The ESD Handbook

26Photomasks

26.1 Photomasks and Reticles

26.1.1 Photomasks

Photomasks are used in the formation of semiconductor devices. A photomask serves as the negative image of the desired structure to be created on the semiconductor wafer for printing purposes. A photosensitive material is formed on the wafer surface to produce the desired shape or structure. The photomask is used to expose the material, and leads to hardening of the photosensitive material. In photolithography, there are both positive tone and negative tone resists. Photomasks are typical formed using a quartz substrate, with chrome shapes on the physical surface. The production of the photomasks must be “defect free.” If a defect occurs, this can lead to latent failure mechanisms and product failure.

As we continue to make smaller linewidths for nanoelectronics, the spacing between lines is also reduced, leading to electrostatic nano-discharges occurring between the mask shapes [1–34]. Electrostatic discharge (ESD) failures are occurring in photomasks and reticles [1–34]. Figure 26.1 shows an example of an ESD discharge as a function of the spacing between two shapes on a photomask.

26.2 ESD Concerns in Photomasks

26.2.1 Electrostatic Charging on Photomasks

One of the key problems is the build-up of electrostatic charge on the mask shapes. Between each chrome shape on the mask, a potential “spark gap” exists that can lead to electrical discharge when the electric potential exceeds the air breakdown. In the formation ...