As electrostatic discharge (ESD) thresholds increase, knowledge of ESD standards and control in cleanrooms is a major concern in both microelectronics and medical device manufacturing.

Background
A cleanroom is a hyper-clean environment achieved through the control of ventilation, filtration, temperature, humidity, air pressure, and ionization as well as materials in the room and other parameters. The quality of cleanliness in a cleanroom is significantly determined by the amount of particles floating in the air. The preferred unit of measurement is the micron, which is equal to one millionth of a meter. One micron equals approximately 0.000039th of an inch.

The method most universally accepted to classify cleanrooms comes from ISO 14644 Standard, which uses 0.1 microns per cubic meter. ISO now replaces the Federal Standard 209-1992 (Table 1), which focuses on particles equal to and greater than 0.5 microns per cubic foot of air. Therefore, a Class 100 cleanroom would have a maximum allowable number of 100 particles equal to or greater than 0.5 microns per cubic foot of air, and ISO classifications would be Class 5, which is 3,520 particles/meter cubed (p/m3) equal to or greater than 0.5 particles per cubic meter.

Electrostatic charge generation control is also necessary in cleanrooms as it can reduce yield, disrupt automatic equipment from electromagnetic interference(EMI), and reduce surface particle collection by electrostatic attraction (ESA). ESA is becoming more of a concern as static charges have been seen to actually bond contaminants to surfaces of products or tooling. For example, for a 4” wafer, charged to 1000 volts, a particle of 1 micron in diameter would have the bonding force of over 830,000 pounds per square inch. Therefore, electrostatic attraction between charged objects and particles can be quite strong compared to gravitational,aerodynamic, or adhesion forces.

With the heavy use of insulating materials such as glass, Teflon, and polymers, items can become highly charged. Grounded workstations are a potential concern, as is the use of stainless steel work surfaces compared to static dissipative work surfaces. Conductive work surfaces can be considered a current-carryinghazard to people and ESD-sensitive devices.

The cleanroom classification is illustrated in Table 2. By U.S. law, Federal Standard 209E can be superseded by new international standards. The 209E standard will be phased out and replaced globally by ISO 14644 [1].