C4: Critical Cleaning For Contamination Control By: John B. Durkee, PH.D., P.E. August 2004
A2C2 --
Water provides three functions in critical cleaning: solvation
of soils; rinsing, flushing, or displacement of undesirable liquids
or solids; and conveyance of cleaning agents to where they are needed.
Water from various cleaning processes is one of the largest waste
streams in industry. This situation is of greater concern where critical
cleaning is done because the last rinse stream contacting the parts
must be pure, as it limits cleaning quality. The burden of supply
of high-purity water, and its disposal, involves investment/operating
cost and environmental/societal consequences.
Bearing that burden has led many users to investigate alternatives
including replacement of water as the conveyor of cleaning agents
or the cleaning agent itself. Organic solvents are one alternative;
ozone, added to water, is another.
It's the Function!
Ozone doesn't provide any of the above three functions. Ozone is
an oxidizing agent, not a solvating agent. The virtue of ozone is
that when it has done its work, the above burden doesn't exist.
The "O" Zone
A simplified description of the very complex chemical reactions
which produce ozone is:
Equation 1: O2 + n , 200nm 2 O *
Equation 2: 2 O2 + 2 O * 2 O3
The * symbol on the oxygen atom means that this specie is not a
complete molecule. The oxygen atom (O*) is very reactive. In the
second step of ozone formation, two oxygen molecules and two oxygen
atoms react to form two ozone molecules. The combined process, with
UV radiation as an active initiator, is:
Equation 3: 3 O2 2 O3
Ozone is produced by three types of generators: an ultraviolet lamp
which is often seen in dentists' offices; corona discharge, where
a tube with a hot cathode is surrounded by a screen anode; and cold
plasma where two glass rods filled with a noble gas produce an electrostatic
plasma field which turns the oxygen into ozone.
Application of ozone is as an oxidizing agent. The generalized example
is described in Equation 4. These reaction products are the reason
why ozone cleaning avoids many negative environmental/societal consequences.
Equation 4: O3 + 2CH22 CO2 + H2O
Sterilization with Ozone
The basic role in cleaning operations is to oxidize organic materials
to harmless byproducts. If the soil contains no organic materials,
ozone may provide no value. Medical operations, such as sterilization,
have long been common. This is because organisms contain hydrogen
and carbon. Other applications involve: food and beverage service,
water purification, and hospital laundry.
In cleaning water, ozone is more efficient even than chlorine. Based
on the time needed to kill 99.99% of all micro-organisms, ozone is
25 times more effective than hypochlorous acid, 2,500 times more
effective than hypochlorite, and 5,000 times more than chloramine.
Particle Removal with Ozone
Oxidation can be of value with more than organics. The oxidation
state of silicon surfaces of semiconductors is of considerable importance
during their manufacture. Application of HF acid in DI water followed
by treatment with ozone first removes a surface oxide layer, and
then replaces it with fresh oxide. Why would this be done? Because
particles on the initial oxide surface can often be removed with
the original oxide.
Ozone is sparingly soluble in water—around 100 ppm. Application
of ozone to semiconductor surfaces can be done with water in which
ozone has been dissolved or by jetting pressurized ozone gas through
flowing DI water onto surfaces.1 The former approach produces longer
cycle times because the oxidation effect is smaller, but may be easier
to control.
Cleaning Up
Cleaning with ozone is not for the masses. It's highly toxic. The
OSHA 24-hour exposure limit is 0.1 ppm. Fortunately, the odor threshold
limit is 0.012 ppm. Yet it is commonly used as a biocide in cooling
towers.
Could the acid/ozone two-step system be used to remove light oils
and particles from stainless steel? That would change applications
from niche to universal! Technically it is possible. But, kinetics
of oxidation on metal surfaces are not well-known.2,3,4 Significant
process development would be required. Would anyone choose to use
acid/ozone to avoid using alkali with water management problems or
solvents with air pollution problems? I don't know.
References
1 M. A. Lester. "Single Wafer Ozone Clean Done Without Megasonics," Semiconductor
International (July, 2003).
2 K. L. Norrod, K. L. Rowlen. "Ozone-Induced Oxidation of Self-Assembled
Decanethiol: Contributing Mechanism for "Photooxidation," J.
Am. Chem. Soc. Vol. 120 (1998) pp. 2656-2657.
3 J-Y Kim, S-H Moon. "A Kinetic Study on Oxidation of Pentachlorophenol
by Ozone," Air & Waste Management Association (April, 2002).
4 S. Ledakowicz, J. S. Miller, D. Olejnik. "Oxidation of PAHs
in Water Solution by Ozone Combined with Ultraviolet Radiation," International
Journal of Photoenergy, Vol.3 (2001).
