Indium electroplating baths for thin layer deposition

Abstract

Indium (In) electroplating solutions which are used to deposit compositionally pure, uniform, substantially defect free and smooth In films with near 100% plating efficiency and repeatability. In one embodiment the plating solution includes an In source, citric acid and its conjugate pair salt and a solvent. At a pH value of below 4.0, sub-micron thick In layers with close to 100% purity at close to 100% plating efficiency are produced. Such In layers are used in fabrication of electronic devices such as thin film solar cells.

1 . An electroplating solution for application of a substantially pure indium (In) film onto a conductive surface at high plating efficiency, comprising:
 a solvent;   an In source providing In ions to the solvent with an In ion concentration of at least about 0.05 M; and   a weak acid and a conjugate pair salt of the weak acid,
 wherein the pH value of the electroplating solution is below about 4.0, 
 and wherein the weak acid is selected from the group consisting of citric acid, acetic acid, formic acid, thioacetic acid, glycolic acid, lactic acid, ascorbic acid, malic acid, butanoic acid, and pentanoic acid. 
   
     
     
         2 . The solution of  claim 1  wherein the weak acid is citric acid and the pH value is below 3.5. 
     
     
         3 . The solution of  claim 2 , wherein the In source comprises at least one of In-chloride, In-sulfate, In-acetate, In-carbonate, In-nitrate, In-perchlorate, In-phosphate, In-oxide and In-hydroxide. 
     
     
         4 . The solution of  claim 3 , wherein the conjugate salt is at least one of sodium citrate, lithium citrate, potassium citrate, and an organically modified citrate, wherein the organically modified citrate comprises a citrate moiety such that one or more organic groups replaces (a) one or more hydrogens that are directly connected to a carbon or oxygen bonded to carbon  2  of the citrate moiety, or (b) the hydroxyl group bonded to carbon  2  of the citrate moiety. 
     
     
         5 . The solution of  claim 2 , wherein the concentration of the In ions is at least 0.1M. 
     
     
         6 . The solution of  claim 5 , wherein the pH value is below 2.5. 
     
     
         7 . The solution of  claim 5 , wherein the solvent comprises water. 
     
     
         8 . The solution of  claim 2  such that it comprises no organic additives. 
     
     
         9 . The solution of  claim 1 , wherein the solvent comprises water. 
     
     
         10 . The solution of  claim 1 , wherein the In source comprises at least one of In-chloride, In-sulfate, In-acetate, In-carbonate, In-nitrate, In-perchlorate, In-phosphate, In-oxide and In-hydroxide. 
     
     
         11 . A method of obtaining a substantially pure and substantially defect-free In film on a surface of a conductor comprising the steps of:
 providing a solution with a pH value below 4 that includes a solvent, an In source providing In ions to the solvent with an In ion concentration of at least 0.05 M, citric acid and a conjugate pair salt of citric acid;   applying the solution onto an anode and the surface of the conductor,   establishing a potential difference between the anode and the conductor, and   electrodepositing the substantially pure In film on the surface of the conductor.   
     
     
         12 . The method of  claim 11  wherein the surface of the conductor comprises copper. 
     
     
         13 . The method of  claim 11  wherein the surface of the conductor comprises gallium. 
     
     
         14 . The method of  claim 11  further comprising the step of controlling the temperature of the solution within the range of 10-60° C. before the step of electrodeposition and wherein the In source comprises at least one of In-chloride, In-sulfate, In-acetate, In-carbonate, In-nitrate, In-perchlorate, In-phosphate, In-oxide and In-hydroxide. 
     
     
         15 . The method of  claim 14  wherein the In source comprises at least one of In-chloride, In-sulfate, In-acetate, In-carbonate, In-nitrate, In-perchlorate, In-phosphate, In-oxide and In-hydroxide. 
     
     
         16 . The method of  claim 15  wherein the conjugate pair salt of citric acid is at least one of sodium citrate, lithium citrate, potassium citrate, and an organically modified citrate.