# Skin-Compatible Amorphous Oxide Thin-Film-Transistors with a Stress-Released Elastic Architecture
## 2 Materials and Methods  
For the sacrificial layer, 15 wt% polyimide (PI) varnish (Polyzen 150, Picomax) was coated on a carrier glass at a spin speed of 4000 rpm for 60 s and was thermally cured at 300 \({}^{\circ}\)C for 1 h in air, resulting in a thickness of \(<\)3 \(\upmu\)m.  
For the solution-processed _a_-IGZO TFTs, a thermal evaporator system was utilized to deposit a 5 nm Cr and a 45 nm Au gate electrode at a 0.1 Asec\({}^{-1}\) deposition rate on the sacrificial PI substrate. By using photolithography with an AZ GXR-601 photoresist (Merck) and wet-etching process, the gate electrode and the following thin film layers were patterned. After patterning the gate electrode, an atomic-layer-deposition system (ALD) was used to deposit a 50 nm Al\({}_{2}\)O\({}_{3}\) gate dielectric layer at 150 \({}^{\circ}\)C using trimethylaluminum as the Al precursor and deionized water as the oxygen source. Before patterning the gate dielectric layer, combustive _a_-IGZO solutions were coated at a spin speed of 2000 rpm for 20 s and a deep ultra-violet (DUV) process was carried out to obtain a 10 nm-thick _a_-IGZO active layer. The combustion _a_-IGZO solution was prepared by dissolving metal precursors in anhydrous 2-methoxyethanol and sequentially adding acetylacetone and ammonium hydroxide. The metal precursors included indium nitrate hydrate (In(NO\({}_{3}\))\({}_{3}\)\(\cdot\)\(\cdot\)H\({}_{2}\)O), gallium nitrate hydrate (Ga(NO\({}_{3}\))\({}_{3}\)\(\cdot\)\(\cdot\)\(\cdot\)H\({}_{2}\)O) and zinc nitrate hydrate (Zn(NO\({}_{3}\))\({}_{3}\)\(\cdot\)\(\cdot\)\(\cdot\)\(\cdot\)H\({}_{2}\)O) [11; 14]. After the DUV annealing process, the _a_-IGZO active layer and Al\({}_{2}\)O\({}_{3}\) gate dielectric layer were sequentially patterned. For the formation of source/drain electrodes, an RF sputtering system was used to deposit a 100 nm-thick IZO layer (power: 150 W, pressure: 1.5 mTorr, time: 3 min). The IZO source/drain electrodes were patterned by a lift-off process using a negative photoresist (NPR) (NR9-300 PY, Futurrex, Franklin, NJ, USA).  
For the formation of the reverse-trapezoid structure, the NPR was spin-coated at a spin speed of 500 rpm for 20 s onto the fully fabricated _a_-IGZO TFTs on a sacrificial PI substrate and patterned by a photolithography process to obtain undercut structures which act as pivotal molds to generate the reverse-trapezoid structure for skin-compatible and stretchable PDMS (Sylgard 184, Dow Corning, Midland, MI, USA). The thickness and undercut structures were controlled by the spin speed and development time, respectively.  
The PDMS solution was then coated at a spin speed of 200 rpm for 60 s and thermally cured at 80 \({}^{\circ}\)C for 12 h in air. The PDMS solution was mixed with 10:1 weight ratio of base agent to curing agent and was degassed in a vacuum chamber for 20 min to eliminate air bubbles. Finally, the fabricated devices were mechanically detached from the carrier glass, and the sacrificial PI layer was eliminated by O\({}_{2}\) plasma treatment at 200 W for 13 min [22]. The remaining NPR layers and by-products were removed by dipping in acetone as shown in Figure S1.  
For finite element analysis (FEA) simulation, a numerical model was validated by commercial COMSOL Multiphysics 5.3. Each thin film layer was modeled by the linear elastic material properties listed in Table 1. The simulated models of conventional and proposed structures resulted in calculation of 2-D planes for simplicity and the uniaxial stretching experiment.  
All the electrical measurements in the experiments were carried out in air ambient darkness. A semiconductor parameter analyzer (4156C, Agilent) was used to characterize the electrical properties of _a_-IGZO TFTs.  
For scanning electron microscopy (SEM), a cross-section view was analyzed by a JSM-7055F Prime (JEOL Ltd.) at an operation voltage of 15 kV. The elastomeric samples were polished by LaboPol-30 (Struers) before the SEM analysis.