Selective Etching of SnO 2 :F Films with a Pulse Programmable Industrial Fiber Laser
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Selective Etching of SnO2:F Films with a Pulse Programmable Industrial Fiber Laser
M. Rekow1, T. Panarello1, W. S. Sampath2 1
EOlite Systems, Inc. 275 Rue Kesmark, Dollard-des-Ormeaux, Québec H9B 3J1, Canada NSF I/UCRC for Next Generation Photovoltaics, Colorado State University, Fort Collins, CO, USA
2
ABSTRACT In our work on laser scribing CdTe solar cells we have found what appears to be an unpublished laser material interaction that allows precise laser etching of SnO2 films to an arbitrary thickness with high uniformity. This precise and efficient laser etching mechanism allows arbitrary reduction of the film thickness in a controlled manner on the scale of tens of nm. In addition to the fine depth selection, we find that there develops a pulse duration dependent microstructure on the surface. This micro microstructure results in a strong diffraction effect in the visible portion of the spectrum. In this work we propose a physical mechanism behind this novel depth selective laser interaction as well as the resultant micro-structure. Finally we demonstrate and propose some possible applications for this process. INTRODUCTION Fluorinated SnO2 is one of the most common transparent conductive oxide (TCO) materials in use to today. Under the Pilkington brand name it is typically referred to as TEC series glass. Its primary use is in architecture glass as a heat reflector due to its low emissivity (Low-E) characteristics. Another large application is as an inexpensive substrate for CdTe solar cells and this material is under exploration for many energy production technologies. This SnO2:F film is typically deposited on soda lime glass in a layer from a few hundred Å to 1 μm in thickness depending on the application. To make a device based on these TCO films typically the film must be patterned by some means. For example, in the case of scribing for CdTe solar cells the film is removed in periodic thin strips along the length of the substrate to create a series of electrically isolated pads. Likewise other applications generally required patterning of electrically isolated structures. This work was originally aimed at demonstrating a novel way to perform the P1 scribe for CdTe solar cells. The intent was that the conductive layer could be removed while preserving the underlying insulating SiO2 and intrinsic SnO2 layers that exist as standard parts of the film deposition. These layers would then form a barrier to Na diffusion from the glass to the semiconductor. Our original work did show that the SiO2 layer was preserved, but subsequent tests indicated that it did not significantly reduce the rates of Na diffusion [1]. The aim of this research then shifted to understanding this novel ablation mechanism and identifying other potential applications. Examples could include direct writing of holographic images, writing passive electronic components, and creating structures for active transparent electronic devices and the like [2]. In addition to precise thickness reduction we observe pulse duration and beam polarizat
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