The Effect of Tungsten on the Nucleation and Growth of Thin Aluminum Alloy Films
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Motorola Inc., Materials Research and Strategic Technologies, Mesa, AZ *Arizona State University, Dept. of Chemical, Bio and Materials Engineering, Tempe, AZ ABSTRACT A variety of alloying elements are currently being investigated for their effects on the mechanical properties and reliability of thin aluminum films. In the present study, scanning electron microscopy and transmission electron microscopy are used to study the nucleation and growth of Al-1.5wt%Cu and Al-1.5wt% Cu-0.2wt% W films. Differences in microstructure, nucleation and growth behavior are observed and are explained in terms of changes in surface energies and atomic mobilities. INTRODUCTION Alloying elements such as silicon and copper are often added in small concentrations to aluminum interconnect materials for use in VLSI devices. Interconnects in VLSI integrated and transistor to power supply connections. Cu is added to circuits make transistor-to-transistor retard electromigration 1 ,2 and stress-induced voiding 3 of the aluminum. Si is added to interconnect aluminum to prevent junction spiking. Si addition can result in the formation of Si precipitates in the Al. Such precipitates can negatively impact contact resistivity 4 ,5 , step coverage 6 ,7 , stress-induced voiding 8,9 and electromigration.1 0 "I Barrier layers consisting of TiN or Ti-W 12' 13 are increasingly being used beneath Al interconnect metallization. Such barriers can prevent junction spiking and therefore eliminate the need for addition of silicon. Despite improvements resulting from the use of barrier layers, electromigration and stress-migration continue to remain problematic. In an effort to further improve the electromigration and stress-migration performance of Al metallization, other alloying elements such as tungsten are being added to aluminum. The effects of such additions on the mechanical properties and reliability of thin aluminum films are being investigated. Nucleation and growth mechanisms can impact the final microstructure of metallization films. The microstructure of the films will ultimately impact electromigration and stress-migration of the metallization. In the present study, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to study the effects of addition of tungsten on the nucleation and growth of thin aluminum alloy films. EXPERIMENTAL Films of two compositions were sputter-deposited at 275°C on oxide-coated silicon substrates. A Varian 3180 dc-magnetron deposition system was used. The base pressure was in the low 10-7 Torr, and the operating pressure was -7 mTorr (argon). A zero bias voltage was used. Deposition sources were alloy targets. Deposition rates were approximately 18 nm/s. No pre-sputter clean was performed. The compositions of the metal films were Al-i.5wt%Cu (AlCu) and Al-l.5wt% Cu-0.2wt% W (AlCuW). Wafers were extracted from the deposition chamber after various deposition times for observation. Deposition times ranged from Is to 20s for both film compositions. Characterization of as-deposited films
