Honors Program Thesis Proposal
Jason A. Smith
The Effects of Pressure on Particle Removal from Substrates with Laser-Induced Plasma
Introduction
Most industrial processes create particles as a side effect of the process. The removal of
these particles during or after the manufacturing process is very important to many industries.
Among these industries are semiconductors, optics, and micro-electromechanical systems. As
time progresses, the size and scale of many of these industrial parts continues to miniaturize. As
the parts being manufactured get smaller, the adverse affect of unwanted particles becomes more
apparent. There is an apparent need to remove smaller particles with improved cleaning methods
and technologies.
The best example of this is in the semiconductor industry. Silicon wafer substrates are the
backbone of the industry's production. Most wafers that are produced are subjected to Chemical
Mechanical Planarization (CMP). CMP is an abrasive process that uses chemical slurries and a
circular action to polish the surface of a wafer smooth. Once complete, some of the particles in
the slurry will have adhered to the surface of the wafer by adhesion. These particles are
undesirable and need to be removed as efficiently as possible while avoiding substrate damage.
The use of a fast, dry, non-chemical, non-contact, and non-damaging particle removal method is
ideal for this situation and many other manufacturing processes.
Background
The Laser-Induced Plasma (LIP) technique is a relatively new method of particle removal.
Because of this, there is not a lot of information or literature about it. However, the body of
knowledge about LIP is growing rapidly. The initial results of LIP removal of 1m tungsten
particles from a silicon substrate were published by Lee and Watkins in 2001 [1]. Similar results
were obtained by Vanderwood and Cetinkaya when they removed silica particles as small as
0.46m from surfaces using the same technique [2,3]. Since not all surfaces in industry are flat,
this study was extended to examine the removal of particles from pinholes and trenches [ 4]. That
particular study had success in removing silica particles from 0.46m to 1.58m in size. All of
these studies were limited to observing removal in a single location. Hooper and Cetinkaya
extended this research further by demonstrating the effectiveness of this process on wafers larger
than 3mm with silica, polystyrene, and ceria particles [5,6]. They also have done research to
determine optimal laser firing distances for removal maximum removal without substrate damage
[7].
Process
The LIP technique is accomplished by firing a laser pulse parallel to the surface of the substrate.
The laser pulse is focused to a point by a lens. The resulting energy density near the focal point
becomes so great that the air breaks down into plasma. This phenomenon has been studied for
decades now.
Figure 1: Schematic of LIP removal setup.
The plasma core is at a very high temperature and as this core expands, it generates a shockwave.
When this shockwave hits the particles on the substrate, it imparts a force to them. The dominant
removal mode is the rolling moment caused by the transient pressure field across the shockwave.
The critical pressure needed for detachment of a particle can be expressed by:
*
prolling

2a( FA  mg)
As ( D cos  2a sin  )
where As is the effective cross-sectional area perpendicular to the pressure field, g is the
gravitational acceleration, and  is the angle between the force vector and a plane parallel to the
substrate surface. The weight force, mg, is negligible for particles of such a small size.
Figure 2: Geometric features of a spherical particle attached to a smooth surface under an
applied pressure field.
The LIP removal process has already been proven to be an effective method for removal. The
next step in making the process more industrially applicable will be to determine how the existing
procedure can be modified to obtain the best results. The experiments to be run for this thesis will
be the removal of silica particles from a silicon substrate. The removal of these particles will be
repeated for several atmospheric pressure variations as these
experiments have only been run at standard conditions. The
initial pressures to be tested at will be 0.25,0.5, 1, 1.5, and 2 atm.
The effectiveness of the removal will be the main area of
concern. Silica particles are chosen for the initial experiments
because of the particle's mostly uniform, spherical shape (as
shown to the left at 30,000x magnification). This shape is the
basis for the equations used in the theoretical model of LIP removal. This will allow a more
careful examination of the differences that pressure has on removal. The main purpose of this
thesis will be a comparison of the effects on removal. This may be expanded to include changing
firing distances from the substrate pending the outcomes and results of initial tests.
References
[1] J.M. Lee and K.G. Watkins, J. Appl. Phys. 89,6496-6500,2001.
[2] R. Vanderwood, M.S. Thesis, Clarkson University, Potsdam, NY , USA (2002).
[3] C. Cetinkaya, R. Vanderwood, and M. Rowell, J. of Adhes. Sci. Tech. 16, 1201-1214, 2002.
[4] Nanoparticle Removal from Trenches and Pinholes with Pulsed-Laser Induced Plasma and
Shockwaves, R. Vanderwood and C. Cetinkaya, in press (# MI370), Journal of Adhesion Science
and Technology, 2002.
[5] T. Hooper Jr., C. Cetinkaya, "Efficiency Studies of Particle Removal with Pulsed-Laser
Induced Plasma", accepted for publication in the Journal of Adhesion Science and Technology,
2002.
[6] T. Hooper Jr., C. Cetinkaya, "Particle Removal with Laser-Induced Plasma over an Extended
Area of a Silicon Wafer", accepted for publication in the Proceedings of the Eighth International
Symposium on Particles on Surfaces, 2002.
[7] T. Hooper Jr., C. Cetinkaya, "Particle Removal from Substrates with Laser-Induced Plasma",
Honors Program Thesis, Clarkson University, Potsdam, NY, USA (2003)