Wire bonding to advanced copper, low-K integrated circuits, the metal/dielectric stacks, and materials considerations

Abstract

There are three areas to consider when designing/implementing wire bonding to advanced ULSI damascene-copper chips having copper metallization and low dielectric-constant polymers embedded beneath them (Cu/LoK). These are: 1) the copper-pad top-surface oxidation inhibitor coatingmetal/organic/inorganic. (Current work involves evaluating the metal and inorganic options); 2) the low dielectric constant materials available; 3) under-pad metal/polymer stacks and support structures necessary for bondability and reliability. There are also various polymer/metallurgical interactions, resulting in long term packaged-device reliability problems, that can occur as the result of the wire bonding process over low modulus, LoK materials with barriers. These include cracked diffusion barriers, copper diffusion into the LoK polymers, cracking/spalling/crazing of the LoK materials, and bond pad indentation ("cupping"). Low-K polymer materials, with high expansion coefficients and low thermal conductivities, can also increase the stress and further extend any existing damage to barriers. Many of the above problems have previously been encountered when bonding to pads over polymers (MCM-D, polymer buildup-layers on PCBs, PBGAs, flex circuits, etc.), and they share some of the same solutions. Well designed LoK and the underpad structures should have no negative effect on bonding parameters and be invisible to the bonding process.

Facing the challenge of designing for Cu/low-k reliability

“…As mentioned before, in the CMOS090 process, copper replaces aluminium and bonding to copper is not an easy thing [15]. In order to fully solve the problems of wire bonding to IC's with copper bondpads, it is necessary to protect the top surface of the Copper from oxidation and sulphating.…”

Characterization of ag nanofilm metallization on copper chip interconnect and its ultrasonic bondability

“…Several methods have been emerged to protect the Cu pad, such as the inert gas shielding, the plasma cleaning and the top surface protective coatings, which is applied most widely. A review on the various form of coating to enhance the bondability of copper pad surface by Harman and Johnson indicated three types of surface coatings, namely metallic top surface, inorganic film and organic layer [5].…”

The Feasibility of Au Ball Bonding on Sn-Plated Cu

“…4 Harman et al 5 used thin inorganic films such as SiN, SiO 2 , and Al 2 O 3 deposited over the Cu surface to successfully prevent oxidation. However, due to the brittle and hard nature of those inorganic films, cracking in the chip might not be reduced, and bond quality is reduced due to the amount of nonmetallic material at the interface.…”