Alexander Diener

Accurate and traceable calibration of two-dimensional gratings

Accurate and traceable calibration of lateral standards (1D and 2D gratings) is a basic metrological task for nano- and microtechnology. Both the mean pitch and the uniformity of the gratings should be measured quantitatively. Although optical diffractometers are effective for measuring the mean pitch, they are not able to measure the uniformity of gratings. In this study, the calibration of gratings is performed using a metrological large range scanning probe microscope with optimized measurement strategies. Two different kinds of data evaluation methods, a gravity centre method and a Fourier transform method, have been developed and investigated. Cosine error, a significant error source of the measurement, is analysed and corrected. Calibrations on several 1D gratings have been carried out. The calibrated mean pitch values have an excellent agreement with those measured by optical diffractometry. Nevertheless, irregularities of the gratings were only deduced from the SPM results. Finally, the usage of the 1D/2D gratings for the calibration of a typical SPM is illustrated.

Multi-wavelength VIS/UV optical diffractometer for high-accuracy calibration of nano-scale pitch standards

A new optical diffractometer has been developed and set up at the Physikalisch-Technische Bundesanstalt (PTB). It offers the possibility of high-accuracy calibrations of the lateral period of gratings (pitch) in the micro- and nanometre scale. The measurement principle is based on a modified Littrow configuration, where the incident and the diffracted laser beams are almost collinear. The grating is mounted on a rotary table, and a high-precision rotary encoder is used to measure its angular positions. The profiles of the diffracted laser beams are recorded by means of a line array image detector. To determine the centre positions of the imaged laser beam profiles, different analysis methods can be applied, among others a new correlation method. A variety of laser wavelengths, ranging from 266 nm to 633 nm, can be used. Due to the optional UV wavelength, the smallest measurable pitch is about 150 nm. Depending on the quality of the sample, the measurement uncertainty can be smaller than 10 pm. For two-dimensional gratings the pitch of the two main and the diagonal directions can be measured and thus, also the angle between the two main grating orientations can be determined.

Picometer-scale accuracy in pitch metrology by optical diffraction and atomic force microscopy

We measured the pitch of a 144-nm pitch, two-dimensional grid in two different laboratories. Optical Diffraction gave very high accuracy for mean pitch and Atomic Force Microscopy measured individual pitch values, gaining additional information about local pitch variation. The measurements were made traceable to the international meter. Optical diffraction gave mean value 143.928 ± 0.015 nm (95% confidence limit, per GUM). AFM gave mean value 143.895 ± 0.079 nm. Individual pitch values had standard deviation 0.55 nm and expanded uncertainty ± 1.1 nm. Mean values measured by the two methods agreed within 0.033 nm. Because this was less than the uncertainty due to random variation in the AFM results, it suggests that the AFM measuring and analysis procedures have successfully corrected all systematic errors of practical significance in microscopy. We also discuss what precision may be expected from the AFM method when it is applied to measure smaller pitches.

Investigations of the influence of common approximations in scatterometry for dimensional nanometrology

Scatterometry is a common tool for the dimensional characterization of periodic nanostructures. It is an indirect measurement method, where the dimensions and geometry of the structures under test are reconstructed from the measured scatterograms applying inverse rigorous calculations. This approach is numerically very elaborate so that usually a number of approximations are used. The influence of each approximation has to be analysed to quantify its contribution to the uncertainty budget. This is a fundamental step to achieve traceability. In this paper, we experimentally investigate two common approximations: the effect of a finite illumination spot size and the application of a more advanced structure model for the reconstruction. We show that the illumination spot size affects the sensitivity to sample inhomogeneities but has no influence on the reconstruction parameters, whereas additional corner rounding of the trapezoidal grating profile significantly improves the reconstruction result.

EUV and DUV scatterometry for CD and edge profile metrology on EUV masks

To test the applicability of scatterometry on EUV masks we measured a prototype EUV mask both with an EUV scatterometer and a conventional scatterometer operated at 193 nm and compared the results with AFM and CD-SEM measurements provided to us by the mask supplier. The results of both CD-SEM and EUV- and DUV scatterometry show a quite good agreement in linearity despite constant CD offsets for these different metrology tools. The influences of the multilayer and Si capping layer on top of the multilayer thickness on EUV scatterometry results have been modelled with the help of FEM based simulations. A strong correlation has been found between the thickness of the capping layer and the sidewall angle. In general these results demonstrate the applicability both of EUV and DUV scatterometry for the characterisation of absorber structures on EUV masks. The application of DUV scatterometry allows to omit any influence from multilayer features and is only sensitive to the absorber structure. In this way EUV and DUV scatterometry complement each other for metrology on EUV masks. For applications in process optimisation and in process control the use of a conventional VIS/DUV-scatterometer may be sufficient in many cases.

Report on an international comparison of one-dimensional (1D) grating pitch

This paper reports results of an international comparison of one-dimensional (1D) grating pitch calibration by optical diffraction. Comparison results are analysed and discussed following the recommended guidelines for the analysis of CIPM key comparisons. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by SIM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

Metrology of nanoscale grating structures by UV scatterometry

In this contribution we demonstrate goniometric scatterometry measurements of gratings with linewidths down to 25 nm on silicon wafers with an inspection wavelength of 266 nm. For each sample, measurements have been performed in four different configurations and the obtained data have been evaluated in parallel. As results we present the reconstruction of the complete cross-section profile. We introduce a novel geometry parameterization which overcomes some limitations of the default parameterization. A co-variance analysis of the parameters is offered to indicate the soundness of the results. A qualitative comparison with cross-section scanning electron microscope (SEM) images shows excellent agreement.

Theoretical modelling and experimental verification of the influence of Cr edge profiles on microscopic-optical edge signals for COG masks

Different types of dimensional metrology instrumentation is in use today for production control of photomasks, namely SEM, AFM as well as optical microscopy and optical scatterometry. High resolution optical microscopy is still important as a reference metrology system, especially because it is sensitive to the optical effects induced e. g. by 2D or 3D details of features on photomasks. Particularly with regard to accurate optical CD measurements a thorough modelling of the optical imaging process on the basis of rigorous diffraction calculation is essential, which accounts for both polarisation effects and the 2D or 3D geometry of the structures. At PTB we use two different rigorous diffraction models to calculate the intensity distribution in the image plane, i.e. the rigorous coupled wave analysis method and the finite elements method. The question arises how accurate the influence of edge details on the microscopic-optical edge signals can be modelled. To answer this question we performed systematic experimental studies on COG test structures with varying height, edge angles and edge profiles. These geometric profile parameters of the test structures have been characterised by AFM measurements. Additionally top CDs of the features have been measured using both a CD-SEM and a metrological AFM. We present UV-optical CD measurements of these test structures and analysed them taking into account the measured profile details and, for comparison, using a simple binary structure model. The CD values determined are compared with the corresponding AFM and SEM values. The good agreement obtained for the optical, AFM and SEM top CD values shows that the optical effects of edge profile details can be modelled correctly with the two models applied. The results again demonstrate the necessity of rigorous model based analysis of the optical measurements, taking into account the edge profile details.

New methods for CD measurements on photomasks using dark field optical microscopy

We present novel methods of dark field optical microscopy for CD measurements on photomasks in the sub lambda regime. Optical CD inspection systems usually suffer from limited resolution and from linear and nonlinear superposition of the light diffracted at both edges of a line structure (optical proximity effects). These disadvantages partly can be overcome using alternating grazing incidence illumination of the specimen, where the angle of incidence of the illumination is perpendicular to the edges or grooves of the specimen. Especially for s-polarised light the diffraction efficiencies of the edges directed towards the illumination are at least an order of magnitude stronger than that of the averted edges. Thus an efficient suppression of optical proximity effects is achieved. Additionally a reduction of the widths of the diffraction limited images of structure edges due to a high pass characteristic of the optical image takes place. Both effects result in a significantly increased resolution power. We developed both a reflection mode (alternating grazing incidence dark field microscopy, AGED) and a transmission mode method (Frustrated internal total reflection microscopy, FIRM). Line width measurements on high quality photomasks will be presented. The experimental results will be compared with theoretical simulations and the applicability for sub lambda structures will be discussed.

Versatile DUV scatterometer of the PTB and FEM based analysis for mask metrology

At PTB a new type of DUV scatterometer has been developed. The concept of the system is very variable, so that many different types of measurements like e. g. goniometric scatterometry, ellipsometric scatterometry, polarisation dependent reflectometry and ellipsometry can be performed. The main applications are CD, pitch and edge profile characterisation of nano-structured surfaces mainly, but not only, on photomasks. Different operation wavelength down to 193nm can be used. The system is not only a versatile tool for a variety of different at-wavelength metrology connected with state-of-the-art photolithography. It allows also to adapt and to vary the measurand and measurement geometry to optimise the sensitivity and the unambiguity for the measurement problem. For the evaluation of the measurements the inverse diffraction problem has to be solved. For this purpose we developed a special FEM-based software, which is capable to solve both the direct diffraction problem and the inverse diffraction problem. The latter can be accomplished using different optimisation schemes. Additionally this software allows also to estimate the quality of the measured data and the model based measurement uncertainty. This paper gives an overview about the PTB DUV scatterometer, its metrological potential and the evaluation methods applied using the software DIPOG2.1.