Nanosecond pulsed laser sintering of carbon nanotube – Silver nanocomposite thin films on a flexible substrate: In-situ temperature measurements and post-process characterizations

Abstract

Cracks could be induced in metallic components of flexible electronics due to repeated large deformations during their service. This could affect their reliability and durability, which may be potentially enhanced by replacing the metal with a metal nanocomposite reinforced by carbon nanotubes (CNTs). Such nanocomposite thin films can be fabricated onto a flexible substrate via laser sintering of metallic nanoparticles (NPs) and CNTs, which is a fabrication method with several potential advantages. Despite previous studies by the authors on continuous-wave (CW) laser sintering, a systematic experimental study on the nanocomposite thin film fabrication by pulsed laser sintering has been seldomly reported in a paper to the best knowledge of the authors'. This current paper has reported experimental study on nanosecond (ns) laser pulse sintering of CNT-reinforced silver matrix nanocomposite thin-film lines on a substrate of polyimide. Both in-process time-resolved measurements of temperatures and post-process characterizations are performed. For the studied conditions, it has been discovered that with the same laser spot scanning speed and time-averaged power, 100 kHz laser pulses can produce much lower nanocomposite electrical resistivity than that by 25 kHz. The fundamental mechanism has been revealed through in-situ measurements of temperatures. It is expected to be related to the pulse-to-pulse heat accumulation effect generated by the laser pulses with the 100 kHz repetition rate, leading to a silver melt pool that is more continuous in time and has a longer lifetime. It has been found that with the same given average laser power, an optimum laser scanning speed exists, which leads to the minimum electrical resistivity of the sintered nanocomposite. At the corresponding optimum speeds and with the same average laser power, compared with the CW laser sintering, the ns laser pulse sintering can produce nanocomposite thin-film lines with similar (slightly smaller) electrical resistivity and much narrower widths at a much faster speed. An interesting “two-region” phenomenon has been discovered in the ns laser sintering. Its fundamental mechanism has been analyzed, which is expected to be related to surface vaporization-induced recoil pressure during laser sintering.