Next generation planar and non-planar complementary metal oxide semiconductor (CMOS) structures are three-dimensional nanostructures with multi-layer stacks that can contain films thinner than ten atomic layers. The high resolution of transmission electron microscopy (TEM) is typically chosen for studying properties of these stacks such as film thickness, interface and interfacial roughness. However, TEM sample preparation is time-consuming and destructive, and TEM analysis is expensive and can provide problematic results for surface and interface roughness. Therefore, in this paper, we present the use of direct measurements of sidewall surface structures by conventional atomic force microscopy (AFM) as an alternative or complementary method for studying multi-layer film stacks and as the preferred method for studying FinFET sidewall surface roughness. In addition to these semiconductor device applications, this AFM sidewall measurement technique could be used for other three-dimensional nanostructures.

Two-dimensional geometric models and experimental data are used to evaluate the effect of tip shape artifacts on atomic force microscopy images, roughness values, and power spectra. The effects of tip size are studied as a function of the surface feature height, spacing, and width. The models demonstrate that the need for sharp tips is dictated by the sample roughness and the size and spacing of surface features. In addition, the magnitude and direction of the error in the measured roughness parameters are related to artifact-induced changes in the skew in the distribution of the data points about the surface mean. Significantly, it is shown that either higher or lower roughness values can be measured using a smaller sized tip, depending on the surface character. These model results are supported by sample data obtained with 2 and 10 nm radii of curvature tipped probes on films used in semiconductor research and development. Sharp-tipped probes are clearly beneficial for imaging the surface microstructure of low-k materials. Tip size has a marked impact on the roughness values obtained on a somewhat smoother metal film sample. However, the high-k sample studied was smooth enough that no benefit was realized with the use of a sharper-tipped probe.

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