Planarization of Fluorine-Doped Tin Oxide for Improved Thin Film CdTe Device Performance

Fluorine-doped tin oxide (FTO) layer is an important transparent conductive oxide (TCO) for thin film photo-voltaics devices fabricated in superstrate configuration. It is used at gigawatt-scale in the manufacturing of thin film CdSeTe/CdTe devices. Commercial FTO is deposited on glass by a highly efficient Atmospheric Pressure Chemical Vapour Deposition process. The resulting FTO films are relatively rough. There is a current focus on the development of thin transparent buffer layers to increase device performance. Intrinsic Tin oxide and Zinc oxide and their alloys are of particular interest. Thin films of these materials are usually deposited by magnetron sputtering and are typically less than 100nm thick. There are concerns that such thin films will not be uniform over the roughness of the FTO. In this paper, we describe how the FTO can be planarized using a chemical-mechanical polishing technique. The process does not introduce microstructural damage to the FTO surface as determined by Transmission Electron Microscopy. The process can also be sufficiently controlled to cause minimal change to the sheet resistance of the FTO. The resulting smooth FTO allows uniformly thin and conformal buffer layers to be obtained by sputter deposition.