Molecular Dynamics Simulation of Megasonic Cavitation in Ammonia Peroxide Solution and its Impact on Silicon Surface
Megasonic cavitation is of importance in the semiconductor industry due to its application in cleaning processes or improving top-to-bottom etch uniformity for nano-scale features, however, our understanding is still limited because the bubble formation/collapse lifetime is very short which makes it difficult to study through experimental methods. Molecular Dynamics (MD) simulations are a powerful tool to simulate such a phenomena at atomic resolution and nanosecond timescale which can provide insights for process and hardware optimizations for megasonic-based recipes.
When megasonic agitation is applied the sound wave in the fluid travels as a pressure wave and gives rise to acoustic streaming and cavitation. Acoustic streaming is motion of fluid caused by the attenuation of sound wave traveling in the viscous fluid. Cavitation is formation of bubbles in the fluid during the low-pressure cycle of propagating wave, followed by violent implosion during high pressure cycle of the wave. Our simulation work focuses on the cavitation and has two parts: 1) Simulating megasonic cavitation in bulk SC1 solution and 2) Simulating the bubble collapse in SC1 solution close to Si surface.
Figure 1 shows our cubic simulation box of SC1 solution, involving 52,190 molecules. We used LAMMPS simulation software to conduct our simulations. To simulate megasonic pressure wave we apply a sinusoidal wave function in our model to change the pressure with time(t):
P(t)=P_0+ΔP sin〖(2πt/τ)〗
Where ΔP = ±180 MPa is the amplitude, τ = 2 ns, is the period, and P0 is the ambient pressure. During the low-pressure region where cavitation happens, we change the external pressure from 0.1 MPa to -185 MPa within 0.5 ns simulation time (500 MHz frequency).
Figure 2 shows the results of our cavitation simulations in SC1. They show that the onset pressure for bubble formation occurs around -175 MPa and happens at 480 picoseconds (ps). Cavitation in the bulk starts with one bubble and appears as multi-bubble formation at the end of the simulation. The total volume of cavitation is estimated to be 449 nm3 at 0.5 ns, which makes 30.3% of the total simulation volume.
In the second part of our work, we simulated bubble collapse in SC1 near the Si surface. Figure 3 shows our simulation setup which involves 844,216 atoms and represents SC1 solution in contact with Si surface. We initially created a bubble close to the surface as shown in the figure. The shockwave is created through the momentum mirror wall method and acts from the top to bottom.
Figure 4 shows the time evolution of bubble collapse in ps. As shockwave reaches the bubble, the bubble starts to collapse and the fluid molecules surrounding the bubble rush toward the center of the bubble which creates a narrowly focused beam with high velocity or nanojet. Figure 5 shows the nanojet velocity as it impacts the surface.
Figure 6. shows the nanojet collision with the surface which creates a shallow pit on the surface. The depth of the damage is found to be less than 2 nm.
Our work provides details on the mechanism of bubble formation during megasonic agitation and quantifies cavitation. Furthermore, it reveals how bubble collapse near the surface can cause damage. These insights can help better understand megasonic agitation from molecular perspective and can aid in designing or optimizing megasonic-based recipes.
BIOGRAPHY
Amir holds a PhD in computational chemistry and brings over 15 years of combined graduate, postdoctoral, and industry research experience specializing in molecular dynamics simulations. Throughout his career, he has developed expertise in applying advanced computational techniques to study the behavior and properties of complex molecular systems. In 2022, Amir joined TEL as a Research Scientist, where he focuses on modeling semiconductor materials and processes. His work contributes to improving the understanding and performance of semiconductor devices through detailed simulations at the atomic and molecular levels.