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Production sequence of Si-spheres and interferometrical determination of the sphere volume

AFM Linewidth Metrology

Working Group 5.23

Atomic resolution traceable CD reference metrology based on HR-TEM

Traceability is a fundamental issue for nano dimensional metrology. The lack of traceability in measurements inhibits the comparison of tools from different manufactures and limits knowledge about the real size of fabricated features. For realising traceable AFM CD measurements, two important issues need to be met, the calibrations of the AFM's displacement scales and the effective tip geometry.

 

Traceable calibrations of the AFM's displacement scales

Traceable calibrations of the AFM's displacement scales is performed via the metrological Met. LR-AFMs developed at PTB shown as figure 19. Currently the scaling factors of AFMs can be calibrated with a relative uncertainty of better than 10-3. Consequently this contribution to the overall measurement uncertainty of CD metrology is typically insignificant, especially for the small feature sizes of the current technology node.

 

Fig. 19 Photo of the Met. LR-AFM developed based on a nano positioning machine (NMM)

 

Traceable calibration of effective tip geometry

The traceable calibration of the effective tip geometry is a challenging task remaining today. We calibrate the effective tip geometry by applying reference structures whose real geometry is accurately and traceably determined by applying transmission electron microscopes (TEM). Two methods have been applied for achieving the measurement traceability, as shown in figure 20. The first method applies the silicon crystal lattice as an internal rule. As shown in figure 20(a), the structure geometry can be calculated by counting the number of the crystal planes inside the structure, N, and the silicon crystal constant d111, which was determined traceably as 313.560 11(17) pm by combined x-ray and optical interferometry from bulk silicon material. This method is highly accurate, however, it demands that the sample material is a single crystal. To solve this problem, an alternative method has been developed, as shown in figure 20(b). Using this method, the pitch L of line features was accurately and traceably calibrated in advance, for instance, by a metrological AFM. After the line features were imaged in a TEM, the pitch (M) and width (N) of the feature pair could be determined in pixel units. Thus, the scaling factor of the TEM image could be calculated as K = L/M nm/pixel and the width of the structure could be evaluated as W = N×L/M. An important idea underlying the proposed method is that unlike the CD metrology the pitch calibration using AFMs is independent of its tip geometry.

The traceability chains of two methods mentioned above are quite different. In the first method, the traceability chain is ensured via the lattice constant in the Si single crystal TEM lamella, x-ray interferometry in Si bulk material, optical interferometry and then to the optical wavelength and the metre definition of the International System of Units, the SI; while in the second method, the traceability chain is ensured via the AFM sample stage position measured by laser interferometer, and then to the optical wavelength and the SI metre definition. Consistency of these two methods has been verified in our previous study.

 

 

Fig.20 Two strategies applied in traceable calibration of the geometry of the reference nano structure based on its transmission microscopic images, (a) via silicon crystal lattice constant and (b) via metrological AFM