Research paperA numerical study on slurry flow with CMP pad grooves
Graphical abstract
Introduction
Chemical mechanical polishing (CMP) is an essential process to planarize inter-level dielectrics and isolate multiple layers in semiconductor manufacturing [1]. The CMP process also is important in the realization of both front-end-of-the-line (FEOL) and the back-end-of-the-line (BEOL) process steps [2]. CMP involves chemical and mechanical interactions of consumables such as a pad, conditioner, and slurry [3]. The pad causes slurry flow and provides contact between the slurry particles and wafer surface [4]. The conditioner has diamond tips, which makes and maintains the pad surface during the CMP process [5]. The slurry flow in the gap between the pad and the wafer affects the CMP performance in terms of removal rate (RR), non-uniformity (NU), and defects [6]. During the CMP process, the pad surface can control slurry flow with pores and grooves [7]. The grooves reduce hydroplaning, which produces microscopic contacts as well as slurry flows [8,9]. In general, a circular groove has been used in many CMP processes. Recently, there are various types of groove depending on the role of the groove, such as circular, radial, offset, and combined groove [[10], [11], [12], [13]]. The grooves supply the slurry, improving the RR and NU, and remove the process by-product, preventing defects and scratches. Thus, the study on the slurry flow with the pad groove is important to overcome performance issues.
In this study, the slurry flow was investigated with the pad grooves. First, the slurry flow was predicted by numerical analysis. The slurry velocity by the pad and wafer rotation was calculated. The velocity streamline and wall shear stress were compared according to groove depth and pattern, and polishing experiments were conducted with two pad grooves. The final results were correlated with those of the initial numerical study. This research can be used to find the optimal pad groove to improve the CMP performance.
Section snippets
Numerical method
Fluid simulations were conducted to observe the slurry flow on the pad surface. A commercial computational fluid dynamics (CFD) code (ANSYS Fluent v. 19.0, USA) was used with approximately 700,000 hexahedral meshes constructed for precise calculation. The flow was treated as steady-state, and the properties of the slurry replaced those of water. Steady-state mass conservation (continuity equation) and momentum equations for Newtonian and incompressible fluid (Navier-Stokes equation) were used
Results and discussion
The CMP process removes the step height on a wafer surface. The removal rate of the wafer surface can be described by Preston's equation, as follows [14]:where Kp is the Preston constant, P is the applied pressure, and V is the relative velocity between pad and wafer. The Kp can be affected by several process parameters, including consumables such as the pad, conditioner, and slurry [15,16]. The pressure (P) is applied to the wafer almost uniformly by the head pressure.
Conclusions
The present work investigated CMP slurry flow in the gap between pad and wafer. First, the slurry flow was calculated and predicted by numerical methods, and then the velocity streamline and wall shear stress were analyzed with two pad groove types, circular and circular plus radial. The slurry flow was found to change depending on groove pattern and depth; as the pad wore down, the shortened groove depth reduced slurry flow into the gap. The circular groove pattern retained the slurry, and the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (NRF2017R1A2B3011222). We would like to thank Samsung Electronics for numerous discussions.
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These authors contributed equally to this work.