Time-resolved studies of femtosecond-laser induced melt dynamics

Abstract

This work presents time-resolved images of femtosecond-laser-induced melt dynamics in 60 nm gold films on glass substrates. Melt dynamics induced by laser radiation with focus diameters of 6 μm and 8 μm (FWHM) at constant laser fluence is investigated with a temporal resolution of 10 ns. In both cases, the formation of the microbumps and gold jets takes at least 250 ns. It is shown that the formation process can be compared to jetting behavior induced by cavitation bubbles near a free liquid surface. This is confirmed by SEM illustrating a re-entrant spike through a hole in the microbump.

Laser-induced forward transfer of pure metals

“…At a later instance (B-images), this ejected dome contracted into a jet like feature, with a contracted droplet at the tip. At these fluence levels the ejection process seems to be dominated by the full melting and the resulting stress relaxation of the heated gold layer, similar to what has been reported for femtosecond LIFT of 60 nm gold layers [84]. Figures 7.2 (f) to 7.2 (g) show the ejection process at higher laser fluence levels.…”

“…Based on the observations made during low fluence nanosecond LIFT, it assumed that a similar mechanism, as proposed by Unger et al [84] can be employed in order to explain the observed ejection mechanism during nanosecond laser-induced forward transfer. A brief description of the stages of this ejection mechanism is illustrated in figure 7.6.…”

“…Based on time-resolved shadowgraphy and scanning-electron microscope (SEM) images Unger et al [84] captured femtosecond LIFT melt dynamics in 60 nm thick gold films with a laser fluence range below the ablation threshold. For laser fluence just above the transfer threshold, the formation of a liquid jet, and the subsequent separation of a gold droplet is documented.…”

“…The expansion of the ejected film is hampered, due to the zero pressure bubble, that is generated inside of the emerging copper bubble. However, the ejection process is still dominated by the inertia of the initial stress wave and liquid motion as described by Unger et al [84] is neglected. As the liquid copper bubble emerges, the elongation of the film depends on the slope of the profile.…”

“…Third, a jet is located at the apex of the bubble. However, a second jet facing from the apex towards the carrier, referred to as a "counter-jet" [42,84], is not identified. Despite the absence of the counter-jet, a significant growth and thickening of the apex is observed.…”