Inkjet printing is one of the key technologies behind the direct writing revolution, spanning an ever-increasing number of applications. During recent years, print-head technology has turned academic research into a viable manufacturing solution. Modern inkjet technology allows accurate printing to form functional components through additive deposition of a variety of materials, including ceramics, conductive polymers and now, metal oxides.
Transparent Conducting Oxides
Patterning Technologies Ltd (PTL), a company at the forefront of several direct writing technologies, is using its know-how to enable the direct patterning of transparent conducting oxide (TCO) films and circuits. These metal oxides are already used for making a broad range of advanced products, including flat panel displays, energy conserving architectural windows, and de-fogging windows for cars and aircraft. Patterned transparent conducting oxides are increasingly being used in displays for making the interconnections between the electronics and the pixel pad.
Transparent Conducting Oxide in Thin Film Transistor (TFT) Flat Panel Displays
Transparent conducting oxide interconnections can be used with any thin film transistor (TFT), including the type being developed by Plastic Logic for plastic electronics. In liquid crystal displays, a transistor switches the liquid crystals off and on by the creation of an electric field around the transparent conducting oxide pixel pad. Display applications such as this typically require a transparent conducting oxide sheet resistance of 125 ohms/sq.
Advantages of Using Inkjet Technology over More Conventional Techniques
In current processing methods, indium oxide doped with tin oxide (ITO) is deposited using sputter coating technology, and so covers the whole area of the substrate. While this is suitable for some applications, others require the patterning of pads and electrical interconnects. Creating a patterned layer is a time-consuming, costly, multi-stage process, involving lithographic masking and etching away of unwanted material. In contrast, the new approach being developed by PTL offers cheaper patterned transparent conducting oxide layers in less time. By using inkjet technology, not only is the processing cheaper, but the materials themselves are less expensive, requiring a much lower concentration of the doping element.
Indium Tin Oxide Circuit Deposition and Properties
Until now, sputtering has been the dominant technology for producing transparent conducting oxide films and circuits, although ITO has been much used to produce circuits, partly because it is etched. The mechanism for conduction in these films requires a high number of available charge carriers, in the form of free electrons and oxygen lattice vacancies. However, this high density of charge carriers also results in an increase in light absorption, which reduces optical transmission. Although the resistance of a sheet of material can be as low as 10 ohms/sq, visible transmission would only be 80% at best. For transmission rates greater than 90%, the resistance would typically be 100 ohms/sq or greater. These and other properties of ITO films are dependent on the deposition parameters and starting composition of the sputter target used.
The Inkjet Printing Process for Transparent Conducting Oxides
However, in the PTL process, additional factors come into play. When inkjet depositions of nanoparticulate transparent conducting oxide materials are made, the particle packing and drying of the films is crucial to the material’s performance. The process needs to include some methods of ‘freezing’ the coalesced droplets on the surface so that the patterned structure and the particle packing is preserved. PTL uses a patented process that controls the fluid on the surface of the substrate using electromagnetic radiation.
In fact, the printing technology integrates the print-head, the ink, the method of image ‘freezing’ and the substrate to achieved optimal deposition. The process takes into account ink ejection mechanics, the viscoelastic behaviour of ink on the substrate, and substrate pre-treatment. In addition to these deposition characteristics, the fluids also have to impart the required functional performance. While the inks are formulated to achieve these characteristics, it is the drying process that has the most influence on the performance of the film. For instance, drying rates affect the particle packing density and the electrical path through the material - the greater the density in the film, the better conductor it makes.
Origins of the Technology
PTL developed its process and materials through a SMART Feasibility Award from the UK Department of Trade and Industry. The aim of the project was to demonstrate the feasibility of printing transparent conducting oxides using inkjet technology to achieve electrical and optical functionality. As part of the project, PTL investigated a number of materials, including ITO. However, there are significant cost advantages in using alternative transparent conducting oxide materials. Inkjet printing offers more freedom of choice of materials because it removes the need for etching to produce patterned layers.
Antimony Tin Oxide – The Material of Choice
The company selected antimony tin oxide (ATO) as the material of choice as it meets the desired criteria of cost and performance. Made by Keeling & Walker (K&W), the antimony tin oxide was processed into a powder and then engineered into an aqueous suspension in which the antimony tin oxide nanoparticles were stable. Various additives were found to assist the lay down of the particles, improving the optical and electrical performance.
PTL researchers then used a drop-on-demand printer to directly write conducting features from data files, using transparent conductive oxide nanoparticle inks, and achieved extremely uniform conducting coatings with thicknesses of about 220nm. Examination of the films before annealing showed them to be made up of clusters of nanoparticles, formed by the coagulation of primary particles ranging in size from 12-40nm. Prior to annealing, the films are of high electrical resistance owing to the porosity and poor conduction path in the material. Annealing creates a much more homogeneous microstructure and increases the electrical conductivity, giving resistance values of approximately 1-2K ohm/sq. Further work on the materials, fluids and deposition is expected to yield even better performance. Some thickness variations in films covering larger areas (greater than 2cm2) have been seen - but these are believed to be associated with jetting deposition and regional mass transfer of particulates during solvent evaporation and forced air-drying.
Areas for Continued Research
The antimony tin oxide thin films produced in this way have already demonstrated conductivity and optical transmission characteristics suitable for some applications. Optical transmission, measured in the green region of the, visible spectrum at 550nm, was greater than 95%. Continuing research has shown that these properties can be improved upon, and PTL and K&W are working to improve the starting nanopowder, processing route and drying process. Work on the drying and annealing technology, being carried out by PTL, in collaboration with Nottingham Trent University, aims to print transparent conducting oxides on a variety of substrates, including glass, silicon and flexible plastics. Improving materials deposition and annealing will improve the electrical transfer paths without adversely affecting the optical performance.
Directly Written ATO Films Compared to Sputtered ITO Films
The bulk films produced are smooth and homogeneous with an average roughness of 3nm. Surface profiles of the films, produced using a Zygo optical interferometer, show that the directly written films are comparable to sputtered ITO films. Clearly, these inkjet printable antimony tin oxide films have the desired properties and could be a significant direct writing materials success, but this depends on PTL’s partnerships with industry. PTL and its many collaborators have been granted a DTI LINK award to develop the application of printed transparent conducting oxide technology for a display developed by The IFM Company. PTL has embarked on a number of other projects, which take advantage of the direct writing of masking layers, the high performance marking system, insulators, medical sensors, coatings and 3D structures.
Potential Use of Inkjet Printed ATO Films in Photovoltaics
One particularly interesting application area is photovoltaics. Thin film solar cell technology is in the early stages of commercial production, and an obvious problem is that the high cost of crystalline silicon wafers dictates the need for cheaper materials to make up the other parts of the cells to keep costs down. A transparent conductive film forms the front contacts of thin film cells, where the current is taken away. This could potentially be inkjet printed using antimony tin oxide ink, which could therefore bring down the capital equipment and materials costs of production, and make the new solar cells a much more viable commercial proposition.