Mechanical integrity of back-end-of-line with Ru nanowires and airgaps

Abstract

In this study, the mechanical integrity of novel back-end-of-line (BEOL) with ruthenium nanowires and airgaps was investigated using a combination of nano-indentation, delamination tests and finite element modelling. The ruthenium interconnects with and without airgaps were benchmarked against standard dual damascene copper BEOLs. The impact of interconnect aspect ratio, adhesion promotion at the metal-dielectric interfaces, airgap implementation and low-k dielectric selection was investigated. The study demonstrates that ruthenium-airgap interconnects of high aspect ratio 3 with standard adhesion promoting layers at the metal-dielectric interfaces and interlevel dielectric's Young's moduli as low as 12 GPa, can provide a mechanical integrity on a par with dual damascene Cu BEOL.

Introduction

Ruthenium is a strong candidate for advanced back-end-of-line (BEOL) metallization given a myriad of favourable characteristics [[1], [2], [3]]. Although Ru's resistivity is higher compared to that of Cu, a significant resistance benefit has been predicted for barrierless Ru nano-interconnects at highly scaled linewidths [4]. Ru has a high melting point which is an indicator of good electromigration (EM) performance. Consistently, an electromigration activation energy of >1.8 eV has been measured for Ru interconnects [5]. Most interestingly, its selective etch is convenient [6], which allows airgaps to be integrated with lower process complexity and cost compared to Cu. In this context, in contrast to Cu-airgap schemes in which it is necessary to encapsulate Cu with a hermetic dielectric liner to prevent oxidization [7], Ru surface oxidization is self-limiting. The relative convenience of airgap integration helps further reduce interline capacitance by abundant implementation across the layer. Thereby, the capacitive penalty of having higher aspect ratio nano-interconnects is minimized and line resistance can be lowered by increasing interconnect height. Moreover, convenience of airgap implementation in Ru-based BEOLs will help avoid interline dielectric breakdown as a major reliability challenge of standard schemes at extremely narrow interline spacing [2].

Nevertheless, reliability under chip–package interaction (CPI) is paramount for the microelectronics industry. As a pre-requisite, any novel interconnect scheme in the BEOL stack should withstand the thermal-mechanical stresses induced during processing and working lifetime in order to be deemed as a viable technological solution. In this context, the integration of porous ultra-low-k dielectrics (ULK) in the standard dual damascene Cu interconnects increased the CPI induced delamination failures and constituted a serious obstacle towards integration of extremely low-k dielectrics given their inferior mechanical stability [[7], [8], [9], [10]]. In Cu-airgap schemes, CPI-aware implementation approaches have been proposed and were shown to be able to mitigate the relative CPI vulnerability [11,12]. Concerning the CPI risks of Ru interconnects with airgaps, the higher aspect ratio together with barrierless implementation can potentially undermine mechanical integrity which motivates evaluation of delamination risk at the Ru-dielectric interfaces. Especially, when the Ru BEOL stack with a relatively higher aspect ratio (AR) is exposed to shear loads. To this end, in this study, a combination of (i) finite element modelling (FEM), (ii) four-point bending delamination tests on blanket films and (iii) nano-indentation experiments on integrated BEOLs are conducted to benchmark Ru interconnects against the standard Cu dual damascene technology.