**5.2. Electrophoretic display driven by all solution process active-matrix oxide TFTs**

Precursor solutions and a solution process were developed for the fabrication of active-matrix amorphous oxide-TFTs [102]. The gate lines (GE), gate insulator (GI), and channel layer (CH) comprised RuO<sup>2</sup> , LZO, and ZIZO films, respectively. The polysilazane-based silicon dioxide (PSZ) film was used for the channel stopper (CS) layer. The source and drain (S/D) electrode had a double-layer structure comprising ITO and RuO<sup>2</sup> films. The silsesquioxane-based silicon dioxide (SQ) and RuO<sup>2</sup> films were used for the passivation layer (PV) and pixel electrodes (PE), respectively (**Figure 11(a)**). Details of the precursor solutions and annealing conditions for the film formations are summarized in **Table 6**. The TFTs exhibited a *μ* of 2.68 cm<sup>2</sup> V−1 s−1, a *SS*-factor of 1.09 V/decade, a *Vth* of 3.06 V, and an "on/off" ratio of 10<sup>5</sup> . Electrophoretic displays (EPDs) with a resolution of 101.6 ppi were successfully fabricated using the all solutionprocessed active-matrix TFTs (**Figure 11(b** and **c)**). Bi-stable black/white images were retained after cutting of the power supply and video signal in these TFT-EPDs (**Figure 11(d)**). It is considered that low-cost electronic paper can be realized using this technology in the near future.

## **5.3. Direct printing of oxide TFT by nano-rheology printing**

Among various solution-based approaches, a direct printing is a promising low-cost technique for fabricating oxide TFTs. The printing technique offers several advantages in manufacturing electronics such as a direct writing of materials, reduction of materials waste, and reproducibility with high resolution, which are not affordable from other solution-based approaches [103–105]. While many printed organic TFTs have been reported, relatively fewer studies, associated with directly printed oxide TFTs, have been pursued [17, 33, 106]. Furthermore, the printing has mainly been applied for metal-oxide semiconductor as the channel layer, while other layers such as GE, S/D, CS, and PV were fabricated by other techniques.

Recently, a new printing method, so-called nano-rheology printing (nRP), for metal oxide patterns within sub-micrometer range, was introduced [33]. The nRP method, which is a type of direct thermal imprinting, uses viscoelastic transformation like glass transition of oxide

used. First, a solution is coated and dried to make a semi-solid thin film (1). It is then loaded onto the heating stage of the imprinting machine, after which a mold is set onto the semi-solid film and pressure is applied (2). At this point, almost no deformation occurs. When the temperature is increased, the semi-solid film will suddenly soften at a certain temperature (2–3). The imprinting temperature (Tim) is maintained to complete the imprinting (3). Next, the temperature is lowered, and then, the mold is detached (4). Although a small amount of residual film is remained, it can be easily removed by etching in atmospheric air and other such simple methods. The etching process slightly reduces the sharpness of the edge but has no other significant influence on the pattern geometry. Well-defined patterns of various functional oxide materials

Low-Temperature Solution-Processable Functional Oxide Materials for Printed Electronics

), semiconductors (IGZO, SnO<sup>2</sup>

ITO) with the size down to several 10 nm were successfully produced by the nRP method [33, 107]. The imprinted patterns show very small shrinkage during post annealing, thereby achieving a high-shape fidelity to the mold. This results from the metal-oxide condensation at imprinting. The viscoelastic transformation and metal-oxide condensation at imprinting constitute the basics for the nRP method, which is closely related to the cluster structure in the precursor gel. Operation of totally rheology-printed oxide TFT (nRP-TFT) has been demon-

and 8.4 cm<sup>2</sup> V−1 s−1, respectively. Furthermore, excellent TFTs with sub-micron channel length could be completely printed by this method in an air ambient [33]. Active-matrix oxide-TFT array totally printed by the nRP method for display application has also been demonstrated

**Figure 12.** Process of the rheology printing. (a) Total process of rheology printing to form a one-layered pattern. (b) The profile of temperature and pressure during the imprinting process together with a schematic illustration of

configurations of the imprinted film and the mold.

[108], indicating great promise for large-area low-cost printed electronics application.

strated (**Figure 13**) [94]. The obtained "on/off" ratio, *SS*-factor, and *μ* were 10<sup>7</sup>

), and conductors (La-Ru-O,

http://dx.doi.org/10.5772/intechopen.75610

95

, 80 mV/decade,

including insulators (ZrO<sup>2</sup>

, SiO<sup>2</sup>

**Figure 11.** Cross-sectional structure of TFT (a), diagram of the pixel layout (b), and photograph of the pixels (c) in the TFT-driven electrophoretic display. The pixel pitch was 250 × 250 μm<sup>2</sup> , which corresponds to a resolution of 101.6 ppi. The aperture ratio was 84.6%.


MEA, 2-aminoethanol; PrA, propionic acid; 2ME, 2-methoxyethanol; acacH, acetylacetonate; NH<sup>4</sup> Ac, ammonium acetate.

**Table 6.** The precursor solutions and annealing conditions for the film formations in all solution-processed TFT-FPD.

precursor gels when imprinted; it softens at a certain temperature during thermal imprinting so that the gel can be rheologically imprinted. The total process for the nRP method is illustrated in **Figure 12**. A thermal nanoimprinting machine (ST-50, Toshiba machine) has been used. First, a solution is coated and dried to make a semi-solid thin film (1). It is then loaded onto the heating stage of the imprinting machine, after which a mold is set onto the semi-solid film and pressure is applied (2). At this point, almost no deformation occurs. When the temperature is increased, the semi-solid film will suddenly soften at a certain temperature (2–3). The imprinting temperature (Tim) is maintained to complete the imprinting (3). Next, the temperature is lowered, and then, the mold is detached (4). Although a small amount of residual film is remained, it can be easily removed by etching in atmospheric air and other such simple methods. The etching process slightly reduces the sharpness of the edge but has no other significant influence on the pattern geometry. Well-defined patterns of various functional oxide materials including insulators (ZrO<sup>2</sup> , SiO<sup>2</sup> ), semiconductors (IGZO, SnO<sup>2</sup> ), and conductors (La-Ru-O, ITO) with the size down to several 10 nm were successfully produced by the nRP method [33, 107]. The imprinted patterns show very small shrinkage during post annealing, thereby achieving a high-shape fidelity to the mold. This results from the metal-oxide condensation at imprinting. The viscoelastic transformation and metal-oxide condensation at imprinting constitute the basics for the nRP method, which is closely related to the cluster structure in the precursor gel. Operation of totally rheology-printed oxide TFT (nRP-TFT) has been demonstrated (**Figure 13**) [94]. The obtained "on/off" ratio, *SS*-factor, and *μ* were 10<sup>7</sup> , 80 mV/decade, and 8.4 cm<sup>2</sup> V−1 s−1, respectively. Furthermore, excellent TFTs with sub-micron channel length could be completely printed by this method in an air ambient [33]. Active-matrix oxide-TFT array totally printed by the nRP method for display application has also been demonstrated [108], indicating great promise for large-area low-cost printed electronics application.

**Figure 11.** Cross-sectional structure of TFT (a), diagram of the pixel layout (b), and photograph of the pixels (c) in the

**Layer Materials Precursor material Solvent Annealing condition**

O, acacH, NH<sup>4</sup>

)<sup>4</sup> PrA 550°C/O<sup>2</sup>

Ac,

, ZnCl<sup>2</sup> PrA, 2ME 550°C/O<sup>2</sup>

MEA, PrA, 2ME

GE RuO<sup>2</sup> Ru(NO)(OAc)<sup>3</sup> MEA, PrA 500°C/10 min

CHOCCH<sup>3</sup> ) 3

CS SiO<sup>2</sup> Polysilazane 500°C/H<sup>2</sup>

PV SiO<sup>2</sup> Silsesquioxane 500°C/O<sup>2</sup>

MEA, 2-aminoethanol; PrA, propionic acid; 2ME, 2-methoxyethanol; acacH, acetylacetonate; NH<sup>4</sup>

PE RuO<sup>2</sup> Ru(NO)(OAc)<sup>3</sup> MEA, PrA 250°C/air/5 min

**Table 6.** The precursor solutions and annealing conditions for the film formations in all solution-processed TFT-FPD.

precursor gels when imprinted; it softens at a certain temperature during thermal imprinting so that the gel can be rheologically imprinted. The total process for the nRP method is illustrated in **Figure 12**. A thermal nanoimprinting machine (ST-50, Toshiba machine) has been

, which corresponds to a resolution of 101.6 ppi.

/10 min

/20 min

/10 min

Ac, ammonium acetate.

500°C/air/10 min

O/30 min

TFT-driven electrophoretic display. The pixel pitch was 250 × 250 μm<sup>2</sup>

C2 H3 )3 , Zr(OC<sup>4</sup> H9

/ITO Ru(NO)(OAc)<sup>3</sup>

SnCl<sup>2</sup>

H9 )4

, In(OCCH<sup>3</sup>

, In(NO<sup>3</sup> )3 ·3H<sup>2</sup>

The aperture ratio was 84.6%.

GI LZO La(O<sup>2</sup>

CH ZIZO Zr(OC<sup>4</sup>

S/D RuO<sup>2</sup>

94 Green Electronics

**Figure 12.** Process of the rheology printing. (a) Total process of rheology printing to form a one-layered pattern. (b) The profile of temperature and pressure during the imprinting process together with a schematic illustration of configurations of the imprinted film and the mold.

Together with advanced printing technologies, it is expected that a truly printing of electronic devices has high potential to replace conventional production line in the near future.

Low-Temperature Solution-Processable Functional Oxide Materials for Printed Electronics

http://dx.doi.org/10.5772/intechopen.75610

97

The author thanks JST-ERATO Shimoda Nano-Liquid Process Project (2007–2014) and JST-CREST (2014–2019) for their financial supports. Special thanks go to members of Green Devices Research Center-JAIST and Center for Single Nano Innovative Devices-JAIST for

School of Materials Science, Japan Advanced Institute of Science and Technology, Nomi-shi,

[1] Yang JJ et al. Memristive switching mechanism for metal/oxide/metal nanodevices.

[2] Chen C-H, Suib SL. Control of catalytic activity via porosity, chemical composition, and morphology of nanostructured porous manganese oxide materials. Journal of the

[3] Gich M et al. Multiferroic iron oxide thin films at room temperature. Advanced Materials.

[4] Vrejoiu I et al. Functional perovskites—From epitaxial films to nanostructured arrays.

[5] Hiroyuki A. Recent advances and future prospects in functional-oxide nanoelectronics: The emerging materials and novel functionalities that are accelerating semiconductor device research and development. Japanese Journal of Applied Physics.

**Acknowledgements**

their fruitful discussions.

**Conflict of interest**

**Author details**

Phan Trong Tue

Ishikawa, Japan

**References**

The author declares that there is no conflict of interest.

Address all correspondence to: phan-tt@jaist.ac.jp

Nature Nanotechnology. 2008;**3**:429

2014;**26**(27):4645-4652

2013;**52**(10R):100001

Chinese Chemical Society. 2012;**59**(4):465-472

Advanced Functional Materials. 2008;**18**(24):3892-3906

**Figure 13.** (a) Cross-sectional structure, (b) optical microscope image, (c) transfer curve, and (d) output curve of the nRP-TFT.
