Mark A. DeSorbo
BERKELEY, CAJust as electro-static discharge (ESD) has rocked circuitry, substrate and disk drive makers, electrostatic attraction (ESA)a static field created from contaminants in the airis increasingly becoming more of a concern, especially in the 300-mm arena.
Larry B. Levit of Ion Systems (Berkeley, CA); Thomas M. Hanley of MEMC Electronic Materials Inc. (St. Peters, MS); and Frank Curran of Analog Devices Corp. (Limerick, Ireland) conclude that ESA “is an important consideration in contamination control.” [See “In 300-mm contamination control, watch out for electrostatic attraction,” p. 25].
Levit, Hanely and Curran say that even in an ISO Class 3 (Class 1) cleanroom, particles have not been eliminated, just kept to a small number. In fact, particles as tiny as 50 nanometers can render a tool useless.
Hanley, a senior research engineer at MEMC Electronics Materials Inc., says charged ions seek each other out, whether they are on the wafer or on particles floating in the air. “Particles are being attracted to the surfaces of wafers and tools, causing a break in the [production] line,” he says.
John T. Kinnear, vice president of the Electrostatic Discharge Association (Rome, NY), agrees, saying neutralizing ESA on particles and surfaces is just as crucial as defusing ESD generated from personnel. In a typical cleanroom, products and fixtures are made of Teflon and other derivatives, which tightly grip static charge, he adds. “If you use that type of material, you'll need an ionizer to eliminate ESA, neutralizing the charges on cassettes and tools.”
ESA, Kinnear says, can be addressed if an ESD-control program is implemented. Together with the American National Standards Institute (New York), the ESD Association penned the ANSI/ESD S20.20 standard, which provides a focus and allows a company to customize a program specific to its needs in combating ESD and ESA.
“If an ESD-control program is implanted, ESA will not be an issue,” he adds.
Copies of ANSI/ESD S20.20 can be obtained free from the ESD Association's Web site, www.esd.org.