Modeling evaporation, ion-beam assist, and magnetron sputtering of thin metal films over realistic time scales

Abstract

A long time scale dynamics technique has been used to model the evaporation, ion-beam assist and magnetron sputtering of thin metal films over realistic time scales. Two fcc metals have been investigated; silver and aluminium. We illustrate how the technique can be used to model growth of these films over experimental time scales, while investigating individual growth mechanisms and surface diffusion events. Long time dynamics is achieved through an on-the-fly Kinetic Monte Carlo method, which determines diffusion pathways and barriers, in parallel, with no prior knowledge of the involved transitions. It was found that Ag has the ability to grow smooth surfaces, using several mechanisms including multiple atom concerted motion, exchange mechanisms and damage and repair systems. Ag {111} and {100} grew dense, complete and crystalline film when sputtering was simulated, whereas evaporation produced incomplete layers. The inclusion of Ar in the ionbeam assisted evaporation of Ag {111} aided growth by transferring more energy to the surface atoms allowing increased diffusion. Al {111} however shows slightly different patterns; growth via evaporation and magnetron sputtering shows only slight differences and the inclusion of the ionbeam assist actually damages the film beyond repair producing subsurface Ar clusters where Al atoms were displaced creating voids throughout the film. Al {100}, similar to Ag {100} grows denser and more complete film when grown via sputtering rather than evaporation. Results show that the energy of the deposition method used, plays a vital role in the resulting thin film and substrate quality.

Growth of silver on zinc oxide via lattice and off-lattice adaptive kinetic Monte Carlo

“…In the case of Ag growth on Ag, it was these concerted motions that could explain the formation of twin boundaries 12 and in the case of ZnO growth how chains of atoms on the surface could move concertedly to form into hexagons. 13 It was therefore surprising to discover that as the silver deposits, there were only a few examples of concerted motion.…”

“…Previous simulations of Ag growth on Ag have also shown that the diffusion barriers in the layers are smaller than the barriers to drop down although with a smaller relative difference than calculated here. 12 …”

“…The basic methodology has been described in previous work 12,13 and allows simulation of metallic systems over several seconds but for the more complex Ag-ZnO ReaxFF potential, the longest simulation time achieved was around 0.1 s of real deposition time on a 512 atom system. The AKMC approach has been used to identify many complex transition pathways and mechanisms for thin film growth.…”

“…The AKMC procedure is slightly modified from that described in our previous work 12 in that the open ended saddle point searches are conducted, not for the entire system but in neighbourhoods (typically all atoms within 6th nearest neighbor distance) of atoms that are not fully co-ordinated compared to typical bulk atoms or atoms in a perfect surface. Such a region will be termed a 'defect volume'.…”

“…First reactive force field potentials [7][8][9] have become accepted as a way to capture many of the effects missing from previous empirical potential descriptions and secondly adaptive KMC methods, introduced by Henkelman and Jónsson, 10 can be used to predict unexpected transitions which would be missed by KMC using predefined event tables. [11][12][13] Originally, a simplified model, using a combination of pair potentials for Ag-Zn and Ag-O interactions fitted to works of separation, 6 was first considered for the simulation of Ag growth on ZnO surfaces. However, the model predicts that a single interstitial Ag atom is energetically favourable compared to a surface Ag adatom on the polar ZnO(000 1) surface by 0.75 eV in contradiction to ab initio results.…”