Stress evolution in platinum thin films during low-energy ion irradiation

Stress evolution of Pt thin films during low-energy ion irradiation is investigated by using wafer bending measurements and molecular dynamics simulations. Noble gas ions ranging in mass from He to Xe and energy from 0.5 to 5 keV are used. Depending on the type and energy of the ion, the change in stress can either be tensile or compressive. Heavier or higher-energy ions tend to create tensile stress, while lighter ions such as He always induce compressive stress. The stress evolution also depends on the initial state of stress in the thin films. The results are explained by a competition between the tensile stress induced by local melting along the ion track and the compressive stress induced by the accumulation of ion-induced interstitials in defect clusters or grain boundaries, often beyond the calculated ion penetration depth. Anisotropic diffusion of interstitials under an external stress field also plays an important role in the stress evolution. Molecular dynamics simulation is employed to evaluate the importance of each of these microscopic mechanisms.

Figure 1

The stress thickness of Pt films grown on Si cantilevers as a function of film thickness. Two different growth temperatures are used. The slope of the curve gives the average initial stress of the films. Data points with error bars represent the average over three or more individual growths.

Figure 2

(a) The change in the stress thickness of Pt films during ion irradiation. These samples have a strong initial biaxial compressive stress of 1.27 GPa. The dotted lines on the graph represent the expected change in the stress thickness due to the sputtering of the highly compressive film. (b) The change in the stress thickness of the highly compressive films irradiated by different ions. Films with thickness from 15 to 30 nm are used in (a) and (b). For these highly compressive films, the stress evolution is independent of the film thickness.

Figure 3

The stress evolution of the highly compressive films irradiated by switching between different energy ions. The arrows on the graph represent the instance when the ion energy is changed. The uppermost and the middle curve represent the switching between 4/1/4 keV Ar and 2/1/2 keV Ar, respectively. The change in stress thickness of a film bombarded by 1 keV Ar is shown for comparison. The initial film thickness is 15 nm.

Figure 4

The evolution of the stress thickness of films irradiated by different ions. These samples have a small initial tensile stress of 0.13 GPa and the initial film thickness is 40 nm.

Figure 5

The change in stress thickness of the slightly tensile films bombarded by 4 keV Ar as a function of ion dose. Each curve is labeled by the initial thickness of the film. The thickness removed by sputtering is indicated on the top axis.

Figure 6

MD simulation of the number of interstitials, bulk vacancy, surface vacancy, and adatom created by a single impact of Ar ion on a Pt(111) surface as a function of energy. The solid and open symbols represent the data taken with initial stresses of $-1.35$ and 0.15 GPa, respectively.

Figure 7

The in-plane and out-of-plane diffusion constants of interstitials measured by MD simulations at 300 K. A biaxial compressive stress, which is varied from 0.1 to 2 GPa, is applied on the (111) planes. A small and constant compressive stress of 0.1 GPa is always maintained in the out-of-plane direction.

Figure 8

The effective volume change induced per ion as a function of the energy deposition density of the ion. Note that all volume changes are negative (volume contraction). A negative sign is multiplied to the volumes so that we can present the data in a log-log plot. The volume change is calculated from the initial slope of the stress-thickness curves in Fig. 2 for the highly compressive films. The solid line on the graph is a fit to Eq. (4).

Figure 9

Atomistic processes occur in the surface region during ion bombardment. (a) In a film with a strong initial compressive stress, (i) the thermal spike process induce tensile stress within the implantation depth of the ion $\left(0<z<a\right)$; (ii) the interstitials have a bias to diffuse in the out-of-plane direction, which leads to recombination with vacancies or loss at the free surface. (b) In a film with a slightly tensile stress, (iii) interstitials can diffuse in the lateral direction as well, which leads to the creation of interstitials loops and attachment of interstitials to the grain boundary. These processes create compressive stress in the region $\left(a<z<d\right)$ beyond the ion implantation depth. [(c) and (d)] Schematics show the stress evolution in the two group of films, from initial state (${\sigma }_i$, dotted lines) to the steady state (${\sigma }_{ss}$, solid lines). The arrows between the profiles indicate the direction of stress change. At the steady state, a stress gradient is present that suppresses the further incorporation of interstitials.