Metrological Atomic Force Microscope Research Articles

Shortened and simplified traceability chain for dimensional metrology based on self-traceable standards

Abstract Nanoscale measurement is an essential task of nanomanufacturing, and measurement traceability is a fundamental aspect of nanoscale measurement. High-precision nanoscale measurement instruments (e.g. atomic force microscopes (AFM) and scanning electron microscopes (SEM)) need to be calibrated by traceable standards to ensure their accuracy and reliability. However, due to the suboptimal accuracy, uniformity, and consistency of existing standards, they need to be calibrated by metrological instruments traceable to primary length standards (e.g. physical wavelength standards) before use. This results in a long traceability chain that leads to error accumulation and significantly reduces calibration efficiency. This paper proposes a novel shortened and simplified traceability chain, where the physical wavelength standard corresponding to the 7S3 → 7P4° transition frequency of chromium atoms is materialized into self-traceable gratings using the atom lithography technology. The self-traceable gratings can then be directly applied for calibrating measurement instruments. To verify this approach, the self-traceable gratings are calibrated using a metrological AFM of the Physikalisch-Technische Bundesanstalt. Measurement results confirmed the feasibility of the approach. Particularly, our results show that the self-traceable gratings have excellent uniformity over different measurement areas and consistency over different samples, both at 0.001 nm level. Finally, the application of the self-traceable gratings for the calibrations of a commercial AFM and SEM is demonstrated. The new traceability chain significantly simplifies the calibration process, providing a more reliable and higher efficient calibration approach for advanced nanomanufacturing than that of the state-of-the-art.

AFM interlaboratory comparison for nanodimensional metrology on silicon nanowires

Abstract Silicon nanowires (NWs) with a cylindrical form are fabricated by means of nanospheres lithography and metal-assisted chemical etching to obtain high aspect ratio nanostructures (diameter of about 100 nm and length of more than 15 µm) on cm2 area.
The nanodimensional characterization of individual NWs is performed by using several techniques, because dimensions at the nanoscale strictly relate to functional performances. In this study, we report the results of an interlaboratory comparison between measurements from a metrological atomic force microscope (AFM) and research AFMs located in different national metrology institutes (NMIs) across Europe and in a university.
The purpose of this study is to characterize two measurands: (i) sidewall roughness (Ra, Rq, Rz, Rsk, Rku parameters), extracted from the top profile measured along the nanowire length, and (ii) diameter of the nanowires measured as top-height. To this goal, the nanowires are spread horizontally on a silicon substrate, which has several areas labelled with a pattern of crosses and letters facilitating the measurement of the same NW, in order to study the reproducibility due to different instruments.
Measurements show a good agreement between the different NMIs, with a combined standard uncertainty of top-height diameter less than 3%, and with a combined standard uncertainty of roughness parameters well within 5% for Ra and Rq values.

Precision of diamond turning sinusoidal structures as measurement standards used to assess topography fidelity

In optical surface metrology, it is crucial to assess the fidelity of the topography measuring signals. One parameter to quantify this is the small-scale fidelity limit T FIL defined in ISO 25 178-600:2019. To determine this parameter, sinusoidal structures are generated, where the wavelengths are modulated according to a discrete chirp series. The objects are produced by means of ultra-precision diamond face turning. Planar areas and regions with slopes below 4° could be produced with form deviations of ≲10 nm. An initial estimate of the cutting tool’s nose radius resulted in a deviation that caused the ridges of the structures to be too narrow by approximately 150 nm, while the trenches were too wide. At the bottom of narrow trenches, deviations are observed in the form of elevations with heights of about 20 to 100 nm. The measurement standard investigated in this study has also been used to characterise optical instruments in a round-robin test within the European project TracOptic, which requires precise knowledge of the geometry of all structures. The geometry of the topography, cosine structures superimposed with form deviations, was measured using the Met. LR-AFM metrological long-range atomic force microscope of the German National Metrology Institute.

Pitch calibration of standard nanoscale for uncertainty reduction of certified reference materials for SEM image sharpness evaluation and magnification calibration

Scanning electron microscopy (SEM) is widely used to observe and analyze nanostructures on surfaces. To obtain accurate and sharp images by SEM, it is necessary to evaluate the sharpness of SEM images and calibrate the magnification simultaneously. The NMIJ-CRM 5207-a is a certified reference material for image sharpness evaluation and magnification calibration. The certified values of NMIJ-CRM 5207-a are the average pitches in the X- and Y- axes. The major source of uncertainty of the NMIJ-CRM 5207-a is the pitch calibration of the commercially available standard nanoscale. To reduce the uncertainty of the pitch calibration value of the standard nanoscale, the pitch calibration was performed using a metrological atomic force microscope and uncertainty was evaluated in this study.

Correction of Interferometric High-Order Nonlinearity Error in Metrological Atomic Force Microscopy

Metrological atomic force microscopes (Met. AFMs) with built-in interferometers are one of the main workhorses for versatile dimensional nanometrology. The interferometric nonlinearity error, particularly the high-order (i.e., 3rd- and 4th-order) nonlinearity errors, is a dominant error source for further improving their metrology performance, which cannot be corrected using the conventional Heydemann correction method. To solve this problem, two new methods were developed. One uses a capacitive sensor embedded in the Met. AFM, and the other applies an external physical artifact with a flat surface. Both methods can be applied very conveniently and can effectively reduce the nonlinearity error. In this paper, the propagation of the (residual) nonlinearity error in step height calibrations is examined. Finally, the performance of the improved tool is verified in the calibration of a highly demanding industrial sample. For the measurements performed at 25 different positions and repeated six times, the standard deviation of the total 150 measured values is 0.08 nm, which includes the contributions from the reproducibility of the metrology tool and sample inhomogeneity. This research has significantly improved our dimensional nanometrology service. For instance, the extended measurement uncertainty (k = 2) is reduced from 1.0 to 0.3 nm for the step height or etching depth calibrations.

Calibration of Step Height Standard Using Metrological Atomic Force Microscope

Calibration of Step Height Standard Using Metrological Atomic Force Microscope

Investigation of a metrological atomic force microscope system with a combined cantilever position, bending and torsion detection system

Abstract. This article presents a new metrological atomic force microscope (MAFM) head with a new beam alignment and a combined one-beam detection of the cantilever deflection. An interferometric measurement system is used for the determination of the position of the cantilever, while a quadrant photodiode measures the bending and torsion of the cantilever. To improve the signal quality and reduce disturbing interferences, the optical design was revised in comparison to the systems of others (Dorozhovets et al., 2006; Balzer et al., 2011; Hausotte et al., 2012). The integration of the MAFM head in a nanomeasuring machine (NMM-1) offers the possibility of large-scale measurements over a range of 25mm×25mm×5 mm with sub-nanometre resolution. A large number of measurements have been performed by this MAFM head in combination with the NMM-1. This paper presents examples of the measurements for the determination of step height and pitch and areal measurement.

Define and measure the dimensional accuracy of two-photon laser lithography based on its instrument transfer function

Two-photon polymerization (TPP) is a powerful technique for direct three-dimensional (3D) micro- and nanomanufacturing owing to its unique ability of writing almost arbitrary structures of various materials. In this paper, a novel method is proposed to quantitatively define and measure the dimensional accuracy of TPP tools based on the concept of instrument transfer function (ITF). A circular linear-chirp pattern is designed for characterizing the ITF. Such a pattern is arranged in a linear chirp function with respect to its radial distance from the pattern centre, thus, well representing a signal with a quasi-flat amplitude over a given spectral bandwidth. In addition, the pattern is rotational symmetric, therefore, it is well suited for characterizing the ITF in different angular directions to detect angular-dependent asymmetries. The feasibility of the proposed method is demonstrated on a commercial TPP tool. The manufactured pattern is calibrated by a metrological large range atomic force microscope (AFM) using a super sharp AFM tip, thus the dimensional accuracy and angular-dependent anisotropy of the TPP tool have been well characterized quantitatively. The proposed method is easy to use and reveals the dimensional accuracy of TPP tools under real manufacturing conditions, which concerns not only the optical focus spot size of the exposing laser but also influencing factors such as material shrinkage, light–matter interaction and process parameters.

A new type of nanoscale reference grating manufactured by combined laser-focused atomic deposition and x-ray interference lithography and its use for calibrating a scanning electron microscope

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.

A metrological atomic force microscope system

A new metrological atomic force microscope (MAFM) with combined deflection detection system that comprises a homodyne interferometer and an optical beam deflection measuring system are presented. The combination allows the simultaneous three-dimensional detection of position, bending and torsion of the cantilever. Two wedge plates with a wedge angle of 0.5° have been integrated to reduce the disturbing interferences. The new measuring system uses two tiltable plane mirrors and a shiftable focus lens to adjust the direction of the focused laser beam and the position of the focus. The integration of the MAFM head in a nanomeasuring machine (NMM-1) creates the possibility of traceable dimensional measurements over a large range of 25 mm × 25 mm × 5 mm with sub-nanometre resolution. This paper introduces its setup, realisation and metrological properties, such as stability of the characteristic curves, noise level and combined measurement uncertainty. Finally, exemplary measurement results are presented.

A novel material measure for characterising two-dimensional instrument transfer functions of areal surface topography measuring instruments

A new material measure for characterising two-dimensional (2D) instrument transfer functions (ITF) of areal surface topography measuring instruments has been designed, manufactured, calibrated and applied. Several innovative ideas are implemented in its design. Firstly, the material measure is designed with circular structure patterns. Such rotational symmetric patterns are advantageous for characterising the ITF in different angular directions, thus for characterising angular-dependent asymmetries of instruments. Secondly, different types of patterns are designed: circular chirp (CC) pattern and circular discreate grating (CDG) pattern. They represent different kinds of spatial signals applicable for characterising ITFs. A material measure consists of 25 different circular patterns with radii from 30 μm to 300 μm, and wavelengths from 0.1 μm to 150 μm. These patterns can be applied complementary and combinedly, offering high application flexibility for calibrating a variety of instruments, e.g. with different optical objectives from 5× to 100× and with different sizes of field of view (FOV). Material measures with heights of 8 nm, 16 nm and 32 nm, respectively, have been manufactured using state-of-the-art e-beam lithography technique. The feature heights are far less than λ/4, thus they are suitable for characterising the ITF of optical tools which can be approximated as linear systems. A metrological large range atomic force microscope (AFM) has been applied in the calibration of the developed material measure, showing good feature quality. The calibrated material measure has been successfully applied in research and industry.

A standard used for probe-tip diameter evaluation in surface roughness measurements using metrological atomic force microscope

Recently metrological atomic force microscopes (metrological AFMs) have been used for surface roughness measurements. The National Metrology Institute of Japan (NMIJ), AIST, provides a profile surface roughness calibration service using a metrological AFM based on ISO 19606:2017. This international standard requires evaluation of probe-tip diameter D and error in roughness measurements by using a standard plate with needle-shaped spikes before conducting surface roughness measurements. However, the conventional standard plate has several problems: (1) the needle-shaped spikes are too high, (2) the distance between spikes is too long, and (3) the spike curvature radius is not small enough, compared with AFM probe-tip size, which may lead to considerable uncertainty derived from probe-tip error evaluation. This study examined a new type of commercially-available standard plate as a candidate for probe-tip diameter evaluation. It features lower spike height and shorter distance between spikes in order to avoid probe-tip wear caused by repeated scanning. This study demonstrated that the overestimated uncertainty derived from probe-tip error evaluation can be corrected by using the new standard plate.

Extension of the range of profile surface roughness measurements using metrological atomic force microscope

The calibration service of profile surface roughness by using a metrological atomic force microscope is now available at National Metrology Institute of Japan (NMIJ), AIST. The calibration method is designed by referring to ISO 19606: 2017 (JIS R 1683: 2014). The scope of the ISO 19606: 2017, however, is limited to roughness measurements of surfaces with an arithmetical mean roughness, Ra, in the range of about 1 nm–30 nm. Currently there is strong demand for the measurement of surface roughness of more than 30 nm in the precision machining industry and for the measurement of surface roughness of sub-nanometer order in the semiconductor industry. In order to meet such demand, it is necessary to extend the range of surface roughness measurements in the NMIJ's calibration service. In this study, authors performed surface roughness measurements using a metrological AFM and evaluated their uncertainties. As a result of a series of measurements and evaluation of their uncertainties, it has been found that the calibration range can be extended to surfaces with an arithmetical mean roughness, Ra, in the range of about 0.2 nm–100 nm. The measurement results and the future challenges are reported in this paper.

Linewidth calibration using a metrological atomic force microscope with a tip-tilting mechanism

The linewidth or critical dimension (CD) of a nanoscale line pattern is calibrated using a metrological atomic force microscope with a tip-tilting mechanism (tilting-mAFM). The tilting tip permits the scanning of the line pattern’s vertical sidewalls. The tilting-mAFM performs two measurements on each side of the line pattern, and two datasets are stitched together to reconstruct the complete shape of the line pattern. Further, the CD is measured based on this pattern. A linewidth standard with a subnanometer-scale uncertainty was used as a target sample to verify this CD calibration procedure. The calibrated CD using the tilting-mAFM was 111.2 nm with an expanded uncertainty of 1.0 nm, which was the smallest uncertainty that was observed among the CD measurements that were reported using the tilting-AFM instruments. Further, the difference between this CD value and the reference CD was only 0.2 nm. The results reveal that the tilting-mAFM can be used for CD metrology with single-nanometer accuracy.

Validation of Single Particle ICP-MS for Routine Measurements of Nanoparticle Size and Number Size Distribution.

Singleparticle inductively coupled plasma-mass spectrometry (spICP-MS) is an emerging technique capable of simultaneously measuring nanoparticle size and number concentration of metal-containing nanoparticles (NPs) at environmental levels. single particle ICP-MS will become an established measurement method once the metrological quality of the measurement results it produces have been proven incontrovertibly. This Article presents the first validation of spICP-MS capabilities for measuring mean NP size and number size distribution of gold nanoparticles (AuNPs). The validation is achieved by (i) calibration based on the consensus value for particle size derived from six different sizing techniques applied to National Institute of Standards and Technology (NIST) Reference Material (RM) 8013; (ii) comparison with high-resolution scanning electron microscopy (HR-SEM) used as a reference method, which is linked to the International System of Units (SI) through a calibration standard characterized by the NIST metrological atomic force microscope; and (iii) evaluation of the uncertainty associated with the measurement of the mean particle size to enable comparison of the spICP-MS and HR-SEM methods. After establishing HR-SEM and spICP-MS measurement protocols, both methods were used to characterize commercial AuNP suspensions of three different sizes (30, 60, and 100 nm) with four different coatings and surface charge at pH 7. Single particle ICP-MS measurements (corroborated by HR-SEM) revealed the existence of two distinct subpopulations of particles in the number size distributions for four of the 60 nm commercial suspensions, a fact that was not apparent in the measurement results supplied by the vendor using transmission electron microscopy. This finding illustrates the utility of spICP-MS for routine characterization of commercial AuNP suspensions regardless of size or coating.

Development of a metrological atomic force microscope with a tip-tilting mechanism for 3D nanometrology

A metrological atomic force microscope with a tip-tilting mechanism (tilting-mAFM) has been developed to expand the capabilities of 3D nanometrology, particularly for high-resolution topography measurements at the surfaces of vertical sidewalls and for traceable measurements of nanodevice linewidth. In the tilting-mAFM, the probe tip is tilted from vertical to 16° at maximum such that the probe tip can touch and trace the vertical sidewall of a nanometer-scale structure; the probe of a conventional atomic force microscope cannot reach the vertical surface because of its finite cone angle. Probe displacement is monitored in three axes by using high-resolution laser interferometry, which is traceable to the SI unit of length. A central-symmetric 3D scanner with a parallel spring structure allows probe scanning with extremely low interaxial crosstalk. A unique technique for scanning vertical sidewalls was also developed and applied. The experimental results indicated high repeatability in the scanned profiles and sidewall angle measurements. Moreover, the 3D measurement of a line pattern was demonstrated, and the data from both sidewalls were successfully stitched together with subnanometer accuracy. Finally, the critical dimension of the line pattern was obtained.

Fast and accurate: high-speed metrological large-range AFM for surface and nanometrology

Low measurement speed remains a major shortcoming of the scanning probe microscopic technique. It not only leads to a low measurement throughput, but a significant measurement drift over the long measurement time needed (up to hours or even days). To overcome this challenge, PTB, the national metrology institute of Germany, has developed a high-speed metrological large-range atomic force microscope (HS Met. LR-AFM) capable of measuring speeds up to 1 mm s−1. This paper has introduced the design concept in detail. After modelling scanning probe microscopic measurements, our results suggest that the signal spectrum of the surface to be measured is the spatial spectrum of the surface scaled by the scanning speed. The higher the scanning speed , the broader the spectrum to be measured. To realise an accurate HS Met. LR-AFM, our solution is to combine different stages/sensors synchronously in measurements, which provide a much larger spectrum area for high-speed measurement capability.Two application examples have been demonstrated. The first is a new concept called reference areal surface metrology. Using the developed HS Met. LR-AFM, surfaces are measured accurately and traceably at a speed of 500 µm s−1 and the results are applied as a reference 3D data map of the surfaces. By correlating the reference 3D data sets and 3D data sets of tools under calibration, which are measured at the same surface, it has the potential to comprehensively characterise the tools, for instance, the spectrum properties of the tools. The investigation results of two commercial confocal microscopes are demonstrated, indicating very promising results. The second example is the calibration of a kind of 3D nano standard, which has spatially distributed landmarks, i.e. special unique features defined by 3D-coordinates. Experimental investigations confirmed that the calibration accuracy is maintained at a measurement speed of 100 µm s−1, which improves the calibration efficiency by a factor of 10.

Size measurements of standard nanoparticles using metrological atomic force microscope and evaluation of their uncertainties

Size measurement methods for particles fixed on a substrate become increasingly important especially in the semiconductor industry, in addition to the conventional methods to measure particle size in the gaseous and liquid phases. In this study, size of standard nanoparticles (polystyrene latex (PSL), gold and silver) was measured using the metrological atomic force microscope (AFM) and its uncertainty was evaluated. Particle deformations were calculated based on the assumption of plastic deformation between particles and a substrate, and uncertainties derived from the particle deformation were also evaluated. Furthermore, the above-mentioned metrological AFM measurement results were compared with those obtained by other microscopies, such as SEM equipped with laser interferometer and AFM calibrated by standard samples, and were compared also with nominal values.

Re-Evaluation of Calibration and Measurement Capabilities of Pitch Calibration Systems Designed by Using the Diffraction Method

One-dimensional grating is one of the most important standards that are used to calibrate magnification of critical-dimension scanning electron microscopes (CD-SEMs) in the semiconductor industry. Long-term stability of pitch calibration systems is required for the competence of testing and calibration laboratories determined in ISO/IEC 17025:2005. In this study, calibration and measurement capabilities of two types of pitch calibration systems owned by a calibration laboratory are re-evaluated through comparison to a reference value and its expanded uncertainty given by a metrological atomic force microscope (metrological AFM) at National Metrology Institute of Japan (NMIJ), AIST. The calibration laboratory’s pitch calibration systems are designed by using the diffraction method (optical and X-ray).

High-precision lateral distortion measurement and correction in coherence scanning interferometry using an arbitrary surface.

Lateral optical distortion is present in most optical imaging systems. In coherence scanning interferometry, distortion may cause field-dependent systematic errors in the measurement of surface topography. These errors become critical when high-precision surfaces, e.g. precision optics, are measured. Current calibration and correction methods for distortion require some form of calibration artefact that has a smooth local surface and a grid of high-precision manufactured features. Moreover, to ensure high accuracy and precision of the absolute and relative locations of the features of these artefacts, requires their positions to be determined using a traceable measuring instrument, e.g. a metrological atomic force microscope. Thus, the manufacturing and calibration processes for calibration artefacts are often expensive and complex. In this paper, we demonstrate for the first time the calibration and correction of optical distortion in a coherence scanning interferometer system by using an arbitrary surface that contains some deviations from flat and has some features (possibly just contamination), such that feature detection is possible. By using image processing and a self-calibration technique, a precision of a few nanometres is achieved for the distortion correction. An inexpensive metal surface, e.g. the surface of a coin, or a scratched and defected mirror, which can be easily found in a laboratory or workshop, may be used. The cost of the distortion correction with nanometre level precision is reduced to almost zero if the absolute scale is not required. Although an absolute scale is still needed to make the calibration traceable, the problem of obtaining the traceability is simplified as only a traceable measure of the distance between two arbitrary points is needed. Thus, the total cost of transferring the traceability may also be reduced significantly using the proposed method.