DEFINE CLEANING
“What do you mean by plasma cleaning?” Bill Moffat, CEO of Yield Engineering Systems, Inc. in Livermore, CA, a manufacturerof plasma cleaning systems, poses this reasonable question. It is in fact a question that ought to be asked in developing processes involving ultrasonic cleaning and in developing any cleaning process.

Bill considers the term plasma etching to be more descriptive than plasma cleaning. Further, he distinguishes photoresist removal, which he describes as “eating organics,” from cleaning. Bill adds, that in many instances manufacturers describe a process as plasma cleaning but what they are really doing is surface modification. For example, in gold wire bonding, argon gas is made electrically active; the gas creates hills and valleys in the surface.

Bill distinguishes cleaning, or swishing a chemical over the surface, from etching, where an organic is removed. For example, if asked to remove a fingerprint with plasma, he explains that while he might remove the oil, in the process, an indelible fingerprint would be etched onto the surface. Mopping a floor is cleaning. In exposing the same floor to plasma, eventually the soils would be removed, but the rest of the surface would be bombarded so that eventually there would be a three dimensional picture of the stain on the floor.

ULTRASONICS OR MEGASONICS
Why do we start this discussion of ultrasonic cleaning with a discourse about plasma? That is because it might lead us to consider the possibility that cleaning and surface modification are a continuum, beginning at the micro level and certainly at the nano level. Are we doing ultrasonic cleaning, or very controlled ultrasonic etching? Or perhaps a very limited, incompletely-defined surface modification? In ultrasonic cleaning as sound waves pass through a liquid, vapor-filled voids form during rarefaction (pulling apart of the liquid). These bubbles implode violently during compression. This provides the cleaning power, and also the potential for surface modification.The power of ultrasonics is used in related techniquesto bond plastics. There are ultrasonic techniques for joiningmany materials including metals to plastics, metalsto ceramics, optics fibers and even biologics.

Are we producing surface modification in ultrasonic cleaning? Ultrasonic cleaning has been used in many applications for decades without observable product damage. At the same time, the functionality of an ultrasonic system is typically tested by observing the impact of ultrasonics on aluminum foil; and this is ultrasonic erosion. We might even speculate that ultrasonics produces benign or perhaps subtly-beneficial surface modification. During development of ultrasonic methods, we occasionally see pragmatic evidence that exposure to ultrasonics produces what might be described as workhardening of the surface. Occasionally, the interaction of cleaning chemistry and ultrasonic power produces unanticipated materials incompatibility. For example, certain metals may show visual changes.