IC Bond Pad Structural Study by “Ripple Effect”

Abstract

IC bond pad structural reliability is studied for a variety of experimental pad designs in a 0.18um technology, having patterned metallization to simulate bond-over-active-circuitry (BOAC) situations in top-metal-minus-one and below. Underlying films deformation after wire bond is studied by optical microscopy after removal of the pad Al, with additional measurements by FIB. Pad designs in this study are rated for robustness to cracking according to an optical “ripple effect” deformation scale. “Ripple” is so named because it has a similar appearance to water ripples. It is due to non-uniform deformation in the underlying Al film(s) in the pad structure. Though the Al material is fully constrained within the SiO2 dielectric body, it is able to migrate plastically into local “hills and valleys” during bonding stress, with the top dielectric film bending in conformance. Cracks can then initiate in the undulating upper dielectric when its tensile strength is exceeded. “Ripple” in these pads is seen to vary depending upon the underlying metallization pattern and density as well as with wire bonding stress, for a fixed pad Al thickness. Traditional-style Al metallization pad structures with full metal plates are least robust, being the most prone to high “ripple” and cracks. Other pad structures more indicative of BOAC designs show varying degrees of improved robustness to cracking as shown by decreasing “ripple”. Results show that significant improvements in pad robustness to cracking are feasible in BOAC designs which include top-metal-minus-one routing in Al-metallization technologies, while providing increased process margin in wire bond, permitting efficient use of die area without extra processing or new issues.