Reticle Inspection

Reticles are transmissive or reflective projection masks patterned with fine features, typically 4x-5x larger than the desired pattern scale on the wafer. They are used with optical illumination systems that image and de-magnify the patterned light to selectively develop photoresists as part of the wafer patterning process.

Arguably, reticle inspection is far more important than either non-patterned or patterned wafer inspection. This is due to the fact that, while a single defect on a bare or patterned wafer has the potential to "kill" one device, a single, undetected defect on a reticle can destroy thousands of devices since the defect is replicated on every wafer processed using that reticle. With EUV this problem is further complicated by the finer resolution of the pattern, the presence of a thin protective membrane, and the reflective design of the reticle.

Reticle inspection systems work on the same principles and have similar physical requirements as wafer inspection tools, with the exception that reticles are normally inspected using transmitted rather than reflected light. Transmitted light is used to locate UV-opaque stains and other transmission defects. Reticle inspection tools employ high resolution imaging optics and either VIS or UV illumination, depending on defect tolerances and/or feature sizes, to find defects on a reticle blank or a patterned reticle. Inspections are routinely performed during the reticle manufacturing process and throughout the reticle usage. Reticle inspection tools employ sophisticated image analysis software and motion control systems similar to those used in wafer inspection tools. The use of conventional optics in reticle inspection systems has been extended down to 90 nm feature sizes by using UV illumination. Reticle inspection at smaller feature sizes is possible using EB since lower throughputs can be tolerated in comparison to patterned wafer inspection. As with wafer inspection, the reticle inspection tools used in sub-100 nm applications (both blank and patterned reticle inspection) employ DUV illumination, typically using a single wavelength at 266 nm or 193 nm. Figure 1 shows a block diagram of a reticle inspection platform. Note that, in addition to the objective optical system, motorized stage, and light source, the platform employs various controllers and data analysis modules. Reticle inspection systems can be configured to employ either transmitted light through the reticle or reflected light from the reticle surface in the inspection process. As with other inspection systems, this reticle inspection tool requires highly accurate and precise motion control for the optical components and the air bearing reticle stage.

Block diagram of the components in a reticle inspection system
Figure 1: Block diagram of the components in a reticle inspection system. Figure is reprinted with permission from Fujitsu Ltd.

Motion Control for Reticle Inspection

The DynamYX Reticle Positioning Air Bearing Stage is designed for use in reticle inspection and repair applications. The DynamYX stage has a much smaller footprint than traditional open-frame solutions and a full open aperture that accommodates flexible optical component integration and ease of service access.

Custom Optical Solutions for Reticle Inspection

We offer optical sub-system design and manufacturing solutions for wafer and reticle inspection tool manufacturers. We have designed and manufactured optical sub-systems for lithography, wafer inspection, excimer and EUV light source, metrology and mask writing applications, among others.

MKS Semiconductor Handbook Cover

For additional insights into photonics topics like this, download our free MKS Instruments Handbook: Principles & Applications in Photonics Technologies

Request a Handbook