Abstract
In recent years, electronics manufacturing has made outstanding progress in fabricating flexible systems, offering several advantages over traditional rigid electronics. Their flexibility comes from the development of new semiconductor technologies and techniques enabling the fabrication of thin-film devices and the suitability of these materials and methods to be used on flexible substrates. This has favored the development of lightweight devices able to withstand mechanical stress and conform to everyday objects, something not possible for standard rigid silicon electronics. Besides the mechanical properties, thin-film systems can benefit from using a wide range of electronic materials with different physical and chemical characteristics, e.g., transparency, biocompatibility, and biodegradability, highlighting their potential for new innovative applications. While the market landscape for thin-film devices is still evolving, indium-gallium-zinc-oxide (InGaZnO or IGZO) has been investigated for its promising properties as a semiconductor material, which have already led to the commercialization of some products. This transparent semiconductor shows good electron mobility (>10 cm2 V−1 s −1 ) while being deposited at room temperature, making its use compatible with the most flexible substrates, i.e., polymers. Indeed, low-temperature deposition and its amorphous phase are key factors in using this active material for flexible electronics. This thesis presents the use of IGZO-based electronics from three different perspectives. First, it demonstrates new fabrication protocols for the integration of thin-film electronics on unconventional substrates. A heat-shrinkable foil was used as a substrate for temperature sensors, demonstrating the adaptability of IGZO devices to irregular surfaces. Similar conformability was also achieved by encapsulating the devices between two sacrificial polymeric layers that can be selectively dissolved to enable substrate-free devices to be transferred onto any sur face. Additionally, IGZO thin-film transistors (TFTs) were integrated into soft micro-robots (1 mm × 5 mm size) that can re-shape and move under external stimuli, demonstrating device functionality after integration and locomotion of the robotic systems. This first part proves the versatility of IGZO-based device fabrication on different substrates. The second part of the thesis focuses on the novel performance and properties of IGZO electronics fabricated on the most commonly used flexible substrate, polyimide. First, fully transparent TFTs and circuits were fabricated by replacing common opaque metals with transparent conductive oxides. These devices provided an electron mobility of ≈20 cm2 V−1 s −1 , and led to low power and low voltage circuits. Next, TFTs with nanoscale dimensions were obtained by combining UV lithography with focused ion beam or electron beam lithography, enabling channel lengths down to 73 nm and gate-to-contacts overlaps <50 nm leading to a transit frequency up to ≈80 MHz. Another approach investigated in this thesis to achieve high-frequency TFTs was the use of a double-gate configuration with independent gates, showing the tunability of the characteristic frequencies up to 170 % with an additional gate bias as well as the possibility to perform analog operations. In the third part of this thesis, the possibility of fabricating hybrid circuits based on two different semiconductors to overcome the lack of a complementary semiconductor type to IGZO is presented. Since IGZO is an n-type semiconductor, organic TFTs based on a p-type semiconductor, 2,9-Diphenyl-dinaphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene (DPhDNTT), were fabricated with small channel dimensions (400 nm), showing a carrier mobility of ≈0.5 cm2 V−1 s −1 enabling a transit frequency of 14 MHz. Afterward, p-type organic TFTs and n-type IGZO TFTs were fabricated on the same substrates. Despite the poor performance compared to state-of-the-art devices of both technologies, the proposed fabrication protocol was a step towards the development of hybrid complementary circuits based on organic and inorganic technologies.