Integration and reliability issues for low capacitance air-gap interconnect structures

As IC technology scales, the performance of ULSI chips is increasingly limited by the capacitance of the interconnects. The interconnect capacitance contributes to RC delay, AC power (CV₂ f), and crosstalk. The use of air-gaps formed between metal lines during SiO₂ deposition has been shown to reduce the capacitance of tightly spaced interconnects by as much as 40% compared to homogeneous SiO₂ (Shieh et al, IEEE Electron Dev. Lett. vol. 19, no. 1, pp. 16-18). This capacitance reduction is comparable to, if not better than, the reduction obtained using low-k materials such as polymers in a homogeneous scheme. Air-gap formation, modeled here using the Stanford SPEEDIE deposition simulator, reduces capacitance for varying feature sizes. However, as with all low-k materials and schemes, a number of process integration and reliability issues must be addressed before air-gaps can be fully incorporated into high performance ULSI interconnects. In this paper, we present and address a number of these issues using a variety of simulation tools and experimental results. Air-gap formation has been simulated using SPEEDIE and resulting geometry input to MARC, a finite element code to simulate electromigration reliability. ANSYS, another finite element code, was used to simulate the thermal performance of interconnect stacks using air-gaps