EUV Scatterometry

Scatterometry, the analysis of light diffracted from a periodic structure, is a versatile metrology for characterizing periodic structures, regarding critical dimension (CD) and other profile properties. When exposing an EUV mask with EUV radiation of 13.5 nm, the radiation is reflected by the multilayer stack which is about 300 nm thick. For EUV radiation, all layers in the stack contribute to the reflection. Therefore, only EUV scatterometry¹ provides direct information on the mask performance comparable to an EUV lithography tool. With respect to the small feature dimensions on EUV masks, the short wavelength of EUV is also advantageous since it provides more diffraction orders as compared to UV. PTB's EUV reflectometer at the storage ring BESSY II² allows mask surface scanning in Cartesian coordinates at 10 μm positioning reproducibility. The probed area (photon beam size) is about 1 mm².

For scatterometry measurements, the detector angle is scanned while the incident beam is kept at a fixed entrance angle of 6˚, see Fig. 1. The measured diffraction orders of EUV masks do not carry direct information about the absorber line profile. The geometrical parameters, top CD, sidewall angle, height, and top corner radius, see Fig. 2, are varied in numerical simulations using the finite element (FEM) method to derive the geometrical properties from the measured diffraction pattern as shown in Fig. 3. The diffraction pattern for different values of the respective parameter is calculated and compared to the experimental results. Table 1 summarizes the results for the 3 parameters top CD, side wall angle and top corner rounding as shown in Fig. 2. The good agreement of the results indicates the robustness of the scatterometry approach.

wavelength	top CD	sidewall angle	height	corner radius
13.40 nm	147.5 nm	89.5˚	76.4 nm	6 nm
13.65 nm	147.3 nm	90˚	75.3 nm	6 nm
13.90 nm	147.8 nm	90˚	77.4 nm	6 nm