# Research Progress on Flexible Oxide-Based Thin Film Transistors
## 3 Approaches to Flexible Oxide-Based TFTs
### Flexible TFTs on Different Kinds of Substrates
#### 3.1.3 Polyimide (PI) Flexible Substrate
have presented two types of IZO TFTs structures and their applications on PI substrates [161, 162]. Figure 9a, b shows the structures of the device, one is a BCE structure and the second an ES one. In the BCE structure, a 100-nm silicon nitride (SiN\({}_{\mathrm{x}}\)) film is deposited by plasma-enhanced chemical vapor deposition (PECVD) on the glass as a surface isolation layer (SIL) prior to the coating of the PI film to prevent the formation of a chemical bond between the PI and the glass. The PI layer with the thickness of 20 \(\upmu\)m is formed by spin coating the poly (amide acid) (PAA) solution on the SIL. Then, it is soft-baked as follows: 80 \({}^{\circ}\)C for 30 min, 200 \({}^{\circ}\)C for 1 h, and 350 \({}^{\circ}\)C for 1 h. The thermal imidization should proceed in the oven with a nitrogen atmosphere for transforming the PAA into PI. The corresponding root-mean-square roughness of the PI film was 2 nm, which may be small enough to avoid affecting the roughness of the IZO film. Then, a stacked structure of SiN\({}_{\mathrm{x}}\) (250 nm)/photoresist (2000 nm)/SiN\({}_{\mathrm{x}}\) (250 nm) was deposited on the PI surface at 150 \({}^{\circ}\)C as gas and water barrier by PECVD and spin coating alternatively. It should be noted that the devices were post-annealed at 300 \({}^{\circ}\)C for 1 h in an air atmosphere, which is the maximum temperature for the TFT process. At last, the PI substrate in company with the TFT devices was separated from the carrier glass when the device formation was completed. The measurement curves of the transfer characteristic with different bending radius are shown in Figure 9c. In ES structure, a surface isolation layer (SIL) is deposited on the carrier prior to the coating of PI film to prevent the formation of a chemical bond between PI and the carrier glass. The poly (amide acid) solution with about a 15-\(\upmu\)m thickness was coated on the SIL. In addition, it was transformed into the PI by the thermal imidization process in an oven with a nitrogen atmosphere. A stacked SiN\({}_{\mathrm{x}}\)/SiO\({}_{2}\) was deposited on the PI as a buffer layer with a total thickness of 300 nm by plasma-enhanced chemical vapor deposition (PECVD). A 200-nm molybdenum (Mo) gate metal film was deposited onto the buffer layer. Then, a 200-nm-thick SiO\({}_{2}\) gate insulator film was deposited by PECVD at 310 \({}^{\circ}\)C. A 30-nm indium zinc oxide active layer with the In:Zn ratio of 1:1 was deposited on the insulator SiO\({}_{2}\) by radio frequency magnetron sputtering, which was pre-annealed in an O\({}_{2}\) atmosphere for 30 min at 350 \({}^{\circ}\)C. Then, an ES layer used for protecting the active layer was fabricated and patterned by dry etching. For the source/drain electrodes, a stacked structure of molybdenum/aluminum/molybdenum (Mo/Al/Mo) was sequentially formed by sputtering and patterned by wet etching. The device was post-annealed  
Figure 8: The fabrication process for IGZO TFTs (top left). Transfer curve of IGZO TFT under bending stress (top right corner). Chemical structure of SA7 [160] (dotted box). at 350 \({}^{\circ}\)C for 30 min in an O\({}_{2}\) atmosphere. Finally, the PI with TFTs was de-bonded mechanically from the glass. Figure 9d shows the measured transfer characteristic (W/L = 10 \(\upmu\)m/10 \(\upmu\)m with Vds = 5.1 V) at the initial state and the bent state with a different curvature radius (R) from 50 mm to 10 mm. Both devices exhibited a high performance and reliability on bending tests and have been employed into the applications shown in Figure 9e,f. We used the BCE TFTs to construct a new gate driver circuit with low power and high stability. ES TFTs were adopted to fabricate flexible AMOLEDs. The experimental results show that the display still works after 10,000 times of testing. As the technology is developing, the application of oxide-based TFTs is not restricted to displays.