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 Interaction σ ε ETM-ETM 1.0 ETM-Particle 1.5 Particle-Particle 10.0 ETM-Substrate Particle-Substrate 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. In these simulations, a linked list is maintained. In this case, a molecule move involves checking molecules in the same cell, and in all the neighboring cells. The boundary conditions are periodic.

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  To obtain significant results, the simulation of the removal process is repeated many times, each with a slightly different initial configuration. Ten different equilibrated configurations are run for each fluid thickness and temperature considered. 3σ 6σ 10σ 25σ 50σ 70σ

Time Evolution – Particle Heated Particle Heated, 25σ Layer, Temperature 2.5 Times are: 0, 5000, 10000, 15000, 20000, 25000, 30000 The particle is rapidly heated. Heat is transferred to the neighboring ETM molecules from the particle. The ETM explodes away from the particle in a circular fashion.

Results – Particle Heated * Although simulation was not run for the given configuration, the particle removal rate is assumed to be nearly 100%.

Time Evolution – Particle & Substrate Heated Particle & Substrate Heated, 25σ Layer, Temperature 1.7 Times are: 0, 5000, 10000, 15000, 20000, 25000, 30000 The ETM is both lifted off the substrate and expelled radially away from the particle.

Results – Particle & Substrate Heated

Time Evolution – Substrate Heated Substrate Heated, 25σ Layer, Temperature 2.5 Times are: 0, 5000, 10000, 15000, 20000, 25000, 30000 The laser energy is absorbed by the substrate and heats the ETM by conduction. Explosive evaporation removes the particles. The ETM is vertically lifted off the substrate.

Results – Substrate Heated Source: K.M. Smith, M.Y. Hussaini, L.D. Gelb, S.D. Allen: Appl. Phys. A 77, 877-882 (2003) No data entry implies that there would be no particle removal for that given configuration based on trends.

Time Evolution – All of ETM Heated All of ETM Heated, 10σ Layer, Temperature 1.5 Times are: 0, 5000, 10000, 15000, 20000, 25000, 30000 The laser heats the ETM directly. In this type of heating, explosive evaporation of the ETM removes the particles.

Results – ETM Heated

A Closer Look – ETM Heating All of ETM heated 10σ, Temp 1.0, Time 30000 Top 2.5σ of ETM heated 10σ, Temp 1.0 | Temp 5.0, Time 30000 When all of the ETM is heated, the majority of the ETM is ejected. The process is generally very efficient. When only the top 2.5σ (7.35 nm) of the ETM is hated, it takes much more heat to fully eject the ETM (and even then, particle removal does not readily occur). The purpose of heating the top 2.5σ of the ETM layer is to examine “surface boiling” which occurs when laser light has a low penetration depth.

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)