Measurements of CD and sidewall profile of EUV photomask structures using CD-AFM and tilting-AFM

Abstract

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°.

Challenges in nanometrology: high precision measurement of position and size

“…10. A systematic comparison between CD-AFM and EUV-scatterometry measurements on an EUV reflection mask was reported in [22]. Figure 11 shows the assumed model topography of the EUV reflection multilayer and the mask absorber stack, while fig.…”

“…For traceable CD metrology on larger substrates such as photomasks or wafers, we are using a Nanostation 300 as the basic positioning machine, a scanning head based on a 6-axis flexure stage and a self-developed operation mode for probing the surface under test, the so-called vector approach probing [30]. Figure 15 shows the reproducibility of repeated measurements of the surface at the top and the sidewall of a lithographic Si line using a flared or reentrant CD tip, made with e-beam assisted deposition techniques (CDR-EBD tip) [31]. …”

Scatterometry reference standards to improve tool matching and traceability in lithographical nanomanufacturing

“…Additionally we have applied different high resolution microscopy tools, namely low voltage SEM, cross section SEM and PTB's 3D AFM [19] to characterize further important structure details such as line edge roughness, edge angles or etch profile details such as corner rounding. Besides the structure height AFM measurements are utilized to characterize line edge roughness, line edge angles and profiles.…”

Comparison of CD measurements of an EUV photomask by EUV scatterometry and CD-AFM

“…This kind of higher order tip effect accounts for the influence of the finite radius of the tip flare when probing sidewalls of different steepness. Details of the procedure are found elsewhere 33 . The measurement with the CD-AFM was performed in the vertical oscillation mode.…”