There is a lot of interest in plating indium bumps and other fine features onto wafers and other substrates. And the desired bump size and pitch is ever decreasing. Here's what you need to know.
When we selectively plate indium onto exact locations, the surface of the work-piece is typically masked (holes in the mask at the locations where the indium is to be plated onto the substrate). Because the desired size of the resulting indium bump is so small, the holes in the mask are, likewise, very small.
Since the indium sulfamate plating solution is an aqueous chemistry, the naturally-occurring surface tension of the solution limits the size of the hole that it will penetrate. Of course, if there is no penetration, there will be little or no contact of the plating solution with the desired locations on the work-piece. In such a case, the ability to plate indium at these locations is severely compromised if not prevented all together.
This white paper discusses a novel technique involving the use of ultrasonic energy to help the solution penetrate the holes in the mask and wet the surface of the work-piece. Of course, this technique could be used to help plate any work-piece where penetration of the solution into any sort of orifice or small opening is required.