New method for the measurement of structure widths with atomic resolution

The term “critical dimensions” (CD) is often used synonymously for “structure width”. CD metrology plays an essential role in process control in the semiconductor industry for ensuring reliable manufacturing of micro- and nano-structures on silicon wafers and photomasks. Due to the progressive miniaturization in the fabrication process (today down to structure widths of 22 nm), the requirements placed on the measurement uncertainties of CD metrology down to the sub-nanometre range are becoming ever more demanding. Therefore, industry has an urgent need for exact and traceable CD reference measurement procedures in order to verify and characterize the diverse CD measuring instruments used in the production lines of the semiconductor industry such as, e.g., optical scatterometers.

The new reference method combines two technologies: transmission electron microscopy (TEM) and atomic force microscopy (AFM). An aberration-corrected, high-resolution TEM is capable of measuring nano-structures on thin, single- crystal layers with atomic resolution. It offers the best accuracies in calibrating the width of a feature on the cross-section polish of line structures by using the atomic spacing in the feature as an internal rule. In this way, the CD can be directly linked to the atomic spacing in the crystal lattice, which can be traceably calibrated by a combined optical and x-ray interferometer. For instance, the lattice spacing d₁₁₁, i.e. the distance between the (111) crystal planes of the material silicon 28Si, was determined precisely to (0.31356011 ± 0.00000017) nm.

Prior to the TEM measurement, two structures on the wafer are measured precisely with the AFM. After that, one of the features is carefully detached and then thinned down to less than 100 nm by means of a focused ion beam (FIB) for the TEM measurements. The disadvantage is that this area of the sample is no longer available for further CD measurements. The TEM measurements, however, allow inherent systematic errors of the AFM method such as, e.g., the probe diameter, to be detected and corrected. As a result, reference CD values can finally be determined on the intact structure with an estimated combined standard measurement uncertainty of 0.81 nm. This has been confirmed by five investigations of the CD of a reference structure which were carried out independently of each other on different TEMs. In addition, the new reference method has been successfully used to measure different structure characteristics of an EUV (extreme ultraviolet light) photomask. With these results, measurements carried out by means of synchrotron radiation at PTB's EUV scatterometer were confirmed. Integrating the combination of FIB and TEM to obtain a more accurate traceability will, furthermore, be a subject at the nano-lab LENA which is currently being set up at the Technische Universität Braunschweig in cooperation with PTB.