Bock Soon Na

Photolithography-Based Patterning of Liquid Metal Interconnects for Monolithically Integrated Stretchable Circuits

We demonstrate a new patterning technique for gallium-based liquid metals on flat substrates, which can provide both high pattern resolution (∼20 μm) and alignment precision as required for highly integrated circuits. In a very similar manner as in the patterning of solid metal films by photolithography and lift-off processes, the liquid metal layer painted over the whole substrate area can be selectively removed by dissolving the underlying photoresist layer, leaving behind robust liquid patterns as defined by the photolithography. This quick and simple method makes it possible to integrate fine-scale interconnects with preformed devices precisely, which is indispensable for realizing monolithically integrated stretchable circuits. As a way for constructing stretchable integrated circuits, we propose a hybrid configuration composed of rigid device regions and liquid interconnects, which is constructed on a rigid substrate first but highly stretchable after being transferred onto an elastomeric substrate. This new method can be useful in various applications requiring both high-resolution and precisely aligned patterning of gallium-based liquid metals.

Stretchable Organic Thin-Film Transistors Fabricated on Wavy-Dimensional Elastomer Substrates Using Stiff-Island Structures

Stretchable organic thin-film transistors (OTFTs) were fabricated on the polydimethysiloxane (PDMS) elastomer substrates by employing the wavy-dimensional and polyimide stiff-island structures. A low-temperature solution process was also designed to obtain high strain profiles. The endurable maximum strains were estimated to be 2.28, 9.70, and 9.32% for the OTFTs formed on the flat, 1D-, and 2D-wavy PDMS elastomers, respectively. The field-effect mobilities were obtained to be 5 ~ 7 × 10-4 cm2 V-1 s-1 for all devices and they did not exhibit any degradation under the stretchable conditions before the fracture. The results suggest that the proposed methodologies were quite suitable for high-performance stretchable OTFTs.

Non-volatile organic ferroelectric memory transistors fabricated using rigid polyimide islands on an elastomer substrate

The authors fabricated stretchable organic ferroelectric memory transistors (OFMTs) on a polydimethylsiloxane substrate using rigid polyimide island structures. The OFMTs exhibited a field-effect mobility of 4 × 10−2 cm2 V−1 s−1 and a current on/off ratio of 105 with a notably low threshold voltage. Furthermore, our memory TFTs exhibit excellent mechanical stability, showing no noticeable change in electrical performance up to a large strain of 50%. These results indicated the feasibility of a promising device for stretchable electronic systems.

Flexible nonvolatile memory transistors using indium gallium zinc oxide-channel and ferroelectric polymer poly(vinylidene fluoride-co-trifluoroethylene) fabricated on elastomer substrate

The authors demonstrated flexible memory thin-film transistors (MTFTs) with organic ferroelectric polymer poly(vinylidene fluoride-co-trifluoroethylene) and an amorphous oxide semiconducting indium gallium zinc oxide channel on the elastomer substrates. The carrier mobility, memory on/off ratio, and subthreshold swing of the flexible MTFTs showed 21 cm2V−1s−1, 107, and 0.5–1 V/decade, respectively. The memory window of 13 V at ±20 V programming was confirmed for the device without any interface layer. These obtained values did not significantly change when the substrate was bent with a radius of curvature of 10 mm. The memory on/off ratio was initially 5 × 104 and maintained at 102 even after a lapse of 3600 s. The fabricated MTFTs exhibited encouraging characteristics on the elastomer that are sufficient to realize mechanically flexible nonvolatile memory devices.

Stretchable copper interconnects with three-dimensional coiled structures

We propose a new scheme of stretchable metal interconnects utilizing the electroplated three-dimensional (3D) coil structure, as a strategy for improving the stretching performance of spring-like electrodes through the suppression of local stress concentration. In this process, the 3D copper coils are fabricated by a multi-step electroplating process, and embedded within an elastomeric substrate forming stretchable configuration. By comparing the stretching behavior of the two-dimensional and 3D coils under static or cyclic deformation, the beneficial effects of the 3D configuration have been demonstrated. The present technique can be regarded as an effective way to enhance the stability of interconnects under cyclic deformation significantly, while maintaining the high mechanical strength and electrical conductivity of electroplated electrodes.

Device characteristics of inkjet-printed ZnO TFTs by solution process

We report on solution-processed n-channel ZnO thin-film transistors. We fabricated by a low-temperature process to improve their performance using inkjet printing under various conditions. The resulting films were inkjet-printed with a resolution 200 dpi using droplets of 50 µm diameter and 35 pl volume. The characteristics of the inkjet-printed TFTs were improved significantly at an annealing temperature of 150 °C. The field-effect mobility, Vth, and on/off current ratio were 3.03 cm2 V−1 s−1, −3.3 V, and 106, respectively. These results indicate that annealing at 150 °C is sufficient to obtain a mobility (μsat) as large as 3.03 cm2 V−1 s−1.

Spontaneously Formed Wrinkled Substrates for Stretchable Electronics Using Intrinsically Rigid Materials

The preparation of a stretchable substrate was carried out by blending poly (ethylene glycol)-block-poly (propylene glycol)-block-poly(ethylene glycol) and polydimethylsiloxane (PDMS) to enhance the stretchability and the surface energy of the elastic film. The deposition of a thin parylene film on this modified PDMS led to the formation of a spontaneously wrinkled surface, resulting in a highly stretchable substrate. Rigid Au conductors were deposited on the resultant wrinkles substrates, and could be stretched up to 20% strain. In addition, stretchable pentacene thin-film transistors were successfully fabricated on the wrinkled substrate without the formation of stiff islands.

Locally-tailored structure of an elastomeric substrate for stretchable circuits

We demonstrate a new process for fabricating a hybrid elastomeric polydimethylsiloxane (PDMS) substrate, which can provide a high ratio (as large as ~50) of the elastic modulus between the active device region and the interconnect area, as well as a locally tailored surface profile for each region. For this process, a Si master mold with a dual surface profile is prepared, where locally flat regions are distributed within a wavy-surfaced area. The stiffer elastomeric islands for active devices are formed on the flat regions by photolithography of a photo-patternable and hard PDMS layer (E ~ 160 MPa), over which a soft PDMS layer (E ~ 2 to 3 MPa) is casted. By releasing the whole PDMS layer from the mold, a hybrid silicone substrate with stiff and flat islands embedded within a soft and wavy matrix is obtained. In this hybrid structure, active devices located on the stiff regions can provide high reliability under stretched conditions, while most strain is accommodated by wavy interconnects within the soft area. Such beneficial effects are demonstrated by organic thin film transistors produced on the hybrid substrate.