High-Performance Flexible a-IGZO TFTs Adopting Stacked Electrodes and Transparent Polyimide-Based Nanocomposite Substrates

Metal oxide semiconductor thin-film transistors for flexible electronics

“…Among the different polymers, polyimide (PI) foils with thicknesses (t S ) between 5 lm and 125 lm are the most frequently utilized substrates, 135,145,152,157,158,160,165,172,177,180,184,188,189 together with PI and nano silica. 142,144 This is because of the numerous advantages of PI (commercially known as Kapton V R ), like a small coefficient of thermal expansion (CTE) of 12 Â 10 À6 K, a small humidity expansion coefficient (HEC) (9 Â 10 À6 %RH), a high T G of %360 C, and a surface roughness in the nanometer range. 114,155 Since standard PI exhibits a yellowish to brownish color, other polymeric substrates have been introduced to benefit from their transparency in the visual wavelength range.…”

“…171 SiN x is only rarely used in the community. 134,142,159,160 Besides metal oxide dielectrics, also organic materials have been used as gate dielectric, such as layers made from olefin polymers, 38 or cross-linked PVP (c-PVP). 165 To this regard, a direct comparison of c-PVP and SiO 2 showed that both materials have a comparable R and result in flexible IGZO TFTs with similar performance parameters, 165 although the thick c-PVP layer (t ox ¼ 280 nm) reduces C ox if compared with the thinner SiO 2 (t ox ¼ 170 nm).…”

“…93,134 Furthermore, duallayers of metal and ITO 40 or IZO have also been employed. 142 As regards source/drain electrodes, the material has to provide a high conductivity and at the same time a small contact resistance with the active layer. Moreover, also other properties like adhesion or transparency need to be considered.…”

“…At the same time, a big variety of multi-layer contacts have been developed to combine the advantageous properties of different materials; recent examples are: Ti/ Au, 59,69,81,84,113,140,141,143,146,150,151,[181][182][183] Ni/Au, 79 Mo/ Al, 166,173 Cr/Au, 114 Mo/AlNd, 156 Cr/Au/Cr, 92 Mo/Al/Mo, 153 Ti/Au/Ti, 162 or Ti/IZO. 142 Regarding transparent source/drain contacts, only ITO 29,78,80,106,139,155,158,177,180,188,189,202,204,205 and IZO have been used. 160,171,184 Finally, contacts based on Ti (drain) and graphene (source) in combination with a VTFT structure have been published.…”

“…162 In alternative to flexible substrates manufactured independently from the TFTs, it is also possible to create the flexible substrate by covering a host substrate with a polymer using either evaporation, 80,81,141 spin, slot or blade coating techniques. 23,39,133,135,137,142,144,147,155,161,164,179 The advantages of these fabrication techniques based on a rigid support are a high compatibility with the standard fabrication processes on Si or glass wafers, a reduction of the expansion of the substrate during the manufacturing process, as well as the possibility to realize devices on very thin (%1 lm) substrates. After the TFT fabrication is completed, the flexible foils or thin deposited polymer layers carrying the devices can be separated from the rigid support using: (1) mechanical peeling, 38,142,155,158,161,164,166,168,179 (2) a low adhesion releasing layer, 133,144 (3) a supporting laser, 137 or (4) a sacrificial layer between the host carrier and the polymer.…”