Experimental quantification of reticle electrostatic damage below the threshold for ESD

Abstract

The damage mechanisms that take place when a reticle is subjected to electrical stress by exposure to an electric field have been investigated by applying voltage directly to the structures in a special test reticle. Surface current was recorded at all levels of stress from 1V to 100V. The current/voltage characteristic was polarity dependent and exhibited increasing non-linearity as the feature spacing was reduced. Atomic Force Microscopy showed that the electrical stress caused EFM (Electric Field induced Migration of chrome), matching the damage seen in reticles stressed through induction by an external electric field. No ESD events were recorded, confirming that EFM is independent of ESD and that it occurs with lower electrical stress. The threshold for EFM was found to be five times lower than the previous estimate, starting at 1V with 1µm spacing. Damage caused by EFM was shown to be continuous, cumulative and the rate of CD degradation was measured to be from 3 to 6 nm per second.

Why SEMI Standard E163 should be followed for the Protection of Extremely Electrostatic-Sensitive Semiconductors and Similar Devices during Manufacturing, Packaging and Handling

“…The measurement, which is shown in Figure 5, reveals that a reticle carried in a static dissipative reticle pod is repeatedly exposed to electrostatic stress from transient electric fields. The level of field penetration into the reticle pod was confirmed to be sufficient to cause cumulative damage in production reticles, following the earlier quantification of reticle sensitivity to EFM [11,12]. Another test carried out with the sensor reticle revealed that static dissipative reticle pods actually generate significant electric field transients through tribocharging during normal use, something that had previously been believed not to happen with static dissipative materials, mainly because of an inappropriate testing methodology using field meters with insufficient temporal response.…”

“…In 2008 further experimental research into EFM was published [11,12] confirming the initial interpretation of the reticle damage mechanism as the field-induced migration of chrome. This study fully quantified the effect and showed that reticles were even more sensitive to electric field than had been estimated five years earlier when EFM was first identified.…”

Current understanding of the electrostatic risk to reticles used in microelectronics and similar manufacturing processes

“…The ambient field strength can be amplified by several orders of magnitude by the reticle's structure, as shown in the circled areas, which means that having even a very low level of electric field in the environment through which a reticle passes could be enough to cause damage within the reticle. [7][8][9][10] As charge is displaced within the reticle under the influence of an applied electric field, it begins to generate an opposite internal electric field to the one that is being applied externally. The charge within the reticle moves until the opposing electric field thus created exactly balances and cancels out the externally applied field, at which time there is no net force acting on the charge and charge redistribution within the reticle stops.…”

“…Under the influence of sufficiently strong electric field, chromium ions can be produced at the edges of the conductive features and these can move across the surface of the reticle in a process called electric field-induced migration (EFM type 2), [7][8][9][10] as shown in Fig. 4(b).…”

“…9,10 It should be noted that EFM is dependent on the local field strength, not the induced voltage between features. However, to make a simplistic comparison between the level of field induction that is needed to produce ESD and that which causes EFM, in modern production reticles with feature separation measured in nanometers, EFM would start with an induced potential difference of very much less than one volt.…”