Issue



A need for reassessing risk management in reticle handling


09/01/2004







For many years, the International Technology Roadmap for Semiconductors (ITRS) has provided guidance to the semiconductor industry about technical problems facing future technology nodes that are defined according to circuit feature dimensions [1]. The ITRS represents a consensus view of those risk areas where development efforts must be focused to ensure reliable semiconductor production in the future, and it undergoes regular reviews. But what would happen if a risk went undetected or underassessed?

One possibly underestimated area is the electrostatic discharge (ESD) damage to reticles caused by exposure to electric fields. In a seminar given at International Sematech in December 2003 [2], data showed that reticles are far more sensitive to damage by electric fields than is widely recognized.

The problem of static electricity

It has been known for many years that sensitive items such as reticles can be damaged by ESD when they are handled. Solutions adopted include replacement of all nonessential insulators with grounded static dissipative alternatives, and using air ionizers that gently neutralize static charges on surfaces before they can build up to hazardous levels. Static electricity in fabs can also disrupt electronic control systems, causing radio-frequency interference that can cause production tools to lock up or robotic systems to go out of control. As a result, it has become standard practice to ground virtually everything and to handle sensitive payloads with static dissipative contacts.

Reticle damage

Some of the widely adopted precautions against static electricity in fabs, however, actually increase the risk of field-induced damage in reticles [2]. It has been demonstrated through computer simulation that a reticle behaves rather like a radio antenna, concentrating and amplifying ambient electric fields through the reticle pattern. The simulations also showed that far from protecting reticles from damage, the industry's standard practice of handling reticles using grounded dissipative contacts increases the "field focusing" effect, which results in greater damage to the reticle pattern. This also has been shown in experiments [3].


Figure 1. Damage seen in test reticles as a function of the voltages induced by an externally applied electric field.
Click here to enlarge image

Images were presented at Sematech to show that reticles are much more easily damaged by electric fields than most people think (Fig. 1). Atomic force microscope (AFM) images reveal astonishing detail of the damage caused in special test reticles that were exposed to an electric field. After analyzing hundreds of such AFM images and re-evaluating the data from other reticle tests, it can be concluded that there are three physically different field-induced damage mechanisms in reticles. ESD is well known, but it requires a much higher electric-field strength around the reticle than the two other newly identified damage mechanisms. Because the damage appears to involve the surface migration of chrome from the reticle features onto the clear quartz, these mechanisms have been described as electric-field induced migration (EFM).

The smallest amount of EFM damage visible in the AFM images (Fig. 1), which looks like a darkening of the edges of the reticle features (the first image), is 10× more damaging to the reticle CD than is allowed by the ITRS 2004 technology node lithography CD budget. Clearly, this type of damage poses a serious risk — it has been found in production reticles for many years, but its origin had previously been identified as "low-level ESD." The damage mechanism for EFM is markedly different from that of ESD and changes the nature of the risk to reticles.

When conductive objects are brought closer together, their interaction with an electric field changes (Fig. 2), so as the spacing between isolated conductive lines in a reticle is reduced in successive technology nodes, the voltage that will be induced between them by an external electric field decreases. The local electric-field strength that will exist between the features in the presence of this external field increases. Since a certain "breakdown" voltage must be present between reticle features for a spark to jump, as lines come closer together in future reticle generations it will become more difficult to induce the voltage needed for a spark; therefore, ESD will become less likely.


Figure 2. Computer modeling of the interaction between an electric field and isolated conductive lines in a reticle as a function of their separation. As feature spacing is reduced, voltage-dependent ESD becomes less likely and EFM more likely.
Click here to enlarge image

Unfortunately, this does not mean that reticles will become more resistant to field-induced damage over time. It has been shown that EFM is dependent on the local field strength between reticle features — not the induced voltage — so this form of damage is likely to become more prevalent in future reticle generations. Although ESD, and not EFM, has been commonplace in production reticles up to now, EFM poses a serious and increasing risk. If the semiconductor industry is only aware of ESD and the occurrence of ESD damage in reticles decreases over time, as the physics predict, it might be concluded that the risk to reticles from electric fields has been adequately controlled. In reality, the risk posed by electric fields is increasing nonlinearly as Fig. 2 indicates, and some current reticle-handling practices are inadvertently increasing these risks.

Conclusion

EFM causes the line shape and spacing in localized regions of susceptible reticles to change. This change is continuous and irreversible, resulting in a gradual degeneration of reticle CD as a function of exposure to electric field. Today's routine metrology techniques are not very sensitive to such changes, so device faults caused by EFM-damaged reticles might escape detection until final test. By contrast, reticle ESD is usually detected before many faulty wafers are processed and the wafers can often be reworked. As ESD gives way to EFM in future reticle generations, the financial consequences of field-induced reticle damage could be orders-of-magnitude greater than ever before.

Current ESD protection methods and reticle-handling practices are insufficient to protect reticles completely from field-induced damage. Since electric fields can never be eliminated completely from the reticle-handling environment of a semiconductor fab, shielding reticles from electric fields and not grounding them are two simple and effective ways to reduce this risk. If future semiconductor production is not to be compromised as a result of field-induced reticle damage, it would be prudent for the industry to take a more detailed look at the unique risks to reticles during their use and handling. A paradigm shift in reticle protection is urgently required.

References

  1. International Technology Roadmap for Semiconductors, http://public.itrs.net.
  2. Reticle ESD Technical Seminar, International Sematech, Dec. 12, 2003, http://.www.sematech.org/.meetings/20031212/index.htm.
  3. Rudack, Pendley, Gagnon, Levit, "Induced ESD Damage on Photomasks: A Reticle Evaluation," EOS/ESD Symposium, Las Vegas, September 2003.

Gavin Rider received his PhD in surface physics from Southampton U., UK, and is now VP of technology and development with Microtome Precision Inc., Colorado Springs, CO; e-mail [email protected], http://www.microtome.com.