Damage Threshold - Surface Swelling

Above an average incident fluence of∼100 mJcm⁻² in experiment an uplift of ma-terial can be observed. This uplift occurs in periodic lines at a corridor where the fluence is exceeding the damage threshold, and is corresponding in periodicity and orientation to the applied fluence line grating. This visible change of the surface

can be related to the ablation threshold for gold at 248 nm, which is given in the literature as 210 mJcm⁻² [130]. This threshold is based on a rather large illumina-tion area. The special attribute here however is the periodical sinusoidal variaillumina-tion of the applied fluence, from zero to peak fluence within a few hundreds of nm. The peak fluence of ∼ 200 mJcm⁻², which is twice as high as the average fluence, cor-responds quite well to the reported value of the ablation threshold, even though no ablation but rather a melting can be observed at the threshold for structures shown in the SEM pictures in Figure 5.5. The melting without material removal might be explained by the localized energy insertion and the distribution of the energy not only in z-direction, but due to the fluence variation also in y-direction, while in x-direction no energy is transfered, due to a constant fluence in this direction.

Consequently, in experiment at∼200 mJcm⁻² peak fluence no ablation is observed, even though the state of matter at the position of the peak fluence would support ablation, the surrounding within a few hundred nm is mainly unaffected, preventing ablation by the laser pulse. The associated mechanisms will be the topic of the discussion below.

Figure 5.5 Sub-structure formation on gold in dependence of the periodicity. At the structure sizes of 270 nm in (a), 350 nm in (b) and compared to 500 nm in (c), with an average incident fluence of ∼120 mJcm⁻². In the first and second row the SEM pictures (gray contrast) are shown. In the first row tilted by 45 ° to increase the contrast, in the second row a top view is shown. In the last two rows AFM surface heights in a orange color scheme are shown, in a top view and a line profile over the surface.

The surface changes for fluences from (120−130) mJcm⁻² for the periodicities of 500, 350 and 270 nm are compared in Figure 5.5. The case discussed here is where after solidification no voids remain beneath the surface, as shown in a cross-section in Figure 5.6. The fluctuations in the fluence lead to a partial overlap of different

mechanism observable in one picture, therefore in Figure 5.5 the melting without void formation is partly overlapping with the formation of voids. The measured structure height difference at the position of the peak fluence and the deepest de-termined position obtained from the AFM profile is about 10−15 nm for all the investigated structure periods. This applies for a regime where presumably no voids are formed beneath the surface. The height from the peak to the assumed location of the previous undamaged surface are highest for the 500 nm structure with about 13 nm and about 10 nm fordp = 270 nm. For the 350 nm structure the height ver-sus the surface where no fluence was applied is only about 5 nm. A difference in the width of the uplifted area can be observed, determined at FWHM from top to lowest depth. For the smallest structure a width of 100 nm is measured, while for dp = 350 nm, the raised structure extent in y-direction is measured to be 120 nm.

Ford_p = 500 nm the uplifted area had an expansion in y-direction of about 145 nm.

All widths are determined from the AFM line profiles, shown in Figure 5.6 and are averaged over three single periods.

100 nm 350 nm

Finc

y

Au Pt

(a) TEM cross-sectiondp= 350 nm

500 nm Finc

y

100 nm

(b) TEM cross-sectiondp= 500 nm

Figure 5.6 Surface swelling in cross-section view, with sketch of incident fluence profile (red) for the periodicities of dp = 350 nm in (a) and dp = 500 nm in (b), both at Finc ∼120 mJcm⁻². In the TEM picture the darker contrast shows the polycrystalline gold, with randomly oriented grains with boundaries, and the (brighter irregular contrast) represents the platinum top layer resulting from the FIB preparation.

Between the elevated areas, in the case of 270 nm and 350 nm a sub-structure in form of depressions next to the uplifted material is visible in Figure 5.5(a) and (b), while for 500 nm periodicity no sub-structure is observed. For d_p = 270 nm in Fig-ures 5.5(a) the sub-structure is most distinct and looks like a ridge, with an extent in y-direction at FWHM of ∼70 nm at a height of about 5 nm. In Figure (b) the sub-structure appears more to be a depression next to the uplifted area as can be observed from the AFM profile. Also in the TEM cross-section in Figure 5.6(a), the lowering of the surface is visible, leading to the assumption that where no flu-ence was applied the surface is completely unchanged. The applied fluflu-ence profile is

shown above the cross-section for both periodicities of 350 nm in Figure 5.6(a) and for 500 nm in Figure 5.6(b), in the latter the rise is minimal or even absent.

The suggested mechanism for the observed uplift and formation of a surrounding depression is a laser induced fast melting leading to a sudden increase of volume.

Liquid gold is less dense than gold in the crystalline phase, the process underlying the uplift of material here is comparable to that reported for Ni by D. S. Ivanov et al. [82]. The density of the crystalline gold is ρ^Au_solid = 19.3^g/cm³, and changes in the MD-TTM simulation for liquid gold to ρ^Au_solid = 18.3^g/cm³, corresponding to a volume increase by about 5 %. A fast volume increase induced by the transfer of heat from the electronic system to the lattice on a few picosecond scale, as discussed in Section 2.3.4, is accelerating the melt and is inducing an uplift. This upwards movement is strongest in the center where the deepest melting into the bulk occurs.

Some small voids might form temporarily, in the overheated melt and increase the upwards directed movement, and release stress/pressure of the material. In the here discussed case sufficiently large voids to sustain or agglomerate cannot be observed.

The suggested mechanism is the following: if not enough energy is present to sustain a minimal energy for a large void to form, molten gold is pulled from the upward moving center towards the middle, slowing the uplift but pulling material from the sides to the center. When recrystallization starts from the bottom and the sides, the original level of the surface at the sides of uplifted material is decreased, and mass conservation is preserved. The process described here is partly derived from the simulation results at slightly higher applied fluence shown in Figure 5.9(e) and (f). The difference is, that here the melt is not strongly overheated and has not enough time to form a stable void. The process with no voids beneath, is not yet simulated. A simulation with d_p = 350 nm and F_inc = 110 mJcm⁻² would allow to observe the development of a non explosive melt, without void formation after laser illumination. In the simulation the cooling process needs to be finished completely to possibly see in the MD-simulation the effect of the decreasing volume and the dip formation at the sides.

If the melt is overheated in a sufficient manner and voids can form before recrystal-lization, also the transiently liquid sides are not pulled down, and no sub-structure is formed. This case, of a void formation is described in the following Section.