Pulsed Laser Planarization of Metals for IC Interconnect
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PULSED LASER PLANARIZATION OF METALS FOR IC INTERCONNECT
Robert J. Baseman, Tung-Sheng Kuan, M. Osama Aboelfotoh, and Joseph C. Andreshak, IBM Research Division, IBM T.J. Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598, Frank E. Turene, IBM General Technology Division, Hopewell Junction, NY 12533, Rosemary A. Previti-Kelly and James G. Ryan, IBM General Technology Division, Essex Junction, VT 05452.
Abstract We consider the feasibility of using an excimer laser to planarize copper deposited over polyimide. Even though the polyimide is thermally sensitive, a 2.8 minthick copper film, deposited over polyimide, was 2 planarized with 248 nm, 25 ns, laser fluences from 1.7 to 2.6 J/cm . Although planar surfaces were readily produced, a variety of defects were observed in the underlying structures. The defects observed include stress induced fractures of the interconnect structure, polyimide surface roughening, and changes in the electrical characteristics of the polyimide. We also describe cross sectional TEM images of cavities formed in interlayer contacts during laser planarization that support recent descriptions of the surface tension driven flow. Excimer Laser Planarization of Copper Deposited over Polyinside Tuckerman initially demonstrated the use of pulsed lasers to planarize thin metal films deposited for integrated circuit interconnect structures. [1] The technique is extremely simple and effective. Briefly, following the deposition of a metal layer by a standard technique, the metal layer is melted momentarily by a short laser pulse. While molten, or nearly so, the high surface tension of the metal, given its relatively low viscosity, rapidly drives the metal to seek a planar metal surface. A number of groups have subsequently explored pulsed laser planarization of gold, Al, Cu, and several Al alloys, deposited with or without barrier. layers, on SiOQ. [2 - 10] New materials for interconnect, in particular, organic insulators and low resistivity metals, are frequently considered for use in integrated circuits in light of increasing integrated circuit performance and manufacturability demands. Organic insulators, such as polyimide, have attractive planarizing abilities, and have lower dielectric constants than SiO 2, which reduce parasitic capacitances and signal delays. Metals with resistivities lower than aluminum, most notably copper, may also improve device performance. A practical laser planarization process requires favorable chemical and physical interactions between the metal being melted and the underlying materials. Presumably, some chemical interaction is desired to promote wetting, but not so much as to lead to extensive chemical reactions between metal and underlying materials. Films may delaminate during rapid heating or cooling , or after solidification due to the incorporation of large stresses, if they have large differences in thermal coefficients of expansion. Underlying materials may degrade thermally. Although organic insulators are attractive for a variety of reasons, u
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