Detlef Bergmann

Characterization of nanoparticles by scanning electron microscopy in transmission mode

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.

Measurement of the detective quantum efficiency (DQE) of digital x-ray imaging devices according to the standard IEC 62220-1

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.

Calibration of CD mask standards for the 65-nm node: CoG and MoSi

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.

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.

An interlaboratory measurement of screen-film speed and average gradient according to ISO 9236-1.

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.

Using DNA origami nanorulers as traceable distance measurement standards and nanoscopic benchmark structures

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).

Photomask linewidth comparison by PTB and NIST

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.

High-definition illuminated table for optical testing

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.