Clarkson University Honors Program Thesis Proposal
Nanoparticle Removal from Substrates with Pulsed Laser Techniques
Thomas Hooper Jr.
Dr. Cetin Cetinkaya, Advisor
April 26, 2001
A major challenge in the fields of microelectronics and nanotechnology is removing
particles deposited on substrates during the manufacturing process. According to SEMATECH's
International Technology Roadmap for Semiconductors, microprocessor minimum feature sizes
in the near future are projected as 130 nm in the year 2002, 100 nm in 2005, and 70 nm in 2008
[1]. As the minimum feature size decreases, smaller particles begin to cause an increase in yield
losses. For the fields of microelectronics and nanotechnology to advance, this problem must
continuously be mitigated.
Current techniques for particle removal are reaching performance limits as the minimum
killer defect size decreases. As the particle size becomes smaller, the adhesion stress between a
particle and a substrate increases greatly. For a 200 nm particle, accelerations on the magnitude of
109 m/s2 are necessary for detachment. A doubling of this particle size leads to an order of
magnitude decrease in the required acceleration for detachment [5]. Many of the current removal
techniques use chemicals and may require physical contact. The use of chemicals increases costs
and may involve agents that are hazardous to people and the environment. Contact based systems
often inflict damage on the substrate during the cleaning process.
Two techniques currently being studied in Professor Cetinkaya's Opto-Acoustics Group
at Clarkson University for the removal of nanoparticles from various substrates are the dry laser
cleaning (DLC) method and the laser induced plasma (LIP) method [4] [5] [6] [7]. In the DLC
technique, a pulsed laser irradiates the surface of the substrate. The energy of the beam is
deposited on the surface and the thermoelastic coupling effect generates elastic waves that
propagate through the media. These elastic waves create surface accelerations of the magnitude
necessary for the removal of micrometer and nanometer sized particles (see Figure 1) [5].
In the LIP technique, a high-powered pulsed laser is focused by means of a convex lens.
Near the focal point of the lens, the energy of the beam is concentrated. The high energy
concentration causes the dielectric breakdown of air and forms plasma (see Figure 2) [2] [3]. This
process creates a shockwave with a high-pressured wavefront that propagates at supersonic
speeds in the near field. When utilized for particle removal, this high-pressured wavefront is
strong enough to effectively remove micrometer and nanometer sized particles.
Both the DLC and LIP techniques have many advantages over current methods of
particle removal. They are both dry, non-contact methods. All the associated chemical costs and
contact related damage issues are avoided. The firing of a pulsed laser is a very rapid process,
allowing for shorter cleaning cycles, which can increase throughput and decrease manufacturing
costs. The removal effectiveness of the LIP technique is independent of optical properties. The
independence of optical properties is especially advantageous for optical cleaning applications,
where the substrates usually have very high transmittance values.
The objective of this research is to investigate the use of the DLC and LIP removal
techniques on flat and contoured surfaces of varying substrates. Experimental data will be
collected in continuation of the work presented in [7] to provide more conclusive evidence of the
effectiveness of these techniques. Scanning electron microscope (SEM) analysis of pulsed
substrates will allow for the quantification of particle removal efficiency. For DLC, the
dependence on laser pulse energy, the number of pulses and the type of substrate will be
investigated. Interferometer measurements of the surface accelerations during DLC will be taken
and compared to the theoretical accelerations needed for particle removal. Current finite element
analysis [5] and substrate excitation models of the DLC process will be used in comparison to
experimental results. For LIP, the dependence on pulse energy, the number of pulses and the
distance from the plasma core to the substrate will be studied. Damage mitigation is a key
concern in both methods. The research will focus on maximizing removal efficiency while
avoiding substrate damage.
Figure 1 (a) The maximum and minimum values of the radial acceleration component ar,
and (b) The maximum and minimum values of the axial acceleration component az over the
simulation period. The wavelength of the laser beam with the diameter of ro=2.5mm is =532nm,
the laser pulse energy is Eo=187mJ, and the absorbed pulse energy is Eo=72.93rnJ (the
reflectivity of copper at =532nm is R = 0.61).
Figure 2. A sequence of shadow grams for the evolution of laser generated plasma, shock
wavefront and hot air core. The original height and width of each frame is 27 mm. The shock
wavefronts are labeled as SW. (Courtesy of H. Sobral et al.).
Works Cited
[1] International Technology Roadmap for Semiconductors (Lithography Roadmap-2000 Update,
http://public.itrs.netlFiles/2000 UpdateFina1/2k UdFinal.htm), (2000) .
[2] M. Villigran-Muniz, H. Sobral, and E. Camps, IEEE Trans. Plasma Sci., 29,613-616 (2001).
[3] H. Sobral, M. Villigran-Muniz, R. Navarro-Gonzales and A. C. Raga, Appl. Phys. Lett.,
77,3158-3160 (2000).
[4] C. Cetinkaya, R. Vanderwood, and M. Rowell, "Nanoparticle Removal From Substrates With
Pulsed-Laser Generated Plasma and Shock Waves ", accepted for publication in the J. of Adhes.
Sci. Technol. (2002).
[5] C. Cetinkaya, J. Lin, R. Vanderwood, and C. Li, "Nanoparticle Removal with a Pulsed Laser:
Thermoelastic Transient Surface Acceleration Simulations for Analyzing Edge Effects", accepted
for publication in the Journal of Adhesion Science and Technology, (2002).
[6] R. Vanderwood, C. Cetinkaya, andJ. Lin, "Laser Cleaning Methods for Sub-micrometer
Particle Removal", to be published in Advanced Applications in Contamination Control, June
(2002).
[7] R. Vanderwood, "Laser Based Methods for Nanoparticle Removal and Dispersion", M.S.
Thesis, Clarkson University, Potsdam, N.Y. (2002).