Controlling Electrostatic Attraction of Particles in Production Equipment
SEMI Document E78-0998 describes the elements of a static control program: these include grounding, static dissipative materials and air ionization. These elements are supported by personnel training and regular program audits.
Among users and manufacturers of semiconductor production equipment, the effects of
electrostatic surface charge are well known. Electrostatic discharge (ESD) damages both
products and reticles. ESD events also result in unwanted electromagnetic interference (EMI),
causing equipment to malfunction. These problems are widely discussed in the literature of the semiconductor, disk drive and flat panel display industries.
Also important are the effects of static charge that impact contamination control. Charged
wafer and reticle surfaces attract particles (electrostatic attraction or ESA) and increase the
defect rate. In this article, we review the issues involved in particle deposition enhanced by static charge. The calculations contained here form the basis for recommendations contained in Semiconductor Equipment and Materials International (SEMI) Document E78-0998, “Electrostatic Compatibility — Guide to Assess and Control Electrostatic Discharge (ESD) and Electrostatic Attraction (ESA) for Equipment,” recently balloted by SEMI Standards.
The charged wafer
The presence of a net electrical charge on a wafer can create an electrostatic field that
accelerates the deposition of particles onto the wafer. Particle deposition velocity (which is equal to the surface particle flux divided by the aerosol particle concentration) increases with growing wafer charge. It has been shown that particle deposition rates onto wafers in a manufacturing environment closely follow theoretical predictions. A clear increase in particle deposition on charged wafers in this kind of environment, exposed in an area of controlled particle concentrations, has been observed and documented.