Laser Fabricated Digital Dewetting Patterns for Self-Alignment of Inkjet-Printed Materials for Organic Field Effect Transistors (OFETs)

Inkjet printing is a versatile technology that enables industry scale production of electronic devices with minimal loss of material, since it’s a digital process using drop-on-demand processing. It is compatible with a wide range of both organic and inorganic materials, can support flexible substrates and even be used for biodegradable devices. Applications include solar cells, photodetectors and medical sensors.<br/><br/>Of particular interest in our group are field effect transistors based on organic materials. It is beneficial to reduce the feature size of printed transistors to reduce operating voltages, power consumption and enable fast switching frequencies. One fundamental limit of inkjet printing is the lateral achievable feature size, that is heavily influenced by the volume of individual ink drops and their interactions with the surface of the substrate. Typically, resolution is restricted to several tenths of µms. We propose the combination of additive as well as subtractive laser-based processing techniques to enhance inkjet-printed devices and achieve resolutions of 10 µm or lower, while at the same time confining the process to one machine, reducing the complexity of processing and alignment. In the past, we already investigated similar ideas using aerosoljet printed structures[1].<br/><br/>One approach relies on the controlled modification of surface free energy (SFE) using laser-based crosslinking of the photoresist material SU-8 with a resolution of 10 µm in the focal spot of a UV fs-laser (additive). The process is fully digital in 2D, allowing for design freedom and flexibility. After crosslinking and washing of residual materials, SU-8 and substrate show contrasts in surface energies. Subsequent deposition of functional silver ink for electrode structures leads to de-wetting and self-alignment along the pre-defined patterns, resulting in the formation of channels that are in the order of 10 µm and only limited by the laser resolution.<br/>Subsequent layers are inkjet-printed on top, forming the field effect transistor. Compared to fully printed field effect transistors, the channel length is reduced by a factor of at least 3.<br/><br/>Another approach relies on the idea of utilizing high power fs-laser ablation to cut transistor channels into previously printed thin film electrode structures (subtractive process). By fine-tuning the laser output power and carfully optimizing the processing, channel lengths below 10 µm can be realised.<br/><br/>Apart from utilizing 1-photon polymerization with a UV-laser as the main crosslinking mechanism in our additive approach, further investigations are intended to be performed using two photon polymerization processes, which would allow to push dimensions to the sub 1 µm regime.<br/><br/>[1] Eckstein et. al., Digitally Printed Dewetting Patterns for Self-Organized Microelectronics , Wiley Advanced Materials 2016, 28, 35, https://doi.org/10.1002/adma.201602082