Laser-Assisted Particle Removal Andrew Jurik, Vanderbilt University

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Laser-Assisted Particle Removal

  • Andrew Jurik, Vanderbilt University

  • Adam Bezinovich, Truman State University

  • Elodie Varo, INSA Lyon

  • Betul Unlusu, Florida State University


Abstract

  • Removal of small particles from solid surfaces is of critical importance for the microelectronic industry where ~50 % of yield losses are due to particle contamination. Laser cleaning is a technique developed in the late 1980s to remove micro and sub-micro scale particles from surfaces. In this study, a two-dimensional molecular dynamics approach is used to simulate the cleaning process. The model approximates laser energy heating the system that includes the particle, a substrate, and an energy transfer medium (ETM), which is a thin liquid film. The particles are removed through the explosive boiling of the ETM.

  • Three methods of heating are tested: (1) heating only the particle, (2) heating both the particle and the substrate, (3) heating the ETM layer. These cases will be compared with a previously analyzed case, that of the substrate absorbing the laser light.



Molecular Dynamics

  • Initialize the system with a set of initial points and parameters.

  • Calculate the forces on each atom. The Lennard-Jones 12-6 potential function is used along with neighbor lists.

  • Integrate the equations of motion. Movement of atoms in a time interval are calculated using initial positions, velocities, and forces.

  • Update the position of each atom.

  • Repeat the process for the next time interval until finished.



Lennard-Jones Potential

  • The Lennard-Jones 12-6 potential is used to model the interaction potential between a pair of molecules.

  • “r” is the distance between two molecules. “σ” is a measure of the molecule’s diameter (the distance where the potential is zero) and “ε” is the depth of the potential well, a measure of the strength of interaction.

  • The parameters σ and ε are chosen to fit the physical properties of the materials.



Lennard-Jones Potential

  • The 1/r12 term models the repulsion of the molecules, especially at short distances.

  • The -1/r6 term constitutes the attractive part, dominating at long distances.



Neighbor Lists

  • The goal of neighbor lists is to improve the speed of the program by maintaining a list of neighbors of the molecules and updating them at intervals.

  • If molecules are separated by distances greater than the potential cutoff (known as the cutoff radius), then the program skips those expensive calculations.



Details of Simulation

  • Cleaning efficiency is defined as the percentage of particles that are removed from the substrate for a given configuration.

  • The simulation is run for 30,000 time steps (0.33 ns) for 10 different initial configurations.

  • The film thicknesses vary from 3σ (1.02 nm) to 70σ (23.8 nm). The particle’s diameter is 19σ (6.46 nm).

  • The temperatures vary from 1.0 (121 K, -152°C) to 5.0 (605 K, 332°C). 0.1 reduced units correspond to 12.1 K.

  • There will be three methods of heating that are tested in this study: (1) heating only the particle, (2) heating both the particle and the substrate, and (3) heating the ETM layer.



Sample Initial Configuration



Time Evolution – Particle Heated



Results – Particle Heated



Time Evolution – Particle & Substrate Heated



Results – Particle & Substrate Heated



Time Evolution – Substrate Heated



Results – Substrate Heated



Time Evolution – All of ETM Heated



Results – ETM Heated



A Closer Look – ETM Heating



Conclusions and Trends

  • Substrate heating keeps the particles intact at higher temperatures and seems to work best between layers 10σ and 50σ.

  • Particle heating works more effectively for thinner film layers than thicker film layers at lower temperatures.

  • Heating of both the particle and the substrate is very efficient for removing particles especially at lower temperatures, but deforms the particles at a faster rate.

  • Heating the ETM completely appears very efficient, but would only be feasible for thinner films.

  • Heating the top 2.5σ of the ETM is not efficient at all, though may be a realistic configuration.



Possible Areas of Future Research

  • Constructing and simulating a three-dimensional model of laser-assisted particle removal.

  • Performing laboratory experiments to corroborate the results from the computer simulations.

  • Exploring different methods of heating (gradual heating, abrupt heating, periodic heating…)

  • Altering ETM fluid properties (viscosity)

  • Altering particle properties (shape)



References

  • K.M. Smith, M.Y. Hussaini, L.D. Gelb, S.D. Allen: Appl. Phys. A 77, 877-882 (2003)

  • M.P. Allen, D.J. Tildesley: Computer Simulation of Liquids (Oxford University Press, New York 1989)

  • F. Ercolessi: A Molecular Dynamics Primer (Available: http://www.fisica.uniud.it/~ercolessi/md/md/)

  • S. Shukla: Optimization of Thickness of Energy Transfer Medium for Laser Particle Removal Process (2003) (Available: http://etd.lib.fsu.edu/theses/available/etd-11242003-113245/unrestricted/manuscript_final_24Nov.pdf)




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