In this experimental evaluation, grazing-incidence small-angle x-ray scattering (GI-SAXS) was selected as the inline reference metrology tool5 and scatterometry was selected as the inline metrology tool. Figure 4 shows a schematic diagram of GI-SAXS. X-rays irradiate the sample with a very low incident angle and scattered x-rays from the sample are detected at a two-dimensional (2-D) detector. On the 2-D detector, the unique scattering signal caused by the surface structure is detected. A measurement model of GI-SAXS is preset and is calculated to fit the experimental scattering signal. GI-SAXS uses the total external reflection signal with very short wavelength radiation. Therefore, the measurement accuracy of GI-SAXS has a high capability. The measurement robustness of the scatterometry and GI-SAXS applied to manufacturing process variations was evaluated. Figure 5 shows the structural models for scatterometry and GI-SAXS: (a) cross-sectional scanning electron microscopy (SEM) image, (b) scatterometry model, and (c) GI-SAXS model. Line-space patterns are etched into a multilayer stack of Si, , and poly-Si films, and the space regions are filled with . A second etching process created line-space patterns of the surface layer only. The measurement target is the line height and because of the variation in Si etching conditions 1 and 2, there are small shape variations of the cross-section of the Si line. For line height measuring, the scatterometry and the GI-SAXS models were prepared as shown Figs. 5(b) and 5(c). Scatterometry is an optical metrology system based on spectroscopic ellipsometry measurement with visible wavelengths. Irradiation light can penetrate into surface and internal structures. Therefore, the scatterometry model should be constructed with a complete two- or three-dimensional structure and set a lot of floating parameters. In Fig. 5(b), this model was constructed by using multitrapezoidal parts for Si and poly-Si line patterns. The number of floating parameters is set as eight parameters for line height measurement. The optical constant of each part was a constant value. GI-SAXS, on the other hand, is sensitive to surface structure only as noted above. In Fig. 5(c), this GI-SAXS model was constructed with single trapezoidal parts with top and bottom rounding. The GI-SAXS model is a simpler structure than the scatterometry model, and the number of floating parameters is five parameters. As the incident x-ray irradiates in the total external reflection condition, GI-SAXS detects only surface structure information. Figure 6 shows the comparison with cross-sectional SEM results. The scatterometry results for etch conditions 1 and 2 have 2.3-nm offset when both are compared with the cross-sectional SEM. Scatterometry can detect under-layer information by using visible light and is also sensitive to under-layer shape changes. The height from GI-SAXS for etching conditions 1 and 2 only has a 0.6-nm offset and a very good measurement linearity when compared with cross-sectional SEM measurements in Fig. 6(b). These results indicate that GI-SAXS has a suitable sensitivity to height and has the additional benefit of being a nondestructive inline reference metrology. However, there is a large gap in throughput between the two techniques; the measurement times of scatterometry and GI-SAXS are 3 and 120 s, respectively. Therefore, by combining scatterometry and GI-SAXS, it is possible to improve the metrology capability for robustness and throughput.