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.
The development of technology infrastructure for EUV Lithography (EUVL) still requires higher levels of technology readiness in many fields. A large number of new materials will need to be introduced. For example, development of EUV compatible pellicles to adopt an approved method from optical lithography for EUVL needs completely new thin membranes which have not been available before. To support these developments, PTB with its decades of experience  in EUV metrology  provides a wide range of actinic and non actinic measurements at in-band EUV wavelengths as well as out of band. Two dedicated, complimentary EUV beamlines  are available for radiometric [4,5] characterizations benefiting from small divergence or from adjustable spot size respectively. The wavelength range covered reaches from below 1 nm to 45 nm  for the EUV beamlines  to longer wavelengths if in addition the VUV beamline is employed. The standard spot size is 1 mm by 1 mm with an option to go as low as 0.1 mm to 0.1 mm. A separate beamline offers an exposure setup. Exposure power levels of 20 W/cm2 have been employed in the past, lower fluencies are available by attenuation or out of focus exposure. Owing to a differential pumping stage, the sample can be held under defined gas conditions during exposure. We present an updated overview on our instrumentation and analysis capabilities for EUV metrology and provide data for illustration.
The throughput of extreme ultraviolet (EUV) lithography systems is presently strongly limited by the available radiant power at the wafer level. Besides increasing the power of EUV sources, also the quality of the optical elements plays a key role. With state of the art multilayer mirrors the main cause of diminished reflectance is surface and interface roughness as well as interface diffusion. Both properties lead to reduced specular reflectance while only the interface roughness causes diffuse scattering. EUV diffuse scatter thus allows to selectively assess the contribution of the interface roughness.
The intensity distribution of diffusely scattered EUV radiation provides information on vertical and lateral correlations of the surface and interface roughness through the appearance of resonant diffuse scattering (RDS) sheets. The study of off-specular scattering thus serves as a natural tool for the investigation of the roughness of the interfaces. However, upon near-normal incidence impinging EUV radiation, dynamical scattering contributions from thickness oscillations (Kiessig fringes) lead to Bragg lines which intersect the RDS sheets. This causes strong resonant enhancement in the scatter cross section which we called “Kiessig-like peak" in analogy to the well known phenomenon of Bragg-like peaks appearing in hard X-ray grazing incidence measurement geometries. Thus for power spectral density studies of multilayer interface roughness, resonant dynamical scattering cannot be neglected. Theoretical simulations based on the distorted-wave Born approximation enable to separate dynamic features of the multilayer from roughness induced scattering. This allows to consistently determine an interface power spectral density (PSD). We have analyzed magnetron sputtered high-reflectance Mo/Si multilayer mirrors with different nominal molybdenum layer thicknesses from 1.7 nm to 3.05 nm crossing the Mo crystallization threshold.
Our off-specular scattering measurements at multilayer samples were conducted at the PTB-EUV radiometry beamline at the Metrology Light Source (MLS) in Berlin. The samples were produced by magnetron sputtering and pre-characterized by Kα X-ray reflectivity at Fraunhofer IWS, Dresden.
EUV Lithography now reaches the fab floor. The technology ramp up and integration with existing processes will require evolutionary steps in many aspects of the technology. For instance will it be necessary to reduce 3D mask effects like shadowing e.g. by introducing a thinner absorber structure. Continuous progress will be based on using new materials, adapted multilayers, and new reticle designs. Many of these developments are based on simulations and computer models for the design of the required structures and thus require data on the optical properties of the materials involved. In particular when addressing the reticle where the optical function is the target value. Using its more than 25 years of expertise in EUV metrology1, PTB operates instrumentation for reflectometry and scatterometry2 in the EUV and adjacent wavelength ranges and can provide the data for the determination of optical material parameters for individual thin layers. The need for sound optical parameter characterization for the development of alternative EUV materials was thoroughly motivated during the 2015 SPIE Advanced Lithography conference3. The data required is not readily available from databases, as thin film properties - depending on their deposition method and interfaces - may deviate significantly from standard bulk data4. Therefore, better optical constants and a continuous availability of the associated measurement tools are vital for further progress in EUV reticle and optical system design. The ability to vary relevant parameters like wavelength, angle of incidence (AOI), the plane of incidence and polarization is a prerequisite to gather sufficient data to model optical constants. We give details on PTB's measurement capabilities and accessible parameter space for optical material parameter characterization and show some representative data and results.
Non-imaging techniques like X-ray scattering are supposed to play an important role in the further development of CD metrology for the semiconductor industry. GISAXS provides directly assessable information on structure roughness and long-range periodic perturbations. The disadvantage of the method is the large footprint of the X-ray beam on the sample due to the extremely shallow angle of incidence. This can be overcome by using wavelengths in the extreme ultraviolet (EUV) spectral range which allow for much steeper angles of incidence but preserve the large range of momentum transfer that can be observed. At the Physikalisch-Technische Bundesanstalt (PTB), the available photon energy range extends from 50 eV up to 10 keV at two adjacent beamlines. PTB commissioned a new versatile Ellipso-Scatterometer which is capable of measuring 6” square substrates in a clean, hydrocarbon-free environment with full flexibility regarding the direction of the incident light polarization. The reconstruction of line profiles using a geometrical model with six free parameters, a finite element method (FEM) Maxwell solver and least-squares optimization yielded consistent results for EUV and deep ultraviolet (DUV) scatterometry. For EUV photomasks, the actinic wavelength EUV scatterometry yields particular advantages. A significant polarization dependence of the diffraction intensities for 0th and +1st orders in the geometry with the grating lines perpendicular to the plane of reflection is observed and the 0th order intensity shows sufficient sensitivity to the line width such that a CD-resolution below 0.1 nm is within reach. In this contribution we present scatterometry data for line gratings using GISAXS, and EUV and DUV scatterometry and consistent reconstruction results of the line geometry for EUV and DUV scatterometry.
Non-imaging techniques like X-ray scattering are supposed to play an important role in the further development of CD
metrology for the semiconductor industry. Grazing Incidence Small Angle X-ray Scattering (GISAXS) provides directly
assessable information on structure roughness and long-range periodic perturbations. The disadvantage of the method is
the large footprint of the X-ray beam on the sample due to the extremely shallow angle of incidence. This can be
overcome by using wavelengths in the extreme ultraviolet (EUV) spectral range, EUV small angle scattering (EUVSAS),
which allows for much steeper angles of incidence but preserves the range of momentum transfer that can be
observed. Generally, the potentially higher momentum transfer at shorter wavelengths is counterbalanced by decreasing
diffraction efficiency. This results in a practical limit of about 10 nm pitch for which it is possible to observe at least the
± 1st diffraction orders with reasonable efficiency. At the Physikalisch-Technische Bundesanstalt (PTB), the available
photon energy range extends from 50 eV up to 10 keV at two adjacent beamlines. PTB commissioned a new versatile
Ellipso-Scatterometer which is capable of measuring 6" square substrates in a clean, hydrocarbon-free environment with
full flexibility regarding the direction of the incident light polarization.
The reconstruction of line profiles using a geometrical model with six free parameters, based on a finite element method
(FEM) Maxwell solver and a particle swarm based least-squares optimization yielded consistent results for EUV-SAS
and GISAXS. In this contribution we present scatterometry data for line gratings and consistent reconstruction results of
the line geometry for EUV-SAS and GISAXS.
The aim of the semiconductor industry to decrease the feature size of integrated circuits poses a huge technological endeavor. Consequently, new challenges are arising for metrology on structures in the nanometer regime. Scatterometry is a fast method which provides non-contact non-destructive characterization of structures on photomasks or exposed wafers. However, the determination of important line structure parameters with subnanometer accuracy still needs further investigation. Grazing incidence small-angle X-ray scattering (GISAXS) is a scatterometry technique to measure both vertical and lateral structural features in the nanometer range with high sensitivity. We apply GISAXS to the investigation of structural parameters such as period length, sidewall angle, linewidth and height on silicon gratings. Our test structures with nominal widths of 35 nm to 100 nm and a pitch from 100 nm to 250 nm were fabricated by electron beam lithography. The diffraction patterns have been analyzed by power spectral density analysis which directly yields periodical modulations of the structured surface such as line width or groove width. We also apply a finite element method (FEM) to the diffraction peak intensity of the grating structure obtained with GISAXS for the geometric reconstruction of the line shape.