Search

The size measurement of nanoparticles contributes to the understanding of their properties and, thus, to the assessment of the risks they pose to health and the environment. For such measurements to be comparable and legally recognized, they must be traceable to the SI unit meter. Recently, interest in traceable measurements of polystyrene particles with sizes up to 500 nm has aroused, e.g. in the aerosol community. To meet this demand, we adapted an established method to traceably measure nanoparticles with high precision in the transmission mode of a scanning electron microscope (STEM-in-SEM or TSEM). Since this method was geared towards smaller particles, we adapted it at two points: New simulations with the program Geant4SEM allow a more accurate modelling, especially of the inelastical scattering processes. In addition, the image evaluation procedure was revised to account for the non-linear signal response at the particle boundary. The measured values obtained in this manner show good agreement with the values of two international intercomparisons.

In this contribution, we present a technique for in situ determination of the numerical apertures (NAs) of optical microscopes using calibrated diffraction gratings. Many commonly practiced procedures use an external setup to determine the objective and condenser NAs. However, these values may become modified in the used microscope systems, e.g., by system intrinsic apertures. Therefore, in our improved technique, determination of the imaging NA is conducted in situ within the corresponding microscope at hand. Furthermore, the method has been extended to yield the microscope’s illumination NA as well. In total, we tested this procedure for determination of the imaging NA for four different microscope objectives with nominal values of 0.55 and 0.9, together with the illumination NAs for four different circular aperture diaphragms with diameters between 10 µm and 500 µm using several gratings of different pitches. All determined NA values agree essentially with their nominal values within their experimental uncertainties, but the uncertainties have been reduced by typically an order of magnitude as compared with the manufacturer’s specifications.

Tip abrasion is a critical issue particularly for high-speed atomic force microscopy (AFM). In this paper, a quantitative investigation on the tip abrasion of diamond-like-carbon (DLC) coated tips in a high-speed metrological large range AFM device has been detailed. Wear tests are conducted on four different surfaces made of silicon, niobium, aluminum and steel. During the tests, different scanning speeds up to 1 mm/s and different vertical load forces up to approximately 33.2 nN are applied. Various tip characterization techniques such as scanning electron microscopy (SEM) and AFM tip characterizers have been jointly applied to measure the tip form change precisely. The experimental results show that tip form changes abruptly rather than progressively, particularly when structures with steep sidewalls were measured. This result indicates the increased tip breakage risk in high-speed AFM measurements. To understand the mechanism of tip breakage, tip-sample interaction is modelled, simulated and experimentally verified. The results indicate that the tip-sample interaction force increases dramatically in measurement scenarios of steep surfaces.

There is relatively little literature concerning the effect of Reynolds number on multi-hole aerodynamic probe performance. In particular, there is almost no discussion in the literature of the underlying mechanisms of Reynolds number (Re) sensitivity for such probes. In order to close this gap, detailed investigations of the effect of Re on a five-hole probe have been performed using both PIV techniques and oil flow visualizations. Wind- and water-tunnels were used to cover a wide range of Re. The open-access Oxford Probe was used for these studies because of the readily available data-sets and processing routines, and to allow future comparisons by other authors. Complex flow dynamics including flow separation and re-attachment were identified, which cause Re-sensitivity of the calibration map at low Re even for low yaw or pitch angles. By comparing calibration maps across a wide range of Re, we demonstrate that the Oxford Probe can be employed without much loss of accuracy at lower Re levels than initially (conservatively) suggested, and quantify the errors in the extreme low-Re regime. Overall we demonstrate the robustness of the Oxford Probe concept across a wide range of Re conditions, we more clearly defined the low-Re limit for the probe design and quantify errors below this limit, and we illustrate the fundamental mechanisms for Re-sensitivity of multi-hole probes.

We report a custom microscope setup whose mechanical and optical components are adjusted by the means of an alignment autocollimator (AAC). Residual centring and angular misalignments of the components towards the microscope’s optical axis are below 500 μm and 1 mrad, respectively. We further perform measurements of dot structures with diameters close to the diffraction limit (nominal diameter = 200 nm; chrome on glass mask) as suitable measures for the evaluation of the microscope’s adjustment and to determine/ visualize the optical aberrations, which affect the image formation of microscopes.

In recent years, DNA origami nanorulers for superresolution (SR) fluorescence microscopy have been developed from fundamental proof-of-principle experiments to commercially available test structures. The self-assembled nanostructures allow placing a defined number of fluorescent dye molecules in defined geometries in the nanometer range. Besides the unprecedented control over matter on the nanoscale, robust DNA origami nanorulers are reproducibly obtained in high yields. The distances between their fluorescent marks can be easily analysed yielding intermark distance histograms from many identical structures. Thus, DNA origami nanorulers have become excellent reference and training structures for superresolution microscopy. In this work, we go one step further and develop a calibration process for the measured distances between the fluorescent marks on DNA origami nanorulers. The superresolution technique DNA-PAINT is used to achieve nanometrological traceability of nanoruler distances following the guide to the expression of uncertainty in measurement (GUM). We further show two examples how these nanorulers are used to evaluate the performance of TIRF microscopes that are capable of single-molecule localization microscopy (SMLM).

Calibration of magnification and nonlinearity of scanning electron microscopy (SEM) is an essential task. In this paper, we proposed a new type of 1D grating sample fabricated by combining laser-focused atomic deposition and x-ray interference lithography as a lateral standard for calibrating SEMs. The calibrations of the grating pattern by a metrological large-range atomic force microscope indicate that the grating sample exhibits outstanding pattern uniformity that surpasses conventional samples fabricated by e-beam lithography: (1) the nonlinear deviation of the grating structures is below +/- 0.5 nm over a measurement range of 5 µm; (2) the maximal variation of the calibrated mean pitch values is lower than 0.01 nm at different locations randomly selected all over the pattern area. The proposed new sample is applied for accurately calibrating the magnification and nonlinearity of a commercial SEM, showing its advantages of easy-of-use and high accuracy. The influence of the defocus level of SEM on the calibration result is also demonstrated. This research offers a feasible solution for highly accurate SEM calibration needed for 3D nanometrology and hybrid metrology demanded in metrology of modern nanoelectronics devices and systems.

We report the initial results of a recent bilateral comparison of linewidth or critical dimension (CD) calibrations on photomask line features between two national metrology institutes (NMIs): the National Institute of Standards and Technology (NIST) in the United States and the Physikalisch-Technische Bundesanstalt (PTB) in Germany. For the comparison, a chrome on glass (CoG) photomask was used which has a layout of line features down to 100 nm nominal size. Different measurement methods were used at both institutes. These included: critical dimension atomic force microscopy (CD-AFM), CD scanning electron microscopy (CD-SEM) and ultraviolet (UV) transmission optical microscopy. The measurands are CD at 50 % height of the features as well as sidewall angle and line width roughness (LWR) of the features. On the isolated opaque features, we found agreement of the CD measurements at the 3 nm to 5 nm level on most features – usually within the combined expanded uncertainties of the measurements.

Dimensional metrology based on optical vision systems is commonly used at the industrial level, for example in optical coordinate measurement machines (CMMs). Measurement uncertainties of well below 1 μm are more and more required. Thus for calibration samples a measurement uncertainty of the order of 100 nm is required. To enable accurate and traceable measurements both unidirectional (e. g. distance) measurements and bidirectional measurements (e. g. width or diameter) have to be performed for system calibrations and the reverification of the performance [1]. However, up to now the national metrology institutes (NMIs) are not able to offer internationally recognized bidirectional calibrations, which are covered by the Mutual Recognition agreement (MRA [2]) and documented in the key comparison database [3]. Currently only one NMI offers optical widths calibrations on suitable user-supplied samples, including line scales, as well.

A deep UV (DUV) microscope equipped with a 193 nm laser source and a high numerical aperture objective (0.9 NA) is currently developed at the Physikalisch-Technische Bundesanstalt (PTB). The system is designed mainly for traceable critical dimension (CD) metrology on state of the art photomasks up to 6-inch and offers "at-wavelength" measurements for current and future 193 nm lithography. The fundamental mechanical system of the DUV microscope has been carefully designed on the basis of finite element analysis, ensuring the microscope to have a 3D stability in the nanometer range. The comprehensive illumination and imaging system of the 193 nm DUV microscope offers four types of functionality: classical transmission microscopy, polarising microscopy, transmission microscopy with structured aperture illumination and structured field illumination. Together with a well-developed CD modelling approach, the 193 nm DUV microscope will have the capability to determine the CD values of micro- and nanostructures with an uncertainty level of < 10 nm for high quality structures. The status of the 193 nm DUV CD metrology tool is detailed in this publication, and an outlook on future measurement extensions will be presented.

A novel optical critical dimension (CD) metrology tool equipped with a 193 nm laser source and a high numerical aperture objective (NA=0.9) is under development at the Physikalisch-Technische Bundesanstalt (PTB), the National Metrology Institute of Germany. The CD tool is designed for characterization of photomasks up to 6-inch and offers "atwavelength" measurements for current and future 193 nm lithography. Design, construction and realisation of the CD metrology tool is presented in this paper. The illumination system, which employs a multi-mode DUV fiber to reduce the lateral coherence of the laser beam, is detailed with numerical simulation and experimental investigation. Combined with precision optical modelling, this optical CD tool will be applicable for quantitative determination of the microstructures on 32 nm node photomasks with uncertainty less than 10 nm.

In preparation for the international Nano1 linewidth comparison on photomasks between nine national metrology institutes, 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.

We report on the traceable calibration of linewidth (CD) photomask standards which are used as reference standards for production masks of the 65 nm node. Two different types of masks with identical layout were produced and calibrated, namely a binary mask (CoG) and a half-tone phase shifting mask (193MoSi PSM). We will in particular describe the applied calibration procedures and cross-correlate the results from different high resolution metrology tools, like SEM, UV microscopy and AFM. The layout of the CD photomask standard contains isolated as well as dense line features in both tones with nominal CD down to 100 nm. Calibration of the standards was performed at PTB by UV microscopy and LV-SEM, supported by additional AFM measurements. For analysis of the measured high resolution microscopy images and the deduced profiles appropriate signal modeling was applied for every metrology tool, which allows a meaningful comparison of geometrical parameters of the measured calibration structures. By this approach, e.g. the deduced feature widths at the top of the structures and the widths at 50% height of the structures can be related to the measured edge angles. The linearity e. g. of the measured top CD on different type of structures on the CoG CD standard was determined to be below 5 nm down to line feature dimensions well below 200 nm.

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 CD's 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.

The DQEs of four digital X-ray detector systems have been measured in accordance with the new international standard IEC 62220-1: two CR detector systems of the same type, a CsI-based indirect flat panel detector and a selenium-based direct flat panel detector. A mobile measurement set-up complying with IEC 62220-1 has been realized. All equipment used was of a specific design, tested and calibrated. A standardized radiation quality (RQA5) was applied, and the air kerma at the detector entrance was varied between about 1 μGy and 20 μGy. The measurements of the two CR detector systems were performed at different sites using different X-ray generators/tubes and were in agreement within 0.02. The maximum DQE values were obtained for the lowest spatial frequency for which the DQE is required to be reported according to the IEC standard, i.e. at 0.5 mm-1: The maximum DQE value measured was 0.21 for the CR systems, 0.42 for the indirect flat panel detector, and 0.31 for the direct Selenium-based detector. It has been demonstrated that the international standard IEC 62220-1 allows accurate and reliable measurements of the DQE to be conducted. It is now possible to objectively measure and compare DQE values of digital X-ray detector systems.

The speed and average gradient of a conventional screen-film system was measured at four European laboratories. This is the first interlaboratory comparison in which the measurement conditions described in ISO 9236-1 were applied. The four laboratories used calibrated measurement equipment. The values obtained by the four laboratories were within a range of 14% for the speed and within a range of 8% for the average gradient. These variations are consistent with the expected measurement uncertainty.

An illuminated table intended for illuminating transmission test charts used for the optical testing of different image capture devices has been developed. It provides a diffusely luminous table face, 300 mm in diameter, in a compact set- up. The spectral distribution of the emitted light is similar to Planck radiation of about 3050 K as required, for example, for testing, digital photographic cameras. The luminance level achievable is about 15000 cd/m2, the standard deviation of which is 1.8 percent within a circle of 300 mm in diameter. The illuminated table consists of an integrating hemisphere, with the large opening covered by an opal glass serving as the illuminated table face. Inside, a set of seven tungsten lamps is posited at locations such that the light output at the table face is as uniform as possible. Residual non-uniformities caused, for example, by slightly differing luminous intensities of the lamps are adjusted by controlling the currents of the seven lamps individually by means of a PC-controlled automatically running optimization process. This closed control loop uses a calibrated digital camera to capture the current spatial light distribution at the table face. The image is analyzed by an algorithm integrated in to the control loop.

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.