Accurate and traceable dimensional metrology with a reference CD-SEM

NIST is currently developing two Reference scanning electron microscopes (SEMs), which are based on FEI Nova 600* variable vacuum, and on FEI Helios* dual-beam instruments. These were installed in the new Advanced Metrology Laboratory at NIST where the temperature variation is under 0.1 C degrees and the humidity variation is under 1%. Both SEMs are equipped with field emission electron guns and are capable of better than 1 nm spatial resolution. The ESEM has large sample capability, allowing for measurements on 200 mm wafers, 300 mm wafers and 150 mm photolithography masks, with a 100 mm by 100 mm measurement area in the center. The dual-beam instrument's laser stage will work on smaller samples and has a 50 mm by 50 mm measurement area. The variable vacuum instrument is especially suitable for measurements on a large and diverse set of samples without the use of conductive coating. These will be among the most scrutinized of SEMs. A detailed, thorough work of combined measurements and optimization of the SEMs themselves is underway, which includes the assessment of resolution, signal transfer characteristics, distortion and noise characteristics in various working modes. Accurate three-dimensional modeling, including all aspects of beam formation, signal generation, detection and processing is under development. Establishment of modeling and measurement methods to ascertain the three-dimensional shape and size of the electron beam is also underway. All these are needed to properly interpret the obtained data in accurate, physics-based measurements and will permit three-dimensional size and shape determination on a scale ranging from a few nanometers up to a few centimeters. Accuracy and traceability will be ensured through calibrated laser interferometry.