Mechanical Integrity Study of Air Gap Structures Assisted by FE Simulations
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1079-N02-02
Mechanical Integrity Study of Air Gap Structures Assisted by FE Simulations Stephane Moreau1, Frédéric Gaillard1, Jean-Charles Barbé2, Raphaël Gras3,4, Gérard Passemard3, and Joaquin Torres3 1 DRT/LETI/LBE, CEA-LETI MINATEC, 17 rue des Martyrs, Grenoble, 38054, France 2 DRT/LETI/LSCDP, CEA-LETI MINATEC, 17 rue des Martyrs, Grenoble, 38054, France 3 STMicroelectronics, 850 rue Jean Monnet, Crolles, 38926, France 4 LTM/CNRS, 17 rue des Martyrs, Grenoble, 38054, France
ABSTRACT In this paper, mechanical reliability of “air gap” structures has been evaluated when a copper line is completely surrounded with air. Different Finite Element (FE) simulation models have been used on a 2-metal level structure to study the M2 copper line bow evolution as a function of its dimensions if complete air cavities are generated underneath (i.e. at via level). Design rules information may therefore be obtained to optimize “air gap” integration considering the 65 nm and 22 nm technology nodes. Thus, we not only highlight that M2 copper line can not collapse considering our failure criterion but that M2 bow variation may also be improved when a tensile SiCN capping layer is deposited on top of the structure. The influence of the interline spacing vs. M2 bow has also been studied and we show that its increase is a beneficial parameter for the air gap structure. In opposite, we demonstrate that buckling can occur when a compressive SiCN layer is used. Finally, we accurately predict the M2 bow variation for air gap structures of the 65 nm and 22 nm technology nodes, but stress and strain distribution can complementary be provided. Those results highlight interesting criteria for designers to build reliable air gap structures. INTRODUCTION In order to meet the ITRS requirements for advanced interconnects (22 nm technology node and below) [1], new materials with ultra low dielectric constant (ULK) or new integration architectures are necessary to improve RC delay, crosstalk and power consumption. Nowadays, the introduction of air cavities as a dielectric material has been considered to achieve those performances. To create “air gaps”, we currently investigate a “full oxide” approach where a sacrificial SiO2 layer is used in the whole stack and further removed by an HF chemical etching agent. This HF chemistry diffuses through out patterned apertures localized in an upper SiCN capping layer deposited at the end of the integration. In such approach, one concern deals with the potential collapse of a M2 copper line on the M1 structure on long M2 line patterns when air cavities are made under the copper line. To prevent this phenomenon, a SiO2 pillar is experimentally left between two adjacent SiCN apertures during the chemical etching process. This idea was successfully processed on a dual damascene integration scheme and exhibits promising results [2]. The intent of this paper is to define design rules to optimize mechanical reliability and, by extension, electrical performances of this previous discussed air gap approach consideri
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