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Technical Brief

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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)

boundary layers illustration

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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