Open air deposition of zinc oxide transistors

Guest article from Kodak

 

Kodak has developed a fast, printing-like process to fabricate high-performance zinc oxide (ZnO) transistors in open atmospheric conditions.

 

ZnO-based devices can be made using a variety of techniques, including sputtering, pulsed laser deposition, solution process methods (e.g. printing) and atomic layer deposition. Of these, only the solution methods can be accomplished in an open atmosphere, and to date solution process based devices have presented relatively low mobility, on the order of 0.1 to 0.5 cm2/Vs.

 

The company has also developed a high-speed Atomic Layer Deposition (ALD) process that works in open air environments. Traditional systems use two gaseous reactants (an oxygen source and a metal source) that are sequentially cycled, separated by a purge gas in an enclosed chamber.

 

In Kodak's process, the spatial distribution of each reactant is confined to controlled regions along the surface of the deposition head, and it is separated from other reactants by an inert gas, as shown in the figure below. The distance between the deposition head and the substrate is approximately 30 microns. The proximity of the substrate to the head ensures that the flow of each gas comes into contact with a well-defined region of the substrate.

 

When the deposition head and substrate remain stationary with respect to each other, each part of the substrate is in contact with a single gas and no deposition occurs. When the substrate moves in a direction perpendicular to the deposition head gas delivery slots, each area on the substrate will receive sequential exposure to the reactant gases, reaction occurs in a manner very similar to that found in traditional chamber-based ALD.

 

Multiple ALD deposition cycles can be implemented using a large head with many sets of gas-delivery channels in single pass mode or using a smaller header with repeat passes. The reactive precursors are presented sequentially to the substrate by means of controlling their spatial positioning. Kodak refers to this process as Spatial-Atomic Layer Deposition (Spatial-ALD).

 

 

The layers are built up as the substrate passes over the deposition head (see below).

 

The process is compatible with in-line and roll-to-roll implementation.

 

Kodak has demonstrated ZnO thin film transistors with an on/off ratio that exceeds 108, near zero turn-on voltage, little or no hysteresis, and mobility that exceeds 10 cm2 /Vs.

 

In addition, Kodak has also demonstrated the ability to selectively deposit materials with the Spatial-ALD process by pre-printing patterns of blocking materials that inhibit the ALD process on the substrate. Fully functional transparent thin-film transistors have been made using Spatial-ALD patterned exclusively with Selective Area Spatial-ALD.

 

 

Guest article from Kodak

 

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