Surface Cleaning Using carbon Dioxide

Technology News

Surface Cleaning Using Carbon Dioxide:
Historical Perspective and Market Approach

The use of CO₂ spray to remove submicron particles from surfaces has been developed for more than 20 years. In the later 70's and 80's, it has emerged to the top of cleaning technology as a viable replacement for Freon cleaning.

So far, it has been shown that an effective delivery of CO₂ will remove the following:

Particulate contamination
Organic films, where the organic contaminant has high solubility in CO₂ (light oils, silicone lubricants, etc.)

. . . but it will not remove the following:

Particulate matter bound to the surface by chemical bonding
Heavy, insoluble oils such as solder flux.

How it works

When a liquid solvent, such as Freon, is used to remove particles from a surface, the solvent itself is passed along such surface at a certain pressure. However, as shown in Figure 1, the layer of solvent closest to the surface (boundary layer) does not move, regardless of the solvent type, velocity or pressure.

However, in general the CO₂ snow is not aggressive enough to remove particles that are chemically bonded to the surface as in product of hydration, like dried-up polishing slurries.

Effect of CO₂ Purity

Organic Purity of the Liquid Phase

Literature data indicates that CO₂ organic impurity - namely the presence of organic contaminants - produces an increase in surface particles (as measured by visual inspection through microscopy) when used to clean a surface. Thus, when using industrial grades of CO₂, the particle population on a surface was measured before and after cleaning, showing as much as a 5 time increase when organic contaminants were present in the CO₂. In fact, when organic contamination is present within the CO₂, upon passing through the orifice of the delivery nozzle, it deposits upon the surface to be cleaned in the form of very small organic droplets, appearing to the observer as small particles.

Figure 1: Solvent speed is reduced to zero as it approaches the surface to be cleaned. This creates an immobile "boundary" layer which will not remove surface residues

The thickness of such boundary layer varies with the solvent and the conditions employed, but for example, for a typical Freon moving across the surface at 10 meters per second (about 23 miles per hour) this speed is reduced to 1 to 4 m/s in the region occupied by a 25 micron particle.

In the 80's, investigators found that using CO₂ snow was an excellent way to clean optical components in space telescope lenses. Later on several papers were published, describing the mechanism of CO₂ snow cleaning and patents were filed for mechanical devices to produce CO₂-snow.

As we realize the importance of a stagnant surface boundary layer, we also realize the intrinsic advantage of CO₂ snow cleaning, in that the fast-moving jet of CO₂ easily passes through the boundary layer and reaches the surface itself.

The CO₂ jet composition needs to be controlled in its combination of solid "pellets" of CO₂ (i.e. dry ice) and gaseous CO₂. This composition is controlled by the design of the nozzle employed as well as by temperature and pressure of the gas. The gas in itself provides the vehicle to deliver the pellets to the surface with enough energy to effectively knock off particles resting on the surface. This is how surface particles are removed, being then transported away from the surface by the gas stream itself.

In addition to removing particles by the physical action described above, the CO₂ stream also removes organic films, as long as the organic itself is soluble in CO₂. This solubilization is therefore connected to the action of liquid CO₂, where the liquid CO₂ phase is obtained as a transient phase at the very surface to be cleaned, as an effect of pressure pulse, resulting in reaching the CO₂ triple point (where all three phases co-exist).

One additional cleaning mechanism for CO₂ is the result of the rapid volumetric change, as CO₂ becomes gas. This effect may contribute to the observed cleaning in very confined spaces, as under surface-mounted devices and similar out-of sight locations.

This effect becomes very important when critical surfaces are cleaned, like semiconductor wafers, magnetic read/write heads and memory disks. The presence of surface organic films or particles (organic droplets) is extremely detrimental to deposition of thin films, where adherence is negatively impacted and it can also provoke long term electrical reliability issues.

Effect of Halocarbon Impurities

Recent reports have indicated that the presence of Halogenated Hydrocarbons within the organic impurities dissolved in the liquid phase can provoke detrimental effects on the performance of electrical devices cleaned with CO₂. In particular, Halocarbon impurities can cause long term corrosion of lead connections such as welds, solder and contact joints, potentially leading to electrical failure of the device.

Scott Products

Scott has developed a system to purify CO₂ in its liquid phase. This proprietary technology allows Scott to produce several grades of CO₂ with extremely low amounts of organics in the liquid phase. For cleaning applications, this liquid phase purity is of paramount importance, since CO₂ grades of lower purity will actually deposit organics on the surface, rather then remove them (as shown in the literature studies referred to above).

Scott offers two main products targeted to surface cleaning applications as follows:

Precision Clean CO₂™(PC CO2): This is the highest liquid-phase purity CO₂ on the market. Scott specification is <0.1 ppmw of nonvolatile organic in the liquid phase.

Continupure™ Delivery System: This is a system that can deliver the same purity to the point of use, guaranteed. This becomes extremely important when considering the fact that CO₂ is an excellent solvent for organics - therefore if it comes in contact with oils, plastic seals or anything containing organics, it could become contaminated. Scott has developed a system that will maintain the purity of PC CO₂ all the way to where it is being used. This system can work either from cylinders, dewars or from a bulk tank supply.

Technology News

Surface Cleaning Using Carbon Dioxide: Historical Perspective and Market Approach

Surface Cleaning Using Carbon Dioxide:
Historical Perspective and Market Approach