Figure 2: TFTs and circuits transferred on a flexible polyimide foil.

(a) Bottom gate transistors fabricated on a parylene/PVAc/PVA/silicon template (ground signal ground, GSG, layout) (scale bar, 500 μm). (b) Device after transfer onto a 50-μm thick polyimide foil. The adhesion between parylene and Kapton is good after baking at 70 °C for 10 min. No wrinkles or cracks are visible (scale bar, 100 μm). (c) Device bent down to 5 mm (ε=0.5%) (scale bar, 100 μm). (d) Transfer characteristic (V_DS=0.1, 5 V) of a typical a-IGZO TFT (channel width/length of 50 μm per 9 μm). The device is measured on Si, transferred onto the foil and then bent to a tensile radius of 5 mm parallel to the TFT channel (dotted lines in the I_DS-V_GS curve represent the gate leakage current). (e) Output (V_GS=1, 2, 3, 4, 5 V) characteristic of the device shown in d. (f–h) After the transfer, 17 devices work out of 20 measured on Si (yield of 85%) and all devices continue to work when bent. Threshold voltage, V_TH, remains almost constant while the mobility, μ, shows an average value of 26 cm² V⁻¹s⁻¹ with an increase when transferred onto polyimide and then a decrease during bending test. Gate leakage, I_G, decreases by more than a decade on polyimide and remains smaller than 1 nA during bending (plot in logarithmic scale). Off-state current, I_off, (plot in logarithmic scale), instead, increases and, consequently, the I_on/I_off ratio decreases while remaining always greater than 10⁶. In addition, an average decrease of the output resistance by 50% and a slight deterioration of the sub-threshold swing are also observed. Errors bars in the plots indicate the s.d. (i) Schematic of a differential amplifier constituted by 7 TFTs, which have a channel length as small as 6 μm. The output signals are measured by two active probes (C⩽2 pF, R=1 MΩ). The supply voltage (V_DD) is 12 V. (j) Photograph of the differential amplifier after fabrication. (k) The bode plot of the amplifier shows a bandwidth of about 1 MHz and the gain is about 3, 2.2 and 1.7 dB, respectively on silicon, on polyimide and under bending. The gain degradation could be due to the TFTs threshold voltage shift and the decrease of the output resistance observed after the transfer.