Laser Direct-Write Techniques for Printing of Complex Materials
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ction Laser forward-transfer techniques have become important direct-write alternatives to lithographic processes for generating high-resolution patterns. In these approaches, a pulsed laser is used to induce the transfer of material from a source film onto a substrate in close proximity to or in contact with the film. The source is typically a laser-transparent substrate that is coated with the material of interest, referred to in the literature as the target, donor, or ribbon. Laser pulses propagate through the transparent substrate and are absorbed by the film. Above an incident laser energy threshold, material is ejected from the source and propelled toward the acceptor, waiting, or receiving substrate. Translation of the source and receiving substrate, or scanning and modulating the laser beam, enables complex pattern formation in three dimensions with speed
typically limited by the laser repetition rate. Commercially available, computercontrolled translation stages or galvanometric scanning mirrors enable rapid motion and high-resolution patterns from the individually written 3D volumetric pixels (voxels) that result from the laser forward-transfer process. Figure 1 shows a schematic illustrating the basic elements required for the laser forward-transfer apparatus. In contrast to many film deposition and patterning techniques, this approach does not necessarily require special vacuum or cleanroom equipment and can be performed under standard laboratory conditions. One may consider these LDW⫹ techniques to be analogous to inkjet deposition of functional materials, without the constraints of a nozzle and with the added benefit of selectively using other laser
MRS BULLETIN • VOLUME 32 • JANUARY 2007 • www.mrs.org/bulletin
processing techniques, such as laser direct-write modification (LDWM) and micromachining, or laser direct-write subtraction (LDW⫺), in the same tool. Also, in contrast to other LDW⫹ techniques, these approaches work on the principle of preserving the properties of the transferred materials rather than relying on a chemical reaction or other material modification to induce the deposition, as in laser chemical vapor deposition (see article in this issue by Stuke et al.). This added versatility in comparison with other printing methods enables LDW⫹ to manage the deposition and transfer of complex materials for which it is critical to maintain the delicate physical, biological, or chemical properties in the resulting patterns. Opportunities in fields ranging from metals deposition and power generation materials to biological and soft condensed matter abound. In this article, we will provide a brief overview of the use of LDW⫹ techniques for depositing complex material systems for applications in microelectronics, power generation, and biomaterials.
Laser-Induced Forward Transfer The use of laser-induced forward transfer (LIFT) was first reported for the deposition of copper metal patterns inside a vacuum chamber.1 Excimer laser pulses (λ⫽193 nm, 15 ns) were focused onto the back surface of a sour
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