NOVELTY - The method comprises plasma oxidizing (506) a back surface of a p-type substrate to form an oxidation layer on the back surface, forming a silicon nitride layer on the oxidation layer, forming a back metal layer on the silicon nitride layer, and depositing an anti-reflection coating layer on a front surface of the substrate. The substrate has a first type of doping atom on the back surface of the substrate and a second type of doping atom on the front surface of the substrate. The plasma oxidization of the back surface comprises exposing the substrate to plasma formed from a gas mixture.    USE - The method is useful for forming a composite passivation layer on a substrate of a solar cell.    ADVANTAGE - The method is capable of economically and efficiently forming the composite passivation layer on the substrate of the solar cell with good passivation film property, reduced film leakage and high film quality and without damage or contamination of the solar cells thus improving the conversion efficiency, photocurrent characteristics and electrical performance of the solar cell.    DETAILED DESCRIPTION - The method comprises plasma oxidizing (506) a back surface of a p-type substrate to form an oxidation layer on the back surface, forming a silicon nitride layer on the oxidation layer, forming a back metal layer on the silicon nitride layer, and depositing an anti-reflection coating layer on a front surface of the substrate. The substrate has a first type of doping atom on the back surface of the substrate and a second type of doping atom on the front surface of the substrate. The plasma oxidization of the back surface comprises exposing the substrate to plasma formed from a gas mixture containing an oxygen containing gas and/or a nitrogen containing gas, and cleaning (504) the back surface of the substrate with hydrogen fluoride or hydrofluoric acid before plasma oxidization of the substrate. The oxidation layer has a thickness of less than 150 Angstrom . The oxidation layer and the silicon nitrogen layer are formed in in-site in a single processing chamber and a decoupled plasma chamber, and form a negatively charged passivation layer on the back surface of the substrate.    DESCRIPTION OF DRAWING(S) - The diagram shows a flow chart of a method to form a passivation layer of a solar cell.    Providing a substrate having solar cell (502)    Cleaning a back surface of the substrate (504)    Plasma oxidizing the back surface of the substrate (506)    Forming a second passivation layer. (508)