Transparent and conducting ITO films: new developments and applications

Tin doped indium oxide (ITO) films are highly transparent in the visible region, exhibiting high reflectance in the infrared region, and having nearly metallic conductivity. Owing to this unusual combination of electrical and optical properties, this material is widely applied in optoelectronic devices. The association of these properties in a single material explains the vast domain of its applicability and the diverse production methods which have emerged. Although the different properties of tin doped indium oxide in the film form are interdependent, this article mainly focuses on the electrical aspects. Detailed description of the conduction mechanism and the main parameters that control the conductivity is presented. On account of the large varieties and differences in the fabrication techniques, the electrical properties of ITO films are discussed and compared within each technique.

/pdf/tin-doped-indium-oxide-thin-films-electrical-properties-1emmfeswwg.pdf

Tin doped indium oxide thin films: Electrical properties

TCO/metal/TCO structures for energy and flexible electronics

A chemical mechanical polishing slurry comprising an oxidizing agent, a complexing agent, an abrasive, and an optional surfactant, as well as a method for using the chemical mechanical polishing slurry to remove copper alloy, titanium, titanium nitride, tantalum and tantalum nitride containing layers from a substrate. The slurry does not include a separate film-forming agent.

Chemical mechanical polishing slurry useful for copper substrates

Organic light-emitting diodes (OLEDs) are promising for full-color, full-motion, flat panel display applications because they offer several advantageous features, for example, ease of fabrication, low costs, light weight, bright self-emission, a wide viewing angle, and the possibility of flexible displays. The basic OLED structure consists of a number of organic semiconductor layers sandwiched between a cathode and an anode. For efficient electron injection into the organic layers, low-work-function materials are required for the cathode. A very thin LiF layer with a thick Al capping is widely used for this purpose. For the anode, indium tin oxide (ITO) is the predominant choice because it offers transparency in the visible range of the electromagnetic spectrum as well as electrical conductivity. However, several aspects of ITO are far from optimal for high-performance OLEDs. It is known that the migration of indium and oxygen from ITO into organic semiconductors during OLED operation causes device degradation. The electrical properties of ITO greatly depend on the film preparation. The rough surface of the deposited ITO film and the work function of ITO, ca. 4.7 eV, limit the efficiency of the hole injection. The typical sheet resistance of a 100 nm thick ITO layer, 20–80 X/ , is still high, which causes a voltage drop along the addressing line, thus limiting the operation of a large-area passive matrix OLED array. Moreover, the cost of ITO has escalated in recent years because of the jump in price of the element indium. Several alternative materials, for example, TiN, Al-doped ZnO, and fluorine tin oxide, have been investigated as anode materials instead of ITO; however, none are optimal as anode in OLEDs because they have either a lower work function or a lower conductivity than ITO. Other transparent conducting oxides, such as Ga–In–Sn–O (GITO), Zn–In–Sn–O (ZITO), Ga–In–O (GIO), and Zn–In–O (ZIO), that have a higher work function and a similar electrical conductivity when compared to ITO have also been examined as OLED anode materials. However, they are potentially problematic because they also contain the element indium that i) may diffuse into the organic layer in the OLED; and ii) has a high price, making these electrodes expensive. Besides these materials several metals with a high work function, such as Au, Ni, and Pt, have been investigated as anodes for OLEDs. In these cases the metal was used to modify the surface of the ITO electrode, or as an anode for top-emitting devices. A surface-modified thin Ag film has been used as a semitransparent electrode instead of ITO, but its transparency was low. Recently, carbon nanotube films have been investigated as transparent, conductive electrodes, but they have a high sheet resistance that may limit the device performance. In this Communication, we report semitransparent metal electrodes fabricated by nanoimprint lithography (NIL), and evaluate their potential as OLED anodes. NIL, an emerging lithographic technique, is well-suited to the area of organic electronics, which requires low-cost and high-throughput fabrication at high resolution. The fabricated semitransparent metal electrode offers several advantages over ITO for OLED applications. First, several problems associated with ITO can be eliminated, such as device degradation by indium diffusion and high costs. Second, efficient hole injection into the organic semiconductor can be realized by choosing metals with a high work function, such as Au or Pt. Third, a semitransparent metal electrode is potentially suitable for topemitting devices and tandem structures. Last, but not least, the output efficiency of the OLED can be enhanced by preventing waveguiding in the ITO layer, which occurs as a result of its high refractive index and is one of the limitations to the external efficiency of OLEDs, and by forming a two-dimensional (2D) hole array with proper periodicity. We demonstrate here that a unique property of such an electrode is that its optical transparency and the electrical conductivity can be tuned separately by changing the aperture ratio and the metal thickness, thereby making it possible to tailor the structures for different applications. To our knowledge, a nanoimprinted semitransparent metal electrode has not been reported before. The semitransparent metal electrodes are in the form of a nanometer-scale periodically perforated dense metal mesh on glass. Two design considerations led to such structures: i) the line width of the metal mesh was designed to be subwavelength, to provide sufficient transparency and to minimize light scattering; and ii) the period of the mesh was chosen to be sub-micrometer to ensure the uniformity of the current injection into the organic semiconductors. Such large-area dense nanostructures can be fabricated by NIL, which is ideal for this application because of its inherently high resolution C O M M U N IC A IO N

/pdf/nanoimprinted-semitransparent-metal-electrodes-and-their-3uoq6663cl.pdf

Nanoimprinted semitransparent metal electrodes and their application in organic light-emitting diodes

The present invention provides a polishing method including the steps of forming a film made of material containing a metal as a main component over a substrate having depressed portions on a surface thereof so as to fill the depressed portions with the film, and polishing the film by a chemical mechanical polishing method using a polishing agent containing a chemical agent responsible for forming a protection film on a surface of the film by reacting with the material containing a metal as a main component, thereby forming a conductive film in the depressed portions. The present invention also provides a polishing agent, which is used in forming a film made of material containing a metal as a main component in depressed portions of a substrate having depressed portions on a surface thereof by using a chemical mechanical polishing method, including a chemical agent responsible for forming a protection film on the surface of a substrate to be polished by reacting with the material containing a metal as a main component.

Polishing agent and polishing method using the same

A copper-based metal polishing solution comprises a water-soluble organic acid capable of reaction with copper to form a copper complex compound which is unlikely to be dissolved in water and has a mechanical strength lower than that of copper. The polishing solution also contains polishing abrasive grains and water. The polishing solution of the particular composition does not dissolve at all copper or a copper alloy when a copper or copper alloy film is immersed in the polishing solution, and permits polishing the copper or copper alloy film at a practical rate in the polishing step.

Copper-based metal polishing solution and method for manufacturing a semiconductor device

We developed low-resistivity transparent conductive films having the structure of indium-tin-oxide/silver/indium-tin-oxide (ITO/Ag/ITO). The thin silver film was sandwiched by ITO films. Our goal was to study the characteristics of the sandwich films and the display characteristics of simple-matrix liquid-crystal displays (LCDs) fabricated using the sandwich film. The electrical and optical characteristics of the sandwich films depended greatly on the thickness of the ITO and silver layers. Low resistivity and high transmittance were obtained when the film structure had a thickness of ITO/Ag/ITO: 40 nm/15 nm/40 nm. The simple-matrix LCD fabricated using a sandwich of ITO/Ag/ITO exhibited 27%–48% reduction in the level of crosstalk compared to the conventionally available simple-matrix LCDs fabricated using a single-layer ITO film; thus, the display performance was improved.

https://iopscience.iop.org/article/10.1143/JJAP.40.3332/pdf

Characteristics of Indium-Tin-Oxide/Silver/Indium-Tin-Oxide Sandwich Films and Their Application to Simple-Matrix Liquid-Crystal Displays

The influence of postdeposition annealing on sputtered indium tin oxide (ITO) film characteristics were investigated. The annealing experiments were carried out in air or vacuum atmosphere. Both air and vacuum annealing decreased the resistivity up to heat treatment of 200° C. Over 300° C treatment, air annealing increased resistivity whereas vacuum annealing decreased it. It was clarified that the resistivity depended on the carrier concentration. The lowest resistivity attained was 1.3×10-4 Ωcm, with film deposited on a 250° C heated substrate and annealed in vacuum atmosphere at 300° C. Transmittance was improved in both air and vacuum annealing. In air and vacuum annealing, crystallinity improved with increasing annealing temperature. The surface topography showed no changes with air or vacuum annealing.

https://iopscience.iop.org/article/10.1143/JJAP.33.302/pdf

Postdeposition Annealing Influence on Sputtered Indium Tin Oxide Film Characteristics

The relationship between micrograin structures and electrical characteristics of sputtered indium tin oxide (ITO) films was investigated. Micrograin structures were observed by a high resolution scanning electron microscope. Electrical characteristics were evaluated by four point probe resistance measurement and Hall effect measurement. Low resistivity ITO films had domain structures. One domain consisted of many sputter grains having the same orientation. The resistivity decreased with increasing domain size. The domain boundary might cause scattering for conduction electrons. Therefore, larger domain ITO films had a higher Hall mobility. The minimum resistivity was 1.8×10−4 Ω cm, deposited at a sputtering voltage of −250 V and a 250 °C deposition temperature. The electron conduction mechanism in domain structured ITO films was taken into consideration.

Masatoshi Higuchi

Papers

Polishing agent and polishing method using the same

Copper-based metal polishing solution and method for manufacturing a semiconductor device

Characteristics of Indium-Tin-Oxide/Silver/Indium-Tin-Oxide Sandwich Films and Their Application to Simple-Matrix Liquid-Crystal Displays

Postdeposition Annealing Influence on Sputtered Indium Tin Oxide Film Characteristics

Micrograin structure influence on electrical characteristics of sputtered indium tin oxide films