[Show abstract][Hide abstract] ABSTRACT: Optical vision systems require both unidirectional and bidirectional measurements for the calibrations and the
verification of the tool performance to enable accurate measurements traceable to the SI unit Metre. However, for
bidirectional measurements up to now the national metrology institutes are unable to provide internationally recognized calibrations of suitable standards. Furthermore often users are not aware of the specific difficulties of these measurements. In this paper the current status and limitations of bidirectional optical measurements at the industrial level are summarised and compared to state-of-the-art optical linewidth measurements performed at PTB on measurement objects of semiconductor industry. It turns out, that for optical widths measurements at an uncertainty level below 1 μm
edge localisation schemes are required, which are based on tool and sample dependent threshold values, which usually
need to be determined by a rigorous simulation of the microscopic image. Furthermore the calibration samples and
structures must have a sufficient quality, e. g. high edge angle and low edge roughness and the structure materials and their material parameters have to be known. The experience obtained within the accreditation process of industrial labs for width calibrations shows that, in order to be able to achieve a desired measurement uncertainties of about 100 nm, the imaging system needs to have a monochromatic Koehler illumination, numerical aperture larger than 0.5, a magnification greater than 50x and the ability to control the deviation of the focus position to better than 100 nm.
[Show abstract][Hide abstract] ABSTRACT: Within the European iMERA-Plus project 'Traceable Characterisation of Nanoparticles' various particle measurement procedures were developed and finally a measurement comparison for particle size was carried out among seven laboratories across six national metrology institutes. Seven high quality particle samples made from three different materials and having nominal sizes in the range from 10 to 200 nm were used. The participants applied five fundamentally different measurement methods, atomic force microscopy, dynamic light scattering (DLS), small-angle x-ray scattering, scanning electron microscopy and scanning electron microscopy in transmission mode, and provided a total of 48 independent, traceable results. The comparison reference values were determined as weighted means based on the estimated measurement uncertainties of the participants. The comparison reference values have combined standard uncertainties smaller than 1.4 nm for particles with sizes up to 100 nm. All methods, except DLS, provided consistent results.
[Show abstract][Hide abstract] ABSTRACT: Traceable size measurements of nanoparticles are accomplished by means of a calibrated scanning electron microscope operated in transmission mode (TSEM). An image analysis tool was developed which individually determines the boundary and size of every particle based on modelled TSEM signals obtained by Monte Carlo simulations. The model relies on first-principle electron scattering theory taking into account particle and instrument properties. A series of TSEM images containing thousands of particles can be analysed in automated batch processing to attain a particle size distribution. As examples, nanoparticles of three different material classes (gold, silica, latex) with sizes ranging from about 5 to 60 nm are analysed. An uncertainty analysis reveals expanded measurement uncertainties (95% confidence interval) of the mean diameter in the range of 1 to 3 nm.
[Show abstract][Hide abstract] ABSTRACT: In this paper we will provide an overview of methods and instruments developed and applied at the National Metrology Institute
of Germany (Physikalisch Technische Bundesanstalt—PTB) for high-precision dimensional (linear and angular) measurements and
discuss some challenges for future developments in this important area of metrology.
Key wordsdimensional (linear and angular) metrology-micrometrology-nanometrology-signal modeling
Optoelectronics Instrumentation and Data Processing 08/2010; 46(4):312-317. DOI:10.3103/S8756699010040023
[Show abstract][Hide abstract] ABSTRACT: Stochastic sub-wavelength structures with lateral sizes of below 100nm and heights of about 250nm were produced on polymer surfaces by plasma treatment. From these master samples, an embossing nickel tool was manufactured, which, in turn, was used to replicate the master surface form by hot embossing in plastic materials. The geometry of the produced nanostructures
was quantified by scanning electron microscopy. The optical surface reflectance of
both the master and the replicated samples were measured in the visible wavelength range. A significant reduction of optical reflectance from the expected 4% of untreated surfaces, to less than 0.5% of the master samples, and to values between 1% and 2% of the replicated samples was achieved. The optical reflectance of the stochastic structures
was calculated by a finite element-based rigorous solution of the Maxwell equations. The modeled results show a good agreement with experimental data.
[Show abstract][Hide abstract] ABSTRACT: In preparation for the international Nano1 linewidth comparison on photomasks between nine national metrology insti-tutes, the National Institute of Standards and Technology (NIST) and the Physikalisch-Technische Bundesanstalt (PTB), initiated a bilateral linewidth comparison in 2008, independent of and prior to the Nano1 comparison in order to test the suitability of the mask standards and the general approach to be used for the Nano1 comparison. This paper reports on the current status of the bilateral comparison. In particular the methods for linewidth metrology applied at NIST and PTB and its major uncertainty contributions will be discussed based on actual measurements results for both of the mask standards chosen for the bilateral comparison.
Proceedings of SPIE - The International Society for Optical Engineering 10/2009; DOI:10.1117/12.831373 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A conventional scanning electron microscope operated in transmission mode (TSEM) was used for imaging silica, gold and latex nanoparticles. Particles were applied to conventional transmission electron microscope (TEM) grids with different supporting films. A semiconductor detector capable of accomplishing both bright-field and dark-field imaging was used to record transmitted electrons. Particle diameter was determined from the images by comparing measured data with the results of corresponding Monte Carlo simulations which took into account particle and instrument properties. Measured and simulated line profiles agreed well; the method is sensitive to changes in diameter in the nano- and sub-nanometre range. It is concluded that TSEM imaging is a promising tool for dimensional characterization of nanoparticles. Necessary extensions to the technique in order to achieve traceable measurements are discussed.
[Show abstract][Hide abstract] ABSTRACT: A new deep UV transmission microscope for traceable micro- and nanometrology is currently being set up at the Physikalisch-Technische Bundesanstalt (PTB), the National Metrology Institute of Germany. The new microscope is especially designed to enable linewidth measurements of micro- and nanostructures with an unsurpassed absolute measurement uncertainty of down to 10 nm (95% confidence interval). The optical resolution is about 100 nm. The main field of this tool will be critical dimension (CD) metrology of photomasks used in optical lithography. In particular, this system offers the possibility of 'at-wavelength' metrology for the currently applied 193 nm lithography technology. The high lateral resolution will be attained by means of 193 nm excimer laser radiation for illumination in conjunction with a high-aperture objective (NA = 0.9). The illumination and imaging system will provide various imaging modalities, ranging from ordinary brightfield to specially structured illumination schemes. Traceability to the SI unit 'meter' will be accomplished by means of laser interferometry. The mechanical set-up is characterized by an ultra-stable bridge construction on a granite base and has been designed with special emphasis on realizing a positioning stability in the nanometer range. The instrument is being set up in the Clean Room Centre of the PTB and will be ready for operation in mid 2009. Simulation calculations, based on a rigorous optical modeling of the expected microscope images, are presented. These simulations are made for the important application of measuring Cr structures on quartz photomasks. Based on these simulations and on available data of the uncertainties of various experimental parameters—including instrument and sample parameters—expected uncertainty budgets for the measurement of the width of Cr lines on quartz substrates are estimated.
[Show abstract][Hide abstract] ABSTRACT: 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.
Proceedings of SPIE - The International Society for Optical Engineering 04/2008; DOI:10.1117/12.768429 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Today, various types of high resolution dimensional metrology instrumentation are in use for a quantitative characterisation of micro-and nanostructures. Although sophisticated ultra high resolution microscopic techniques like SEM and AFM are available, optical methods like microscopy and scatterometry are still of interest and are important because they are non-destructive, fast and have a good in-line capability. At PTB different optical tools are used for high-resolution metrology. Our standard instrument for CD metrology is a special UV transmission microscope. A new 193nm microscope is currently under development which will meet future requirements at least for the 32nm node as specified in the ITRS roadmap. A special alternating grazing incidence dark field microscope is used to measure the width of single features down to 100nm. For grating structures, we developed an optical diffractometer for pitch calibrations with an uncertainty down to 10 pm. Recently we realised a DUV scatterometer and ellipsometer capable to measure accurately CD, edge profile, layer thickness, and optical parameters. A versatile EUV scatterometer can be used to characterise absorber structures e. g. on EUV photomasks. For accurate measurements a thorough modelling on the basis of rigorous diffraction calculation is essential, which accounts for both polarisation effects and the 3D geometry of the structures. We use the rigorous coupled wave analysis (RCWA) method and the finite elements (FEM) method. We present an overview of PTB's current and future activities in optical high-resolution metrology and how these systems compare to ultra-high resolution microscopy like SEM or AFM.
Proceedings of SPIE - The International Society for Optical Engineering 01/2008; DOI:10.1117/12.814531 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In preparation of the international Nano1 linewidth comparison on photomasks between 9 national metrology institutes, NIST and PTB have started a bilateral linewidth comparison in 2008, independent of and prior to the Nano1 comparison in order to test the suitability of the mask standards and the general approach to be used for the Nano1 comparison. This contribution describes the rationale of both comparisons, the design of the mask comparison standards to be used and the measurement methods applied for traceable photomask linewidth metrology at NIST and PTB.
Proceedings of SPIE - The International Society for Optical Engineering 01/2008; DOI:10.1117/12.801435 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have developed a lateral standard on the nanometre scale for use with high-resolution optical microscopy techniques [U. Huebner, in: Proceedings of the 5th International Euspen Conference, 2005, pp. 185–188; U. Huebner, in: Optical Fabrication Testing, and Metrology II, Proc. SPIE 5965 (2005) 59651W]. The so-called nanoscale linewidth/pitch standard contains structures in the submicron- and sub-100 nm scale and meets the metrological requirements for accurate, easy and traceable optical microscopy measurements. The optical contrast of the pattern in the UV wavelength range makes it suitable for transmission and reflection UV-microscopy and for confocal laser scanning microscopy. The standard can be used for pitch- and linewidth measurements and for quick resolution testing of these instruments. Additionally, the integrated circular gratings provide a new way of calibrating scanning probe microscopes.
[Show abstract][Hide abstract] ABSTRACT: A new algorithm for SEM CD evaluation of trapezoidal line structures is presented. It is based on the physical modeling of SEM image formation and allows the assignment of top and bottom structural edge positions to the SEM signal. The SEM image profile is described by a set of piecewise continuous functions which is convoluted with the electron probe intensity profile. The resulting function is fitted to the measured signal profile by a least squares algorithm. The fit returns both top and bottom edge positions as well as the electron probe diameter. The algorithm is verified against three different Monte Carlo simulation programs using different physical models of elastic and inelastic electron scattering and secondary electron generation and transport. The effect of the physical modeling on the evaluated critical dimension is discussed and the absolute CD deviation of the algorithm is determined for different sets of specimen and tool parameters like edge slope angle, beam energy, and electron probe diameter.
Proceedings of SPIE - The International Society for Optical Engineering 03/2007; DOI:10.1117/12.712503 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Measurement and control of edge profiles and edge angles is increasingly important in advanced lithography. Especially for critical dimension metrology a sophisticated multi-dimensional shape metrology is highly beneficial. Different types of dimensional metrology instrumentation are in use today for edge profile and edge angle measurement. While destructive cross section SEM measurements often serve as reference, AFM and optical scatterometry systems are commonly used for day-to-day or in-line control. Due to the limitations of these metrology systems (AFM: slow, scatterometry: only integral measurements of periodic structures), the evaluation and modelling of top down SEM images is increasingly considered, too. At the PTB both SEM and AFM as well as optical scatterometry are applied for edge angle and/or edge profile metrology, supported by optical transmission microscopy. At the PTB we have realised a new DUV hybrid scatterometer for measurements over the full range of 6025 format masks which combines essential elements of a reflectometer, an ellipsometer, and a diffractometer. In addition to scatterometric measurements this set-up allows to measure the complete Müller-matrix including transmission, polarisation and depolarisation. This new set-up will be presented in detail. Finally we study the possibilities of evaluating high resolution top down SEM images to determine edge angles. The potential of edge angle evaluation using these new analysis procedures will be discussed. We present an overview of the PTB measurement capabilities with an emphasis on newly developed metrology methods and systems.
Proceedings of SPIE - The International Society for Optical Engineering 03/2007; DOI:10.1117/12.712137 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A new algorithm is presented that evaluates the critical dimension (CD) of nanostructures from scanning electron microscopy (SEM) images. The algorithm is based on the physical modelling of the SEM image formation and evaluates both top and bottom CD. The SEM intensity profile is modelled by a piecewise-defined continuous function which is approximated to the measured profile extracted from images by means of a least-squares fit. The algorithm is tested in a series of Monte Carlo simulations with respect to a variation of the edge-slope angle and the electron probe diameter. The maximum deviation observed is smaller than 3 nm for both top and bottom CD. As an application example, CD measurements of silicon line structures are presented.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: We have developed a new lateral standard on the nanometre scale for use with the recently introduced high-resolution optical microscopy techniques such as deep ultraviolet microscopy (DUVM) and confocal laser scanning microscopy (CLSM). The standard provides structures in the submicron-and sub-100 nm scale, and meets the metrological requirements for accurate and traceable optical microscopy measurements. It can be used as a length measurement standard (for pitch and linewidth measurements) and for quick resolution and astigmatism testing of all these instruments. Additionally, circular gratings provide a new way for the calibration of scanning probe microscopes.