IC Bond Pad Structural Study by “Ripple Effect”

摘要

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-oneand below. Underlying films deformation after wire bond is studied by optical microscopy after removal of the padAl, with additional measurements by FIB. Pad designs in this study are rated for robustness to cracking accordingto an optical “ripple effect” deformation scale. “Ripple” is so named because it has a similar appearance to waterripples. It is due to non-uniform deformation in the underlying Al film(s) in the pad structure. Though the Almaterial is fully constrained within the SiO_2 dielectric body, it is able to migrate plastically into local “hills andvalleys” during bonding stress, with the top dielectric film bending in conformance. Cracks can then initiate in theundulating upper dielectric when its tensile strength is exceeded. “Ripple” in these pads is seen to vary dependingupon the underlying metallization pattern and density as well as with wire bonding stress, for a fixed pad Althickness. Traditional-style Al metallization pad structures with full metal plates are least robust, being the mostprone to high “ripple” and cracks. Other pad structures more indicative of BOAC designs show varying degrees ofimproved robustness to cracking as shown by decreasing “ripple”. Results show that significant improvements inpad robustness to cracking are feasible in BOAC designs which include top-metal-minus-one routing in Almetallizationtechnologies, while providing increased process margin in wire bond, permitting efficient use of diearea without extra processing or new issues.