Accurate and traceable measurements of critical dimension (CD) and sidewall profile of extreme ultraviolet (EUV) photomask structures using atomic force microscopes (AFMs) are introduced in this paper. An instrument complementarily applied with two kinds of AFM techniques, the CD-AFM and the tilting-AFM, has been developed. High measurement stability of the instrument is demonstrated, for instance, the long-term CD stability is better than 1 nm over 500 successive measurements over 55 h. To traceably calibrate the effective tip geometry, transmission electron microscopes-based method is applied, which uses either the silicon crystal lattice or the structure pitch value calibrated by metrological AFMs as an internal scale. Several grating patterns with different nominal CDs and line/space ratios of an EUV photomask have been measured using the developed methods. A data evaluation method with considered higher order tip effect due to the non-vertical sidewall is introduced. Detailed measurement results of a test EUV photomask, such as middle CD, left and right sidewall angle, feature height, line edge roughness and edge profiles are given. Finally, the AFM results are compared to that of a PTB EUV scatterometer. The comparison of the middle CD yields a linear relation within a spread of only about ±2 nm and an offset of the absolute values below 3 nm. For the sidewall angle, both methods yield consistent results within a range of about 2°.
International comparisons between National Metrology Institutes are important to verify measurement results and the associated uncertainties. In this paper, we report a comparison of the line width calibration of a crystalline silicon line width standard, referred to as IVPS100-PTB standard, between the Physikalisch-Technische Bundesanstalt in Germany and the National Institute of Standards and Technology in the United States. Critical dimension atomic force microscopy was the measurement method used for this comparison. Both institutes applied generally the same but independently developed traceability pathways: the scaling factor of the atomic force microscope (AFM) scanner was calibrated by a set of step height and lateral standards certified by metrological AFMs, while the effective tip width was ultimately traceable to the lattice parameter of silicon via high resolution transmission electron microscopy. Good agreement has been achieved in the comparison: For two groups of line features with nominal critical dimensions (CDs) of 50 nm, 70 nm, 90 nm, 110 nm and 130 nm that were compared, the observed deviations of CD results were between −1.5 nm and 0.3 nm. The deviations are well within the associated measurement uncertainty.
A new method for accurately characterizing the tip geometry of critical dimension atomic force microscopy (CD-AFM) has been introduced. A sample type IVPS100-PTB whose line features have vertical sidewall, round corner with a radius of approx. 5 ∼ 6 nm and very low surface roughness has been applied as the tip characterizer. The geometry of the line features has been accurately and traceably calibrated to the lattice constant of crystal silicon. In this paper, detailed measurement strategies and data evaluation algorithms have been introduced, particularly concerning several important influence factors such as the line width roughness of the tip characterizer, measurement noise, measurement point density, and the calculation of the averaged tip geometry. Thorough experimental studies have been carried out, indicating high measurement accuracy of the developed method. For instance, tip geometry of a probe type CDR120 with a nominal tip diameter of 120 nm is reconstructed using two different tip characterizers before, during and after it is applied for a calibration of a user sample. The agreement of all 20 obtained tip profiles reaches 0.4 nm, confirming the high measurement stability, low tip wear as well as the high measurement consistency between two tip characterizers. Furthermore, the results of a nanofeature of the user sample after correcting the tip contribution show a repeatability of approximately 0.3 nm when it is repeatedly measured by a same tip, and a reproducibility of 0.9 nm when it is measured using two different tips, confirming the good performance of the tip correction method as well.
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