| AVS 60th International Symposium and Exhibition | |
| Electronic Materials and Processing | Thursday Sessions |
| Session EM-ThA |
| Session: | Materials and Process for Advanced Interconnects II |
| Presenter: | G. Karbasian, University of Notre Dame |
| Authors: | G. Karbasian, University of Notre Dame H. Xing, University of Notre Dame A.O. Orlov, University of Notre Dame P.J. Fay, University of Notre Dame G.L. Snider, University of Notre Dame |
| Correspondent: | Click to Email |
In today’s IC technology, the size of the transistors is constantly shrinking and interconnect RC delay is a dominant factor in determining the speed of circuits. Gold is a promising replacement for copper and aluminum due to its low resistivity and high chemical stability. A key feature of a successful Au CMP is the use of a relatively soft adhesion layer beneath the gold, along with proper slurry additives. Ni is a soft metal compared to other common adhesion layer metals such as Ti and Cr, and was therefore used as the adhesion layer. Ultra Sol A20 from Eminess Technologies Inc., an alumina based slurry with added potassium iodide, was used to polish the Au/Ni metal overlay on the patterned dielectric substrate. The slurry, at pH=4, was modified by adding hydrogen peroxide as an oxidizer. Previously, Ishii et al.[1] observed that adding 30% H2O2 to an alumina based slurry with added potassium iodate at pH=4, in 1:1 ratio resulted in the maximum polish rate of gold, and proposed it as the optimum concentration. However, this slurry composition results in high static etch rate, i.e. chemical dissolution, of gold, approximately 95nm/min. This high static etch rate increases edge recess and dishing in the interconnect trench as shown in Figure1. Moreover, this concentration of H2O2 exceeds the optimal oxidizer concentration needed for the highest Ni removal rate[2]. The lower polish rate of adhesion layer compared to gold, which was lowest for Cr and highest for Ni due to their different hardness, made adhesion layer removal the limiting factor in determining the total polish time. Our experiments show that reducing the concentration of 30% H2O2 solution from 50% to 25% improves the selectivity between the adhesion layer and gold such that at this concentration, Ni is polished approximately 3 times faster than gold, and this allows us to reduce the total polish time by a factor of 3. Additionally, the chemical etch rate of gold dropped from 90nm/min to 30nm/min, which reduces the dishing of Au during the Ni removal step.
Adding SDS and PVP enhanced the stability of the colloidal slurry that was otherwise prone to agglomeration leading to nonuniform polishing and microscratches. A combination of H2O2 and UltraSol A20 in1:3 volume ratio along with added SDS and PVP resulted in a stable slurry giving a successful CMP Damascene process, Figure2, with 37nm of dishing across a 150 µm wide contact pad.
We have also studied the evolution of the coefficient of friction during the CMP process, and observed a decrease in this parameter when the metal overlay is polished away, suggesting that this can be used for end-point detection.