Nanomaterials: Processing and Characterization with Lasers

In conjunctionwith the increasing availability of cost-efficient laser units during the recent years, laser-based micromachining techniques have been developed as an indispensable industrial instrument of ‘‘tool-free’’ high-precision manufacturing techniques for the production of miniaturized devices made of nearly every type of materials. Laser cutting and drilling, as well as surface etching, have grown meanwhile to mature standard methods in laser micromachining applications where a well-defined laser beam is used to remove material by laser ablation. As an accurately triggerable nonmechanical tool, the ablating laser beam directly allows a subtractive direct-write engraving of precise microscopic structure patterns on surfaces, such as microchannels, grooves, and well arrays, as well as for security features. Therefore, laser direct-write (LDW) techniques imply originally a controlled material ablation to create a patterned surface with spatially resolved three-dimensional structures, and gained importance as an alternative to complementary photolithographic wet-etch processes. However, with more extended setups, LDW techniques can also be utilized to deposit laterally resolved micropatterns on surfaces, which allows, in a general sense, for the laser-assisted ‘‘printing’’ of materials [1, 2]. As outlined in Figure 5.4.1, the basic setup for an additive direct-write deposition of materials is the laser-induced forward transfer (LIFT), where the laser photons are used as the triggering driving force to catapult a small volume of material from a source film toward an acceptor substrate. In contrast to the classic laser-ablation micropatterning setup where material is removed from the top surface (Figure 5.4.1a), for LIFT applications the laser interacts from the inverse side of a source film, which is typically coated onto a nonabsorbing carrier substrate. The incident laser beam propagates through the transparent carrier before the photons are absorbed by the back surface of the film. Above a specific threshold of the incoming laser energy, material is ejected from the target source and catapulted toward a receiving surface that is placed either in close proximity to or even in contact with the donor film. With adequate tuning of the applied laser energy the thrust for the forward propulsion is generated within the irradiated film volume. The absorbed laser photons cause a partial ablation of the film material, which induces a sharply triggered volume expansion coupled with a pressure jump that catapults the overlying solid material away. However, the energy conversion processes as well as the phase transitions involved in the LIFT process are complex and affected by a large number of diverse parameters. Therefore, the highly dynamic interactions between the laser and the transferred material are not easy to describe in fundamental models. Apart from laser parameters such as emission wavelength,