# Skin-Compatible Amorphous Oxide Thin-Film-Transistors with a Stress-Released Elastic Architecture
## 3 Results and Discussion  
To investigate the effect of the proposed reverse-trapezoid architecture on mechanical stress distributions in stretchable _a_-IGZO TFTs, FEA simulations were performed on conventional and reverse-trapezoid-structured PDMS substrates by using COMSOL Multiphysics. The corresponding models for _a_-IGZO TFTs on elastomeric PDMS are shown in Figure 1. In this modelling, a 30% lateral strain was applied to the _a_-IGZO TFT by considering the maximum strain of the epidermis [23; 24]. As shown in Figure 1, the highest stress was induced in the gate dielectric layer for the conventional PDMS substrate because of the large Young's modulus of Al\({}_{2}\)O\({}_{3}\) (Table 1) [25]. Importantly, this simulation suggests that the reverse-trapezoid structure can efficiently lower the stress distributions in the whole region of the stretchable _a_-IGZO TFTs compared to conventional PDMS, as shown in Figure 1a,b. In particular, the maximum stress induced in the _a_-IGZO TFT on a reverse-trapezoid PDMS substrate was 0.86 \(\times\) 10\({}^{8}\) Nm\({}^{-2}\), which is lower than that on a conventional planar PDMS substrate (1.28 \(\times\) 10\({}^{8}\) Nm\({}^{-2}\)). For the reverse-trapezoid structure, the thickness and the undercut depth were varied to achieve the most effective architecture in FEA simulation. We assumed that the underlying mechanism of stress relaxation was the presence of the undercut, transforming the lateral force of strain to a vertical direction without a deteriorative effect on the _a_-IGZO TFTs. As a result, the reverse-trapezoid structure for stretchable _a_-IGZO TFTs had the effect of releasing the stress induced by the strain. Additionally, undercut depths over the specific value prevented most advantages of the reverse-trapezoid structure, as shown in Figure S2. Based on the FEA simulation results, the reverse-trapezoid architecture was utilized to fabricate solution-processed _a_-IGZO TFTs by a simple route using a photolithography process. As shown in Figure 2a, the cross-sectional view of the SEM image confirms that pouring onto the mold generated a trapezoidal edge as a copy of the NPR undercut by fully filling the undercut space. Here, we used an epoxy material instead of PDMS to prepare the SEM sample because the PDMS was sheered and squashed during the polishing process, providing a distorted cross-sectional view. The thickness and undercut depth of \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Materials** & **Cr** & **Au** & **Al\({}_{2}\)O\({}_{3}\)** & **IGZO** & **IZO** & **PDMS** \\ \hline Density [kg/m\({}^{3}\)] & 7150 & 19,300 & 3965 & 5600 & 7120 & 970 \\ Young’s modulus [GPa] & 279 & 70 & 400 & 137 & 137 & \(750\times 10^{-6}\) \\ [PERSON]’s ratio & 0.21 & 0.44 & 0.22 & 0.36 & 0.36 & 0.49 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Material properties of stretchable _a_-IGZO TFTs for the FEA simulation. the fabricated architecture were set to values almost identical to the simulation data, where the thickness was 12 \(\upmu\)m and the undercut was 6 \(\upmu\)m. The optical microscope image of the stretchable _a_-IGZO TFTs is shown in Figure 2b. In contrast with the device on a PI substrate, the _a_-IGZO TFTs on a stretchable PDMS had top-gate, bottom-contact, and self-passivated structures after the mechanical transference and O\({}_{2}\) plasma processes. The effect of mechanical transference and O\({}_{2}\) plasma processes on the electrical characteristics of _a_-IGZO TFTs was investigated as shown in Figure 3a,b. Because the _a_-IGZO TFTs were mechanically detached from a carrier glass and exposed to O\({}_{2}\) plasma to etch the sacrificial PI layer, there were concerns about the harmful effects on _a_-IGZO TFT devices by mechanical stress and plasma bombardment during the fabrication. The stretchable _a_-IGZO  
Figure 1: FEA modelling for stretchable _a_-IGZO TFTs. Stress distributions of _a_-IGZO TFTs under 30% strain on a PDMS substrate having (**a**) conventional structure and (**b**) reverse-trapezoid structure.