We investigated the interaction between femotsecond laser and polyimide with a high repetition femtosecond fiber laser and a precisely motorized 3D stage. We have found that high repetition femtosecond laser pulse train can effectively fabricate double-layer electrical conductive tracks inside a polyimide (PI) sheets by a single-time irradiation. This interaction comprised multi-photon absorption, dissociation of polymer molecules and the thermal accumulation. The experiment unveiled that dual-layer carbonization was a consequence of an inside micro-lens formed instantly as laser was just focused into the inside of polyimide. This micro-lens further focused the subsequent laser pulse to carbonize the polymer through multi-photon excitation, bond breaking and graphite layer reformation and eventually form the second electronic conductive layer. The second conductive layer was generated below the focal point. With the laser irradiating is kept at the same height, the top layer at the focused plane continued to absorb laser energy then carbonized into the conductive layer. We called the process as a kind of self-focusing phenomenon. We study the focus effect of inside microlenses under different laser powers and irradiation times. The gap of double electronic tracks embedded in the polyimide matrix can be adjusted with the laser processing parameters. When the gap is more than 30 micrometer, two conductive layers are electrically insulating. While the gap is smaller than 10 micrometer, two conductive layers are electrically connected. Various applications, such as, supercapacitors, capacitive sensors and the field effect transistors were investigated in the flexible PI sheets using this 3D double-layer electrical conductive architecture.
Shutong Wang, Yongchao Yu, Guoying Feng, and Anming Hu, "Laser manufacturing for transparent, flexible and stretchable electronics (Conference Presentation)," Proc. SPIE 10093, Synthesis and Photonics of Nanoscale Materials XIV, 100930J (Presented at SPIE LASE: February 02, 2017; Published: 21 April 2017); https://doi.org/10.1117/12.2253190.5389862303001.
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