Immersion¹ cleaning with liquid or supercritical CO₂, can be advantageous. CO₂, raises no concern about health issues and is non-flammable, virtually inert, and not an ozone-depleting compound. Nor does it generate concern about global warming It also has a low price! After cleaning, the only waste generated is the contaminants that were removed from the cleaned part. There are no large volumes to treat no liquid (a sin aqueous cleaning) or air (as in some solvent cleaning systems). For these reasons, various US government agencies have evaluated CO₂, immersion cleaning. Some are using it. But many customers are disappointed. Why isn’t every one using this technology?

High Cost

Immersion CO₂, technology requires significant investment at any level of cleaning quality. It costs several hundred thousand dollars to contain, control, let-down, and attain the pressures associated with supercritical(>l,OOO psi) and liquid (XOO psi) CO₂. This work is by necessity batch. Interlocks for continuous operation between high and atmospherinnc pressure are too expensive to be justified.

Unmatched Solvency

In a sense, CO, is the wrong solvent. It is a unique molecule Applying the principle of “like dissolves like,” few soils are similar to liquid CO₂. This can be shown with Hansen Solubility Parameters:

Liquid or supercritical (fluid) CO₂, can be of value in dissolving light oils. Fluid CO₂, doesn’t have adequate polar or hydrogen-bonding character² to match many other soils. But fluid CO₂, does have near-zero surface tension and a very low viscosity. That makes it an excellent candidate for flushing or particle removal.³

Commercial Response

Developers try to change the solvency of fluid CO₂, by adding surfactants to the pressurized cleaning bath.⁴ The excitement now is with added gases and/or chemistries to enhance soil match up and removal. Additions of linear alcohol or THPA are increasing the applications for removal of resin flux for hybrids. Injection of per-oxide and flash lamp treatment are creating opportunities for wafer production. Of course, this affects process control, rinsing, and performance; and increases the already high investment.

Others have created unique processes to increase value to clients.⁵ Particle removal has become an area of activity. Recently, a major supplier of products and services to the those doing critical cleaning announced collaboration with a developer of super- critical CO₂, technology⁶ to “... further develop and commercialize technology for dense and supercritical CO₂, semiconductor wafer processing....” as well as MEMS-based products.

What Of The Future?

Today, I doubt $5 million of equipment and processes are sold in the US. That will increase with the announced collaboration. Marketing has not been focused because of the difficulty in identifying applications where customers will value CO₂, immersion cleaning. The problem hasn’t been marketing. The problem has been that valued applications for fluid CO₂, cleaning have been scarce. While I am not currently involved in this area⁷ I believe valued applications are more likely to be found where we haven’t looked rather than where we have. These new work areas might be nanotechnology, manufacture of medical devices, printing devices, or (to think more broadly) manufacture in a “liquid cleanroom.” Of course I could be wrong!

References:

1. Not to be confused with non-immersion cleaning with CO, where fragments of solid CO2, impinge upon soil at the part surface.

2. CO2, has no hydrogen atoms, so it shouldn’t have hydrogen-bonding character.

3. A knowledgeable friend writes “... I have NOT found liquid CO2, particularly good at removing anything but very small and loose particles...”

4. Recent patents include: 6,613,157 Methods for removing particles from microelectronic structures, and 6,200,943 Combination surfactant systems for use in carbon dioxide-based cleaning formulations.

5. Patents of interest include: 6,596,093 – Methods for cleaning microelectronic Structures with cyclical phase modulation, 6,506,259 - Carbon dioxide cleaning and separation systems, and 5,344,493 - Cleaning process using microwave energy and centrifugation in combination with dense fluids and 5,377,705 - Precision cleaning system.

6. In 1997, your author recommended these activities to one of the participants in this collaboration.

7. Or I couldn’t write about this topic.