November, 2008
CLEANROOM NEWS
Process Spotlight: Megasonic Glass Cleaning
Last month we discussed ultrasonic cleaning of glass, which uses cavitation to remove small particles
from the surface of a substrate. Ultrasonic cleaning is a simple and cheap way to clean parts, but is
somewhat limited for high end display applications or semiconductor work. The main drawbacks to
ultrasonics for high end applications are the possibility of damage to sensitive parts/patterns, and the
decreased efficiency of removal of small particles (less than several microns in size).
Megasonic cleaning is a particularly useful technology for removal of submicron particles from finestructured substrates. In the simplest terms, megasonic differs from ultrasonic cleaning mainly by using
higher frequencies: whereas ultrasonic cleaners use frequencies mainly between 40 and 100 kHz,
megasonic cleaners use frequencies of 1-2 MHz.
Figure 1. Concept of megasonic cleaning. Cavitation and acoustic streaming remove particles and sweep
them away from the substrate (from http://www.prosysmeg.com/ )
Figure 1 shows the basic concept of a megasonic batch cleaning system. A piezoelectric transducer is
mounted to the bottom of a process tank, and couples acoustic waves into the cleaning fluid, which can be
water, or different cleaning chemistries such as SC-1 (ammonium hydroxide/hydrogen peroxide). As in
ultrasonic cleaning, the pressure oscillations cause cavitation, or formation and destruction of small
bubbles. In megasonic cleaning, however, this cavitation is not as pronounced, as the higher frequencies
result in smaller bubble size and lifetime. The acoustic streaming phenomenon is much more
pronounced, and will sweep contaminants away from the surface. The boundary layer is much reduced,
as well (Fig 2).
Figure 2. Illustration of boundary layer differences in megasonic vs. ultrasonic cleaning (from
http://www.prosysmeg.com/ )
What is the mechanism for cleaning?
This acoustic streaming is generally thought to be the more important process in megasonic cleaning.
However, Manish (ref 1) has shown that removal of charged particles from the surface can be enhanced
by electric field effects. Effectively, oscillating electric potentials can be caused by the acoustic waves
traveling through an ion-containing solution, and these potentials can be important in removal of charged
particles.
Practical considerations
For semiconductor manufacturers, single substrate processing has become more widespread, as crosscontamination issues are reduced, and yield is increased. This is critical when the cost of a large wafer is
considered. For displays, tank processing can be a more economical and higher throughput way to go.
Single substrate processing usually involves a quartz transducer bar held close to a spinning substrate,
with jet nozzles to spray cleaning solution on the substrate. The acoustic energy is directed from the
transducer bar to the substrate. If properly designed, these systems can very efficiently deliver energy to
the surface to dislodge particles, which are then swept off the substrate edge by spinning.
Figure 3. Single substrate megasonic cleaner, with quartz arm to couple acoustic energy to substrate
(http://www.verteq.com/ )
Figure 4. Portable probe for megasonic characterization ( from ppb Megasonics, Inc. )
Several companies now manufacture probes which can be used to map frequency and power in ultrasonic
and megasonic systems.
References
• Manish, Keswani, Semiconductor Wafer Cleaning Using Megasonics, Verlag Publishing, 2008, ISBN10: 3639090306
• Lester, Maria, “A Glimpse into Megasonic Cleaning,” Semiconductor International, Jan 2003.
• http://www.techsonic.fr/megatheory.htm
• Azar, Lawrence, “Understanding and Evaluating Ultrasonic and Megasonic Cleaners,” Process
Cleaning Magazine, Sept/Oct 2007, pp. 40-43.
Quite a bit of useful information is available from equipment manufacturer websites:
• http://www.prosysmeg.com/
• http://marteqprocesssolutions.com/ (formerly Verteq, then Goldfinger: tank and single substrate
cleaners)
• http://www.pctsystems.com/
• http://www.nanomaster.com/ (single substrate cleaners)
• http://www.akrionsystems.com/ (production type systems)
• http://www.fsi-intl.com (production type systems)
Some sources for ultrasonic / megasonic probes / meters:
• ppb Megasonics: http://www.megasonics.com/products.html
• Techsonic SA: http://www.techsonic.fr/
EQUIPMENT UPDATE
The Brewer Cee Spincoater underwent repair last month. The shaft and bearings were replaced, as well
as the motor controller board. The system runs much more smoothly now. The LCI recently purchased
two new Delrin chucks for this spinner, to allow for better spinning of large (up to 7”) substrates. The
Headway Large Area Spincoater is still available to spin substrates larger than this, up to 14-16”.
The MRC 603III Sputter Coater is currently undergoing repair, but is still functional in the interim. A
valve solenoid failed, and the main mechanical pump requires repair. A substitute pump is currently
being used, and a non-critical solenoid was swapped to provide a working system for now. Repairs
should be completed within the next few weeks.
LCI NEWS
Prof. Hiroshi Yokoyama joins LCI as Ohio Research Scholar on July 1, 2009
The LCI is pleased to announce that Prof. Hiroshi Yokoyama has become an Ohio Research Scholar and
Professor of Chemical Physics at the Liquid Crystal Institute. He most recently held the position of
director of Nanotechnology Research Institute (NRI), National Institute of Advanced Industrial Science
and Technology (AIST), one of the largest government-funded research organizations in Japan, which is
responsible for strategic planning and implementation of research programs over the whole spectrum of
nanotechnology.
Prof. Yokoyama is a world expert in the field of liquid crystal physics with a focus on the surface
properties. His primary research interests are in the areas of liquid crystals, surface and colloid science,
organic thin films and scanning probe technology. His research effort at the LCI will be associated with
the recently funded by the State of Ohio’s Third Frontier Project entitled Research Cluster on Surfaces in
Advanced Materials (RC-SAM). RC-SAM is the partnership of Kent State University, Case Western
Reserve University, Youngstown State University, AlphaMicron, Inc., Cleveland Botanical Garden,
CoAdnaPhotonics, Inc., Kent Displays, Inc., Kent Optronics, Inc., and Liquid Crystal Displays, Inc.
Prof. Yokoyama will join the LCI on July 1, 2009 when he and his family will relocate to Kent in the
Summer of 2009.
Professor Kumar appointed to Editorial Board of Europhysics Letters
Kent State Professor Satyendra Kumar has been appointed to the editorial board of Europhysics Letters,
an international journal that publishes letters across the whole of physics. Recently, the Board has taken
steps to increase publication of papers from the liquid crystal community. Kumar will serve as a co-editor
of the journal and handle review of papers in the general field of liquid crystals and soft-condensed matter
physics. He’ll be working closely with his colleagues in the Liquid Crystals section of the journal to lead
the future direction and quality of this important area of research. The journal’s website is:
http://www.iop.org/EJ/journal/EPL
New Faces at LCI
Chunzhen Fan will be working for one year with Dr. Qi-Huo Wei. She is from Fudan University in
Shanghai, China.
Dr. Rameshbabu Krishnamurthy is the new LCI Senior Chemist. He previously worked as a postdoctoral
fellow in Dr. Quan Li’s Chemical Synthesis lab.
Recent CPIP Ph.D. Defenses
October 23, Xiaoli Zhou, “Synthesis and Characterization of Novel Discotic Liquid Crystal Porphyrins
for Organic Photovoltaics”
November 14, Mitya Reznikov, “Effect of surface alignment layer on electro-optical properties of
ferroelectric liquid crystal displays”
Recent LCI Seminars
November 5, Prof. Peixuan Guo, Dane and Mary Louise Miller Endowed Chair in Biomedical
Engineering, University of Cincinnati, "Single molecule detection of six pRNAs and direct observation of
phi29 DNA-packaging motor with customized single molecule dual-view system".
Note: This was a joint seminar with the Department of Biological Sciences.
November 17, Prof. Margaret W. Frey, Department of Fiber Science & Apparel Design, Cornell
University, Ithaca, NY, "Formation and functions of high surface area fabrics"
November 19, Prof. Nongjian Tao, Department of Electrical Engineering & School of Materials
Research, Arizona State University, "Molecular Electronics and Sensors"
Upcoming LCI Seminars
December 3, Prof. Michael Rubinstein, John P. Barker Distinguished Professor, Department of
Chemistry, University of North Carolina at Chapel Hill, "Physics of a Lung: A Simplified View of Airway
Surface Layer of a Lung"
December 10, Prof. Nader Engheta, H. Nedwill Ramsey Professor of Electrical and Systems Engineering,
and Professor of Bioengineering, University of Pennsylvania, "Circuits with Light at the Nanoscale:
Metananocircuits and Metactronics.
Be sure to let us know if you would like more detailed information about any activities at the LCI.
Phil Bos
pbos@lci.kent.edu
330-672-2511