The problem of electrostatic discharge (ESD) resulting from charges on wafers is a serious concern in IC manufacturing processes.

Even though micro-environments such as a front opening unified pod (FOUP) or a Standard Mechanical Interface (SMIF) pod provide a grounded path to conduct away charges on wafers, this method cannot remove charges on the insulative features on a workin- progress wafer. In addition, as the feature size of ICs becomes smaller, the ICs and photomasks become more prone to damages caused by ESD. Because higher electrostatic charge levels usually cause stronger ESD, the International Technology Roadmap for Semiconductors’ (ITRS) requirement on allowable electrostatic charge levels on wafers gets lower with each new technology node.^1,2Hence, reducing or removing charges on wafers will benefit semiconductor fabrication in both yield and cost.

Electrostatic charges on a wafer are generated either by direct contact with anything it touches followed by separation, or directly from the fabrication processes. Currently, two popular methods are used to reduce charges on wafers. One method dissipates the accumulated charges on a wafer by providing a path to ground where electrostatic dissipative materials are used in a wafer handling mechanism to create such a path. For example, in a front opening unified pod (FOUP), carbon filled polymer materials are used to achieve an appropriate conductivity. Unfortunately, the charges on any isolated non-conductive island of the wafer surface cannot be dissipated.

Alternatively, airborne ions can be used to neutralize charges in-situ on wafers. Air ionizers have been broadly used in cleanrooms and mini environments and have proved to be an effective way of neutralizing charges; however, in most advanced fabs, wafers are seldom exposed to the ambient environment of the cleanroom. In addition, all process tools do not necessarily provide sufficient neutralization to wafers before putting them back into a FOUP or a SMIF pod (microenvironment). Due to these shortcomings, there exists a need and an opportunity to further remove the charges on wafers while in a micro-environment.

Many fabs are starting to purge micro-environments with clean dry nitrogen or extreme clean dry air (XCDA) between certain critical processes so as to reduce oxygen, moisture, and other airborne molecular contaminants.^3-7 Micro-environments with features such as inlet and outlet ports, filters, and check valves have been manufactured to facilitate an automatic purge in fabs; however, there is an opportunity to add one more benefit: electrostatic charge neutralization. By adding an ionization module to the purge gas supply line, fabs can fill the micro-environment with ionized gas to neutralize the charges on wafers.