Gallium indium eutectic

To minimize mechanical stress, the whole wafer without a dicing process was used for all the electrochemical experiments. For the electrical connection to Si/SiO₂, the oxide layer on the back of the silicon wafer was removed by scratching with a diamond point pen by ~1 cm² and casting a droplet of 48% hydrofluoric acid solution. This area was covered by gallium-indium eutectic (≥99.99% trace metals basis from Sigma-Aldrich) and then attached by ~10-cm-long conductive adhesive tape. The tape was connected to the working electrode cable of the electrochemical analyzer. Tens-of-microliters of PBS solution were dropped onto an exposed SiO₂ area to form an electrochemical cell (Figure S8). Electrochemical characterization via a conventional electroanalytical technique was performed using an electrochemical analyzer (CHI 750, CH Instrument). Pt wire and a saturated calomel reference electrode (SCE) with a saturated-KNO₃ double junction were employed as the counter and reference electrodes, respectively. All potentials in this paper are referenced to SCE. Linear sweep voltammetry (LSV) and chronoamperometry (CA) at a constant applied potential (−4 V) were carried out in 0.1 M PBS solution (pH 3) to see characteristic electrochemical behavior of Si/SiO₂ during DB and post-breakdown.

The terminals of the printed pattern of the keypad and the copper wires were connected using liquid metal (Gallium-Indium eutectic, Sigma-Aldrich) and a NaOH 3% diluted solution (NaOH, 99%, Sigma-Aldrich:DI water 3 wt%) [29 (link)]. The connected wires were fixed using Ecoflex (00-30), linking the keypad to a microcontroller unit (Arduino Leonardo), which could detect and send changes in the capacitance signals to a PC. A custom developed Arduino program analyzed the output signals and activated specific keypad values. When the value of each channel of the keypad increased above a threshold, the PC executed a corresponding, specific value. If the value did not exceed the threshold, a keypad ‘release’ was performed.

I–V measurements were conducted using a semiconductor analyzer (SCS-4200, Keithley) with tungsten probe tips (model: T20, straight needle shape, tip-end diameter of 1 µm). The liquid metal (Gallium–Indium eutectic, Ga 75%, In 24.5% >99.99% trace metals basis, Sigma Aldrich) was utilized to lead out SS-RRAM from the tungsten tips to prevent undesired damage to the switching region of the SS-RRAM. The LABVIEW (National Instrument) program was customized to control the SCS-4200. Initially, the SS-RRAM was placed in a probe station (Tp-802x manual Probestation, Trip). A DC bias was applied to the top electrode of the RRAM while the bottom electrode was grounded. In the Set process, a compliance current of 500 µA was pinned to prevent undesired strong electrical breakdown or the formation of irreversible filaments. In the Reset process, the compliance current was limited to 0.1 A. The resistance in the HRS and LRS during the retention test was measured by applying a voltage of 0.4 and 0.15 V to the conducting upper layer of the polymer, respectively.

Static and cyclic bending tests were conducted to evaluate the flexibilities of the fibers. For static bending, 5 cm-long fiber samples were wrapped around circular objects with different radii. For the cyclic bending test, the 5 cm-long fiber was repeatedly subjected to bending by linearly translating one end of the fiber by 2 cm at a rate of 5 cm/s with the opposite end of the fiber fixed. This motion resulted in a bending radius of 7 mm at the fiber center. For both tests, the change in electrical resistance caused by bending was evaluated by using the digital multimeter. A strip of copper tape was attached to both ends of the fiber, and a small amount of silver paste (ELCOAT P-100, CANS Co., Tokyo, Japan) and liquid metal (gallium–indium eutectic, Sigma-Aldrich, St. Louis, MO, USA) was applied to the silver–copper interface to enhance electrical contact.

To minimize mechanical stress, we used the whole wafer without a dicing process for all electrochemical experiments^{17 (link)}. For electrical contact with Si/SiO₂, Si/Si₃N₄ and Si/HfO₂, the insulating layer on the back of silicon wafer was removed by scratching an approximately ~ 1 cm² area with a diamond point pen and casting a droplet of 48% hydrofluoric acid solution^{17 (link)}. This area was covered by gallium-indium eutectic (≥ 99.99% trace metals basis from Sigma-Aldrich) then covered by ~ 10-cm-long conductive adhesive tape^{17 (link)}. The tape was connected to the working electrode cable of the electrochemical analyzer (CHI660, CH Instrument, US)^{17 (link)}. 3 μL of 0.5 M aqueous electrolyte was dropped on the exposed SiO₂ area^{17 (link)}. To complete the electrochemical cell Pt wire and Ag/AgCl reference electrode (3 M KCl) with a double junction filled with 1 M KNO₃ were employed as the counter and reference electrodes, respectively. All potentials in this paper are referenced to Ag/AgCl reference electrode (3 M KCl)^{17 (link)}. Linear sweep voltammetry (LSV) was carried out to see characteristic dielectric breakdown behavior of Si/SiO₂, Si/Si₃N₄ and Si/HfO₂. LSV initial potential for SiO₂ and Si₃N₄ was—3 V, and for HfO₂ was 0 V. The scan rate was 50 mV/s. Potential at which dielectric breakdown occurred (V_db) was determined as the most positive potential with 100 nA during LSV.

To measure the RIS substrate’s Young’s modulus, strain–stress curve, and durability, a force gauge (M7-2, Mark-10, Mark-10 Corporation, Copiague, NY, USA) and a motorized test stand (ESM 303, Mark-10, USA) were used. To quantify the adhesion force between each section, we use a customized adhesion tester consisting of the above force gauge and a motorized test stand controlled using motion control software. The adhesion force was measured by following the standard test procedure (ASTM D3330). To measure the electrical properties during stretching, a uniaxial stretching machine (Jaeil Optical system), a digital multisource meter (2450, Keithley Instruments Inc., Cleveland, OH, USA), a step motor controller (NMC-201N, COPIA), liquid metal (Gallium-Indium eutectic, Sigma-Aldrich), and copper wire were used. The morphology was characterized by means of scanning electron microscopy (SEM, Quanta FEG 650, FEI. Ltd., Natural Bridge Station, VA, USA) and optical microscopy (OM, BX51).

Stretching tests were performed to characterize the electrical performance of the stretchable surface electrodes under strain using the following protocol: A piece of a polyethylene terephthalate film was attached to the auto-stretching stage (Motorizer X-translation Stages, Jaeil Optical System) to hold both edges of the stretchable device patch. The sample was loaded onto the stretching stage and robustly fixed using tape (3M Co., Ltd.) and silicon epoxy (Sil-poxy, Smooth-On Co., Ltd.). A droplet of liquid metal (gallium indium eutectic, Sigma Aldrich) was applied to the electrode channel and contact pad, followed by wiring cables up to a measurement instrument. A source meter (Keithley 2450 SourceMeter, TEKTRONIX, Inc.) was used to measure the electrical performance of the stretchable electrodes. The resistance of the stretchable electrode was measured according to the stretching range using LabVIEW customized software (National Instrument Corp.).