ITO thin films prepared by synchronal pulsed RF-DC sputtering
Indium tin oxide (ITO) is used as transparent front contact layer in thin film solar cell systems and other applications. Commonly used (pulsed) DC sputtering of ITO provides high quality transparent conductive layers and is easy to use on large area substrates. The approach of RF superimposed DC sputtering lowers the resistivity of ITO thin films in comparison to DC or pulsed-DC sputtering. Thus the film thickness required and therewith the material costs can be reduced. Furthermore the near infrared transmission of the deposited films can be enhanced. ITO was deposited using a (pulsed) RF and a (pulsed) DC plasma excitation simultaneously. With increasing RF power ratio the resistivity of ITO thin films decreases, even without additional heat exposure to the substrate. However, adding RF power to a (pulsed) DC sputtering process leads to an increase of arc events. A Pulse synchronization module (PSM), which connects the Pulse DC power generator and the RF power generator, was implemented to synchronize both pulsing patterns. The PSM can adjust the pulse-on and -off times (duty cycle) of both generators individually and optimize the sputtering process. This leads to a reduced arcing probability. The ITO films obtained are characterized by a very low resistivity and a high electron mobility. The ITO thin films were analyzed by UV-VIS and IR-spectroscopy as well as Hall-measurements.