Contact Resistance Measurements for Electrodes on Silicon Prepared by Autocatalytic Electroless Metallization Using Metal Nanoparticles

We reported the adhesive metal-film formation by autocatalytic electroless metallization on silicon (Si) by using gold (Au) nanoparticles¹⁾. Autocatalytic electroless deposition, which is a conventional method to metalize nonmetallic substrates, has several advantages including simplicity of process, uniformity of films, and the covering of complicated structures. So, the process is expected to replace sputtering of back metal of power devices and screen-printing of thick film electrodes of solar cells. In this study, we determined contact resistance (R_c) between electrolessly deposited metal electrode and Si semiconductor by using TLM (transfer length model) method shown in Fig.1.

Figure 2 shows photo and dimension of test sample, and Fig.3 shows schematic process flow of preparing two test samples; 1. Photoresist pattern formation: 2. Au nanoparticles formation by displacement reaction using a mixture solution of HAuCl₄ and HF: 3. Autocatalytic electroless deposition of nickel-phosphorus alloy (Ni-P) film for sample (A), and: 4. Electron-beam silver (Ag) deposition on electrolessly deposited Ni-P film for sample (B). The thickness of Ni-P films was 0.25 mm and Ag thickness on Ni-P film was 1.15 mm.

We measured R_t of samples and determined ρ_c. R_t and ρ_c are given by

R_t = 2R_m + 2R_c+ R_s

ρ_c  = R_m + R_c.

Figure 4 shows TLM result of samples (A) and (B). The values of ρ_c were 140 mΩ cm² in (A) and 13 mΩ cm² in (B). These results indicate that the ρ_c is mainly determined by resistance of bulk metal (R_m) and the contact resistance (R_c) of these samples is lower than 13 mΩ cm². The R_c of screen-printed-and-fired Ag electrodes of crystalline Si solar cells was reported as 2~3 mΩ cm² lower than ρ_c of sample (B)²⁾. Contact resistance (R_c) is important for devices rather than ρ_c³⁾. The R_c of electrode prepared by the autocatalytic electroless metallization using Au nanoparticles is approaching the value of conventional electrodes without any heat treatment. We also review interface structure and contacting mechanism between Au nanoparticles and Si semiconductor⁴⁾.

ACKNOWLEDGEMENTS

The present work was partly supported by JSPS KAKENHI (23560875) and A-STEP from JST.

REFERENCE

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