Deposition and characterization of thin electroless palladium films from newly developed baths

Thin palladium films of about 100 nm thickness have been successfully deposited on hydrogenated amorphous silicon using an electroless deposition process from palladium-ammine baths. The d.c. magnetron reactive sputtered 18% hydrogenated amorphous silicon (a-Si:H) possessed a hydrogen-passivated surface, necessitating an activation step prior to the electroless deposition to obtain a film with good uniformity. The specially prepared hypophosphite-based dilute metal ion bath exhibited good stability at low operating temperatures of 35-50 °C. The composition and microstructure of the active palladium aggregates were characterized by transmission electron microscopy and energy-dispersive X-ray spectroscopy, while the palladium films deposited from the citrate, NH ₃ NH ₄ Cl, ethylnediamminetetraacetic acid (EDTA) and triethyleneammine (TEA) baths were examined by scanning electron microscopy. In the activation step, marked differences in morphology and distribution of the palladium aggregates on the activated a-Si:H, a-Si, and crystalline silicon substrates were observed and discussed. The electroless palladium films with higher coverage from the NH ₃ NH ₄ Cl, EDTA, and TEA baths were more uniform and crack free, while those from the citrate bath suffered from damage due to incorporated hydrogen, resulting in bare spots, microcracks and even local a-Si film peeling. Auger electron spectroscopy revealed that codeposited phosphorus varied from a maximum of 14 at.% in the EDTA bath films to a minimum of 2-3 at.% in the citrate and TEA bath films.