Metrological Atomic Force Microscope Research Articles

Comparison of line width calibration using critical dimension atomic force microscopes between PTB and NIST

International comparisons between National Metrology Institutes are important to verify measurement results and the associated uncertainties. In this paper, we report a comparison of the line width calibration of a crystalline silicon line width standard, referred to as IVPS100-PTB standard, between the Physikalisch-Technische Bundesanstalt in Germany and the National Institute of Standards and Technology in the United States. Critical dimension atomic force microscopy was the measurement method used for this comparison. Both institutes applied generally the same but independently developed traceability pathways: the scaling factor of the atomic force microscope (AFM) scanner was calibrated by a set of step height and lateral standards certified by metrological AFMs, while the effective tip width was ultimately traceable to the lattice parameter of silicon via high resolution transmission electron microscopy. Good agreement has been achieved in the comparison: For two groups of line features with nominal critical dimensions (CDs) of 50 nm, 70 nm, 90 nm, 110 nm and 130 nm that were compared, the observed deviations of CD results were between −1.5 nm and 0.3 nm. The deviations are well within the associated measurement uncertainty.

Determination of the shape of indenters for nanohardness testers via interferometry

A method for determination of the contact-area functions for diamond indenters of nanohardness testers using a metrological atomic-force microscope with three-coordinate laser interferometer is proposed. Face shapes of a number of indenters of Berkovich pyramid type are measured. The precision of the indenter surface coordinates measurement is 1 nm. It is demonstrated that the indenter tip shape changes in the course of its use; in particular, for the first 100 nm the deviation from the ideal pyramid can exceed 30 nm. Thus, one of the methods for verification of the contact-area function for an indenter is its periodic calibration using a metrological atomic-force microscope.

Two approaches for realizing traceability in nanoscale dimensional metrology

Traceability is a fundamental issue for nanoscale dimensional metrology. The lack of traceability in measurements inhibits the comparison of tools from different manufacturers and limits knowledge about the real size of fabricated features. Two approaches for realizing traceability in nanometrology, referred to as a top-down approach and a bottom-up approach, are presented. Following the top-down approach, for instance, realized using metrological atomic force microscopes, the dimension of nanostructures is derived from the displacement of the scanner, which is directly measured by laser interferometers whose optical frequency is calibrated to an iodine frequency-stabilized laser. Thus, the measurement result is directly traceable with an unbroken chain to the International System of Units—the meter. However, to achieve subnanometer measurement accuracy, which is far smaller than the optical wavelength (632.8 nm in this study), the subdivision of the interference fringe is essential for obtaining desired measurement resolution and accuracy. On the contrary, with the bottom-up approach, the dimension of nanostructures is determined using the silicon crystal lattice as an internal ruler. Due to the small dimension of the crystal lattice constant (e.g., d111=0.313 nm), the bottom-up approach offers measurements with potential highest accuracy. The crystal lattice constant can be traceably calibrated to the meter by, e.g., a combined optical and x-ray interferometer; thus, the traceability of the bottom-up approach is also ensured. The consistency of the two approaches is experimentally confirmed in this paper.

Development and characterisation of a new line width reference material

A new critical dimension (CD, often synonymously used for line width) reference material with improved vertical parallel sidewalls (IVPSs) has been developed and characterised. The sample has a size of 6 mm × 6 mm, consisting of 4 groups of 5 × 5 feature patterns. Each feature pattern has a group of five reference line features with a nominal CD of 50 nm, 70 nm, 90 nm, 110 nm and 130 nm, respectively. Each feature pattern includes a pair of triangular alignment marks, applicable for precisely identifying the target measurement position, e.g. for comparison or calibration between different tools. The geometry of line features has been investigated thoroughly using a high-resolution transmission electron microscope and a CD atomic force microscope (CD-AFM). Their results indicate the high quality of the line features: the top corner radius of <7 nm, vertical sidewall (slope mostly within 90° ± 0.5°) and very small line width variation (LWR down to 0.36 nm). The application of the developed sample for calibrating the scaling factor and effective tip geometry of the CD-AFM are demonstrated. The scaling factor of the CD-AFM is calibrated to be 0.9988, coinciding well with the theoretical value 1 as the tool was calibrated to a traceable metrological atomic force microscope. The effective width of a CDR120-EBD tip is calibrated as 128.32 nm. Finally, a strategy for the non-destructive calibration of the developed sample is introduced, which enables the application of the reference material in practice.

Quantitative measurements of electromechanical response with a combined optical beam and interferometric atomic force microscope

An ongoing challenge in atomic force microscope (AFM) experiments is the quantitative measurement of cantilever motion. The vast majority of AFMs use the optical beam deflection (OBD) method to infer the deflection of the cantilever. The OBD method is easy to implement, has impressive noise performance, and tends to be mechanically robust. However, it represents an indirect measurement of the cantilever displacement, since it is fundamentally an angular rather than a displacement measurement. Here, we demonstrate a metrological AFM that combines an OBD sensor with a laser Doppler vibrometer (LDV) to enable accurate measurements of the cantilever velocity and displacement. The OBD/LDV AFM allows a host of quantitative measurements to be performed, including in-situ measurements of cantilever oscillation modes in piezoresponse force microscopy. As an example application, we demonstrate how this instrument can be used for accurate quantification of piezoelectric sensitivity—a longstanding goal in the electromechanical community.

Profile surface roughness measurement using metrological atomic force microscope and uncertainty evaluation

Surface roughness measurements are sometimes performed using an atomic force microscope (AFM) in order to evaluate conditions of thin film fabrication and material surface treatment. Recently precise and reliable surface roughness measurement has been required in order to further improve quality of both thin films and material surfaces. Evaluation method of AFM tip shape is a key technology in the surface roughness measurement using an AFM. An evaluation method of AFM tip shape using a probe examination sample and its evaluation criteria are stipulated in the Japanese Industrial Standard (JIS) R 1683: 2007 “Test method for surface roughness of ceramic thin films by atomic force microscopy”. In this study, profile surface roughness measurements were performed based on JIS R 1683: 2007 and the measurement results are reported.

Nano-Traceability Study of a Cr Standard Grating Fabricated by Laser-Focused Atomic Deposition

A dimensional artifact is developed, which is a chromium (Cr) deposition grating fabricated by a laser-focused atomic deposition technique. The mean pitch of the grating is measured by using a metrological atomic force microscope with a large range, where a series of reference signs have been performed to locate the deposition area. Cosine error of the measurement result is analyzed and eliminated by the iterative angle calibration. The measurement result shows that the mean pitch of the grating is 212.66 ± 0.02 nm, which is very close to half of the standing laser wavelength (λ = 425.55 nm). This means that the grating has traceability with high accuracy and can substitute the laser interference technology for instrument calibration. Moreover, using the Cr deposition grating as a nano standard can shorten the traceability chain and improve the practical application.

A novel pitch evaluation of one-dimensional gratings based on a cross-correlation filter

If one-dimensional (1D), p-period and arbitrarily structured grating position-related topographical signals coexist with noise, it is difficult to evaluate the pitch practically using the center-of-gravity (CG) method. The Fourier-transform-based (FT) method is the most precise to evaluate pitches; nevertheless, it cannot give the uniformity of pitches. If a cross-correlation filter—a half period of sinusoidal waveform sequence (pT period), cross-correlates with the signals, the noise can be eliminated if pT ≈ p. After cross-correlation filtering, the distance between any two adjacent waveform peaks along the direction perpendicular to 1D grating lines is one pitch value. The pitch evaluation based on the cross-correlation filtering together with the detection of the peak's position is described as the peak detection (PD) method in this paper. The pitch average and uniformity can be calculated by using the PD method. The computer simulation has indicated that the average of pitch deviations from the true pitch and the pitch variations are less than 0.2% and 0.2% for the sinusoidal and rectangular waveform signals with up to 50% uniform white noise, less than 0.1% and 1% for the sinusoidal and rectangular waveform signals and 0.6% and 2.5% for the triangular waveform signal if three waveform signals are mixed with Gaussian white, binomial and Bernoulli noise up to 50% in standard deviation, one probability and trial probability, respectively. As examples, a highly oriented pyrolytic graphite (HOPG) with a 0.246 nm distance between second nearest neighbour atoms and a 1D grating with 3000 nm nominal pitch are measured by a ultra-high vacuum scanning tunneling microscope (UHV STM) and a metrological atomic force microscope (AFM), respectively. After the position-related topographical signals are cross-correlation filtered, the 0.240 and 3004.11 nm pitches calculated by using the PD method are very close to the 0.240 and 3003.34 nm results evaluated by the FT method.

Measurements of CD and sidewall profile of EUV photomask structures using CD-AFM and tilting-AFM

Accurate and traceable measurements of critical dimension (CD) and sidewall profile of extreme ultraviolet (EUV) photomask structures using atomic force microscopes (AFMs) are introduced in this paper. An instrument complementarily applied with two kinds of AFM techniques, the CD-AFM and the tilting-AFM, has been developed. High measurement stability of the instrument is demonstrated, for instance, the long-term CD stability is better than 1 nm over 500 successive measurements over 55 h. To traceably calibrate the effective tip geometry, transmission electron microscopes-based method is applied, which uses either the silicon crystal lattice or the structure pitch value calibrated by metrological AFMs as an internal scale. Several grating patterns with different nominal CDs and line/space ratios of an EUV photomask have been measured using the developed methods. A data evaluation method with considered higher order tip effect due to the non-vertical sidewall is introduced. Detailed measurement results of a test EUV photomask, such as middle CD, left and right sidewall angle, feature height, line edge roughness and edge profiles are given. Finally, the AFM results are compared to that of a PTB EUV scatterometer. The comparison of the middle CD yields a linear relation within a spread of only about ±2 nm and an offset of the absolute values below 3 nm. For the sidewall angle, both methods yield consistent results within a range of about 2°.

Metrological atomic force microscope and traceable measurement of nano-dimension structures

&lt;span style="font-family: &amp;quot;Times New Roman&amp;quot;; font-size: 10pt; mso-fareast-font-family: 宋体; mso-ansi-language: EN-GB; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA; mso-bidi-font-size: 12.0pt;" lang="EN-GB"&gt;The quantity assurance in semiconductor industry development requires dimensional measurement with nanometer accuracy. A metrological AFM is designed to establish a traceable standard with nanometer uncertainty. The principle and design of the instrument is introduced in this paper. The displacement of the sample is traced to the SI unit by interferometers. The metrological AFM is applied to step height and line width measurement. The results are compared with optical instrument and profilometer. The metrological AFM is used to step height measurement in an international comparison and the result shows an uncertainty less than &lt;/span&gt;&lt;span style="font-family: &amp;quot;Times New Roman&amp;quot;; font-size: 10pt; mso-fareast-font-family: 宋体; mso-ansi-language: EN-CA; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA; mso-bidi-font-size: 12.0pt;" lang="EN-CA"&gt;2 nm. The application of metrological AFM in line width and pitch are also introduced.&lt;/span&gt;

Design of a sample approach mechanism for a metrological atomic force microscope

In order to obtain the high accuracy required for a metrological atomic force microscope, the sample approach mechanism meets strict specifications. The design presented in this paper offers a stiff construction, which limits the influences of floor vibrations on the measurement. Next to this, thermal considerations in the design decrease the uncertainties introduced by temperature variations of the environment. Uncertainties can also be caused by misalignment of the sample holder with respect to the measurement system of three interferometers. To limit these uncertainties, the approach mechanism provides sufficient alignment possibilities. The performance of the sample approach mechanism was evaluated by means of a finite element simulation of its dynamic stiffness. A series of experiments provide the unknown parameters to the simulation model. The dynamic stiffness lies around 395Hz, which is sufficiently high to provide accurate measurements.

Evaluation of a vertical piezoelectric transducer stage using a large range metrological atomic force microscope

A method is described for the calibration of vertical piezoelectric transducer (PZT) stages using a large range metrological atomic force microscope (LRM-AFM). A vertical PZT stage is mounted onto the system andanoptical-flat sampleisattachedonthetopofthePZTstage.TheAFM probe is operated in contact mode and used as a null indicator to measure the movement of the optical-flat as it is driven by the PZT stage. As the PZT stageisscannedvertically,theAFMprobeisincontactwiththetestsurface without lateral scanning. The displacement of the vertical stage of the LRM-AFM is measured by the integrated laser interferometer, and the corresponding laser interferometer readings and the nominal position of the PZT stage are recorded. This body of data is then used to establish the relationship between the laser interferometer reading and the nominal displacement of the PZT stage. Our results show that this method can be used to calibrate PZT stages up to a range of 250 μm with an expanded uncertainty of less than 5 nm. © 2012 Society of Photo-Optical Instrumentation Engi-

Measurement strategies and uncertainty estimations for pitch and step height calibrations by metrological atomic force microscope

Gratings and step height standards are useful transfer standards for lateral and vertical length scale calibration of atomic force microscopes (AFMs). In order to have traceability to the SI-meter, the standards must have been calibrated prior to use. Metrological AFMs (MAFMs) with online laser interferometric position measurements are versatile instruments for the calibrations. The developed task-specific measurement strategies for step height and pitch calibrations with the Centre for Metrology and Accreditation's (MIKES's) metrological AFM are described. The strategies were developed to give high accuracy and to reduce measurement time. Detailed uncertainty estimations for step height and grating pitch calibrations are also given. Standard uncertainties are 0.016 and 0.018 nm for 300 and 700 nm pitch standards, respectively, and 0.21 and 0.44 nm for 7 and 1000 nm step height standards.

Quantitative characterisation of nanoimprinted structures using metrological large range AFM and CDAFM

Nanoimprint lithography (NIL) is a promising method for low-cost, high throughput patterning of nanoscale devices and systems. Quantitative characterisation of nanoimprinted structures is of high importance not only to ensure the quality of the structures but also to monitor/control the manufacturing processes. In this paper, two atomic force microscopes (AFMs) are presented which are applicable for the related measurement purpose. One is a metrological large range AFM (met. LR-AFM) with a measurement volume of (25 × 25 × 5) mm3. The other instrument is a critical dimension AFM (CDAFM) with a specially developed vector approach probing (VAP) strategy for 3D measurements of nanostructures. The met. LR-AFM had assisted in diagnosing the quality issues of NIL manufacturing processes. Furthermore, the calibration of two nanoscale standards fabricated by NIL is presented, taking into account influences of defects and stitching errors. Other parameters of the nanoimprinted structures like e.g. CD, corner rounding, sidewall slope as well as line edge roughness were quantitatively determined.

25 nm pitch comparison between a traceable x-ray diffractometer and a metrological atomic force microscope

A one-dimensional grating (1D grating) is one of the most important reference standards to calibrate nanometrological instruments, such as a critical dimension scanning electron microscope. Recently, the resolutions of nanometrological instruments have become higher, and 1D grating standards with smaller pitches are required. Based on this demand, 1D gratings with 25 nm pitch consisting of Si/SiO2 multilayer thin-film structures have been developed. Furthermore, pitch calibrations using metrological atomic force microscopes (metrological AFMs) and an x-ray diffractometer have been applied. In this study, the results of a 25 nm pitch comparison between a length-and-angle-standards-traceable x-ray diffractometer and a metrological AFM are reported.

Directional Repetitive Control of a Metrological AFM

Atomic force microscopes (AFMs) are used for sample imaging and characterization at nanometer scale. In this work, we consider a metrological AFM, which is used for the calibration of transfer standards for commercial AFMs. The metrological AFM uses a three-degree-of-freedom (DOF) stage to move the sample with respect to the probe of the AFM. The repetitive sample topography introduces repetitive disturbances in the system. To suppress these disturbances, repetitive control (RC) is applied to the imaging axis. A rotated sample orientation with respect to the actuation axes introduces a nonrepetitiveness in the originally fully repetitive errors and yields a deteriorated performance of RC. Directional repetitive control (DRC) is introduced to align the axes of the scanning movement with the sample orientation under the microscope. Experiments show that the proposed directional repetitive controller significantly reduces the tracking error as compared to standard repetitive control.

Development of a metrological atomic force microscope with minimized Abbe error and differential interferometer-based real-time position control

A metrological atomic force microscope (mAFM) has been developed at LNE. It will be dedicated to traceable dimensional measurements and calibrations of transfer standards with a maximum size of 25 mm × 25 mm × 7 mm. The displacement range is 60 µm for the X and Y axes and 15 µm for the Z axis. The instrument uses four laser differential interferometers in an original Abbe-compliant arrangement to measure the position of the tip relative to the sample and to be directly traceable to the SI. The expected uncertainty for the measurement of the tip/sample relative position is 1 nm for the whole range without taking into account the tip contribution. To fulfill this specification, the design of the instrument has been optimized to minimize Abbe errors, to reduce the metrology loop length, to limit the drifts due to thermal dilatation and to improve the stability of interferometric position measurement in ambient air. To limit Abbe errors, a dedicated three-axis flexure stage has been developed to reduce parasitic rotational motion at the level of 1 µrad for the whole range. This stage is driven by piezo-actuators. The instrument controller is based on a FPGA combined with an embedded PXI controller to perform real-time control of the XYZ position. We present the design of the instrument and the very first results.

Measurement of deep groove structures using a self-fabricated long tip in a large range metrological atomic force microscope

Metrological atomic force microscopes are widely used in national metrology institutes for measuring step height, lateral pitch and surface roughness. However, the maximum measurable depth or height variation is limited by both the vertical scanning range of the AFM and the tip height and sharpness of the tip at the end of the cantilever. A normal commercial AFM typically has a vertical scanning range less than 10 µm and a tip height of the cantilever only up to 15 µm so that it can be used to detect only relatively smooth surfaces or shallow structures up to several micrometres. To overcome these limitations, we have successfully integrated a long diamond tip of up to 120 µm developed at Namiki Precision Jewel Co., Ltd of Japan onto our large range metrological atomic force microprobe (LRM-AFM) for deep groove structure measurement. The LRM-AFM is based on a nano-measuring machine with a large scanning range of 25 mm in the X and Y axes and 5 mm in the Z axis. This paper describes the long diamond tip fabrication process and especially its application in the LRM-AFM for measuring deep groove structures of a step height of up to 100 µm. In addition, the mechanical quality factor (Q) of the diamond tip micro-cantilever was detected in the system. The results demonstrate that the system is capable of calibrating and measuring the surface structure with deep groove in tens of micrometres.

Compact field programmable gate array (FPGA)-based multi-axial interferometer for simultaneous tilt and distance measurement in the sub-nanometre range

Quantitative determination of microstructure and nanostructure properties is essential in research, development and in control of the production process. In instruments for dimensional nanometrology the positioning stage is the key component, since the characteristics of the position acquisition and control determine directly the achievable precision of the complete system. Compact commercial positioning systems usually employ capacitive or inductive position sensors. Both technologies not only offer a resolution in the nanometre range, but also require a (periodic) calibration. To achieve traceable measurements, interferometric sensors need to be implemented into the metrology system. However, currently available commercial interferometers turned out to be too large to be installed in miniature positioning stages easily. Therefore, a new highly compact interferometer to allow for traceability and an uncertainty in the Ångstrom range was developed. One of its first applications will be the new metrological low-noise atomic force microscope at the Physikalisch-Technische Bundesanstalt. The new interferometer is based on a modified homodyne Twyman–Green interferometer concept. It uses a novel signal processing approach based on a field programmable gate array, whereby a spatial interferogram is acquired by a high-speed CMOS line sensor and transformed into its frequency spectrum through a discrete Fourier transform. The spectral representation is analyzed for its major components; the phase information bears a direct connection to the displacement of the positioning unit. Furthermore, a possible stage tilt during the scan gives rise to a shift of the peak magnitude in the frequency spectrum. In addition, the developed system proved to be easily extendable to multiple axes by superimposing multiple interferograms on a single line sensor.

Grating Pitch Measurement Beyond the Diffraction Limit with Modified Laser Diffractometry

The demand for accurate measurements of nanostructures is increasing for the microprocesses and nanotechnology used in the semiconductor industry. In this study, we present an improved method for measuring gratings with a pitch size lower than one-half of the laser wavelength by using a modified laser diffractometer (LD). In experiments, the modified LD with a 633 nm laser is used to measure a grating with a 288 nm pitch size. This result is compared with results measured by both a metrological atomic force microscope and the traditional LD with a 543 nm laser. The validity of this method is demonstrated. The difference between the pitch size of 288 nm obtained by these three methods is approximately 0.17 nm.