Sequential electrodeposition of metals using modulated electric fields for manufacture of circuit boards having features of different sizes
A continuous layer of a metal is electrodeposited onto a substrate having both hydrodynamically inaccessible recesses and hydrodynamically accessible recesses on its surface by a twostep process in which the hydrodynamically inaccessible recesses are plated using a pulsed reversing current with cathodic pulses having a duty cycle of less than about 50% and anodic pulses having a duty cycle of greater than about 50% and the hydrodynamically accessible recesses are then plated using a pulsed reversing current with cathodic pulses having a duty cycle of greater than about 50% and anodic pulses having a duty cycle of less than about 50%.
1. A method for depositing a continuous layer of a metal onto a substrate having small recesses in its surface comprising:
immersing an electrically conductive substrate having a generally smooth surface having small recesses therein in an electroplating bath containing ions of a metal to be deposited onto said surface, said electroplating bath being substantially devoid of at least one member selected from the group consisting of levelers and brighteners,
immersing a counter electrode in said plating bath,
passing an electric current between said electrodes,
wherein
said electric current is a modulated reversing electric current comprising pulses that are cathodic with respect to said substrate and pulses that are anodic with respect to said substrate,
the charge transfer ratio of said cathodic pulses to said anodic pulses is greater than one, and
the on-time of said first cathodic pulses ranges from about 0.83 microseconds to about 50 milliseconds and the on-time of said first anodic pulses is greater than the on-time of said cathodic pulses and ranges from about 42 microseconds to about 99 milliseconds,
in a second electroplating step, passing a second modulated reversing electric current between said electrodes, wherein
said second modulated reversing electric current comprises second cathodic pulses and second anodic pulses,
said second cathodic pulses have a duty cycle greater than about 50% and said second anodic pulses have a duty cycle less than about 50%,
the charge transfer ratio of said second cathodic pulses to said second anodic pulses is greater than one, and
the frequency of said second pulses ranges from about 10 Hertz to about 5000 Hertz.
2. The method of claim 1 wherein an interval of no electric current flow is interposed between said cathodic pulses and succeeding anodic pulses.
3. The method of claim 1 wherein an interval of no electric current flow is interposed between said anodic pulses and succeeding 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, in said first electroplating step, 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, in said first electroplating step, said cathodic pulses and said anodic pulses form a pulse train having a frequency between about 50 Hertz and about 10000 Hertz.
7. The method of claim 1 wherein, in said first electroplating step, said cathodic pulses and said anodic pulses form a pulse train having a frequency between about 100 Hertz and about 6000 Hertz.
8. The method of claim 1 wherein, in said first electroplating step, said cathodic pulses and said anodic pulses form a pulse train having a frequency between about 500 Hertz and about 4000 Hertz.
9. The method of claim 1 wherein, in said first electroplating step, said cathodic pulses have a duty cycle of from about 30% to about 1%.
10. The method of claim 1 wherein, in said first electroplating step, said cathodic pulses have a duty cycle of from about 30% to about 15%.
11. The method of claim 1 wherein, in said first electroplating step, said cathodic pulses have a duty cycle of from about 30% to about 20%.
12. The method of claim 1 wherein, in said first electroplating step, said anodic pulses have a duty cycle of from about 60% to about 99%.
13. The method of claim 1 wherein, in said first electroplating step, said anodic pulses have a duty cycle of from about 70% to about 85%.
14. The method of claim 1 wherein, in said first electroplating step, said cathodic pulses have a duty cycle of from about 70% to about 80%.
15. The method of claim 1 wherein said metal is selected from the group consisting of copper, silver, gold, zinc, chromium, nickel, tin, lead, bronze, brass, solder, and alloys thereof.
16. The method of claim 1 wherein, in said first electroplating step, a layer of metal of substantially uniform thickness is deposited on said surface and within said hydrodynamically isolated recess.
17. The method of claim 1 wherein, in said first electroplating step, the thickness of the metal layer deposited within said hydrodynamically isolated recess is greater than the thickness of the metal layer deposited on said surface.
18. The method of claim 1 wherein, in said first electroplating step, said hydrodynamically isolated recess is substantially filled with metal.
19. The method-of claim 1 wherein said hydrodynamically inaccessible recess has at least one transverse dimension not greater than about 350 micrometers.
20. The method of claim 1 wherein at least one transverse dimension of said hydrodynamically inaccessible recess is from about 5 micrometers to about 350 micrometers.
21. The method of claim 1 wherein at least one transverse dimension of said hydrodynamically inaccessible recess is from about 10 micrometers to about 250 micrometers.
22. The method of claim 1 wherein at least one transverse dimension of said hydrodynamically inaccessible recess is from about 25 micrometers to about 250 micrometers.
23. The method of claim 1 wherein at least one transverse dimension of said hydrodynamically inaccessible recess is from about 50 micrometers to about 150 micrometers.
24. The method of claim 1 wherein in said first step said first cathodic pulses have a duty cycle less than about 50% and said first anodic pulses have a duty cycle greater than about 50%.
25. The method of claim 1 wherein in said first step said first cathodic pulses have an on-time and current density selected to produce electrodeposition under predominantly tertiary control and said anodic pulses have an on-time and current density selected to produce electroremoval of metal under predominantly primary and secondary control.
26. The substrate of claim 24 wherein said recesses are filled with metal.
27. The method of claim 1 wherein, in said second electroplating step, said cathodic pulses and said anodic pulses form a pulse train having a frequency between about 10 Hertz and about 4000 Hertz.
28. The method of claim 1 wherein, in said second electroplating step, said cathodic pulses and said anodic pulses form a pulse train having a frequency between about 10 Hertz and about 3000 Hertz.
29. The method of claim 1 wherein, in said second electroplating step, said cathodic pulses and said anodic pulses form a pulse train having a frequency between about 10 Hertz and about 1500 Hertz.
30. The method of claim 1 wherein, in said second electroplating step, said cathodic pulses have a duty cycle of from about 60% to about 99%.
31. The method of claim 1 wherein, in said second electroplating step, said cathodic pulses have a duty cycle of from about 70% to about 95%.
32. The method of claim 1 wherein, in said second electroplating step, said cathodic pulses have a duty cycle of from about 85% to about 95%.
33. The method of claim 1 wherein, in said second electroplating step, said anodic pulses have a duty cycle of from about 40% to about 1%.
34. The method of claim 1 wherein, in said second electroplating step, said anodic pulses have a duty cycle of from about 30% to about 5%.
35. The method of claim 1 wherein, in said second electroplating step, said cathodic pulses have a duty cycle of from about 85% to about 5%.