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A Bond Pad's View of Wirebonding
Keywords: wire bonding, bond pad, aluminum
Many IC bond pads must endure the stress of wirebonding, and these days it's copper (Cu) wirebonding. This study asks what stresses the bond pad experiences, and how do the bond pad films and features react? Mechanical stresses of wirebonding from the point of view of various bond pad structure are detailed, using a combination of experimental and finite element simulation data. The focus is on the traditional Al-SiO2 bond pads that have been in use in semiconductor products for decades. Cu wirebond tends to cause more stress than many bond pads were designed for. Factors that play a role in a bond pad's mechanical resilience to bonding are studied in detail, including pad aluminum (Al) thickness, barrier films thickness, intermetal dielectric (IMD) thicknesses, and sub-layer metallization thicknesses and patterning. A bond pad’s ability to dissipate both lateral and downforce stresses in the pad Al is important, and having a thick top IMD also improves its robustness to cracking. The challenge for many bond pads is that the films are too thin to provide this strength, as bond pads inherit design characteristics relating to the electrical requirements of the low cost, small die size IC. Bond pads may have very thin pad Al, thin IMD layers, circuitry under pad (CUP), and might even be 'pad anywhere' above circuitry. Methods to increase a bond pad's robustness to wirebonding stress are shown through experimental data and further confirmed by stress modeling techniques.
Stevan Hunter,
Idaho State University (ON Semiconductor)
Pocatello, ID

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