Page 264 - Semiconductor Manufacturing Handbook

Geng(SMH)_CH18.qxd 04/04/2005 19:58 Page 18.5

WET CLEANING

WET CLEANING 18.5

number for the determination of the effect of the hydrodynamic removal force on the cleaning and a
moment balance approach to determine the affect of the direct moment transfer from the brush to the
particle. The flow particle Reynolds number is defined as
r
dV
Re = p
p
m
where d = particle diameter
r = fluid viscosity
V = relative velocity between the fluid and the particle at the center of the particle
p
m = fluid viscosity
The critical particle Reynolds number (Re ) is the Reynolds number at which the particle will
pc
begin to roll on the surface. In order for the particle to be removed, Re must be greater than Re .
p
pc
Typically in a scrubbing system, the distance between the brush and the wafer surface is such that
brush-particle interaction occurs that has the effect of either lowering Re due to adhesion to the
pc
9
brush or a momentum transfer between the brush and the particle. Recent work has concluded that
the brush must come into contact with the particle for complete removal and that the particle shape
10
has a great effect on the efficiency of the scrubbing system. The optimum wafer surface for this
type of clean is a hydrophobic wafer (particles are transferred to the brush) without patterning (the
bristles cannot penetrate into small line widths because of their size). The benefit of scrubbing is its
highly efficient removal of particles. The problems associated with scrubbing are maintenance inten-
siveness as brushes must be changed or cleaned frequently, the poor cleaning ability on patterned
wafers, and the potential to damage the wafer surface.
Pressurized Jet Cleaning. Pressurized jet cleaning is the application of a liquid, usually water, by
a high pressure nozzle. Air or nitrogen has been used for many years to remove particles from a sur-
face. A more refined approach is a pressurized liquid jet to remove small particles from the wafer
surface. The phenomenon that governs this type of cleaning is that the kinetic energy of the moving
fluid imparted to the particle is greater than the force of adhesion of the particle on the surface there-
by removing the particle from the wafer surface. The use of a liquid instead of gas for the jet has
some fundamental benefits. The force imparted from one body to another body is based on the
momentum of the moving object. Therefore, at comparable velocities, a liquid will have a much
greater momentum than a gas due to its higher density. Another benefit of using a liquid instead of
a gas is that a liquid develops a smaller boundary layer than a gas thereby allowing the jet stream to
interact with much smaller particles. Furthermore, a pressurized jet has the ability to remove small
particles from a patterned wafer. Disadvantages of this cleaning method are the potential for damage
of the wafer surface and the propensity for the moving fluid to create a static charge that can lead to
device damage.

Sonic Cleaning. Sonic cleaning can be divided into two categories—ultrasonics and megasonics.
The principle for sonic cleaning is to use the energy of sound waves to initiate cavitation, in the case
of ultrasonics, or acoustic streaming (high-velocity pressure waves), in the case of megasonics, in a
liquid system. Sonic energy is used to overcome the particle adhesion force and dislodge the parti-
cle from the wafer surface. The ultrasonic frequency range is from 20 to 800 kHz, while the mega-
sonic frequencies are greater than 800 kHz.
Megasonic cleaning is dependent on the reduction of the boundary layer and on acoustic stream-
ing. The acoustic boundary layer is very small when compared to the hydrodynamic boundary layer.
The acoustic boundary layer is given as

n 
d ac =  2 w  12 /


Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.