An Electric-field (E-field) exposure tool for Photomasks was designed, assembled, then utilized to subject 250 nanometer technology node reticles to variable electric fields. A similar study had been demonstrated using the Canary Reticle. The goal was to induce an Electrostatic Discharge (ESD), and attempt to damage the reticle's chrome structures via the Field Induced Damage Model. Electrostatic Discharge emits a radio wave in the 100 MHz to 2.0 GHz frequency range, which can be detected using a Digital Sampling Oscilloscope and antenna. Once detected via radio wave sampling techniques, the Field Induced Damage is evaluated on a KLA STARlight inspection tool, and a damage map provided. A Digital Instruments Atomic Force Microscope utilizes the damage map to locate defects for further evaluation.
An evaluation of a Photolithography Mask damaged by Electrostatic Discharge (ESD) is presented, using pictures and data from the toolset at International SEMATECH's Advanced Technology Development Facility. The Photomask used in the printability evaluation is the Canary (DuPont TM) Reticle, demonstrating various degrees of ESD-induced damage to a repeating structure contained in the chrome-on-quartz pattern. Levels of damage to the chrome structures vary from non-existent, to barely detectable, to moderate, to catastrophic. The ESD-induced damage is then measured and compared through an assortment of Mask Metrology tools.
Electrostatic charge on reticles and adjacent objects is a growing and significant source of damage to reticles . Next generation exposure tools will likely employ vacuum ultraviolet light (157 nm.) to achieve improvements in spatial resolution. Because, air is opaque such light due to the presence of oxygen and water vapor , these exposure tools will need to operate in a nitrogen atmosphere. In order to benchmark the magnitude of static charge in 157 nm. exposure tools, the efficiency of tribocharging was measured in a test chamber with atmospheres of 10-60 percent RH air and in nitrogen. The data shows that the dependence of tribocharging on humidity is generally higher as humidity is decreased. Tribocharging in a nitrogen environment is different than in 10 percent RH air and is substantially higher than in the environment of a conventional air-filled stepper. That suggests that maintaining the safety of reticles in the 157 nm. lithography era will require a rigorous electrostatic management program.
Reticle damage due to ESD is a well-known phenomenon.. A defective reticle prints defective dies1. Unfortunately, ESD damage to reticles is a common problem and becoming more common as time goes on. We have developed a diagnostic tool called the Canary Reticle for the purpose oflocating the sources of ESD damage within a photo bay. Over the past year, we have used it to study the effects of ESD on reticles. In this article, we summarize the findings ofthe study2. Extremely high levels of electrostatic charge develops in cleanrooms in general and particularly so for semiconductor cleanrooms. The absence of surface contamination, the action ofihe HEPA filters in sweeping ions out ofthe room and low humidity results in much higher voltage levels on objects as compared to conventional rooms. Many ofthe objects used must be excellent insulators or are insulators, but have been selected for other physical properties. Examples are quartz reticles and Teflon wafer cassettes and plastic reticle pods. The chrome structures on reticles have sharp corners, which concentrate the electric fields, making breakdown ofthe air between them easy to occur. The voltage on a conductor is the same at all points. Thus, in the presence of a voltage gradient (Electric field), the potential differences are focused into the gaps between the chrome structures, again making discharge easy. It is hoped that by understanding the mechanisms for ESD damage photo engineers will better be able to avoid it.