In my previous two columns, I described current “wet” and “dry” technologies for particle removal, sorted by my estimate of particle size at which their use is limited. (Please refer to the tables in each article.) In this third column I will comment on the general transition from “wet” to “dry” critical cleaning. A previous literature article¹ shows how approaches have changed over 15 years.

Reasons for Change

Critical cleaning is gradually evolving from “wet” to “dry” cleaning for some basic reasons. Most importantly, “wet” cleaning doesn’t remove small particles well, partly because the boundary layer blocks access.² “Wet” cleaning is useful for particles whose size is above or somewhat below 1000 nanometers (1 micron).

Secondly, many of the chemicals employed (e.g., HF, H₂O₂, H₂SO₄, NH₄OH) are hazardous.

Thirdly, “wet” cleaning produces significant quantities of waste and uses lots of water to do that.

Location, Location, Location

Ultrasonic transducers, useful only in wet cleaning, are omni-directional: we don’t have to know the location of any particle to use the technology. However, megasonic transducers are uni-directional: we do have to know in which direction the particles are located. So the location of one or many particles isn’t really an issue with “wet” cleaning.

“Dry” cleaning, on the other hand (except for Argon aerosol and perhaps cluster beam technologies), is very location-sensitive. “Line-of-sight” is a key limitation for all the laser-based technologies. You have to know either roughly or specifically where the particle is so you can aim the laser or the tweezers at it.

Thinking about the Future

“Dry” cleaning is a very different paradigm than “wet” cleaning. In a sense, we are moving from cleaning in a tank of water to cleaning in a conditioned-environment chamber.

Critical cleaning is becoming more than putting the items on a rack, immersing the rack in three tanks of warm water-based chemicals with each tank containing sonic transducers, and afterward blowing the parts dry with nitrogen or using a vacuum environment to remove moisture.

This technology transition makes me recall one of my favorite maxims: “Whenever something gets clean, something else gets dirty.” Said in other ways: for “wet” cleaning, the parts will be no cleaner than the last rinse fluid; for “dry” cleaning, the parts will be no cleaner than the level of dirt in the clean room.

Predictions you should consider:

A significant problem with the new “dry” cleaning technologies is that, despite claims, no single technology is proven to rid surfaces of organic films, trace metallic elements, or particles simultaneously. Thus, as sonic-based technology is the tool of choice for removal of all micron-sized particles, removal of nano-sized particles will be done using “dry” technologies customized for each application.

*Critical cleaning is going to get more expensive. Remember that cleaning to a higher standard always costs more. The increase in cost, with decrease in the amount of residue, is more exponential than linear.³ So as we seek to eliminate identified nano-sized particles, we will be paying significantly more to remove each microgram of residue.

* Pre-cleaning will become more important. We’ll use cheaper “wet” technology to remove the micron-sized debris, reserving the “dry” technologies for the nano-sized particles.

* Cleaning with nanotechnology may become a rate-limiting step.

The Ultimate Transition

Critical cleaning is or will soon be done with the same tools and techniques used in the clean room for processing the parts. In other words, critical cleaning will become processing. The technology used for cleaning will transition into the technology used for processing (manufacturing).

After all, the final two items mentioned in the previous column (Microscopic Gripping Mechanism and the “Laser Tweezers”) are used for manipulating objects in the size range of 10 to 100 nm. Why can’t they be used for manipulating (removing) dirt particles in the same size range?

References:

1 J. Bardina, “Methods For Surface Particle Removal: A Comparative Study,” Particulate Science Technology, Vol. 6, No. 121 (1988).

2 J. Baker, J. Durkee, “C4: Hiding Particles in the Boundary Layer: Part 1,” A2C2 Magazine, (September, 2001).

3 J. Durkee, “Now Cost is Becoming Critical Part 1: The Cost/Quality Tradeoff,” A2C2 Magazine, (March, 2003).