Figure - available from: Applied Physics A
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a The temporal laser peak intensity I (r = 0, t, z) contours and b the spatial plasma density ρ (r, t = 0, z) contours with no avalanche ionization
Source publication
Characteristics and formation mechanism of filamentary plasma string induced by single picosecond laser pulse in sapphire are studied experimentally and numerically. Relative brightness and spatial distribution of the filamentary plasma string are characterized by time-resolved luminescence images. The whole filamentary plasma string is composed of...
Citations
... Lasers are seen as a preferred tool to dice brittle materials as they are contactless. Lasers have been engaged in various sapphire wafer cutting techniques such as dual-wavelength double-pulsed laser irradiation [4], laser-induced plasma-assisted ablation [5,6], controlled thermal cracking [7], and filamentation [8,9]. Depending on the laser dicing technique, the surface roughness of sidewalls ranges from 50 nm to 2000 nm [3]. ...
With multi-foci laser cutting technology for sapphire wafer separation, the entire cross-section is generally scanned with single or multiple passes. This investigation proposes a new separation technique through partial thickness scanning. The energy effectivity and efficiency of the picosecond laser were enhanced through a two-zone partial thickness scanning by exploiting the internal reflection at the rough exit surface. Each zone spanned only one-third thickness of the cross-section, and only two out of three zones were scanned consecutively. A laser beam of 0.57 W and 50 kHz pulse repetition rate was split into 9 foci, each with a 2.20 μm calculated focused spot diameter. By only scanning the top two-thirds sample thickness, first its middle section then upper section, a cleavable sample could result. This was achieved with the lowest energy deposition at the fastest scanning speed of 10 mm/s investigated. Although with partial thickness scanning only, counter intuitively, the cleaved sample had a previously unattained uniform roughened sidewall profile over the entire thickness. This is a desirable outcome in LED manufacturing. As such, this proposed scheme could attain a cleavable sample with the desired uniformly roughened sidewall profile with less energy usage and faster scanning speed.
The influence of the picosecond (ps) pulsed burst with a nanosecond scale of temporal separation (50 ns) on filamentary traces in sapphire substrate is investigated. The spatiotemporal evolution of the filamentary plasma string induced by sub-pulses of the burst-mode is revealed according to the analysis of the instantaneous photoluminescence images. Due to the presence of residual plasma, the energy loss of sub-pulse during the balancing of self-focusing effect is reduced, and thus refreshes the plasma via refocusing. The refreshed plasma peak generated by the subsequent subpulse appears at relatively low density positions in the formed filamentary plasma string, which results in more uniform densities and less spatial overlap among the plasma peaks. The continuity and uniformity of the filamentary trace in sapphire are enhanced by the burst-mode. Besides, the burst filamentary propagation can also remain effective when the sub-pulse energy is below the self-focusing threshold. Based on this uniform and precise energy propagation mode, the feasibility of its use for the laser lift-off (LLO) process is verified.
