By Robert P. Donovan
Static electricity receives a lot of press in many diverse publications. Most of this publicity, however, is unfavorable, portraying static electricity as an objectionable condition that can often be hazardous to both product and personnel.
Much of this unfavorable portrayal is deserved, as when static electricity is identified as an ignition source causing catastrophic consequences. For example, grain silo explosions have been triggered by a static discharge in a confined atmosphere consisting of dense aerosol clouds made up of small, combustible grain particles.
The same vulnerability applies to vapor emission from many volatile organic baths as defined by the flash point—the lowest temperature at which the vapor of a combustible liquid can be made to momentarily ignite in air. The temperature in a spark created by a discharge of static electricity can easily exceed the flash point of virtually all combustible liquids. Thus, a static discharge during aircraft refueling, or even automobile refueling, can have fatal consequences.
Static discharges in cleanrooms do not generally rate the dramatic headlines describing catastrophes of the type just described. I don't recall any cleanroom explosion being attributed to a static discharge. Yet, contemporary semiconductor cleanrooms almost universally include air ionizers hanging from the ceiling or elsewhere. The sole function of these devices is to neutralize incipient static charge buildups within the manufacturing area.
The undesired consequences of static that ionizers are designed to avoid include:
- Electrostatic discharges (ESD) between closely spaced but dielectrically isolated electrodes on a chip or a photomask, resulting in the destruction or degradation of the isolating dielectric material;
- Electromagnetic pulses (EMP) created by electrostatic discharges, introducing temporary or permanent upsets in equipment control circuits;
- Enhanced electrical attraction between aerosol particles and surfaces having excessive static charge, resulting in enhanced particle deposition on those surfaces.
The trend toward smaller devices, ever more closely packed on chip surfaces, and the photomasks used in their manufacture increases chip vulnerabilities to ESD. The near universal use of solid-state components in equipment control circuitry implies lower thresholds for safe operation in EMP environments than similar thresholds for predecessor vacuum tube circuits—and consequences can degrade photomasks and chips prior to packaging.
So, what creates these static charges? Part of the answer is that airborne-charged species occur naturally due to cosmic radiation, radioactive decay of airborne species and charge exchange between dissimilar materials in contact. Liu 1 measured the airborne densities of electrically charged species in various environments, as shown in Table 1.
Note that the HEPA/ULPA filters of a cleanroom remove charged species from the air passing through them, so that the charge density in cleanroom air immediately downstream of the ceiling filters is lower than in the other environments sampled. This helpful filter action, however, does not eliminate the static problem in a cleanroom. Charged species can still be created by contact and subsequent charge transfer between any two electrically neutral but dissimilar materials—or even between two similar bulk materials with differing surface constituents. Electrons transfer across the interface formed by the two contacting materials until the electrochemical potential (the fermi level) becomes the same on both sides of the interface. When these two materials are then subsequently separated, one has acquired excess electronic charge and the other has lost electrons.
The materials making up the charge transfer interface do not have to be solids. Any air-solid interface, for example, represents an interface between two dissimilar materials, so that solid surfaces in a cleanroom presumably can be electrically charged simply by the air flowing through the cleanroom. High voltages can also induce electrical charge on neighboring surfaces.
When these materials and surfaces are not grounded, the electrical charges—arising from any mechanism—remain trapped as static charge and act as potential contributors to the objectionable consequences listed above.
The role of the air ionizers in cleanrooms is to flood the manufacturing area with high concentrations of bipolar ions (Table 1), which neutralize these static charges. Air ionizers protect a semiconductor fab from the perils of static, and no modern semiconductor fab should be without them.
Robert P. Donovan is a process engineer assigned to the Sandia National Laboratories and a monthly columnist for CleanRooms magazine. He can be reached at [email protected].
1. Liu, B. Y. H., D. Y. H. Pui, W. O. Kinstley and W. G. Fisher, “Aerosol Charging and Neutralization and Electrostatic Discharge in Clean Rooms,” J. Environmental Sci. 30(2), March/April 1987, pp. 42-46