To address the increasing requirements of the semiconductor industry for accurate measurement of small features, Critical Dimensions (CD) Grazing-Incidence Small-Angle X-ray Scattering (GISAXS) is being considered. GISAXS offers fast, surface sensitive measurements from which relevant structural parameters can be reconstructed. However, an essential limitation for the practical application is the large measurement spot size resulting from the shallow incidence angle. Here we show that despite a large spot size, GISAXS measurements of small targets are possible if the targets are designed such that the scattering from the targets is separable from the scattering of other structures in the measurement spot. For grating targets, in particular rotation of the targets in the sample plane such that the grating lines are not parallel to the predominant direction of the surroundings allows reliable measurement of the target scattering irrespective of the X-ray beam size. We discuss the effect of X-ray source, rotation angle, target size and grating pitch on the minimum measurement times and number of measurable grating diffraction orders and conclude that a single 50 μm × 50 μm area grating target can be measured with high-end laboratory sources in tens of seconds if it is rotated by at least 0.2°. For smaller targets, significantly longer measurement times and larger rotations are necessary.
Lamellar gratings are widely used diffractive optical elements and are prototypes of structural elements in integrated
electronic circuits. EUV scatterometry is very sensitive to structure details and imperfections, which makes it suitable for
the characterization of nanostructured surfaces. As compared to X-ray methods, EUV scattering allows for steeper angles
of incidence, which is highly preferable for the investigation of small measurement fields on semiconductor wafers. For
the control of the lithographic manufacturing process, a rapid in-line characterization of nanostructures is indispensable.
Numerous studies on the determination of regular geometry parameters of lamellar gratings from optical and Extreme
Ultraviolet (EUV) scattering also investigated the impact of roughness on the respective results. The challenge is to
appropriately model the influence of structure roughness on the diffraction intensities used for the reconstruction of the
surface profile. The impact of roughness was already studied analytically but for gratings with a periodic pseudoroughness,
because of practical restrictions of the computational domain. Our investigation aims at a better
understanding of the scattering caused by line roughness. We designed a set of nine lamellar Si-gratings to be studied by
EUV scatterometry. It includes one reference grating with no artificial roughness added, four gratings with a periodic
roughness distribution, two with a prevailing line edge roughness (LER) and another two with line width roughness
(LWR), and four gratings with a stochastic roughness distribution (two with LER and two with LWR). We show that the
type of line roughness has a strong impact on the diffuse scatter angular distribution. Our experimental results are not
described well by the present modelling approach based on small, periodically repeated domains.
As the industry continues to progress along the ITRS roadmap, not only the device dimensions shrink, but the architectures also increase in 3D complexity. Therefore, new metrology approaches for small structures are required. Small angle X-ray scattering has the potential for fast in line metrology but suffers from the large spot size needed in grazing incidence reflection and the low signal for the transmission geometry. Turning the incidence angle closer to normal and tuning the wavelengths into the EUV spectral range allows to decrease the spot size while keeping the signal high. We present an exploration of soft X-ray and EUV-scatterometry from grazing to near normal incidence, including a new approach in the design of metrology targets to avoid the footprint problem in grazing incidence geometry. Measurements were performed on e-beam written silicon gratings. The reconstructed geometrical line shape models are statistically validated by applying a Markov-Chain Monte Carlo sampling technique. Experimental data and simulation results provide a first insight into the potential of EUV Scatterometry.