Atomic processes at interfaces created by wafer bonding determine the physical properties of the semiconductor materials. Therefore molecular dynamics simulations (MD) based on empirical potentials are used to investigate the elementary steps of wafer bonding. The system is coupled elastically to the bulk wafers, and the energy dissipation is controlled by the transfer rates of the kinetic energy at the borders of the model. The applicability of the method is demonstrated by studying the interaction of perfect wafer surfaces, which corresponds to UHV bonding conditions. Related UHV-experiments demonstrate that large area, self-propagating, room-temperature covalent wafer bonding is possible.
However, calculated bonding energies and forces strongly depend on the surface termination, native oxides, adsorbates, and the process control. Thus calculations covering the influence of surface steps, rotational mis-orientations and adsorbates are being carried out to correlate atomic properties with macroscopic ones. Applications and software descriptions may be found in "Detailed Information".
The simulations lead to a better understanding of the physical processes at the interfaces and support the experimental investigations, especially the electron microscope structure analysis. Suitably fitted many-body empirical potentials are necessary to simulate sufficiently large number of particles and relaxation times. However, the electronic structure and the nature of the covalent bonds can only be described indirectly. Therefore it is of importance to find physically motivated semi-empirical potentials as, e.g., bond order potentials (BOP), based on the moments of the electron density and using tight-binding representations. Such potentials allow to predict the bondability of diamond, where the tight-binding level is necessary to describe correctly the π-bonds.
The Figure shows a typical defect structure after energy dissipation during bonding at steps (red color indicating higher potential energy above ground state, which is blue) and the electron density of the dreidl defect core reconstructions as the structural unit of a 90° twist boundary. DFT-LDA band structures of the dreidl interface versus a bulk silicon crystal demonstrates possible tailoring of the electronic properties.