Method to achieve robust solder bump height

A process for solder bump formation, comprising:

• on an upper surface of a semiconductor wafer, depositing, and then sputter cleaning, a seed layer;
• applying a layer of negative photoresist, having a first thickness, on said seed layer and then exposing said layer of photoresist through a first pattern that defines a first mold having a first inside diameter and a floor;
• developing said layer of photoresist in such a manner as to form said first mold while leaving a layer of unexposed photoresist, having a second thickness that is less than said first thickness, fully covering said floor;
• then exposing said layer of unexposed photoresist through a second pattern that defines a second mold located inside said first mold and having a second inside diameter that is less than said first inside diameter;
• then fully developing all photoresist, thereby forming a final mold;
• by means of electroplating, growing a layer of UBM and then a layer of solder inside said final mold until said final mold is filled with solder;
• removing all photoresist, thereby forming a solder bump having the form of a cylinder, having said first diameter, coaxially disposed above a cylinder having said second diameter;
• then removing all exposed UBM;
• and
• heating said wafer whereby said solder bump assumes the shape of an oblate spheroid whose long axis extends vertically upwards from said upper surface.