Method to improve reliability for flip-chip device for limiting pad design

A method of fabricating a solder bump, comprising the steps of:

• providing a UBM over a substrate having an exposed pad portion;
• the UBM in electrical contact with the pad portion;
• forming a first patterning layer over the UBM;
• the first patterning layer being comprised of a photosensitive material sensitive to light having a first wavelength;
• forming a second patterning layer over the first patterning layer;
• the second patterning layer being comprised of a photosensitive material sensitive to light having a second wavelength;
• the second wavelength not being the same as the first wavelength;
• selectively exposing the first patterning layer with the light having the first wavelength, leaving a first unexposed portion substantially centered over the pad portion between first exposed portions;
• selectively exposing the second patterning layer with the light having the second wavelength, leaving a second unexposed portion wider than, and substantially centered over, the first unexposed portion of the exposed first patterning layer;
• the second unexposed portion of the exposed second patterning layer being between exposed portions;
• removing the second unexposed portion of the exposed second patterning layer and the first unexposed portion of the exposed first patterning layer to form opening;
• the opening including a lower opening having substantially vertical walls within the exposed first patterning layer and an upper opening having substantially vertical walls within the exposed second patterning layer;
• forming a solder plug within the opening;
• removing the exposed portions of the exposed first patterning layer and the exposed portions of the exposed second patterning layer;
• and
• reflowing the solder plug to form a solder bump.

A method of fabricating a solder bump, comprising the steps of:

• providing a UBM over a substrate having an exposed pad portion;
• the UBM in electrical contact with the pad portion;
• forming a first patterning layer over the UBM;
• the first patterning layer being comprised of a photoresist material sensitive to light having a first wavelength;
• forming a second patterning layer over the first patterning layer;
• the second patterning layer being comprised of a photoresist or a dry film material sensitive to light having a second wavelength;
• the second wavelength not being the same as the first wavelength;
• selectively exposing the first patterning layer with the light having the first wavelength, leaving a first unexposed portion substantially centered over the pad portion between first exposed portions;
• selectively exposing the second patterning layer with the light having the second wavelength, leaving a second unexposed portion wider than, and substantially centered over, the first unexposed portion of the exposed first patterning layer;
• the second unexposed portion of the exposed second patterning layer being between exposed portions;
• removing the second unexposed portion of the exposed second patterning layer and the first unexposed portion of the exposed first patterning layer to form opening;
• the opening including a lower opening having substantially vertical walls within the exposed first patterning layer and an upper opening having substantially vertical walls within the exposed second patterning layer;
• forming a solder plug within the opening;
• removing the exposed portions of the exposed first patterning layer and the exposed portions of the exposed second patterning layer;
• removing the UBM not under the solder plug before reflowing the solder plug;
• and
• reflowing the solder plug to form a solder bump.