Water provides three functions in critical cleaning: solvationof 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 + v<200nm → 2 O *

Equation 2: 2 O₂ + 2 O * → 2 O₃

The * symbol on the oxygen atom means that this specie is not acomplete 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 lampwhich 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 + —CH₂— → 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 hydrogenand 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 withthe 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," SemiconductorInternational (July, 2003).

2 K. L. Norrod, K. L. Rowlen. "Ozone-Induced Oxidation of Self-AssembledDecanethiol: 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 Pentachlorophenolby Ozone," Air & Waste Management Association (April, 2002).

4 S. Ledakowicz, J. S. Miller, D. Olejnik. "Oxidation of PAHsin Water Solution by Ozone Combined with Ultraviolet Radiation," InternationalJournal of Photoenergy, Vol.3 (2001).