Investigating the Role of Fluid Dynamics on Cut Width Accuracy in Wet Bevel Cleaning Techniques
Authors: Dong Rencheng , Fujita Akira , Bassett Derek , Shinohara Kazuyoshi , Takahashi Shuhei , Simms Ihsan 
Published: December 2024

As the size of advanced technology nodes keeps scaling down, IC manufacturers continue to push the active die area to the wafer edge. Defects originating from the wafer edge can result in substantial yield loss. Etching and cleaning bevel films and particles is increasingly critical in improving wafer yield performance near the wafer edge. Even though backside center dispense spin (BCS) and bevel nozzle dispense spin (BVS) methods have been developed and employed as two main wet bevel cleaning techniques in high volume manufacturing, the comparison of cut width accuracy between different wet bevel cleaning techniques has not been fully studied. The goal of this study is to investigate and understand the effects of fluid dynamics on cut width accuracy in different wet bevel cleaning techniques (i.e. BCS and BVS). In this study, computational fluid dynamics (CFD) numerical models were developed to study the chemical liquid flow in wet bevel cleaning processes.

In the BCS process, the chemical liquid is dispensed through a nozzle below the center of wafer backside while the wafer is spinning. In order to characterize the cut width profile after BCS process, the liquid flow was modeled under the coordinate system of the rotating wafer instead of the regular stationary coordinate system. This special type of model allows understanding how the liquid is distributed on the wafer surface with respect to the rotating wafer, which will be more straightforward to be compared with the measurement of cut width profile.

For the BCS process, the simulation results of liquid flow on the wafer backside surface are shown in Fig. 1. Due to the increasing centrifugal force towards the wafer edge, the liquid flow starts to develop some branching patterns. The liquid velocity (Fig. 1(a)) and liquid film thickness (Fig. 1(b)) are not axisymmetric anymore. Fig. 2 shows a vertical cross section of liquid flow at the wafer edge. The frontside etching in BCS relies on this liquid wrap-around flow. Due to the inertia and capillary force effects, the liquid first climbs from wafer backside to frontside as shown in Fig. 2(a). The liquid volume keeps increasing as liquid accumulates around the wafer edge as shown in Fig. 2(b). When the liquid volume at wafer bevel is greater than a certain threshold, the extra liquid will be stripped from the continuous liquid film as shown in Fig. 2(c). Because this wrap-around flow is unstable and periodic, the leading edge of the liquid on wafer frontside always fluctuates during BCS process as shown in Fig. 3. The non-axisymmetric backside flow leads to non-axisymmetric wrap-around flow on the wafer frontside. Because the liquid wrap-around flow is spatially non-axisymmetric and temporally unstable, the cut width profile over the whole perimeter of wafer bevel region can be quite non-uniform as shown in Fig. 4(a).

In the BVS process, the chemical liquid is dispensed through a nozzle above the frontside bevel region as shown in Fig. 5. After the liquid jet lands on the rotating wafer, the liquid will develop into a stable circular flow along the wafer edge as shown in Fig. 5(a). A vertical cross section of the liquid film flow is plotted in Fig. 5(b). The inner cut position of the liquid film flow dictates the cut width in the BVS process. Because this circular liquid flow during BVS process is more stable, the cut width profile is more uniform in the BVS process as shown in Fig. 4(b).

In practice, the cut width profile after the BCS process is also dependent on the properties of wafer surface such as wafer surface wettability. If the wafer surface is hydrophobic, the liquid flow on the wafer backside surface can break up, which results in very limited wrap-around flow and limited etching at the wafer edge. Therefore it can be difficult to use the BCS technique to etch the hydrophobic bevel films. On the other hand, the circular liquid flow in the BVS process is less affected by the properties of wafer surface because the liquid is directly dispensed over the bevel region. It is much easier to achieve the cut width target using the BVS technique by adjusting operation conditions such as the nozzle position. Since the numerical model developed in this study can accurately represent the flow physics in the BVS process, this CFD model can be adopted as a digital twin of the BVS tool to optimize the BVS process conditions based on the film material properties (e.g., contact angle) to meet the cut width target.

Figure 1