigated. In a first step, highly conductive cobalt thin films are demonstrated by means of high-power impulse magnetron sputtering. The high conductivity of the cobalt films is owing to big grains, high density, high purity, and smooth interfaces. In a second step, the potential of high-power impulse magnetron sputtering for the metallization of nanostructures is further explored.