Fabrication of dual damascene BEOL structures using a multilevel multiple exposure (MLME) scheme, part 1: lithographic patterning

Abstract

In this work, the conventional via-first dual damascene (DD) patterning scheme is replaced by a cost-efficient Multi-Level Multiple Exposure (MLME) patterning and etching approach. A two-layer positive-tone photoresist stack is sequentially imaged using 193 nm dry lithography, to produce a DD resist structure that is subsequently transferred into an auxiliary dual organic underlayer stack, and then further into a dielectric layer. This novel integration approach eliminates inter-tool wafer exchange sequences as performed in a conventional litho-etch-litho-etch process flow, while simultaneously being applicable to all back-end-of-the-line (BEOL) levels, ensuring throughput increase. The top and bottom resist layers are chemically designed in such a way that they feature differential solubility in organic solvents making it possible to coat the top photoresist onto the bottom resist layer without intermixing to enable a strict litholitho- etch processing sequence. Independent registration of the via and trench structures in the bottom and top resist layers is achieved by selective photospeed decoupling of the respective layers, so that the bottom resist is largely insensitive at nominal resist exposure dose for the top resist. Imaging performance evaluation of the newly introduced MLME technology includes the resist materials selection process and their required properties (solvent compatibility, adhesion, photospeed, defectivity and correction of via dose bias due to trench exposure) as well as metrology work. Image transfer of the patterned DD resist structure into an underlying transfer layer stack and then further into a dielectric layer using Reactive Ion Etching (RIE) followed by electroplating, polishing and electrical testing was also thoroughly investigated and is described in detail in an accompanying paper.