Soon Won Jung

Organic Thin-Film Transistors with Short Channel Length Fabricated by Reverse Offset Printing

We report on the fabrication of organic thin-film transistors (OTFTs) with a reverse-offset-printed Ag metal source/drain (S/D) electrode pattern. The printed electrodes had a channel length of less than 5 μm and resistivity of 3 × 10−6 Ω cm. The OTFTs were fabricated from regioregular poly(3-hexylthiophene) as the semiconductor and poly(methyl methacrylate) as the gate insulator. The transfer and output characteristics of a top-gate OTFT with a channel length of 5 μm were evaluated. Here we discuss in detail the technological challenges encountered with reverse offset printing and the failure modes.

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.

Top-gate staggered poly(3,3″′-dialkyl-quarterthiophene) organic thin-film transistors with reverse-offset-printed silver source/drain electrodes

Here, we report on high-performance top-gated poly(3,3″′-dialkyl-quarterthiophene) (PQT-12) organic thin-film transistors (OTFTs) with reverse-offset-printed (ROP) silver (Ag) source/drain (S/D) electrodes. OTFT devices with ROP S/D electrodes using Ag nanopaste show higher performance (∼0.01 cm2/Vs) than those fabricated by vacuum electron beam evaporation with conventional photolithography and a standard lift-off process (∼1 × 10−3 cm2/Vs). This dissimilarity is attributed to the higher work function (−4.9 eV) of the ROP Ag electrode due to AgO formation on the Ag surface during thermal annealing. This results in a low interfacial hole injection energy barrier between the S/D electrodes and the PQT-12 semiconductor.

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.

Nonvolatile programmable metallization cell memory switching element based on Ag-doped SbTe solid electrolyte

We report that industrially qualified SbTe chalcogenide film can be applied to programmable metallization cell memory switching device. To fabricate the switching device, Sb35Te65, Ag, and W (top electrode) were consequently sputtered on TiW (bottom electrode)/SiO2∕Si substrate, and Ag diffusion process was not added. During Ag sputtering, it is apparent that Ag is diffused into Sb35Te65 film to form Ag-doped Sb35Te65 solid electrolyte, and that some of the diffused Ag reacts with Te to form Ag–Te bond in the solid electrolyte.

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.

A resistive switch device based on SbTeN chalcogenide film

To meet the needs of the next-generation programmable switch circuit, we propose a SbTeN-based resistive switch device, which unifies the functions of the pass transistor and the SRAM-cell, has non-volatility, and passes endurance specification. By adding an optimum nitrogen-content in a SbTe film, the on-resistance reduction is successfully demonstrated, which is considered to be the first solution to meet the needs for low-power consumption.