Ricoh Inkjet Printing High-Resolution Electrode Patterning Technology

Technical Overview

Ricoh employs various low-temperature printing methods to develop fine TFT arrays on flexible substrates. The technology uses 300 ppi organic thin-film transistors (OTFT) arrays, where the TFT consists of a pixel circuit, a transistor, and a capacitor. Surface energy controls the inkjet printing method on flexible substrates.

The TFT array pitch is 85μm, and the minimum source electrode width is 15μm, much finer than traditional inkjet printing resolutions. The minimum electrode spacing is 0.8μm, which is even smaller than the spacing using a photolithographic aligner.

The performance and high-resolution 150 ppi OTFT array, where the TFT has a pixel circuit with two transistors and a capacitor, is made through printing methods and through-hole laser processing. A surface energy-controlled inkjet printing method using direct laser patterning, rather than UV exposure with a photomask, was developed. This hybrid technology enhances digital processing and online demand handling, meeting the distinctive characteristics of surface-controlled inkjet printing processes.

TIPS

TFT (Thin Film Transistor) refers to thin-film transistors. TFT-based displays are the mainstream display devices in various laptops and desktops. Every liquid crystal pixel on such a display is driven by the thin-film transistor integrated behind the pixel, making TFT a form of active matrix liquid crystal display (LCD). It is one of the best LCD color displays, offering high responsiveness, brightness, and contrast, with display quality close to CRT displays. TFT screens are also widely used in mid-to-high-end color mobile phones, with 65,536 colors, 16 million colors, and 160 million colors. Their display performance is excellent. TFT ensures high-speed, high-brightness, and high-contrast displays. TFT-LCD (Thin Film Transistor-Liquid Crystal Display) is a common type of LCD used in most displays.

Printing Display Technology

"Printed Electronics" refers to the technology that builds electronic devices using printing methods such as inkjet printing, flexographic printing, screen printing, gravure printing, and offset printing, with metal and semiconductor inks. Compared to traditional semiconductor processes that require etching to thin functional materials, this additive manufacturing process offers material efficiency and low-temperature processing advantages. It is a low-cost, environmentally friendly processing technology and has been gaining attention.

Using unique technologies like surface energy-controlled inkjet printing (creating hydrophilic and hydrophobic areas on new polyimide film surfaces, where ink is applied to hydrophilic areas), the resolution is better than traditional inkjet printing.

Electrode Printing with Ricoh's Unique Surface Energy-Controlled Inkjet Technology

• The printed electrodes (a) are much more precise than (b), without interference fringes, ensuring a smooth surface.
  
  

Through surface energy-controlled inkjet printing, ink formulation, inkjet control, and multiple printing methods, finer transistor circuits are created on plastic substrates at low temperatures.

Integration of Printing Process and Laser Processing

A through-hole formation process has been developed. First, laser ablation forms through-holes to connect two electrodes, and then inkjet printing fills the holes when creating the upper electrodes.
Using laser ablation to form through-holes, our unique full-printing TFT manufacturing requires only one additional step. We connect two TFTs with finer patterns and can manufacture higher functionality (as shown in the diagram below).

Printed Circuit Flexible Display Demonstration

By laminating a 200 ppi full-printed OTFT backplane on a plastic substrate, a thin and light high-resolution display is achieved. We achieved the best resolution (200 ppi = pixels per inch) as a display driven by transistor circuits, where each component (electrode, semiconductor, insulator, and wiring) is printed separately.

Innovative Technology

As a voltage-driven transistor circuit with one capacitor and one transistor (1T1C), we achieved the best resolution (300 ppi = pixels per inch, TFT pitch: 80µm), where all components (electrode, semiconductor, insulator, and wiring) are manufactured using printing processes.

We also achieved the best resolution (150 ppi = pixels per inch, TFT pitch: 169µm) for a current-driven transistor circuit with one capacitor and two transistors (2T1C), where all components (electrode, semiconductor, insulator, and wiring) are made through printing and laser processing.

Application Fields

This low-cost, low-environmental impact, high-resolution, and low-temperature process can be applied to various large-area flexible devices (not limited to displays), including sensors and RFID tags, which are essential for a low-carbon society.

Ricoh's developed technology is expected to promote continuous progress in the printed electronics field.