Contents:
Film Properties
Refractive Index
Material Behavior
Electron-Beam Deposition Parameters
Physical Properties of Solid ITO Material
Sputter Deposition
Sputtered Film Applications
Forms and Sizes Available from CERAC

ITO, TIN-DOPED INDIUM OXIDE
FOR OPTICAL COATING

Introduction
Indium oxide doped with tin oxide, ITO, is used to make transparent conductive coatings. Thin film layers can be deposited by electron-beam evaporation or sputtering. Roll or web coating on polymer substrates is done by magnetron or other techniques of sputtering.

Typical applications of ITO-coated substrates include touch panel contacts, electrodes for LCD and electrochromic displays, energy conserving architectural windows, defogging aircraft and automobile windows, heat-reflecting coatings to increase light bulb efficiency, gas sensors, antistatic window coatings, wear resistant layers on glass, etc.

Film Properties
The optical and electronic properties of ITO films are highly dependent on the deposition parameters and the starting composition of evaporation material used. The deposited film layer must contain a high density of charge carriers for it to conduct. These carriers are free electron and oxygen vacancies, and an excessive population produces absorption. High conductivity (or low sheet resistance) is balanced against high transmission in the visible region. Sheet resistance can be less than 10 Ohms/sq. with a visible transmission of >80%. To obtain transmission near 90%, sheet resistance must be >100 Ohms/sq. ITO films behave as metals to long wavelength light because of the presence of a plasma wavelength above 1 µm. At longer wavelengths, the film becomes reflecting, and the IR reflectance is related to the sheet resistance of the film; sheet resistance must be <30 Ohms/sq. to obtain IR reflectance >80%.

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Material Behavior
The deposition parameters play interdependent roles in the optimization of film properties. Principal among the deposition parameters are partial pressure of oxygen, substrate temperature, rate of deposition and material composition. Some processes require post deposition baking at 300-500° C in air to oxidize residual fractionated metal component and improve conductivity. For sputter processes, a high energy plasma can be substituted for a high substrate temperature.

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Electron-Beam Deposition Parameters
Best results are obtained using reactive evaporation at a slow rate and relatively high pressure because the starting material is reduced (loses oxygen) during evaporation. Recommended preconditioning consists of slowly sweeping a low power electron beam to gradually and uniformly fuse the surfaces of the material and avoid hole drilling by the beam. Monitor the pressure and crucible to minimize outgassing and spitting while slowly increasing the power to just below evaporation temperature.

Evaporation Temperature	~600 ° C
Oxygen Partial Pressure	Near 5 x 10-5 Torr. Range
Rate	~2 Å/sec.
Substrate Temperature	Near 300° C.

Film thicknesses should be in the range 1000-2000 Å for high IR reflectivity, but there is little dependence of conductivity on thickness.

The specific geometry of the evaporation system can influence the results, so the parameters listed should be considered guidelines. Post baking in air might be required to adjust the transmission or to obtain minimum sheet resistance.

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Physical Properties of Solid ITO Material

Molecular Weight	Varies with composition
Melting Point	~1900° C
Color	Light yellow to gray, depending on degree of oxidation
Crystal Density	~7.14 g/cc

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Sputtering onto polymer substrates such as PET requires attention to the following parameters. The substrate surface must be free of water and other surface contaminants. Low substrate temperatures for coating polymers are possible with magnetron sputtering. High deposition rates are achieved in roll or web coating systems. Presputtering of the target surface to remove adsorbed gases and partially reacted layers is essential for success. The addition of a small percentage of hydrogen has been found beneficial in achieving low sheet resistances at low substrate temperature.

Figure 1 gives the dependencies of carrier concentration and resistivity on substrate temperature for a RF sputtered CERAC target. The carrier concentration increased substantially probably due to the creation of more oxygen vacancies at the hotter temperatures. Typically, resistance is traded with transmission, and these are directly related to the degree of oxidation of the film.

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Forms and Sizes Available from CERAC

Item No.	Purity	Description
I-2019	99.99%	91In2O3-9SnO2 (mol%) 3-12 mm sintered pieces
I-2009	99.99%	90In2O3-10SnO2(wt%) 3-12 mm sintered pieces

CERAC offers other particle sizes for evaporation as well as sputtering targets of various compositions. To view pricing on the items listed above, please visit our on-line catalog and look-up by item number or chemical name. If you require a custom manufactured item, please contact our sales department at 414-289-9800 or ceracsales@beminc.com with your specific requirements. You can also fill out our quotation request form.

References

1. Ray Swati, R. Banerjee, N. Basu, A. K. Batabyal, and A. K. Barua, J. Appl. Phys. 54(6), 3497 (1983).

Phone:  414-289-9800 /  FAX: 414-289-9805  /   ceracinfo@beminc.com