Effect of Dielectric Capping Layer on TDDB Lifetime of Cu Interconnects in SiOF

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Effect of dielectric capping layer on TDDB lifetime of copper interconnects in SiOF Jeff Gambino, Fen Chen, Steve Mongeon, Phil Pokrinchak, John He, Tom C. Lee, Mike Shinosky, Dave Mosher IBM Microelectronics, 1000 River Street, Essex Junction, VT, 05452 ABSTRACT In this study, intralevel dielectric breakdown is studied for copper interconnects in an SiOF dielectric, capped with either SiN or SiCN. The leakage current is higher and the failure time of dielectric breakdown is shorter for an SiCN capping layer compared to an SiN capping layer. It is proposed that the dielectric breakdown of the integrated structure is limited by the interface between the capping layer and the SiOF dielectric. Lower lifetime for dielectric breakdown is observed for structures with an SiCN cap compared to structures with an SiN cap, due to higher leakage current in the SiCN. The higher leakage for an SiCN cap is consistent with results from planar metal-insulator-semiconductor capacitors. INTRODUCTION Copper interconnects have gained wide acceptance in the microelectronics industry due to improved resistivity and reliability compared to Al interconnects [1]. More recently, low-k dielectrics such as SiCOH have been introduced, to further reduce the circuit delay [2]. Because of the lower modulus and lower fracture toughness of SiCOH compared to SiO2, the Cu wires in the last dielectric layers are generally fabricated in SiO2 or SiOF. The line-to-line capacitance of the upper wiring layers can be reduced by using an SiCN capping layer rather than an SiN capping layer, similar to what is done at the lower wiring layers [3]. The reliability of the dielectric surrounding the copper interconnects is assessed using time dependent dielectric breakdown (TDDB) measurements on comb-serpertine structures [4]. For damascene Cu integration, the main leakage path between Cu metal lines, and hence the location of the dielectric breakdown, is typically at the interface between the inter-level dielectric and the capping layer. Therefore, the processing of this interface is critical for achieving a robust TDDB reliability. For example, the TDDB lifetime is increased by using plasma treatments in H2 or NH3 prior to SiN cap layer deposition on SiO2 [4,5]. Unfortunately, the H2 plasma treatment also increases the resistance of Cu, converting the surface to a high resistivity CuSiCON layer [6]. Hence, it is desirable to use an alternate plasma clean, such as an Ar plasma, to minimize the increase in resistance [7]. There have been relatively few studies comparing the effect of SiN versus SiCN capping layers on TDDB. For capping layers on SiCOH dielectric, it has been observed that an SiN capping layer has either lower TDDB lifetime [8] or similar TDDB lifetime [9] as an SiCN capping layer. In contrast, for an SiOF dielectric, the TDDB lifetime was longer for an SiN cap than for an SiCN cap [10]. In this study, the dielectric breakdown is studied for different capping layers on an SiOF dielectric, comparing an Ar versus H2 preclean, and SiN versus