MATERIALS PROCESSING: Picosecond fiber laser targets pulsed laser deposition

Ultrashort-pulsed-laser deposition performed with the plume of a picosecond fiber laser is one of many promising potential applications for emerging fiber-laser technology. Conventional materials-processing lasers, based on carbon dioxide, Nd:YAG, copper-vapor, or excimer technology, introduce thermal stresses that permanently damage heat-affected zones of a processed material. Amplified ultrashort pulses, however, form critical-density plasma that expands away from the surface of a material, caused by depositing laser energy on a short time scale compared to the transfer time through the bulk of the material. The result is a relatively high ablation efficiency and minimal collateral damage due to heat or shock (see www.laserfocusworld.com/articles/31475). Much research and product development has focused on making the most of this potential using femtosecond-laser technology (initially bulk solid-state lasers and now increasingly with fiber lasers). Corelase (Tampere, Finland), however, has focused on developing picosecond fiber-laser technology.

Conventional excimer pulses deliver energy on the order of 0.1 J per pulse at repetition rates up to about 1 kHz. Femtosecond-laser pulses deliver energy on the order of 0.1 to 1 μJ per pulse at repetition rates up to about 100 kHz. The Corelase modelocked fiber laser introduced earlier this year (see www.laserfocusworld.com/articles/250387), and upgraded to a 20 W version in May, delivers energy on the order of 5 μJ per 20 ps, 1064 nm pulse at repetition rates up to about 10 MHz and is targeted primarily at micromachining applications.

The lower shocks and pressures due to lower pulse energies are easier on the target material and allow even more precise cutting, according to Matt Rekow, North American business-development manager at Corelase. In addition, the high repetition rate, potentially climbing as high as 100 MHz, yields high process speed and controllability. Initial applications are surfacing across a wide range of materials, cutting low-k dielectric materials in the semiconductor industry, for instance, where the process enables maintaining the same ablation rate through adjacent layers containing different materials.

Pulsed-laser deposition of thin-film coatings is an important potential application area because of the difficulty of coating either large areas or temperature-sensitive materials using current methods such as thermal evaporation, ion sputtering, or chemical-vapor deposition. While eliminating the need to heat the substrate, pulsed-laser deposition is, however, still plagued by disadvantages that include uneven coverage and high defect rates. The relatively low pulse energies and high process controllability of picosecond fiber lasers may help to address these disadvantages, Rekow said.