What is the Boundary
Layer as it relates to Megasonic cleaning?
During Megasonic cleaning, the cleaning solution rushes past
the substrate being cleaned, forcing chemistry onto contaminant
particles, removing them from the surface, and carrying them
away. On a microscopic scale, the fluid friction at the surface
causes a very thin layer of solution to move more slowly than
the bulk solution. This layer of slow-moving fluid at the
substrate surface is called the Boundary Layer. The Boundary
Layer effectively shields the substrate surface from fresh
chemistry and shields contaminants from the removal forces
of the bulk fluid. *
Boundary layer thickness depends on the following factors:
- Frequency of the acoustic wave
- Energy (acoustic intensity) effectively transmitted to
the solution
- Fluid properties (kinematics viscosity, density)
Why is the Boundary Layer important?
Minimizing the Boundary Layer is essential to optimal particulate
removal and transport, high chemistry refresh rates, and chemical
access to surface features.
The reduced fluid flow in the Boundary Layer negatively impacts
the processes in the following manner:
- Particles are shielded from the flow of the cleaning solution,
and remain on the substrate
- Fresh chemistry does not effectively reach the interface
of small surface features leaving strip residues or under-etched
devices
- Cleaning times may be unacceptably long
How is the Boundary Layer reduced?
The Boundary layer can be reduced in the following ways:
- Increase the acoustic frequency. Boundary layer thickness
decreases with increased frequency (0.5µ at megasonic
frequencies as compared to 2.5µ at ultrasonic frequencies)
This effect is especially important in removing small particles
and accessing small surface features.
- Transmit more energy (acoustic intensity) to the solution
Higher energy results in a thinner Boundary Layer. Pulsed
wave megasonics transmits significantly more energy than
continuous wave megasonics.
- Increase acoustic streaming
Acoustic streaming is the fluid motion induced by the velocity
gradient near a small bubble under megasonic vibration.
Acoustic streaming is a function of frequency and delivered
intensity.
Acoustic streaming has several important effects:
- The primary effect is the strong localized flow of cleaning
solution, whose shear force is the primary particle removal
agent. Boundary Layer thickness decreases with increase
in this fluid velocity.
- The small, controlled cavitation bubbles generated by
pulsed wave megasonics remove contaminants with in the thinner
Boundary Layer. Acoustic streaming enhances the transport
of particles once they are detached (both within and outside
the Boundary Layer).
Benefits of Decreasing the Boundary Layer
Decreasing the thickness of the Boundary Layer has the following
benefits:
- Increased removal of sub 0.5(µ particles (particles
previously protected by the Boundary Layer )
- Increased particle removal overall
- Increased transport of removed particles through increased
acoustic streaming
- Higher chemistry refresh rate at the substrate surface
resulting in faster cleaning
- Increased chemical access to small surface features for
enhanced etch or strip applications
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