Development of the photolithography process for the fabrication of flexible thin film thermoelectric generator
The thermoelectric generators (TEG) are characteristic of high reliability, long term operation and require no maintenance. Flexible TEG can be the alternative to chemical current sources. The development of the technology of computer-tunable photomask for the fabrication of flexible thin film TEG is the relevant objective. In this work, a setup for pattern exposure on a photoresist without preparing a photomask is presented. It was demonstrated that the developed setup allows forming elements with a minimum size of 50 × 50 µm<sup>2</sup>. Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> and Bi<sub>2</sub>Te<sub>3</sub> thermoelectric materials were used for the formation of the p - and n -type legs, respectively, in the flexible thin film TEG. Results of the investigations of the temperature dependences of electrical conductivity and Seebeck coefficient for the Bi<sub>2</sub>Te<sub>3</sub> and Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> thin films were used to calculate the power factor. It was found that power factors for these films are weakly dependent on temperature in the studied temperature ranges and at room temperature are equal to 2.9 and 3.0 mW/m·K<sup>-2</sup> for Bi<sub>2</sub>Te<sub>3</sub> and Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub>, respectively. Based on the results of the adhesion study, a polyimide substrate and a contact system based on a conducting Cu layer and a Ni barrier sublayer were chosen for the fabrication of the flexible thin film TEG. A technological process has been developed and a flexible thin film TEG with a vertical arrangement of thermoelement legs has been fabricated. For the formation of the topological patterns for all layers, explosive photolithography (5 photolithography processes) was carried out using the developed setup for pattern exposure on the photoresist.