Frequently Asked Questions
CO2 Snow Cleaning is simple and straightforward. We hope these questions and answers assist in understanding the process. More detailed information can be found via the links to technical pages. The links below are our most common questions.
There are four different CO2 cleaning Methods
Macroscopic hard and dense dry ice pellets
Softer microscopic "snow" particles
Liquid CO2 Washing Systems
Supercritical fluid carbon dioxide (SFCO2).
Whichever process is used, cleaning depends on either the liquid carbon dioxide solvent properties or the energy and momentum transfer by the impacting solid phase, or a combination of solvent properties and momentum or energy transfer. Pellet systems rely upon the thermo-mechanical impact stresses related to the high impact velocity by macroscopic pellets for contamination removal - a momentum and energy transfer process. Snow sprays rely upon a combination of the momentum transfer of high velocity microscopic snow particles and the solvent action of liquid CO2. The liquid based CO2 washing systems rely on the CO2 liquid phase solvent properties. Finally, the SFC systems rely exclusively upon carbon dioxide's unique supercritical fluid properties.
More details can be found on each method at Different CO2 Cleaning Methods.
Applied Surface Technologies wrote a short review paper for the 1999 McGraw-Hill Annual Review of Technology. A preprint is available, please email request to Applied Surface Technologies
What are the CO2 snow cleaning details?
Snow Cleaning systems rely on the expansion of either gaseous or liquid carbon dioxide. The output stream is usually a high velocity solid and gas mix and focused at the surface for cleaning. Cleaning is accomplished by a combination of momentum transfer and solvent action between the dry ice and surface contamination. Cleaning mechanisms are discussed elsewhere on the site. CO2 Snow Cleaning can remove particles of all sizes, even submicron and nanometer sized particles, and hydrocarbon based deposits and films. The most common commercial approach to the snow cleaning technology involves single expansion nozzles with high velocity outputs. The best nozzles aim to have a constant enthalpy expansion and a high velocity stream. The asymmetric venturi nozzles (supersonic nozzles) can yield these conditions. Other nozzle geometries give rise to high velocity snow streams but may be less focused, may need nitrogen boosting, or can compromise organic removal abilities. Carbon Dioxide Snow Cleaning units from Applied Surface Technologies use the asymmetric venturi nozzle design and generate a high velocity snow stream. With this selection, the snow spray systems can remove both particulates and organic residues and can be formed with either a liquid or gas CO2 source.
What CO2 Snow Cleaning Can Do For You?
CO2 Snow Cleaning removes both particulates and organic based residues. Particles of all sizes can be removed, from visible to nanometer. To date, the smallest particles removed have been about 3 nanometers. This is shown in our Atomic Force Microscopy Section for the AFM step height standard. CO2 Snow cleaning also removes organics, especially hydrocarbon based contamination. Oils, fingerprints, facial grease, machine shops lubricants, and many others hydrocarbon based contaminants come off quickly. Even silicone based contamination can be removed. The particle and organic removal steps are simultaneous. The cleaning process leaves no residues, is nondestructive and there is no chemical waste to handle after cleaning. Simple example of particle and organic removal from the exact same areas can be seen at Cleaning Examples or Applications, or the AFM page.
How is CO2 Snow Formed?
When either liquid or gaseous CO2 is expanded from high pressures, such as from the cylinder pressure of 800 psi (55 bars), the large pressure and temperature changes lead to solid CO2 nucleation. Factors that determine the effectiveness of the phase changes, output velocities, organic removal, the nondestructive nature of the cleaning process, and details on the thermodynamics of snow formation can be found at snow formation page First year college Physical Chemistry is useful to understand the page.
What are the two CO2 snow cleaning modes?
There are two CO2 snow cleaning modes, which I refer to as the high velocity and low velocity methods. In the high velocity method, small dry ice crystals are formed and strike the surface at a high velocity. In this mode, particle removal is over a large range, from visible down to 3 - 5 nanometers. The high velocity mode is also capable of removing hydrocarbon, organic, silicone residues, fingerprints, and even water and solvent spots. The high velocity mode is used for precision cleaning for substrates, optics and many other items. Spot size ranges from 1/8 to 1/4 inch, or 3 to 6 mm
In the low velocity mode, the initial high velocity stream exits into a larger diameter expansion zone and the small dry ice stream changes to a large snowflakes at lower velocities. This mode is most common for telescope cleaning. Application may also exist for optics and art conservation. Larger particle removal is the primary goal and hydrocarbon or organic removal is unlikely. The stream size exiting the expansion zone is either 1 inch round (25 mm) or a 2 inch slot (50 mm).
How Does CO2 Snow Cleaning Remove Particles and Organics?
The simplest description of particle removal mechanisms is described by momentum transfer between the incident CO2 Snow and surface particle. The snow's collisions with surface particulates impacts momentum, breaking the weak van der Waals particle adhesion bonds to the surface, and frees the particles from the surface. The blowing gas just sweeps them away. Strongly bound items by covalent, ionic or metallic bonds will stay. Essentially, CO2 snow cleaning can remove physically adsorbed contamination, and is unlikely to remove chemisorbed items.
Organic removal is by two possible mechanisms, solvency and freeze fracture. When the dry ice snow strikes the surface, the impact leads to pressure increases between the surface and the dry ice. Once this pressure exceeds 78 psi., the triple point pressure, the solid phase reverts back to a liquid phase. Liquid CO2 is an excellent solvent and can absorb large quantities of hydrocarbons.
CO2 Snow cleaning can also remove silicones, and flux residues under certain conditions. In this process, the desired species are not soluble in liquid CO2 and thus another mechanism is present. Here, the removal is by freeze-fracture, just by freezing the debris off.
There are more details to the cleaning mechanisms and the reader can get the details at Cleaning Mechanisms.
How is CO2 Snow Cleaning Different from the other CO2 Cleaning Methods ?
Essentially, CO2 Snow Cleaning is a hybrid method, a mix of impact energy of pellets and solvency action of liquid CO2. It is the only CO2 cleaning process that removes particles and organic residues simultaneously without any other assistance. It is a continuous process, not batch like supercritical or liquid CO2. Furthermore, it is nondestructive to well bonded over layers, opposite the abrasive power of pellets. Another major difference is the price, CO2 snow cleaning equipment starts are about $1800 for a full system, the other processes require a factor of 10 greater costs to get going.
Where Does CO2 Snow and Contamination Go?
The debris and snow just fly away. Since it does, the user has to make sure that proper venting or capture is done and no clean regions get re-contaminated.
What Are The Process Parameters?
Proper cleaning using CO2 Snow Cleaning requires attention to the set up and details. Proper cleaning using CO2 Snow Cleaning requires attention to the set up and details. A proper CO2 Snow cleaning set up has methods to:
Avoid recontamination once cleaned
Avoid recontamination from cleaning equipment and CO2
Methods and Procedures
Prevent Moisture Condensation
Avoid Static Charge Buildup
We have a separate page discussing these in greater detail; please go to Cleaning Issues Page
Does CO2 Snow Cleaning Cause Damage?
In most all cases, no damage. CO2 Snow cleaning, because of its high velocity, can't clean powders or individual fibers unless they are well supported. In the past, we managed to remove particles off a 1.0 mil Au wire bonded to a pad and are willing to try again. We have cleaned the ends of fiber optics and can even remove particles from a lubricated hard drive without removing the lubricant. As for damage, soft polymers such as PTFE, can show damage but it requires an AFM or high power SEM to find. Damage is discussed more in Cleaning Issues.
What are some Simple Examples?
How did the CO2 Snow Cleaning Technology Originate?
Most all people credit Stuart Hoenig of University of Arizona, Tucson, for introducing CO2 Snow. His first papers date to 1985-86. Prof. Hoenig, now retired, went about the country demonstrating the process and trying to get interest among high technology companies and the industrial gas companies. The BOC Group, formally Airco Industrial Gases and now Linde, saw the talk and developed nozzles based upon the venturi design. Other companies including Hughes Aircraft, developed nozzles based upon a straight tube. Both of these companies are no longer in the business of CO2 Snow Cleaning. As an aside, Applied Surface Technologies is from the BOC efforts.
What Are it's Best Applications?
Many applications have been developed and the best way to summarize the best application for CO2 Snow Cleaning is when an inline, continuous or occasional process is needed for particle removal or organic removal or both. Some Applications include:
contamination removal from metals, ceramics, polymers, and glasses
particle and stain removal from Si, InP, and GaAs wafers
cleaning optics, i.e., coated lenses, laser, IR and UV optics
sample preparation before surface analysis (Auger, XPS, SIMS) and AFM
cleaning vacuum systems components, bellows, electron and ion optics
general laboratory, production, and cleanroom cleaning
manufacturing of many metal and ceramic parts and assemblies
removing particles from microelectronic and hybrid circuits
art restoration, fire recovery, telescopes, and many many more.
Is The Process Safe To Use? Noisy?
Safe and quiet, see Safety Issues!!!