Effects of focused-ion-beam irradiation and prestraining on the mechanical properties of FCC Au microparticles on a sapphire substrate

Abstract

We have studied the effects of focused-ion-beam (FIB) irradiation and prestraining on the mechanical properties of nearly defect-free Au microparticles on a sapphire substrate. The Au microparticles, which were produced by a solid-state diffusion dewetting technique, were FIB-irradiated and/or prestrained, the latter using a nanoindenter with a flat ended punch operating under a nanohammering mode. Also, the prestrained Au microparticles were exposed to FIB to examine the effects of ion-beam damage on the properties of crystals containing mobile dislocations. We found that both FIB irradiation and prestraining reduced the yield strength of pristine Au microparticles significantly and made the stress–strain curves jerky. However, FIB irradiation does not affect the mechanical properties of prestrained Au microparticles very significantly. Once a microparticle contains mobile dislocations, its mechanical properties are not influenced much by the defects generated by FIB irradiation, even at the submicrometer scale.

Appendix

We used the Stopping and Range of Ions in Matter (SRIM) simulation to study the penetration depth and the number of implanted Ga⁺ ions. According to the SRIM calculation, the penetration depths of Ga⁺ ion beam are ~30 nm for {111} plane and ~25 nm for {100} plane for Au microparticles. Figure Al shows that the penetration depth at 4° incidence (the side surface of micropillars) is about one-third of penetration depth at 90° incidence (the top surface of microparticles). The simulation results also provide the number of implanted and backscattered ions. For a given number of applied ions, the total number of implanted ions for the normal incidence is about three times higher than that of 4° incidence. Thus, our microparticle appears to be more damaged than the micropillar. However, we have to consider the current density of the beam since we used the large circular pattem to reduce the current density. The schematic in Fig. A2 shows the geometry of micropillar and microparticle for the calculation of current densities. Suppose that the Au microparticle with a top diameter of 400 nm and a central diameter of 1 µm is exposed to the 10 pA beam for 7 s. Since we used a 5-µm diameter circular pattem, the current density of the beam is (10 pA)/ (π × (2.5 µm)2) = 0.51 C/(m² s). Then, the total dose of Ga⁺ ions per unit area can be obtained by multiplying the current density by the beam exposure time. So, the total dose per unit area is (0.51 C/(m² s))/(1.6 × 10⁻¹⁹ C) × (7 s) = 2.23 × 10¹⁹ ions/m² for 7 s. Assume that the concentric circular pattem is used for the micropillar with D ~ 400 nm, a similar top diameter. For the final milling process, suppose that the inner diameter of concentric pattern is 420 nm and the outer diameter is 460 nm. In this case, the current density is (10 pA)/[π × ((230 nm)² − (210 nm)²)] = 362 C/(m² s). For 1 s final thinning, the total ion dose per unit area is 362 C/(m² s)/(l.6 × 10⁻¹⁹ C) × (1 s) = 2.3 × 10²⁰ ions/m² for 1 s.

Suppose that the spherical shape of microparticle with the top and center diameters of 400 nm and 1 µm, respectively. Then, the side surface area, which is exposed to FIB, can be obtained by subtracting the cap area from the area of hemisphere. The beam exposed area of microparticle is the sum of the top surface and the side surface, which is π(200 nm)² + [2π(500 nm)²− π((200 nm)² + (42 nm)²)] = 1.57 × 10⁶nm². In the case of pillar geometry with the same top diameter of 400 nm, let us assume that the aspect ratio (L/D) is three, there is no tapering, and only the side surface is exposed to FIB. Then, the side surface area is 2π(200 nm)(3 × 400 nm) = 1.51 × 10⁶ nm². Thus, the beam exposed area is similar for the both geometries. Consequently, for the given top diameter, the ion dose of micropillars is about 10 times higher than that of microparticles due to the 10 times higher current density and the similar surface area. Here, the calculations were done for D ~ 400 nm, but even for the different diameters, the difference in beam exposed areas does not differ significantly.

FIG. A1.

Results of Stopping and Range of Ions in Matter (SRIM) calculations for 30 keV Ga ion implantation into pure Au at (a) glancing incidence (35.3°) for the {100} top plane, (b) normal incidence (90°) for the {111} top plane, and (c) glancing incidence (4°) for the side surface of nanopillar. The total 10,000 ions collided with the gold surface in these simulations.

FIG. A2.

The schematic of the Au microparticle and the Au nanopillar with a top diameter of 400 nm for the calculation of the total ion doses. The dimensions of ion-beam patterns are shown above samples. The shaded areas are the surfaces which are exposed to Ga⁺ ion beam.