Photomask dimensional metrology in the scanning electron microscope, part I: Has anything really changed?

Photomask dimensional metrology in the scanning electron microscope has not evolved as rapidly as the metrology of resists and integrated circuit features on wafers. This has been due partly to the 43 (or 53) reduction in optical steppers and scanners used in the lithog- raphy process, and partly for the lesser need to account for the real three dimensionality of the mask structures. So, where photomasks are con- cerned, many of the issues challenging wafer dimensional metrology at 13 are reduced by a factor of 4 or 5 and thus could be temporarily swept aside. This is rapidly changing with the introduction of advanced masks with optical proximity correction and phase shifting features used in 100 nm and smaller circuit generations. Fortunately, photomask metrology generally benefits from the advances made for wafer metrology, but there are still unique issues to be solved in this form of dimensional metrology. It is likely that no single metrology method or tool will ever provide all necessary answers. As with other types of metrology, resolu- tion, sensitivity and linearity in three-dimensional measurements of the shape of the lines and phase shifting features in general (width, height and wall angles) and departure from the desired shape (surface and edge roughness, etc.) are the key parameters. Different methods and tools differ in their capability to collect average and localized signals at acceptable speed, but in any case, application of thorough knowledge of the physics of the given metrology is essential to extract the information needed. This paper will discuss the precision, accuracy and traceability in SEM metrology of photomasks. Current and possible new techniques utilized in the measurements of photomasks including suppression of charge and highly accurate modeling for electron beam metrology will also be explored to answer the question, Has anything really