Wet Metal Recess Process for Enabling Scaling Boosters
Authors: Akanishi Yuya , Kesters Els , Le Quoc Toan , Pacco Antoine , Holsteyns Frank 
Published: February 2020

Accompanied with the downscaling of semiconductor devices, many challenges arise in the semiconductor fabrication process. One of the biggest challenges is securing patterning margin as dimension size of semiconductor structures shrink. To overcome these challenges, scaling boosters are proposed, such as FSAV (Fully Self-Aligned Via) and SAGC (Self-Aligned Gate Contact) [1-2]. The common requirement for these scaling boosters is the metal line recess prior to upper interconnect layer formation. For these metal recess processes, the recess amount uniformity is required (within wafer, pattern to pattern), since recess amount nonuniformity results in electrical resistance variation. Also, surface roughness control is important to minimize electron scattering at the surface, which also results in resistance increase.
The concept of Cu recess using a cyclic process comprising of wet oxidation and oxide removal steps for controlled metal recess was previously reported [3]. In this study, the process for applying the above-mentioned requirements for advanced interconnects has been further optimized.
In a cyclic process comprising of wet oxidation and metal oxide removal, 3% H2O2 and 0.05% HF (both at room temperature) were used. It was found that the Cu and Co recess amounts were impacted significantly depending on the pattern width when the H2O2 – HF cyclic process was applied. For Cu, the recess amount was around 10 nm for 440 nm Cu line width; however, the recess amount increased to 35 nm for 20 nm Cu widths. This phenomenon can be explained by the difference of grain sizes as a function of pattern widths. Cu grain size is known to be smaller when patterned in smaller trenches [4], which results in a higher density of the grain boundaries. It is believed that an increase in grain boundary results in a faster reaction rate due to the greater lattice disorder compared to the bulk structure; therefore, Cu and Co recesses are estimated to proceed faster in smaller trenches compared to larger trenches. This leads to nonuniformity of the recess amount when different trench widths exist on the same surface. To mitigate this effect, different etchant chemistries were evaluated. By using organic acids like acetic acid or citric acid, the recess amount uniformity between different CD sizes improved drastically. Since acetic acid and citric acid have higher molecular volumes compared to HF, they are less susceptible to grain boundary lattice disorders. As a result, the recess amount range between different CD sizes (20 nm ~ 440 nm) reduced from 26 nm (H2O2 - HF) to 3 nm (H2O2 – Acetic acid). This process is effective not only for Cu recess, but also for Co recess.
Another factor for recess amount uniformity control is dissolved oxygen level in etchant chemistry. It has been reported that dissolved oxygen control is a key factor for controlling metal etching amount in acidic chemistry [5]. It will also be shown that both dissolved oxygen and atmospheric oxygen impact on Cu and Co recess during the organic acid step.
Finally, the optimized metal recess process for Cu/Co enabling scaling boosters for advanced process nodes will be proposed.
References
[1] G. Murdoch et al., “Feasibility study of fully self aligned vias for 5nm node BEOL”, Proc. IEEE Int. Interconnect Technol. Conf., pp. 1-4, (2017)
[2] Benjamin D. Brigg et al., “Fully Aligned Via Integration for Extendibility of Interconnects to Beyond the 7 nm Node”, IEDM, 17-341, (2017)
[3] A. Iwasaki, et al., “Study of Copper Surface Preparation by Sequential Atomic Layer Wet Etching and Laser Annealing Treatments”, ECS Trans. 2017 80(2): 233-241, (2017)
[4] W. Steinhögl, et al., “Comprehensive study of the resistivity of copper wires with lateral
dimensions of 100 nm and smaller”, J. App. Phys. 97, 023706, (2005)
[5] Y. Akanishi et al., “Corrosion of Co in BEOL interconnects in dilute HF solution”, Solid State Phenomena, Vol. 282, pp. 256-262, (2018)