Christian Laubis

Pre-flight calibration of LYRA, the solar VUV radiometer on board PROBA2

Aims. LYRA, the Large Yield Radiometer, is a vacuum ultraviolet (VUV) solar radiometer, planned to be launched in November 2009 on the European Space Agency PROBA2, the Project for On-Board Autonomy spacecraft. Methods. The instrument was radiometrically calibrated in the radiometry laboratory of the Physikalisch-Technische Bundesanstalt (PTB) at the Berlin Electron Storage ring for SYnchroton radiation (BESSY II). The calibration was done using monochromatized synchrotron radiation at PTBs VUV and soft X-ray radiometry beamlines using reference detectors calibrated with the help of an electrical substitution radiometer as the primary detector standard. Results. A total relative uncertainty of the radiometric calibration of the LYRA instrument between 1% and 11% was achieved. LYRA will provide irradiance data of the Sun in four UV passbands and with high temporal resolution down to 10 ms. The present state of the LYRA pre-flight calibration is presented as well as the expected instrument performance.

Radiometric characteristics of new diamond PIN photodiodes

New PIN photodiode devices based on CVD diamond have been produced showing high responsivity in a narrow bandpass around 200 nm. A set of measurement campaigns was carried out to obtain their XUV-to-VIS characterization (responsivity, stability, linearity, homogeneity). The responsivity has been measured from the XUV to the NIR, in the wavelength range of 1 nm to 1127 nm (i.e. 1240 to 1.1 eV). The diamond detectors exhibit a high responsivity of 10 to 30 mA W−1 around 200 nm and demonstrate a visible rejection ratio (200 nm versus 500 nm) of six orders of magnitude. We show that these PIN diamond photodiodes are sensitive sensors in the 200 to 220 nm range, stable under brief irradiation with a good linearity and homogeneity. They will be used for the first time in a solar physics space instrument LYRA, the Large Yield RAdiometer.

The influence of line edge roughness and CD uniformity on EUV scatterometry for CD characterization of EUV masks

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. For extreme ultraviolet (EUV) masks, only EUV radiation provides direct information on the mask performance comparable to the operating regime in 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. First measurements using PTBs EUV reflectometer at the storage ring BESSY II showed that it is feasible to derive information on the line profile in periodic areas of lines and spaces by means of rigorous numerical modeling. A prototype EUV mask with a matrix of test fields each divided into subfields containing among others test fields with lines & spaces was used for the measurements. In this contribution we summarize our present results in determining line profile parameters using scatterometry and reflectometry to provide the input data for the determination of CD and side-wall geometry using rigorous calculations of EUV diffraction. Particularly, we present a first investigation on the influence of line edge roughness and CD uniformity by correlating in-plane scatterometry data for the discrete diffraction orders corresponding to the pitch of the structure to out-of-plane measurements of diffusely scattered light induced by line edge roughness and CD uniformity. We demonstrate the influence of diffuse scattering on the determination of CD and side-wall geometry using only the discrete in-plane diffraction orders. To this aim we perform finite element (FEM) simulations on 2D computational domains.

Status of EUV reflectometry at PTB

With several pre-production EUV tools already in the field, the development of EUV Lithography has gained momentum and the first production tools are scheduled for delivery in 20131. Consequently, the demand for EUV radiometry has grown as well with respect to volume and variety of the measurement requirements. Building on almost two decades of experience, PTB offers a wide range of actinic EUV measurements, like reflectance and transmittance, to characterize optical elements and sensitivity measurements to characterize detection devices. Based on these core competences, new applied measurements can be offered. Diffusely scattered light can be used to assess surface roughness (PSD) in the otherwise difficult to measure spatial frequency region of 1 μm to 10 μm. PTB performs EUV metrology at two dedicated complementary beamlines covering the wavelength range from 0.65 nm to 30 nm with particularly well-collimated radiation and the range from 5 nm to 50 nm with higher radiant power, variable degree of polarization and adjustable spot size on the sample. Both beamlines are optimized to achieve temporally stable normalized radiant power and excellent wavelength reproducibility at high spectral purity with out-of-band radiation in the range of 10-3 relative, as a prerequisite for low measurement uncertainties and long-term reproducibility. We present an updated overview of our capabilities with a focus on the long-term stability of our instrumentation and methods.

Evaluation of EUV scatterometry for CD characterization of EUV masks using rigorous FEM-simulation

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. For extreme ultraviolet (EUV) masks, only EUV radiation provides direct information on the mask performance comparable to the operating regime in an EUV lithography tool. With respect to the small feature dimensions on EUV masks, the short wavelength of EUV is also advantageous since it increases the sensitivity for small structural details. Measurements using PTBs EUV reflectometer at the storage ring BESSY II showed that it is feasible to derive information on the absorber line profile in periodic areas of lines and spaces by means of rigorous numerical modeling with the finite element method (FEM). A prototype EUV mask with fields of nominally identical lines was used for the measurements. In this contribution we correlate the scatterometry data to CD-SEM and surface nano probe measurements of the line profiles as provided by the mask supplier. We discuss status of the determination of CD and side-wall geometry by scatterometry using rigorous FEM calculations of EUV diffraction and directions for further investigations.

Metrology of EUV masks by EUV-scatterometry and finite element analysis

Extreme ultraviolet (EUV) lithography is seen as a main candidate for production of future generation computer technology. Due to the short wavelength of EUV light (≈ 13 nm) novel reflective masks have to be used in the production process. A prerequisite to meet the high quality requirements for these EUV masks is a simple and accurate method for absorber pattern profile characterization. In our previous work we demonstrated that the Finite Element Method (FEM) is very well suited for the simulation of EUV scatterometry and can be used to reconstruct EUV mask profiles from experimental scatterometric data. In this contribution we apply an indirect metrology method to periodic EUV line masks with different critical dimensions (140 nm and 540 nm) over a large range of duty cycles (1:2, ... , 1:20). We quantitatively compare the reconstructed absorber pattern parameters to values obtained from direct AFM and CD-SEM measurements. We analyze the reliability of the reconstruction for the given experimental data. For the CD of the absorber lines, the comparison shows agreement of the order of 1nm. Furthermore we discuss special numerical techniques like domain decomposition algorithms and high order finite elements and their importance for fast and accurate solution of the inverse problem.

Rigorous FEM simulation of EUV masks: influence of shape and material parameters

We present rigorous simulations of EUV masks with technological imperfections like side-wall angles and corner roundings. We perform an optimization of two different geometrical parameters in order to fit the numerical results to results obtained from experimental scatterometry measurements. For the numerical simulations we use an adaptive finite element approach on irregular meshes. This gives us the opportunity to model geometrical structures accurately. Moreover we comment on the use of domain decomposition techniques for EUV mask simulations. Geometric mask parameters have a great influence on the diffraction pattern. We show that using accurate simulation tools it is possible to deduce the relevant geometrical parameters of EUV masks from scatterometry measurements. This work results from a collaboration between AMTC (mask fabrication), Physikalisch-Technische Bundesanstalt (scatterometry) and ZIB/JCMwave (numerical simulation).

High accuracy EUV reflectometry at large optical components and oblique incidence

The development of EUV lithography is critically based on the availability of suitable metrology equipment. To meet industrys requirements, the Physikalisch-Technische Bundesanstalt (PTB) operates an EUV reflectometry facility at the electron storage ring BESSY II. It is designed for at-wavelength metrology of full-sized EUVL optics with a maximum weight of 50 kg and a linear dimension of up to 1 m. With the development of EUV lithography tools, the requirements for lower measurement uncertainty are steadily increasing. For small test samples at near normal incidence, a total uncertainty of 0.10 % for peak reflectance is achieved with a reproducibility of 0.05 % and the uncertainty in the center wavelength of 2 pm is mainly given by the uncertainty for the reference wavelength of the Kr 3d5/2-5p resonance. For real optical elements like PO-box mirrors and collectors for EUV pulsed plasma sources it is also essential to measure at the exact location on the mirror because of gradients in the layer thickness and also to measure at the correct local angle of incidence (LAOI) which may deviate significantly from normal. Thus alignment becomes critical for achieving low measurement uncertainties. Here we present PTBs experience in measuring large EUV optical components.

Use of EUV scatterometry for the characterization of line profiles and line roughness on photomasks

Scatterometry is a versatile metrology for characterizing periodic structures, regarding critical dimension (CD) and other profile properties. With respect to small feature sizes on future lithography photomasks, the short wavelength of extreme ultraviolet (EUV) radiation is advantageous since it minimizes diffraction phenomena and increases the sensitivity to roughness. The advantage of the short wavelength can also be used for investigations of DUV photomasks. For EUV masks, only EUV radiation provides direct information on at-wavelength mask performance. The intensity of the measured diffraction orders carries information about the absorber line profile like top CD, sidewall angle, height, and top corner radius. It is shown that it is feasible to derive information on the absorber line profile in periodic areas of lines and spaces by means of rigorous numerical modeling with the finite element method (FEM). We demonstrate the determination of line profile parameters for chrome on glass (CoG) and EUV masks. EUV Reflectometry on CoG masks is successfully used to determine absorber line heights. A good correlation (0.2 nm rms) between CD values determined using EUV scatterometry and CD-SEM is demonstrated for an EUV mask. A clear correlation between diffuse scatter intensity and CD uniformity, respectively line roughness, is also obtained.

Characterization of large off-axis EUV mirrors with high accuracy reflectometry at PTB

CZ SMT AG produced large off-axis EUV mirrors as they are used e.g. in ASMLs alpha demo tools, the predecessor for Extreme Ultraviolet Lithography (EUVL) production tools by ASML. The coating development and a large part of the actual coatings were done by the FOM-Institute. The Physikalisch-Technische Bundesanstalt (PTB) operates an EUV reflectometry facility at the electron storage ring BESSY II for at-wavelength metrology of full-size EUVL optics with a weight of up to 50 kg and a diameter of 550 mm. Critical issues for EUVL mirrors are the high reflectivity close to the theoretical limit, the matching of the period to the operating wavelength of the stepper (13.5 nm) and the imaging properties of the EUV optics. The full multilayer stack needs to be controlled laterally to such extend that the initial sub-nanometre surface figure of the substrate is preserved. The so-called added figure error should not exceed 100 pm in order to ensure faultless imaging at 13.5 nm wavelength. Here, we discuss representative results obtained at large off-axis EUV mirrors. We especially discuss the challenges of measurements at higher local angles of incidence according to the optical design and the accuracy needed in sample alignment for measurement of the coating profiles. PTB has shown excellent reproducibility for measurements of the near normal incidence reflectance of flat homogeneous mirrors over several years. For large off-axis EUV mirrors, measurements have to be done at angles significantly off normal, which dramatically increases the influence of angular alignment errors of the sample on the measured peak wavelength. Furthermore, according to the optical design, these optics have gradients of the coating thickness which require exact knowledge of the measurement position in the mirror coordinates. Extensive studies were done to estimate and validate the uncertainties connected to the sample alignment. Our results clearly show that it is possible to meet and verify the tight specifications for the lateral coating profiles of EUV multilayer mirrors. The non-correctable added figure error is significantly better than required and the overall reflectance of the coatings with a special protective capping layer is 65%.