Jaakko Leppäniemi

Flexography-Printed In2 O3 Semiconductor Layers for High-Mobility Thin-Film Transistors on Flexible Plastic Substrate.

Industrially scalable and roll-to-roll-compatible fabrication methods are utilized to fabricate high-mobility (≈8 cm(2) V(-1) s(-1) ) nanocrystalline In2 O3 thin-film transistors (TFTs) on an flexible plastic substrate. Flexographic printing of multiple thin In2 O3 semiconductor layers from precursor-solution is performed on a Al2 O3 gate dielectric obtained via atomic layer deposition. A low-temperature post-contact-annealing step allows control of the TFT device turn-on voltage to ≈0 V for enhancement-mode operation.

Substrate-facilitated nanoparticle sintering and component interconnection procedure

Room temperature substrate-facilitated sintering of nanoparticles is demonstrated using commercially available silver nanoparticle ink and inkjet printing substrates. The sintering mechanism is based on the chemical removal of the nanoparticle stabilizing ligand and is shown to provide conductivity above one-fourth that of bulk silver. A novel approach to attach discrete components to printed conductors is presented, where the sintered silver provides the metallic interconnects with good electrical and mechanical properties. A process for printing and chip-on-demand assembly is suggested.

Far-UV Annealed Inkjet-Printed In2O3 Semiconductor Layers for Thin-Film Transistors on a Flexible Polyethylene Naphthalate Substrate

The inkjet-printing process of precursor solutions containing In nitrate dissolved in 2-methoxyethanol is optimized using ethylene glycol as a cosolvent that allows the stabilization of the droplet formation, leading to a robust, repeatable printing process. The inkjet-printed precursor films are then converted to In2O3 semiconductors at flexible-substrate-compatible low temperatures (150-200 °C) using combined far-ultraviolet (FUV) exposure at ∼160 nm and thermal treatment. The compositional nature of the precursor-to-metal oxide conversion is studied using grazing incidence X-ray diffraction (GIXRD), X-ray reflectivity (XRR), and Fourier transform infrared (FTIR) spectroscopy that indicate that amorphous, high density (up to 5.87 g/cm3), and low impurity In2O3 films can be obtained using the combined annealing technique. Prolonged annealing (180 min) at 150 °C yields enhancement-mode TFTs with saturation mobility of 4.3 cm2/(Vs) and ∼1 cm2/(Vs) on rigid Si/SiO2 and flexible plastic PEN substrates, respectively. This paves the way for manufacturing relatively high-performance, printed metal-oxide TFT arrays on cheap, flexible substrate for commercial applications.

Printed Low-Voltage Fuse Memory on Paper

A printed lateral resistive fuse-type write-once-read-many (WORM) memory on paper substrate is demonstrated. The memory writing process is based on breaking of a silver nanoparticle conductor. Low-voltage and low-current writability demonstrated with printed batteries are enabled by a μm2-range cross-sectional bit area that are achieved by super-fine inkjet technology. Supported by the statistical distribution of the writing times, the bit writing process is attributed to electromigration of silver and the required current density for fusing is found to be 34 mA/μm2. The results show an improvement in memory retention time when compared with structurally similar printed antifuse-type WORM memories.

Gravure printed sol–gel derived AlOOH hybrid nanocomposite thin films for printed electronics

We report a sol–gel approach to fabricate aluminum oxyhydroxide (AlOOH)-based inks for the gravure printing of high-dielectric-constant nanocomposite films. By reacting 3-glycidoxypropyl-trimethoxysilane (GPTS) with aluminum oxyhydroxide (AlOOH) nanoparticles under constant bead milling, inks suitable for gravure printing were obtained. The calculated relative dielectric constants based on the measured capacitances and film thicknesses for the gravure-printed GPTS:AlOOH nanocomposites varied between 7 and 11 at a frequency of 10 kHz. The dielectric constant depended on the mixing ratio of the composite and was found to follow the Maxwell-Garnett ternary-system mixing rule, indicating the presence of micro/nanopores, which affect the electrical properties of the fabricated films. An increasing leakage current with increasing AlOOH content was observed. The high leakage current was reduced by printing two-layer films. The double-layered gravure-coated films exhibited a similar capacitance density, but a clearly lower leakage current and fewer electrical breakdowns compared to single-layered films with comparable film compositions and film thicknesses. The best composite yielded a capacitance density of 109 ± 2 pF mm−2 at 10 kHz frequency and a leakage current density of 60 ± 20 μA cm−2 at a 0.5 MV cm−1 electric field as a single layer. The calculated relative dielectric constant at 10 kHz frequency for this composition was 11.2 ± 0.5.

In 2 O 3 Thin-Film Transistors via Inkjet Printing for Depletion-Load nMOS Inverters

Ability to digitally control the amount of a deposited material is one of the many advantages of inkjet printing. In this letter, we demonstrate the applicability of inkjet printing for the fabrication of depletion-load nMOS inverters based on metal oxide thin-film-transistors (TFTs) from printed metal oxide precursors where the threshold voltage of the TFTs is controlled by adjusting the thickness of the deposited semiconductor layer. Enhancement- and depletion-mode n-type In2O3 TFTs were fabricated from In-nitrate precursor using two printing strategies: 1) multilayer multinozzle printing and 2) single-layer single-nozzle printing in perpendicular or parallel to the TFT channel. TFTs with saturation mobility up to ~2.4 cm2/(V · s) and the ON/OFF-ratio of 107 were obtained after annealing at 300 °C. Devices connected as depletion-load nMOS inverters showed gain up to ~26 on a Si/SiO2 substrate, and an inverter on a flexible polyimide substrate with atomic layer deposited Al2O3 dielectric was demonstrated with a maximum gain of ~45.

Rapid Electrical Sintering of Nanoparticle Stuctures

A method for rapid electrical sintering (RES) of nanoparticle structures on temperaturesensitive substrates is presented. For an inkjetted silver nanoparticle conductor, a conductance increase of five orders of magnitude is demonstrated to occur in a timescale that typically varies between a few and one hundred milliseconds depending on process parameters. Furthermore, most of the conductance change takes only a few microseconds. The achievable final conductivities are within a factor of two from the bulk silver conductivity, as calculated using the external geometric dimensions of the structure ignoring porosity. The method is also applicable to other inorganic conductors such as indium-tin-oxide (ITO). More generally, the method offers a versatile tool in nanotechnology for electrical functionalization of nanoparticle structures. The method is also potentially suited for mass production.

High performance solution processed oxide thin-film transistors with inkjet printed Ag source–drain electrodes

This letter reports the fabrication of inkjet-printed thin-film transistor devices employing an electron transport layer incorporating polyethyleneimine for engineering of the interface between In2O3 semiconductor and Ag source/drain contacts, resulting in significant enhancement of saturation mobility by two orders of magnitude, from ∼0.03 cm2 (V s)−1 to ∼3.0 cm2 (V s)−1. The improvement is assigned to the lowered contact resistance between the source/drain electrodes and the underlying layers. The results are of importance for fully-printed oxide TFTs and circuits.

Reverse Offset Printing of Semidried Metal Acetylacetonate Layers and Its Application to a Solution-Processed IGZO TFT Fabrication

The submicrometer resolution printing of various metal acetylacetonate complex inks including Fe, V, Mn, Co, Ni, Zn, Zr, Mo, and In was enabled by a robust ink formulation scheme which adopted a ternary solvent system where solubility, surface wettability, and drying as well as absorption behavior on a polydimethylsiloxane sheet were optimized. Hydrogen plasma in heated conditions resulted in bombarded, resistive, or conductive state depending on the temperature and the metal species. With a conductivity-bestowed layer of MoO x and a plasma-protecting layer of ZrO x situated on the top of an IGZO layer, a solution-processed TFT exhibiting an average mobility of 0.17 cm2/(V s) is demonstrated.

Effect of UV light and low temperature on solution-processed, high-performance metal-oxide semiconductors and TFTs

This paper focuses on ultra-violet (UV)-curing of solution-processed metal-oxide (MO) semiconductors for application in thin film transistors (TFTs). The goal is to combine low-temperature thermal annealing with UV exposure and achieve printable transistors on flexible plastic substrates. In this paper we focus on the use of two different wavelengths of UV and clarify their effect on the performance of the metal-oxide semiconductors. The electrical properties of TFTs made with these semiconductors are characterized. The results show that wavelength of the UV exposure is critical for optimized performance of the semiconductor and the TFTs.