Stay on top of process variables and the evolving understanding of sonics when setting up and maintaining ultrasonic and megasonic cleaning systems
by Barbara Kanegsberg and Ed Kanegsberg
Cleaning by the interaction of sound with liquid—ultrasonics and megasonics—is an ongoing source of inspiration and controversy.
Ultrasonics and megasonics are not distinct or competing techniques but are actually a continuum. Both are based on sound waves traveling through liquid, producing cycles of compression and rarefaction. Vapor-filled bubbles result from tears in the liquid.
The differences between ultrasonics and megasonics are in the frequencies of these sound waves. At lower ultrasonic frequencies, cavitation bubbles (actually vacuum voids) are relatively transient, growing then imploding with tremendous localized force and heat. At the higher frequencies associated with megasonic cleaning, bubbles are smaller and more stable.
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While those involved in wafer fabrication emphasize the particulate removal attributes of these techniques, they are also useful in removal of thin-film contamination by continuously reducing the soil loading of the cleaning solution adjacent to the surface.
In general, lower frequencies are associated with more aggressive removal of soil, whereas higher frequencies are associated with substrate protection and with fine-particle removal. For example, a classic, industrial metals cleaning process might utilize 18 KHz or 25 KHz. Such frequencies are suitable for applications where there is heavy soil loading or where substrate erosion is not an issue.
Higher frequencies are associated with lower potential damage to the substrate. For example, components associated with inertial navigation systems might be cleaned at 38 KHz or 68 KHz.
While ultrasonics usually involves cleaning at frequencies of 18 KHz to 100 KHz and megasonics is often considered to be in the range of 360 KHz to 2,000 KHz, higher-frequency ultrasonics emphasizes the continuum. Requirements for cleaning fine-geometry products without modifying the surface have been a driver toward ultrasonics equipment with higher and higher frequencies. This trend, which began perhaps three to four years ago, has resulted in availability of systems with frequencies of 300 KHz to 400 KHz.
There still remains constraints to using sonics, probably due to historical reminders of when ultrasonics were less controllable and product was damaged during processing. However, there are many processes where, because of product configuration, expected end use and stringent product standards for particle removal, it might be irresponsible to be using anything but sonics.
Standards development
A major breakthrough effort is underway to develop cleaning standards, including standards for ultrasonic cleaning for implantable medical devices.
There are other third-party standards related to ultrasonics cleaning. Examples of ASTM documents include standard practices for cleaning high-value components (G131-96); use of ultrasonics in extraction procedures for monitoring particulate residue in gloves and other cleanroom-related items (G136-96) and preparation of coupons for evaluation of cleaning agents (G121-98).
Challenges to the development of standards include the array of process variables and the evolving understanding of sonics. In addition, ultrasonics techniques and metrics is a developing, somewhat contentious science.
There are many competing theories, laboratory studies and calculations, each having “the definitive” answer to the relative importance of frequency, boundary layer, shape of the cavitating bubble, etc. For the components manufacturer, the pragmatic result is the telling one; and there may be several ultimate truths. Further, each components manufacturer or assembler may somewhat justifiably be reluctant to publish process details, so it is difficult to reconcile theory with practice.
However, company-specific standards are increasingly prevalent. This means, of course, that each one of us must re-discover the same secrets of ultrasonic and megasonic cleaning.
Process development
An ultrasonic and megasonic system must be developed, just as with any other cleaning system. Table 1. summarizes important considerations in setting up and maintaining sonic-based cleaning systems; a few of these—product, substrate, soils, level of cleanliness, surface quality attributes, custom wave form and fixturing—are discussed in greater detail below.
Product
Given the number of variables, as with other surface-preparation activities, it is important to consider the interaction of product attributes, including the substrate, the soils, the level of cleanliness, and desired surface quality.
Substrate
This includes the materials of construction and the product configuration. The size and shape of the component must be considered in choosing whether or not to clean with sonics, as well as the nature of the system.
For example, if tubular parts are to be cleaned, it may be necessary to adapt a system for both immersion and flushing. Exceedingly large, heavy parts may not be technically or economically amenable to immersion cleaning with sonics.
Soils
There are many sources of organic and inorganic contamination; and contaminates may be thin-film and particle contamination. Minimizing contamination at previous process steps can improve efficacy of cleaning with ultrasonics or megasonics.
For example, particles that have dried on to a surface tend to be particularly difficult to remove. Further, small changes in the soils, such as oils, polishing compounds, photoresists and even cleaning agents used at earlier stages of fabrication, as well as process methods that change particle size, can impact the ultrasonic or megasonic cleaning step.
Level of cleanliness
We automatically assume that cleaner is better. Even assuming unlimited process time and company resources, with additional cleaning comes the potential for product damage or undesirable surface modification.
Surface quality attributes
Cleaning, surface preparation and the quality and nature of the surface are separate issues which, in practice, are inextricably entwined. All manufacturing processes, including cleaning with or without sonics, have the potential for modifying the surface.
Undesirable modification is thought of as surface damage; specific, desirable modification may be necessary for coatings or product functionality. Surface cleanliness and surface quality must be considered in terms of subsequent process requirements and product performance.
Custom wave form
Ultrasonic systems are now available with an array of design options to assist in soil removal while minimizing the potential for product damage. These are sometimes referred to as “designer wave forms.”
Sweeping across a relatively narrow frequency range has proven helpful in improving cavitation of some of the newer organic solvents, particularly for viscous solvents. The range of the sweep as well as the frequency, pattern and randomness of the sweep can be specified. Power and amplitude can be varied.
It is also possible to have a range of harmonic frequencies, two different frequencies or several sequential frequencies in a given tank.
Options vary among equipment suppliers. The relative importance for a given process is left to the engineers at the fabrication facility. Given the many available options, it is prudent to discuss cleaning agents and cleaning requirements with the equipment supplier, signing confidentiality agreements where appropriate.
Fixturing
Particularly with ultrasonics, it is often assumed that the product will automatically be uniformly exposed to cavitation. In fact, product shadowing may result in uneven, incomplete cleaning. Parts are ideally racked or fixtured; and they may also be rotated or agitated to improve consistency.
The fixtures themselves may have negative impacts. To avoid particle generation and product contamination, materials of construction of the fixtures must be such that they do not degrade on long-term exposure to the cleaning chemistry and the process conditions.
Further, fixture materials must be such that they do not dampen or mask sonic action. One group, for example, had designed rubberized clips to hold miniature devices during ultrasonic cleaning. By using aluminum foil as witness samples, fixturing them in the suspect device holders, it was possible to demonstrate absence of the characteristic “orange-peel” erosion pattern.
Monitor your process
Ultrasonic and megasonic systems have an ever-increasing array of design options; the selection of such systems is much akin to that of a sophisticated sound system.
It is important to select those features of prime importance; then, carefully document features, selected equipment settings and any process changes. Reports, theoretical studies and analytical testing are very helpful.
However, expected performance cannot replace empirical testing. Once a reasonable process is determined, the variables must be unambiguously documented and process performance must be monitored. III
Acknowledgements: The authors would like to thank John Fuchs, Blackstone Ney Ultrasonics; John Harman, Branson Ultrasonics; Bob Moss, Moss Consulting; Steve Spiegelberg, Polymer Technology; and Carole LeBlanc, U. Mass Lowell Toxics Use Reduction Institute.
Barbara and Ed Kanegsberg, co-editors of the “Handbook for Critical Cleaning,” CRC Press, are with BFK Solutions, LLC (Pacific Palisades, Calif.), an independent consulting company which helps components and device manufacturers set up cleaning processes and achieve contamination control of the product. Barbara is a biologist, biochemist and clinical chemist with 25 years of experience in process development. Ed is a chemical physicist and engineer with 30 years of production experience. They can be reached at [email protected].
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