Grain growth and void formation in dielectric-encapsulated Cu thin films
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V. Kumar and K. Barmak Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213
K.R. Coffey Advanced Materials Processing and Analysis Center, University of Central Florida, Orlando, Florida 32826 (Received 23 January 2008; accepted 21 April 2008)
Grain growth in 40-nm-thick Cu films encapsulated by over- and under-layers of SiO2, Al2O3, Si3N4, and MgO was investigated. The films were magnetron sputter deposited onto cooled SiO2/Si substrates in an ultrahigh vacuum purity environment. Ex situ annealing was performed at 400 and 800 °C in 1 atm reducing gas. Films deposited at −120 °C exhibited more extensive grain growth after annealing than films deposited at −40 °C. Films annealed at room temperature had grain sizes less than 35 nm. All films exhibited some void formation after annealing at 400 and 800 °C, but the films encapsulated in Al2O3 exhibited the lowest area fraction of voids. The mean grain sizes of the Al2O3-encapsulated films, as measured by the linear intercept method, were 86 and 134 nm after annealing at 400 and 800 °C, respectively.
I. INTRODUCTION
In recent years, Cu has been the conductor of choice for interconnects in complementary metal–oxide semiconductor (CMOS) technology because of its low resistivity and longer electromigration life time.1 However, the resistivity of Cu thin films increases greatly as film thickness is reduced to (and below) the mean free path of conduction electrons. For Cu, this mean free path is 39 nm at room temperature. The widely observed increase in resistivity has been attributed to four mechanisms for increased electron scattering, namely, impurity scattering, surface scattering from a flat interface, surface scattering from a rough interface, and grain boundary scattering.2 A number of investigations have focused on grain growth as a means to reduce interconnect resistivity.3–5 Grain growth in thin films, induced by annealing, is driven by surface and elastic strain energy reduction, in addition to grain boundary energy reduction.6–9 Also, it has been shown that deposition at reduced temperatures
II. EXPERIMENTAL PROCEDURE
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0254 J. Mater. Res., Vol. 23, No. 7, Jul 2008
http://journals.cambridge.org
can result in a larger postdeposition grain size. For example, a 40% enhancement in room temperature grain growth is seen for films deposited at −40 °C.3 However, it has also been observed that grain size is limited by film thickness, a phenomenon known as the specimen thickness effect.10 When films reach this limiting grain size, further increases in annealing time and temperature do not promote grain growth but instead result in void formation and film agglomeration.11,12 Agglomeration is a thermally activated process, and, as in grain growth, it results in the reduction of surface energy, interfacial energy, and strain energy within the film.13 Film defects, such as grain boundaries14 and pinholes15 can act
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