Laser-Assisted Particle Removal

1
Laser-AssistedParticle Removal

• Andrew Jurik, Vanderbilt University
• Adam Bezinovich, Truman State University
• Elodie Varo, INSA Lyon
• Betul Unlusu, Florida State University

2
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.

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Molecular Dynamics

1. Initialize the system with a set of initial
   points and parameters.
2. Calculate the forces on each atom. The
   Lennard-Jones 12-6 potential function is used
   along with neighbor lists.
3. Integrate the equations of motion. Movement of
   atoms in a time interval are calculated using
   initial positions, velocities, and forces.
4. Update the position of each atom.
5. Repeat the process for the next time interval
   until finished.

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Lennard-Jones Potential
Interaction s e
ETM-ETM 1.0 1.0
ETM-Particle 1.0 1.5
Particle-Particle 1.0 10.0
ETM-Substrate 1.0 1.5
Particle-Substrate 1.0 1.5

• 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. s is
  a measure of the molecules diameter (the
  distance where the potential is zero) and e is
  the depth of the potential well, a measure of the
  strength of interaction.
• The parameters s and e are chosen to fit the
  physical properties of the materials.

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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.

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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 areperiodic.

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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 3s (1.02 nm) to
  70s (23.8 nm). The particles diameter is 19s
  (6.46 nm).
• The temperatures vary from 1.0 (121 K, -152C) to
  5.0 (605 K, 332C). 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.

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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.

3s 6s 10s
25s 50s 70s
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Time Evolution Particle Heated
Particle Heated, 25s Layer, Temperature 2.5Times
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.

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Results Particle Heated
Although simulation was not run for the given
configuration,the particle removal rate is
assumed to be nearly 100.
11
Time Evolution Particle Substrate Heated
Particle Substrate Heated, 25s Layer,
Temperature 1.7Times are 0, 5000, 10000, 15000,
20000, 25000, 30000

• The ETM is both lifted off the substrate and
  expelled radially away from the particle.

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Results Particle Substrate Heated
13
Time Evolution Substrate Heated
Substrate Heated, 25s Layer, Temperature
2.5Times 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.

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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.
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Time Evolution All of ETM Heated
All of ETM Heated, 10s Layer, Temperature
1.5Times 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.

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Results ETM Heated
17
A Closer Look ETM Heating
All of ETM heated10s, Temp 1.0, Time 30000
Top 2.5s of ETM heated10s, 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 2.5s (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
  2.5s of the ETM layer is to examine surface
  boiling which occurs when laser light has a low
  penetration depth.

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Conclusions and Trends

• Substrate heating keeps the particles intact at
  higher temperatures and seems to work best
  between layers 10s and 50s.
• 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 2.5s of the ETM is not efficient
  at all, though may be a realistic configuration.

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Possible Areas ofFuture 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)

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References

1. K.M. Smith, M.Y. Hussaini, L.D. Gelb, S.D. Allen
   Appl. Phys. A 77, 877-882 (2003)
2. M.P. Allen, D.J. Tildesley Computer Simulation
   of Liquids (Oxford University Press, New York
   1989)
3. F. Ercolessi A Molecular Dynamics Primer
   (Available http//www.fisica.uniud.it/ercolessi/
   md/md/)
4. 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)