High density indium bumping through pulse plating used for pixel X-Ray detectors

High density indium bump bonding is in high demand for devices which operate under cryogenic environments, such as pixellated X-ray detectors for high energy physics, due to the outstanding ductility of indium even at liquid helium temperatures. For these assembly applications, the connection pitch size is shifting to below 50 mum, such that the packaging density, i.e. I/Os, may exceed 40,000/cm². Electrodeposition is a promising approach to enable a low-cost and high yield bump bonding process, compared with conventional sputtering or evaporation which is currently utilized for small-scale production. Previous studies have shown the capability of electrodeposition to achieve high yield and high density indium bumps. The challenge exists to improve the bump height uniformity and consistency of electroplated indium bumps across the wafer at ultra-fine pitches with the highest yield. This paper is an initial investigation of the application of pulsed plating to the indium plating process and considers the influence of various current waveforms on the morphology and uniformity of the bumps. The results indicated that change in frequency and duty cycle did not have a significant influence on the indium bump morphology, but, together with the addition of a thief ring to the wafer design, pulse plating did have a noticeable impact on the bump height uniformity.