Low Temperature Oxidation and Spalling of Electrodeposited and Vapour-Deposited Copper
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ABSTRACT Thermal cycling of silicon solar cells heated with incandescent light or using an in-situ heating stage on an optical microscope caused unexpectedly rapid oxidation of the electrodeposited copper current collecting grid. Unusually fine buckling and spalling of the oxide film also occurred at temp. 0 as low as 180 C. These effects appear to stem from the very fine columnar grains and high defect content of the copper ribbons and some Cl residue from the electrolyte. With vapour-deposited Cu, similar relatively rapid oxidation and spalling were also observed. Origins of the high oxidation rates, the large compressive stresses within the oxide and the lowtemperature spalling are discussed. INTRODUCTION Electroplating, vapour-deposition and screen-printing of Cu are used extensively in the manufacturer of semiconducter chips, circuit boards and electronic devices. Any subsequent oxidation of the thin films and ribbons of Cu can degrade circuit characteristics or device reliability. During studies of the thermal fatigue of silicon solar cells [1], unexpected and rather striking oxidation of the surface of the electroplated Cu current collecting grid was noted. The probable reasons for rapid oxidation at low temperature have been studied in greater detail with relevance to possible deterioration of electronic devices or components during processing or in service. EXPERIMENTAL METHODS The initial experiments were performed on single crystal silicon, dendritic web solar cells produced by Westinghouse. During manufacture a Cu current-collecting grid and a contact pad 10 um in thickness are applied to the front (i.e. active) surface of these cells by electroplating onto a grid formed by evaporating Ti and then Pd through a mask (the Cu grid arrangement may be noted in Fig. 1). The Pd serves as a Cu diffusion barrier and the Ti provides good adhesion to the Si. TiO2 is then applied to the Si as an anti-reflection coating. 0 Some of the solar cell samples were heated to 180-200 C with an incandescent lamp and allowed to cool to room temperature in 10 minute cycles. They were examined by optical or scanning electron microscopy after 0 1 to 20 cycles. Other samples were examined in-situ during heating to 300 C others were heated using a heating stage on an optical microscope; still isothermally for various periods in an oven and examined by light microscopy Similar studies were carried out on vapour-deposited Cu or with an SEM. The films, 0.5 - lum thick, on glass or single crystals of silicon. composition of the electroplated Cu on one solar cell was determined by Auger spectroscopy.
Mat. Res. Soc. Symp. Proc. Vol. 54. 1986 Materials Research Society
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OXIDIATION AND SPALLING OF CU GRID ON SI SOLAR CELLS The surface oxide on an electroplated Cu current collecting grid after 20 cycles under a heat lamp is shown in Fig. 1. A variety of forms of oxide delamination and buckling occur, with the sinusoidal "worms" being the most characteristic. The sinusoidal buckles form by growth from a few isolated origins. Occasion
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