Manufacturing - Lithography
wafer size to allow for handling and titling. Such photomasks are often described as master photomasks, and can either be used directly or can be used to make contact print copy photomasks. The alternative is to manufacture photomasks called Reticles when stepper or scanner technology is used, where each reticle will typically be 5” or 6” in size and the features will be 2X, 2.5X, 4X, 5X, 10X the size required on the final silicon wafer. The requirements may also include the need for one or two pellicles to be attached. A pellicle protects the surface of the photomask and also moves any particles on the pellicle surface out of the focal plane of the system using the photomask. The minimum feature size that appears on the photomask will dictate which exposure system needs to be used to write it. Most writing systems have a number of different writing modes, and the mode determines the minimum feature size which can be exposed, as well as the typical tolerance which can be achieved. Commercial manufacturers are making hundreds of photomasks per week and
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maintaining SPC data about the key aspects of their process equipment so will have a good understanding of their process capabilities. For the semiconductor industry, most photomasks will have CD tolerances of less than 0.25 microns, so any requirement which is slacker than this can be ordered on the basis of the SPC results. Where machines are being used as part of a research environment, or are cheaper photo plotters rather than the laser/ebeam based writing systems used in merchant photomask manufacturers, there is likely to be more process variation between photomasks.
Photomasks can also be checked for the positional accuracy of feature placement within the photomask, either by comparing the overlay of the photomask with another photomask or by comparing points measured from the photomask to a standard grid. Any photomask being produced for the semiconductor industry will typically have registration and overlay results better than 0.25 microns, and for most non- semiconductor applications SPC results can be
www.euroasiasemiconductor.com Issue I 2012
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