Ultrasonic Cavitations and Precision Cleaning
(Revised 5/2013)

"Precision Cleaning - The Magazine of Critical Cleaning Technology"

 by  Sami B. Awad, Ph.D.

Precision or critical cleaning  is currently in great demand and is expected to increase in the future. The rapid advancements in various current technologies and the constant trend in miniaturizing of components have created a need for higher cleanliness  levels. Contamination in the level of monolayers can drastically alter surface properties such as wettability, adhesion, optical or electrical characteristics. Particles in the range of few microns down to submicron levels, trace contaminants such as non-volatile residues (NVR) in the  range of micrograms/cm2 and pictogram/cm2, ionics in the same range or traces of corrosion have become part of the daily concerns of the manufacturing  engineers in major industries such as semiconductors, automotive, disk drive,  optics, ophthalmic, glass, medical, aerospace, pharmaceuticals and tool  coatings, among others.

The specifications on trace contaminants and particle sizes are being tightened periodically to reflect the new technology trends. Every  industry has its own set of cleanliness specifications and the focus differs. For  example, while NVR has not been an automotive industry issue until now, it has  been crucial for the semiconductor and the disk drive industries for years.  Trace contaminants are not acceptable in the carbide, optics and ophthalmic industries, as they may cause adhesion failures in a multi-coating  process that follows cleaning. For obvious reasons, absolutely clean surfaces are an extremely critical requirement in cleaning medical devices. Concern  about particles has become a common denominator among all industries.

Precision Cleaning

Precision or critical cleaning of components or substrates is the  complete removal of undesirable contaminants to a desired preset level. The  preset level is normally the minimum level at which no adverse effects take place in a subsequent operation. To achieve this level, it is critical not to  introduce new contaminant(s) into the cleaning process. For example, if the  cleaning of organic and ionic contaminants is achieved by an aqueous process, it is important to have high quality water and the proper parameters in the  rinsing stages. Otherwise, residual detergent and/or ionics from the rinsing  water will be the new contaminants. If drying is slow, deionized rinse water may  react with some metallic surfaces at high temperatures and create undesirable  stains or marks. Re-contamination of cleaned parts with outgassed residues  produced from packaging or storing materials is another big concern.

To select an effective cleaning method, the three essential factors  directly influencing cleaning results are the cleaning chemistry, the scrubbing  method and the process parameters. The subject of examining various  combinations of available cleaning methods and their effectiveness, or lack thereof, is  massive and well-explained in the current literature (see references). The focus in this article will be on ultrasonic cavitations and the ultrasonic  cleaning mechanism. Ultrasonic technology is proven to be a versatile method for cleaning various organic, inorganic and particle contaminants from  various metallic and nonmetallic surfaces.

Ultrasonic Cavitations and Surface Cleaning

Cleaning with ultrasonics offers several advantages over other  conventional methods. Ultrasonic waves generate and evenly distribute cavitation  implosions in a liquid medium. The released energies reach and penetrate deep into crevices, blind holes and areas that are inaccessible to other cleaning methods. The removal of contaminants is consistent and uniform,  regardless of the complexity and the geometry of the substrates.

                                                                                      

                                                                            Figure 1

Ultrasonic waves are mechanical pressure waves formed by actuating  the ultrasonic transducers with high frequency, high voltage current  generated by electronic oscillators (power generators) (Figure 1). A typical industrial high power generator produces ultrasonic frequencies ranging  from 20 - 120 kHz. Typical PZT transducers are normally mounted on the bottom  and/or the sides of the cleaning tanks or immersed in the liquid. The generated ultrasonic waves propagate perpendicularly to the resonating surface.  The waves interact with liquid media to generate cavitation implosions. High  intensity ultrasonic waves create micro vapor/vacuum bubbles in the liquid medium, which grow to maximum sizes proportional to the applied ultrasonic frequency  and then implode, releasing their energies. The higher the frequency, the smaller the cavitation size. The high intensity ultrasonics can also grow cavities  to a maximum in the course of a single cycle.

Cavitation Generation and Abundance

                                                   

                                                                           Figure 2

At 20 kHz the bubble size is roughly 170 microns in diameter (Figure 2). At a higher frequency of 68 kHz, the total time from nucleation to implosion  is estimated to be about one third of that at 25 kHz. At different  frequencies, the minimum amount of energy required to produce ultrasonic cavities  must be above the cavitation threshold. In other words, the ultrasonic waves  must have enough pressure amplitude to overcome the natural molecular bonding  forces and the natural elasticity of the liquid medium in order to grow the  cavities. For water, at ambient, the minimum amount of energy needed to be above the threshold was found to be about 0.3 and 0.5 watts/cm2 per the transducer radiating surface for 20 kHz and 40 kHz, respectively.

The energy released from an implosion in close vicinity to the  surface collides with and fragments or disintegrates the contaminants, allowing  the detergent or the cleaning solvent to displace it at a very fast rate.  The implosion also produces dynamic pressure waves which carry the fragments away from the surface. The implosion is also accompanied by high speed micro streaming currents of the liquid molecules. The cumulative effect of  millions of continuous tiny implosions in a liquid medium is what provides the  necessary mechanical energy to break physically bonded contaminants, speed up the hydrolysis of chemically bonded ones and enhance the solubilization of  ionic contaminants. The chemical composition of the medium is an important  factor in speeding the removal rate of various contaminants.

                                                              

                                                                          Figure 3

The ultrasonic cleaning model (Figure 3) illustrates the generating cavitations through at least three steps: nucleation, growth and violent collapse or implosion. The transient cavities (or vacuum bubbles or  vapor voids), ranging 50-150 microns in diameter at 25 kHz, are produced  during the sound waves' half cycles. During the rarefaction phase of the sound  wave, the liquid molecules

are extended outward against and beyond the liquid natural physical elasticity/bonding/attraction forces, generating a vacuum nuclei that  continue to grow. A violent collapse occurs during the compression phase of the  wave. It is believed that the latter phase is augmented by the enthalpy of the  medium and the degree of mobility of the molecules, as well as the hydrostatic pressure of the medium. Cavitations are generated in the order of  microseconds. At the 20 kHz frequency, it is estimated that the pressure is about  35-70 K Pascal and the transient localized temperatures are about 5000°C, with  the velocity of micro streaming around 400 Km/hr (Figure 2).

Several factors have great influence on the cavitation's intensity  and abundance in a given medium. Among these factors are the ultrasonic wave form, its frequency and the power amplitude. Other determining factors are the colligative properties of the liquid medium, including viscosity,  surface tension, density and vapor pressure; the medium temperature and the  liquid flow, whether static or dynamic or laminar; and dissolved gases.

In general, at low frequencies (20-30 kHz), a relatively smaller  number of cavitations with larger sizes and more energy are generated. At higher frequencies (60-100 kHz), much denser cavitations with moderate or lower energies are formed. Low frequencies are more appropriate for cleaning  heavy and large-size components, while high frequency (60-80 kHz) ultrasonics  is recommended for cleaning delicate surfaces and for the rinsing step.

For example, at 68 kHz, the cavitation abundance is high enough and  mild enough to remove detergent films and remove submicron particles in the  rinsing steps without inflicting damage on surfaces. The 35-45 kHz frequency  range was found to be appropriate for a wide range of industrial components and materials.

Estimates of cavitation abundance at various ultrasonic frequencies  have shown that the number of cavitation sites is directly proportional to  the ultrasonic frequency. For example, about 60 to 70 percent more  cavitation sites per unit volume of liquid are generated at 68 kHz than at 40 kHz. The  average size of cavities is inversely proportional to the ultrasonic frequency.

Therefore, one would expect that at the higher frequency, at a given  energy level, the scrubbing intensity would be milder, particularly on soft and thin or delicate surfaces, and more penetration and surface coverage into the recessed areas and small blind holes would be expected.

Ultrasonic Frequency and Particle Removal

Recent investigations have confirmed that higher frequencies are more effective for the removal of certain contaminants. Reports on particle  removal efficiency have shown that the removal efficiency of one micron and  submicron particles in deionized water has increased with the higher frequency. At 65 kHz, the removal efficiency of a one micron particle is 95 percent,  versus 88 percent at 40 kHz. A similar increase in efficiency results was reported for 0.7 and 0.5 micron particles. It was also reported that there was zero  or little difference in the removal efficiency of particles at the  ultrasonic frequency of 65 kHz and at the megasonic frequency of 862 kHz. Both  frequencies showed 95 percent removal efficiency of one micron particles and 87/90,  84/85 for 0.7 and 0.5 micron particles, respectively.

Aqueous and Semi-Aqueous Ultrasonic Cleaning

Cavitations generated in plain water can clean limited numbers of  certain contaminants. However, cleaning is more complex in nature than just  extracting the contaminants away from the surface. Consistency and reproducibility  of results are the key, particularly in industrial production lines.  Cleaning chemistry, as part of the overall cleaning process, is a crucial element in achieving the desired cleanliness. First, the selected chemistry must cavitate well with ultrasonics. Also, compatibility of the chemistry with the substrates, wettability, stability, soil loading, oil separation, effectiveness, dispersion of solid residues, free rinsability and chemistry disposal are all crucial issues that must be addressed when deciding on  the proper chemistry. Chemistry is needed to do one or multiple tasks - to  displace oils or solvents, to solubilize or emulsify organic contaminants, to encapsulate particles, to disperse and prevent redeposition of  contaminants after cleaning. Special additives in cleaning chemistries can assist in  the process of breaking chemical bonding, removal of oxides, preventing  corrosion or enhancing the physical properties of the surfactants.

For example, we have found that ultrasonic cavitations enhanced the  removal efficiency of hydrophobic solvent cleaning films by about 30 to 40  percent versus using a spray rinse technique, when coated metallic and  nonmetallic surfaces were treated with aqueous displacement solutions (ADS). The ADS material is chemically designed to be compatible with the substrate and  to rapidly displace hydrophobes. All tested surfaces were rendered  solvent-free and hydrophilic.

Particles, in general, are not spherical and have irregular shapes.  Some of the adhesive forces that influence detachment of a particle are van der  Waals, electrical double layer, capillary and electrostatic. One would expect  that small particles are easier to remove. The fact is that the smaller the particles, the more difficult they are to remove. The weight of a  particle is another factor greatly influencing a particle detachment. Kaiser has  recently reported that although the force between a particle and an adjacent  surface decreases with particle size, it becomes more difficult to remove a  solid particle from a solid surface because of the value of the ratio, Fa/W,  where Fa is the force of attraction and W is the weight of the particle. The  value of Fa/W increases rapidly as the diameter of a particle decreases.

Ultrasonic Systems

Typical ultrasonic aqueous batch cleaning equipment consists of four  steps: ultrasonic cleaning, two ultrasonic reverse cascade water rinses and  heated re-circulated filtered clean air for drying. The number and the size of  the stations are determined based on the required process time. A  semi-aqueous cleaning system includes an extra station for solvent displacement,  connected to a phase separation/recovery system. Typical tank size ranges from 20  liters to 2,000 liters, based on the size of the parts, production throughput  and the required drying time. The cleaning process can be automated to include computerized transport systems able to run different processes for  various parts simultaneously. The whole machine can be enclosed to provide a  clean room environment meeting class 10,000 down to class 10 clean room  specifications. Process control and monitoring equipment consists of flow controls,  chemical feed-pumps, inline particle count, TOC measurement, pH, turbidity, conductivity, refractive index, etc. The tanks are typically made of  corrosion resistant stainless steel. However, other materials are also used - such as quartz, PVC, polypropylene or titanium - to construct tanks for special applications. Titanium nitride coating is used to extend the lifetime of the radiating surface in tanks or immersible transducers.

Automation of a batch cleaning system is an integral part of the  system. Advantages of automation are numerous. Consistency, achieving  throughputs, full control on process parameters, data acquisition and maintenance of  process control records are just a few.

Mechanism of Cleaning

Two main steps take place in surface cleaning. The first step is  contaminant removal and the second is keeping those contaminants from re-adhering to the surface. The removal of various contaminants involves different  mechanisms, based on the nature and/or the class of the contaminant.

Three general classes of common contaminants are organic, inorganic  and particulate matter. Particles do not necessarily belong to a certain  class and can be from either class or a mixture. Contaminants of any class could  be water soluble or water insoluble. Organic contaminants in most cases will be hydrophobic in nature, such as oils, greases, waxes, polymers, paints,  print, adhesives or coatings.

Most inorganic materials are insoluble in solvents that are water-immiscible. Water is the best universal solvent for ionic  materials, organics or inorganics. However, water insoluble inorganics, such as  polishing compounds made of oxides of aluminum, cerium or zirconium, require a  more complex cleaning system.

Organic contaminants can be classified into three general classes -  long chain, medium chain and short chain molecules. The physical and chemical characteristics are related to their structure and geometry.

Organic contaminants are removed by two main mechanisms. The first is by solubilization in an organic solvent. Degree of solubilization in  various solvents is directly related to their molecular structure. The second  mechanism is by displacement with a surfactant film followed by encapsulation and dispersion.

                                                       

                                                                          Figure 4

In aqueous cleaning, the detergent contains surfactants as essential  ingredients. Surfactants are long chain organic molecules with polar and non-polar  sections in their chains. Surfactants can be ionic or non-ionic in nature, based  on the type of functional groups attached to or part of their chains. When  diluted with water, surfactants form aggregates called micelles (Figure 4) at a level above their critical micelle concentration (CMC). The circle in  Figure 4 represents the ionic or the hydrophilic moiety and the line represents  the nonionic or hydrophobic portion of the surfactant molecule. The  equilibrium tends to shift to the right.

                                                      

                                                                          Figure 5

The mechanism of removal of organic contaminants by detergent involves  wetting of the contaminant as well as the substrate. According to Young's equation, this will result in increasing the contact angle between the contaminant and  the surface, thus decreasing the surface area wetted with the hydrophobe,  reducing the scrubbing energy for removal (Figure 5).

cos theta = gamma SB -  gamma SO / gamma OB       

The ultrasonic cavitations play an important role in initiating and finishing the removal of such hydrophobic contaminants. The shock wave  (the micro streaming currents) greatly speed up the breaking of the hanging contaminants, enhancing displacement with the detergent film. The  removed contaminants are then encapsulated in the micellic aggregates, thus  preventing their redeposition. The net result is that ultrasonic cavitations  accelerate the displacement of contaminants from the surface of the substrate and  also facilitate their dispersion throughout the cleaning system.

Particles, in general, have irregular shapes. All the adhesion forces - van der Waals, electrical double layer, capillary and electrostatic - in  theory are directly proportional in magnitude to the size of the particle. One  would expect that the energy of detachment would decrease with the size of  particles. However, the smaller particles are always more difficult to detach. This is mainly due to the lodging effect. Smaller particles tend to get trapped  in the valleys of a rough surface.

                                                        

                                                                           Figure 6

The mechanism of particle removal involves shifting the free energy of  detachment to be near or smaller than zero, according to Gibbs adsorption equation (Figure 6). Surfactants play a very important role in decreasing  by adsorbtion at particle and substrate interfaces and with the bath.

Delta G = gamma SB + gamma OB - gamma OS

The wettability of the surface plays an important role in achieving  this step. The ultrasonic cavitation's role is to provide the necessary  agitation energy for the detachment (i.e. the removal force). At high frequency  (60-70 kHz) ultrasonics, the detachment or the removal efficiency of one micron particles, measured in deionized water, was found to be 95 percent,  equaling the efficiency obtained by using the megasonics at about 850 kHz, versus 88 percent at 40 kHz. This is expected in light of the fact that cavitation size is smaller at higher frequencies and can reach deeper into the surface  valleys. One would then anticipate that by using a combination of the high  frequency ultrasonics at 65-68 kHz and the appropriate chemistry, the removal  efficiency of various particles can be further optimized.

Redeposition of Contaminants

Redeposition of contaminants is inhibited by another mechanism, by  forming a barrier between the removed contaminant and the cleaned surface. In  solvent cleaning, the adsorbed solvent layers on the substrate surface and the contaminants provide a film barrier. In aqueous cleaning, a good  surfactant system is capable of encapsulating contaminants inside their micelle structure as depicted in Figure 7. Thus, redeposition of the encapsulated contaminants (soils) onto the surface is prevented via steric hindrance for nonionic surfactants, while anionic surfactants prevent redeposition via electrical repulsive barrier.

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                                                                         Figure 7

Encapsulation can be permanent or transient, based on the nature of the used  surfactants. Transient encapsulation is superior to emulsification, as it allows  better filtration and/or phase separation of contaminants. The potential of  reversing the redeposition step by the sonic shock waves on loaded micelles  results in partial re-adhesion. Therefore, allowing the increase in

the soil load in a cleaning solution to reach saturation point,  without good filtration, will result in a significant decrease in the detergent  cleaning efficiency, at which point the cleaning action may cease. To ensure  steady cleaning efficiency, the dispersed contaminants must be removed by  means of continuous filtration or separation of contaminants, along with  maintaining the recommended concentration of the cleaning chemical.

References

1. S.B. Awad, "An Ultrasonic Semi-Aqueous Alternative to Vapor Degreasing," Precision Cleaning, I (1), p. 75, 1993.

2. S.B. Awad, "Method for Cleaning and Drying of Metallic and Nonmetallic Surfaces," U.S. Patent, 5,397397, 1995.

3. A.A. Busnaina, G. W. Gale and I. I. Kashkoush, "Ultrasonic and Megasonic Theory and Experimentation," Precision Cleaning, 13, II (4), 1994.

4. D. Deal, "Coming Clean: What's Ahead in Silicon Wafer Cleaning Technology," Precision Cleaning, II (6), 24, 1994.

5. J.B. Durkee, The Parts Cleaning Handbook, Gardner Pub. Inc.,  Cincinnati, OH, 1994

6. EPA/400/1-91/016, "Aqueous and Semi-aqueous Alternatives For  CFC-113 and Methyl Chloroform Cleaning of Printed Circuit Board Assemblies,"  1991.

7. EPA/400/1-91/018, "Eliminating CFC-113 and Methylchloroform In Precision Cleaning Operations," 1991.

8. R. Kaiser, Particles on Surfaces 2, p. 269, K.L. Mittal, Editor,  Plenum Press, NY, 1989.

9. R.C. Manchester, "Precision Aqueous Cleaning System and Process Design," Precision Cleaning, II (6), p. 11, 1994.

10. K.L. Mittal, Editor, Particles on Surfaces 3, Plenum Press, New  York, 1991.

11. K.L. Mittal, "Surface Contamination Concepts and Concerns," Precision Cleaning, III (1), p. 17, 1995.

12. R. Nagarajan, R. W. Welker and R. L. Weaver, "Evaluation of  Aqueous Cleaning for Disk Drive Parts," Microcontamination Proceedings, p. 113, 1990.

13. M. O'Donoghue, "The Ultrasonic Cleaning Process," Microcontamination, 2 (5), 1984.

14. S.S. Seelig, "The Chemical Aspects of Cleaning," Precision Cleaning, III (10), p. 33, 1995.

15. K.S. Suslick, Editor, Ultrasound, Its Chemical, Physical and  Biological Effects, VCH Publishers, Inc., 1988.

16. K.S. Suslick, "The Chemical Effects of Ultrasound," Sci. Amer., 80, 1989.

About the Author (updated)

Dr. Sami B. Awad is a Scientist and Technologist. He has more than 25  years of industrial experience in the areas of Surface Treatment and  Ultrasonic Applications Technology

With his team of expert chemists, they developed tens of successful ultrasonic cleaning  processes for small and large International companies. Many ultrasonic processes were developed for precision cleaning of components used in aerospace, electronics, disk drives, semiconductors, medical devices, medical and dental implants, Pharmaceutical, optical devices, flat  panel display, automotive, aerospace, injection molds, various metal  & carbide tools, weapons, metal surface finishing and filters and heat exchangers for the oil industry and for a special projects in the areas of Sonochemistry and Biodiesel and Decontamination of Produce. 

Dr. Awad co-founded NovChem Technology to fill in the need for highly reliable quality ultrasonic cleaning chemicals.  The company is dedicated to help engineers, managers  and chemists by offering them recommendations with solutions to their  complex cleaning and / or corrosion issues and supply the necessary  chemicals to do so. The chemicals are a newly developed line of high quality industrial chemical cleaners and corrosion inhibitors for general &  critical ultrasonic cleaning. The the brand name is Novchem™. 

Through his involvement with engineers and chemists worldwide, Dr. Awad has helped them in resolving challenging operational and chemical issues. He assisted many companies in successfully switching from using hazardous solvents in their cleaning operations to cleaning with aqueous chemistries.

He authored more than 25  academic papers in organic reaction mechanisms and has about 20 Patents. He made numerous  presentations in different conferences and professional societies. Published many technical articles and authored individual book chapters on the applications and mechanism of Ultrasonics and Megasonics Precision Cleaning and nano-Particle Removal.

Dr. Awad is formerly VP of Technology and Lab Director at Crest Ultrasonic Corp., Trenton, NJ.  He was a Principle Scientist at Henkel Corporation, USA, Surface Technology Group, Madison Hts, MI. Prior to his industrial career, Dr. Awad was a Professor of Chemistry at Cairo University and  Drexel University, Philadelphia. Dr. Awad has a Ph.D. in Organic  Chemistry and been a member in the professional societies  ACS, UIA,  IDEMA, ASM, ASTM and SME. He served on several ultrasonic cleaning forums and on the board of advisors for a USDA project.

Contact: sawad@ultrasonicapps.com

             sawad@novchem.com

             Phone: 713-575-8782

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Novchem Technology  PO Box 450064 Houston, TX 77045
Phone: 713-575-8782      technology@novchem.com