Characterization of new CD photomask standards

Abstract

We report on the metrological characterization of photomask standards which were developed within a project with industrial partners in Germany to be used as reference standards for different type of linewidth or CD metrology instruments. It was the objective to develop high quality mask standards which would allow to perform CD calibrations with smallest possible uncertainties and to use the standards within the project consortium as well as to make the standards available to interested third parties. The design of the standards consists of line as well as box structures, isolated as well as group patterns, each in different tones and in x/y-orientation. The structures are all placed on a regular measurement grid to be easily accessible by automated CD metrology systems. Target CD values are going down to 0.2 μm with smallest CD steps of 20 nm and line to space ratio within groups is varied between 1:1 and 1:5. Additionally, there are larger CD structures and clear fields and different pitch structures on the mask. This contribution will concentrate on the description of the methods and instrumentation used at the PTB for investigation and calibration of the standards. Different methods of optical transmission microscopy as well as low energy scanning electron microscopy and scanning probe microscopy were applied which all were developed to provide traceable linewidth calibration values. This implies, that suitable physical models for the interaction of the different probes with the photomask structures had to be developed for a meaningful interpretation of the measurement signals for all type of CD metrology methods. This analysis also has to take into account the influences of structure imperfections, like edge slope and line edge roughness.