Metal lift failure modes during fine pitch wire bonding low-k devices with Bond Over Active (BOA) design

Abstract

The size of IC device has been reduced resulting from the advancement of silicon fabrication technology in reducing the transistor gate length. For wirebonded devices with high IO count, the final die size is principally determined by the size and layout configuration of the IO cells and wire bond pads. Traditional design of wire bond pads would consist of a top metal layer with no active circuitry or back-end-of-line (BEOL) interconnecting routing underneath the metal bond pad. The keep-out zone underneath the wire bond pad is typically filled in with back-end-of-line tiling for fabrication planarity and/or structural robustness. As BEOL metal layers increases, this keep-out zone represents white space on silicon and becomes a huge penalty against die size reduction. Bond Over Active (BOA) technology enables the use of the keep-out zone and moves active devices, ESD circuitry, power and ground busses underneath the wire bond pads. Finer pitch wire bonding capability for smaller wire bond pad dimensions and tighter pitches further enables IO cell resizing and optimization for die size reduction. The combination of BOA technology, fine pitch wire bonding and low-k / copper technology have thus become a new development scope for wire bonding technology development works. Several new metal lift failure modes were reported during wire bonding or during wire bonding destructive tests such as ball shear and wire pull. The Aluminum Cap Lift failure mode denotes the delamination between the copper bond pad and the Tantalum barrier layer of the aluminum cap bonding pad region. The Copper Metal Lift failure mode describes the delamination between BEOL dielectric layer of FSG (fluorine-doped silicon glass) and the Tantalum barrier layer in the BEOL stacks. The specific delamination interfaces for Aluminum Cap Lift and Copper Metal Lift failures were determined by several failure analysis techniques. Some of these techniques are standard failure analysis tools such as TEM (Transmission Electron Microscope) and Auger depth profiling, and others are more dedicated such as nano-indentation. The last failure mode called Tilted Metal Lift was also observed during wire pull test as the ball bond diameter was reduced for fine pitch wire bonding. Multiple layers are pulled off and the break is deeper in the area closer to the die edge than to the die core region. Root cause analysis for each failure mode and its corresponding corrective actions will be disclosed in this paper.