Electroless Ni Layer: Influence of Growth Steps and Annealing Temperature on NiSi Formation

Nowadays in the photovoltaic field, the research aim is to develop cheaper solar cells with efficiency more and more higher. For both the silicon and thin film solar cells, one of enhancement ways concerns the front-side collect lines. Traditionally made by screen-printed silver paste, the current development of these lines are focused on the reduction of the silver quantity inside the paste [1,2], the modification of the deposit techniques [3,4], the contact design [5] or several modifications in the same time [6], as for our work. Indeed, our objective is to replace silver lines by nickel/copper lines, respectively deposited by electroless and electrolytic processes.

From this method, the electroless nickel ensures a barrier role against the copper diffusion throughout the silicon, which would damage the solar cell efficiency. This nickel layer is first annealed to obtain specifically NiSi composite, necessary to guarantee the desired contact adherence and the low contact resistance (14 µΩcm). However, the thickness and the composition of this composite must be monitored.

This research work is focused on the nickel annealing under Ar/5% H₂ atmosphere. In a first part, the Ni coverage ratio as a function of the plating time and the influence of the initial Ni thickness on the diffusion depth throughout the silicon are studied. A second part is devoted to the importance of the annealing duration on the diffusion depth, including the sample cooling and the furnace extraction. Moreover, the composition of the Ni_xSi_y layer depends on the annealing temperature [7,8,9]. The NiSi film is obtained for a temperature between 350°C and 700°C. Therefore, in a final part, the impact of the annealing temperature on the Ni_XSi_Ycomposition is presented.

This work has been made on polished silicon substrate [100] and monitored using complementary surface equipments: SEM (imaging), EDS (bulk chemical analyses in surface and cross-section) and XPS (surface chemical information and composition profiling). Figure 1 shows an example of this analysis complementarity. SEM micrographs of a Ni layer deposited during 1 min are presented in surface (a) and on cross-section (b). Figure 1 c) shows the surface sample after annealing at 370°C under Ar/5%H₂atmosphere. EDS analyses in cross-section (d and e) prove the Ni diffusion throughout the Si substrate, generating the NiSi layer, and allow its thickness measurement. XPS profile analyses (f) confirms these results.

ACKNOWLEDGMENTS

This work has been carried out in the frame of the BIFASOL project supported by the “Agence Nationale de la Recherche” (ANR-11-PRGE-004). The authors would like to thanks INES (Institut National de l’Energie Solaire) for providing silicon substrates.

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