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.

Use of Wire Bonding to Study Bond Pad Damage from Wafer Probe

“…The deformed sub-layer Al films and bending SiO 2 from ball bonding often give the appearance of water ripples in a pond. The sharpest ripples illustrate where the stresses to the pad sub-layers were highest, with significant sub-layer Al films deformation and greatest potential for cracking of the top SiO 2 [5,6]. Figure 9 shows the large ripple associated with the traditional bond pads in this experiment.…”

“…Probe cracks from a particular tip of a particular probe card on the same pad structure can then be compared for visible damage after probe vs. after bonding. Only pad structures having top vias or high pattern density in the MT(-1) layer are expected to crack from the harsh probing [3,5]. Our expectation was that probe cracks, and even hidden damage from probing, may be "enhanced" by the bonding stress to become more easily detected.…”

“…Ripple is easily seen in dark field and lower magnifications, whereas cracking can be more difficult to observe, requiring high magnification. [5,6].…”