Laser-Induced Forward Transfer of Functional Microdevices

Laser-induced forward transfer (LIFT) technique is an emerging micro additive manufacturing (AM) technique that has been widely used to print a variety of materials. Distinguished from other nozzle-based AM techniques, LIFT operates without the existence of the nozzle. This unique feature enables the transfer of prefabricated microstructures, and many works demonstrated successful transfers of different microcomponents. However, few studies have investigated the transfer of microstructures with functionality. In addition, no studies have ever explored the transfer of complex or fragile microstructures. Therefore, this thesis aims to tackle these challenges by transferring prefabricated SU-8-supported microdevices and proving their post-transfer functionality. Furthermore, the transfers of two-photon lithography (TPL) fabricated complex and fragile microstructures are demonstrated.In this thesis, we start by performing the LIFT experiments using pure SU-8 microdisks. Four different variables, including the laser energy (laser fluence), the polyimide (PI) thickness, the SU-8 thickness and the donor-to-receiver gap distance, are systematically studied. Successful and damage-free transfer of SU-8 microdisks with high transfer precision can be achieved with the optimized parametric combination (e.g., laser energy = 5 µJ, PI thickness of 3 & 5 µm, SU-8 thickness of 50 µm, and small gap distance).Based on the experimental results in the previous chapter, we take one step further, transferring SU-8-supported functional microdevices. Two types of functional devices, the SU-8-supported metal quick response (QR) codes and temperature sensors, are transferred to different receivers. For the QR codes, their readability is verified after the transfer by the QR scanner. Further analysis of the adhesion between the transferred SU-8-supported QR codes and the receiver demonstrates that such SU-8 devices adhere well to the receivers and they can survive in harsh conditions. As for the temperature sensors, they are transferred onto receivers with prepatterened electrodes. Our transfer guarantees good electrical contact between the temperature sensor and the electrodes on the receiver, which is confirmed by the subsequent electrical measurement. Apart from these two functional microdevice transfer demonstrations, we extend LIFT for the realization of large-scale transfer using customized programming codes. A 40 × 40 array of SU-8 microdisks are transferred onto a PDMS-coated 4-inch wafer. Furthermore, SU-8 microdisks are assembled to form three-dimensional (3D) structures and are transferred to different nonplanar surfaces to reveal the non-contact transfer ability of LIFT.In the end, we extend the transfer to TPL-fabricated microstructures. Microscaffolds with different pore sizes and dimensions are transferred without damage. Moreover, by adding a supporting layer, we achieve the transfer of fragile microscaffolds and large scaffolds. Subsequently, three transfer examples are demonstrated. The first demonstration is the transfer of a T-shape microstructure into the PDMS channel with high precision. In the second example, microscaffolds loaded with Rhodamine B ink are transferred, and the ink release ability is qualitatively assessed. The last demonstration makes use of the ability of LIFT for upscaled transfer, and a 7 × 7 microscaffold array is placed on a 4-inch wafer.