Atmospheric pressure plasma reduction of copper oxide to copper metal
Copper oxide films can be reduced to copper with an atmospheric pressure argon and hydrogen plasma at temperatures between 25 and 300° C. A 50-nm-thick CuO layer on a Cu-coated Si wafer, 200 mm in diameter, can be fully reduced by the plasma in 200 seconds at 200° C. The activation energy for the reaction can be approximately 3.7 kcal/mol. X-ray photoelectron spectroscopy can show the copper oxide reduced to metallic copper.
1 . A method of producing an ionized gas plasma comprising:
directing gas flow comprising an inert gas and hydrogen from an inlet through a flow path within a housing to an outlet for plasma comprising reactive hydrogen species;
directing the gas flow within the housing between a powered electrode surface of a power electrode and a grounded electrode surface of a ground electrode, the grounded electrode surface closely spaced from the powered electrode surface;
delivering radio frequency power coupled to the power electrode and the ground electrode from a power supply to ionize the gas flow and produce the plasma comprising the reactive hydrogen species from the hydrogen; and
disposing a copper material within an enclosure near the outlet of the housing to receive the reactive hydrogen species;
wherein copper oxide on the copper material is removed by reaction with the reactive hydrogen species to produce copper metal and water vapor.
2 . The method of claim 1 , wherein the outlet of the housing for plasma comprises a linear opening.
3 . The method of claim 2 , wherein the linear opening is at least as wide as the copper material and the copper material is passed at a constant speed relative to and contacting the plasma comprising the reactive hydrogen species.
4 . The method of claim 1 , further comprising translating the housing with the outlet for the plasma comprising the reactive hydrogen species relative to a surface of the copper material such that the entire surface of the copper material is uniformly treated with the reactive hydrogen species from the plasma.
5 . The method of claim 1 , wherein the radio frequency power is delivered at 13.56 or 27.12 MHz.
6 . The method of claim 1 , wherein the power supply includes an auto-tuning network that impedance matches the radio frequency power supply to the plasma to minimize reflected power.
7 . The method of claim 1 , wherein the hydrogen gas is added to the gas flow at a concentration between 0.1 to 5.0 volume % and a fraction of the hydrogen gas dissociates into atoms inside the plasma, and then flows out of the outlet.
8 . The method of claim 1 , wherein the inert gas is selected from the group consisting of argon and helium.
9 . The method of claim 1 , wherein the copper material is a lead frame strip.
10 . The method of claim 1 , wherein the copper material is used in bonding an integrated circuit to a package.