Porosity and its Effect on Barrier Performance of Thin Electroless Cobalt Alloy Caps
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Porosity and its Effect on Barrier Performance of Thin Electroless Cobalt Alloy Caps Qingyun Chen1, Jun Liu2, Elizabeth Walker2, Richard Hurtubise1, Daniel Stritch1, and Xuan Lin1 1 R & D, Cookson Electronics/Enthone Inc., 350 Frontage Road, West Haven, CT, 06516 2 R & D, ATMI, 7 Commerce Drive, Danbury, CT, 06810
ABSTRACT Pinhole densities of electroless cobalt alloy films on copper substrate are characterized using optical and electrochemical methods. The impact of pits or pinholes on electroless cobalt alloy film barrier property is discussed. The improved film barrier property is shown by reduction of pit formation through deposition process optimization. INTRODUCTION Electroless cobalt alloys are proven to be superior candidate as the capping layer because of higher electromigration resistance, good barrier property and thermal stability, and possible reduction of effective k value of the dielectric stack1-3. However, film defectivity is still too high to meet the reliability specifications for future ULSI fabrication lines of 32 nm technology node and beyond. Continuity and uniformity of the cobalt alloy thin film as well as selectivity are critical for this application 4-8. Porosity is potentially one of the characteristic defects of an electroless cobalt deposition process, recognizing that the co-evolution of hydrogen during cobalt deposition can lead to entrapment of the hydrogen gas in the film. It is also observed that cobalt nucleates differently at copper facets and grain boundaries, which may cause higher porosity at the interfaces due to liquid entrapment during the process. Those pores can connect to each other and form a ìthrough holeî in the film, performing as a diffusion channel for copper to travel though, thus decreasing the barrier effectiveness. The through holes, e.g. pits or pinholes, can be enlarged during the following integration process steps such as dielectric deposition and resist strips, and eventually affect the barrier performance at greater level on devices. For future ULSI applications that require the thickness of electroless CoWP caps to be 10 nm or below, pits or pinhole formation must be suppressed in order to maximize the barrier performance of the film. The purpose of this study is to investigate the impact of pitting on the barrier property of electroless cobalt alloy caps, explore analytical methods for possible quantification of pinhole density, optimize the plating process to reduce pitting and maximize the barrier performance of the capping layers. EXPERIMENT Self-initiated electroless CoWBP films were deposited on copper wafer substrates using alkaline ion free baths with hybrid reducing agents such as dimethylamine borane and hypophosphorous acid. Before deposition of CoWBP films, copper substrates were pretreated in
an acid solution followed by deionized water rinse. Pd-activated electroless CoWP processes were also investigated. CoWP films were deposited on copper wafer substrates using alkaline ion free baths with hypophosphorous acid as a reducin
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