For Better Contacts Special Plasma Cleans Copper and Silver Surfaces

An atmospheric pressure plasma process studied at Innovent e.V. has been proven to reduce oxide and sulfide layer components on copper and silver surfaces...

Copper and silver are well known for their high electrical conductivity and are used in numerous electronic applications. However, oxide or sulfide layers that form in air can interfere with processing as they impair electrical conductivity and solderability. Unfortunately, the removal of these disruptive layers with conventional methods is rather labor-intensive. Wet chemical processes require the use of strong chemicals, and thermal reduction methods operate at elevated temperatures, resulting in energy-intensive conditions. As for so-called low-pressure plasmas, they require vacuum infrastructure. Against this backdrop, researchers from Innovent Technology Development in Jena have investigated the extent to which such surface layers can be selectively reduced using a plasma that operates under atmospheric pressure.

Atmospheric Pressure Plasma with Forming Gas under Scrutiny

The focus of the investigations, as further reported from Jena, was an atmospheric pressure plasma jet of the Tigres CAT600 type, operated with forming gas consisting of 95 percent nitrogen and 5 percent hydrogen. The research aimed to determine whether it is suitable for the reduction of oxidized copper layers and silver sulfide deposits on silver layers. For this purpose, defined metal layers were prepared on glass, silicon wafers, and plasma-chemically oxidized (PCO) aluminum substrates, which were then selectively oxidized or sulfided. The copper oxide layers were produced by thermal treatment in air, while the silver sulfide layers were generated through controlled exposure to in situ formed hydrogen sulfide. The results show that both copper oxide and silver sulfide layers can be effectively reduced using the atmospheric pressure plasma system.

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Drastic Reduction of Oxide and Sulfide Layers Confirmed

So-called ellipsometric measurements on the copper samples demonstrate, for example, that the thickness of the oxide coating layer can be reduced from around 95 to approximately six nanometers. At the same time, the electrical sheet resistance returned to the initial level of the metallic copper layer, as emphasized. On the plasma-treated surface, the dark oxide layer was thus removed, revealing a metallic, shiny, pink copper surface again. A similar behavior was observed in the silver samples. The previously created silver sulfide layers, with thicknesses of up to approximately 70 nanometers, were significantly reduced by plasma treatment. They decreased to seven nanometers, it is stated. XPS analyses further confirmed the chemical transformation of the surface. After treatment, the sulfur content significantly decreased while the silver content increased. The restoration of a functional silver surface was thus proven both visually and analytically.