Effect of Working Distance on Properties of Sputtered Molybdenum Films
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Effect of Working Distance on Properties of Sputtered Molybdenum Films Shirish A. Pethe1, Ashwani Kaul1, Neelkanth G. Dhere1 1 Florida Solar Energy Center, University of central Florida, 1679 Clearlake Road, Cocoa, FL 32922, U.S.A. ABSTRACT Molybdenum back contact deposition is a bottleneck in high volume manufacturing due to the current state of art where multi layer molybdenum film needs to be deposited to achieve the required properties. In order to understand and solve this problem experiments were carried out. The effect of working distance (distance between the target and the substrate) on film properties was studied and is presented in this work. Earlier work carried out at Florida Solar Energy Center reflected on the effect of the sputtering power and working gas pressure on the film properties. This work is continuation of that effort in understanding effects of various sputtering parameters and determining the possible route to develop single layer molybdenum films with the required properties of near zero stress, low resistivity and good adhesion to substrate. INTRODUCTION With the increasing cost of electricity from conventional sources and the effect of global warming, it is of paramount importance to shift to renewable energies to meet the global energy demand. Photovoltaic (PV) is the most promising of all the renewable technologies. Among the various PV technologies such as c-Si, a-S:H, CdTe and copper-indium-gallium-selenide-sulfide (CIGS), CIGS absorber based heterojunction solar cell has the greatest potential for cost reduction with respect to power generation efficiency. The reported highest efficiencies of CIGS solar cell and module are 20.3% and 16.7% respectively [1, 2]. Several candidates have been experimented with for ideal back contact material for CIGS thin film solar cells. Molybdenum is found to be the most appropriate back contact material because of its inertness and high conductivity. Generally, the molybdenum film is deposited using DC magnetron sputtering. It is well known that sputtering process parameters such as sputtering power, working gas pressure and working distance i.e. target to substrate distance control the properties of sputter-deposited Mo thin films [3, 4, 5]. For example, films deposited at lower working gas pressures generally have poor adherence to the substrate, higher conductivity, and are under compressive stress, whereas those deposited at higher gas pressures tend to have good adherence, high resistivity, and are under tensile stress. Hence, it is very difficult to achieve a well adherent and low resistive single Mo thin film layer, despite the fact that both characteristics are essential for stable and highly efficient solar cell fabrication. The state of art technology solves this problem by deposition of multi-layer molybdenum back contact. However, because a multi-layer process requires several deposition steps, it is not an ideal process in terms of both time and cost. The motivation of this investigation is to determine the influence of DC magnetron sp
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