Electrodeposition of chromium from trivalent chromium using modulated electric fields

Abstract

A layer of chromium metal is electroplated from trivalent chromium onto an electrically conducting substrate by immersing the substrate and a counter electrode in a electroplating bath and passing a modulated electric current between the electrodes. In one embodiment, the current contains pulses that are cathodic with respect to said substrate and in another embodiment the current contains pulses that are cathodic and pulses that are anodic with respect to said substrate. The cathodic pulses have a duty cycle greater than about 80%.

What is claimed is: 
     
         1 . A method for depositing a layer of chromium metal onto a substrate comprising:
 immersing an electrically conductive substrate in an electroplating bath containing trivalent chromium ions,   immersing a counter electrode in said plating bath,   
       passing an electric current between said substrate and said counter electrode, wherein
 said electric current is a modulated current comprising pulses that are cathodic with respect to said substrate, 
 said cathodic pulses have a duty cycle greater than about 80%. 
 
     
     
         2 . The method of  claim 1  wherein an interval of no electric current flow is interposed between said cathodic pulses. 
     
     
         3 . The method of  claim 1  wherein an interval of no electric current flow is interposed between said anodic pulses and cathodic pulses. 
     
     
         4 . The method of  claim 1  wherein an interval of no electric current flow is interposed between said cathodic pulses and succeeding anodic pulses and between said anodic pulses and succeeding cathodic pulses. 
     
     
         5 . The method of  claim 1  wherein said cathodic pulses and said anodic pulses succeed each other without intervening intervals of no electric current flow. 
     
     
         6 . The method of  claim 1  wherein said cathodic pulses and said anodic pulses form a pulse train having a frequency less than about 500 Hertz. 
     
     
         7 . The method of  claim 1  wherein said cathodic pulses form a pulse train having a frequency between about 100 Hertz and about 6000 Hertz. 
     
     
         8 . The method of  claim 1  wherein said cathodic pulses have a duty cycle of at least about 80%. 
     
     
         9 . The method of  claim 1  wherein said anodic pulses have a duty cycle of less than about 10%. 
     
     
         10 . The method of  claim 1  wherein said electroplating bath may additionally include a metal selected from the group consisting of copper, silver, gold, zinc, nickel, bronze, brass, and alloys thereof. 
     
     
         11 . The method of  claim 1  wherein a layer of metal of substantially uniform thickness is deposited on said surface. 
     
     
         12 . The method of  claim 1  wherein said plating bath includes chromium sulfate, ammonium sulfate, boric acid, formic acid, sodium n-dodecyl sulfate, chromium(II) chloride, and potassium hydroxide. 
     
     
         13 . The method of  claim 12  wherein the bath has a pH of about 2.5. 
     
     
         14 . The method of  claim 1  wherein the bath comprises: 
       
         
           
                 
                 
                 
               
                     
                     
                 
                     
                   Compound 
                   Approximate Range 
                 
                     
                     
                 
                     
                 
                 
                 
                 
                 
               
                     
                   Sodium Gluconate 
                   0 to 0.5 
                   mol/l 
                 
                     
                   Triton X 100 
                   0 to 1000 
                   ppm 
                 
                     
                   Citric Acid 
                   0 to 0.5 
                   mol/l 
                 
                     
                   400 Mw Polyethylene Glycol 
                   0 to 1000 
                   ppm 
                 
                     
                   Ethylenediaminetetraacetic acid 
                   0 to 1000 
                   ppm 
                 
                     
                   8000 Mw Polyethylene Glycol 
                   0 to 1000 
                   ppm 
                 
                     
                   Chrometan Powder (75% w/w 
                   100 to 300 
                   g/l 
                 
                     
                   chromium sulfate) 
                     
                     
                 
                     
                   Ammonium Sulfate 
                   25 to 500 
                   g/l 
                 
                     
                   Boric Acid 
                   5 to 40 
                   g/l 
                 
                     
                   Sodium n-dodecly sulfate 
                   0.01 to 1.0 
                   g/l 
                 
                     
                   Chromium (II) chloride 
                   0 to 1.0 
                   g/l 
                 
                     
                   Potassium Hydroxide 
                   15 to 50 
                   g/l 
                 
                     
                     
                 
             
                
                
                
               
               
                
               
            
             
                
                
                
                
                
                
                
                
                
                
                
                
                
                
               
            
           
         
       
     
     
         15 . The method of  claim 1  wherein the bath comprises: 
       
         
           
                 
                 
                 
               
                     
                     
                 
                     
                     
                   Approximate Range 
                 
                     
                   Compound 
                   (when present) 
                 
                     
                     
                 
                     
                 
                 
                 
                 
                 
               
                     
                   Sodium Gluconate 
                   0.05 to 0.2 
                   mol/l 
                 
                     
                   Triton X 100 
                   100 to 500 
                   ppm 
                 
                     
                   Citric Acid 
                   0.5 to 0.2 
                   mol/l 
                 
                     
                   400 Mw Polyethylene Glycol 
                   100 to 500 
                   ppm 
                 
                     
                   Ethylenediaminetetraacetic acid 
                   100 to 500 
                   ppm 
                 
                     
                   8000 Mw Polyethylene Glycol 
                   100 to 500 
                   ppm 
                 
                     
                   Chrometan Powder (75% w/w 
                   140 to 180 
                   g/l 
                 
                     
                   chromium sulfate) 
                     
                     
                 
                     
                   Ammonium Sulfate 
                   50 to 200 
                   g/l 
                 
                     
                   Boric Acid 
                   15 to 30 
                   g/l 
                 
                     
                   Sodium n-dodecly sulfate 
                   0.2 to 0.6 
                   g/l 
                 
                     
                   Chromium (II) chloride 
                   0.15 to 0.5 
                   g/l 
                 
                     
                   Potassium Hydroxide 
                   20 to 32 
                   g/l 
                 
                     
                     
                 
             
                
                
                
                
               
               
                
               
            
             
                
                
                
                
                
                
                
                
                
                
                
                
                
                
               
            
           
         
       
     
     
         16 . The method of  claim 6  wherein said cathodic pulses and said anodic pulses form a pulse train having a frequency of about 10 to 200 Hertz. 
     
     
         17 . The method of  claim 7  wherein said cathodic pulses form a pulse train having a frequency between about 200 Hertz and about 2000 Hertz. 
     
     
         18 . The method of  claim 16  wherein the duty cycle is about 85 to 95%. 
     
     
         19 . The method of  claim 17  wherein the duty cycle is about 85 to 95%.