2008

Patterning of Indium Tin Oxide Films

The patterns are formed by laser printing directly onto the films.

A relatively rapid, economical process has been devised for patterning a thin film of indium tin oxide (ITO) that has been deposited on a polyester film. ITO is a transparent, electrically conductive substance made from a mixture of indium oxide and tin oxide that is commonly used in touch panels, liquid-crystal and plasma display devices, gas sensors, and solar photovoltaic panels. In a typical application, the ITO film must be patterned to form electrodes, current collectors, and the like. Heretofore it has been common practice to pattern an ITO film by means of either a laser ablation process or a photolithography/etching process. The laser ablation process includes the use of expensive equipment to precisely position and focus a laser. The photolithography/etching process is time-consuming.

Image
A Pattern of Interdigitated Electrodes wasformed directly on the ITO-coated surface of atransparent polyester film by use of a laser printer.After etching away of the ITO not covered bythe toner, the toner was washed away, leavingthe polyester film with the patterned ITO.
The present process is a variant of the direct toner process — an inexpensive but often highly effective process for patterning conductors for printed circuits. Relative to a conventional photolithography/ etching process, this process is simpler, takes less time, and is less expensive. This process involves equipment that costs less than $500 (at 2005 prices) and enables patterning of an ITO film in a process time of less than about a half hour.

In the direct toner process as practiced heretofore, a laser printer or copier is used to print the desired pattern on a water-soluble paper, from whence the pattern is transferred to a circuit board in a sequence of laminating, liftoff, and etching steps. In the present variant of the direct toner process, there is no need for transfer paper: instead, an ITO-coated polyester film is fed directly into a laser printer or copier, where the pattern is printed directly onto the ITO. The patterned polyester film (see figure) is then immersed in a 10-percent (2-normal) solution of H2SO4 in water. The sulfuric acid etches away the ITO in the non-printed areas, while the toner in the printed pattern serves as a mask to prevent etching of the underlying ITO. After etching, the toner is washed away by use of acetone, leaving the patterned ITO.

This work was done by Christopher Immer of ASRC Aerospace Corp. for Kennedy Space Center.

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