The decreasing size of semiconductor features and the increasing structural complexity of advanced devices have placed continuously greater demands on manufacturing metrology, arising both from the measurement challenges of smaller feature sizes and the growing requirement to characterize structures in more than just a single critical dimension. For scanning electron microscopy, this has resulted in increasing sophistication of imaging models. For critical dimension atomic force microscopes (CD-AFMs), this has resulted in the need for smaller and more complex tips. Carbon nanotube (CNT) tips have thus been the focus of much interest and effort by a number of researchers. However, there have been significant issues surrounding both the manufacture and use of CNT tips. Specifically, the growth or attachment of CNTs to AFM cantilevers has been a challenge to the fabrication of CNT tips, and the flexibility and resultant bending artifacts have presented challenges to using CNT tips. The Korea Research Institute for Standards and Science (KRISS) has invested considerable effort in the controlled fabrication of CNT tips and is collaborating with the National Institute of Standards and Technology on the application of CNT tips for CD-AFM. Progress by KRISS on the precise control of CNT orientation, length, and end modification, using manipulation and focused ion beam processes, has allowed us to implement ball-capped CNT tips and bent CNT tips for CD-AFM. Using two different generations of CD-AFM instruments, we have evaluated these tip types by imaging a line/space grating and a programmed line edge roughness specimen. We concluded that these CNTs are capable of scanning the profiles of these structures, including re-entrant sidewalls, but there remain important challenges to address. These challenges include tighter control of tip geometry and careful optimization of scan parameters and algorithms for using CNT tips.
Critical dimension atomic force microscopy (CD-AFM) is a widely used reference metrology technique. To characterize modern semiconductor devices, small and flexible probes, often 15 to 20 nm in diameter, are used. Recent studies have reported uncontrolled and significant probe-to-probe bias variation during linewidth and sidewall angle measurements. To understand the source of these variations, tip-sample interactions between high aspect ratio features and small flexible probes, and their influence on measurement bias, should be carefully studied. Using theoretical and experimental procedures, one-dimensional (1-D) and two-dimensional (2-D) models of cylindrical probe bending relevant to carbon nanotube (CNT) AFM probes were developed and tested. An earlier 1-D bending model was refined, and a new 2-D distributed force (DF) model was developed. Contributions from several factors were considered, including: probe misalignment, CNT tip apex diameter variation, probe bending before snapping, and distributed van der Waals-London force. A method for extracting Hamaker probe-surface interaction energy from experimental probe-bending data was developed. Comparison of the new 2-D model with 1-D single point force (SPF) model revealed a difference of about 28% in probe bending. A simple linear relation between biases predicted by the 1-D SPF and 2-D DF models was found. The results suggest that probe bending can be on the order of several nanometers and can partially explain the observed CD-AFM probe-to-probe variation. New 2-D and three-dimensional CD-AFM data analysis software is needed to take full advantage of the new bias correction modeling capabilities.
Critical Dimension AFM (CD-AFM) is a widely used reference metrology. To characterize modern semiconductor
devices, very small and flexible probes, often 15 nm to 20 nm in diameter, are now frequently used. Several recent
publications have reported on uncontrolled and significant probe-to-probe bias variation during linewidth and sidewall
angle measurements [1,2]. Results obtained in this work suggest that probe bending can be on the order of several
nanometers and thus potentially can explain much of the observed CD-AFM probe-to-probe bias variation. We have
developed and experimentally tested one-dimensional (1D) and two-dimensional (2D) models to describe the bending of
cylindrical probes. An earlier 1D bending model reported by Watanabe et al.  was refined. Contributions from several
new phenomena were considered, including: probe misalignment, diameter variation near the carbon nanotube tip (CNT)
apex, probe bending before snapping, distributed van der Waals-London force, etc. The methodology for extraction of
the Hamaker probe-surface interaction energy from experimental probe bending data was developed. To overcome
limitations of the 1D model, a new 2D distributed force (DF) model was developed. Comparison of the new model with
the 1D single point force (SPF) model revealed about 27 % difference in probe bending bias between the two. A simple
linear relation between biases predicted by the 1D SPF and 2D DF models was found. This finding simplifies use of the
advanced 2D DF model of probe bending in various CD-AFM applications. New 2D and three-dimensional (3D) CDAFM
data analysis software is needed to take full advantage of the new bias correction modeling capabilities.
The National Institute of Standards and Technology (NIST), Advanced Surface Microscopy (ASM), and the National Metrology Centre (NMC) of the Agency for Science, Technology, and Research (A*STAR) in Singapore have completed a three-way interlaboratory comparison of traceable pitch measurements using atomic force microscopy (AFM). The specimen being used for this comparison is provided by ASM and consists of SiO2 lines having a 70-nm pitch patterned on a silicon substrate. For this comparison, NIST used its calibrated atomic force microscope (C-AFM), an AFM with incorporated displacement interferometry, to participate in this comparison. ASM used a commercially available AFM with an open-loop scanner, calibrated with a 144-nm pitch transfer standard. NMC/A*STAR used a large scanning range metrological atomic force microscope with He-Ne laser displacement interferometry incorporated. The three participants have independently established traceability to the SI (International System of Units) meter. The results obtained by the three organizations are in agreement within their expanded uncertainties and at the level of a few parts in 104.
The National Institute of Standards and Technology (NIST), Advanced Surface Microscopy (ASM), and the National
Metrology Centre (NMC) of the Agency for Science, Technology, and Research (A*STAR) in Singapore have
completed a three-way interlaboratory comparison of traceable pitch measurements using atomic force microscopy
(AFM). The specimen being used for this comparison is provided by ASM and consists of SiO<sub>2</sub> lines having a 70 nm
pitch patterned on a silicon substrate.
NIST has a multifaceted program in atomic force microscope (AFM) dimensional metrology. One component of this
effort is a custom in-house metrology AFM, called the calibrated AFM (C-AFM). The NIST C-AFM has displacement
metrology for all three axes traceable to the 633 nm wavelength of the iodine-stabilized He-Ne laser - a recommended
wavelength for realization of the SI (Système International d'Unités, or International System of Units) meter. NIST
used the C-AFM to participate in this comparison.
ASM used a commercially available AFM with an open-loop scanner, calibrated by a 144 nm pitch transfer standard. In
a prior collaboration with Physikalisch-Technische Bundesanstalt (PTB), the German national metrology institute,
ASM's transfer standard was calibrated using PTB's traceable optical diffractometry instrument. Thus, ASM's
measurements are also traceable to the SI meter.
NMC/A*STAR used a large scanning range metrological atomic force microscope (LRM-AFM). The LRM-AFM
integrates an AFM scanning head into a nano-stage equipped with three built-in He-Ne laser interferometers so that its
measurement related to the motion on all three axes is directly traceable to the SI meter.
The measurements for this interlaboratory comparison have been completed and the results are in agreement within
their expanded uncertainties and at the level of a few parts in 104.
In 2004, the National Institute of Standards and Technology (NIST) commissioned the Advanced Measurement
Laboratory (AML) - a state-of-the-art, five-wing laboratory complex for leading edge NIST research. The NIST
NanoFab - a 1765 m<sup>2</sup> (19,000 ft<sup>2</sup>) clean room with 743 m<sup>2</sup> (8000 ft<sup>2</sup>) of class 100 space - is the anchor of this facility
and an integral component of the new Center for Nanoscale Science and Technology (CNST) at NIST.
Although the CNST/NanoFab is a nanotechnology research facility with a different strategic focus than a current high
volume semiconductor fab, metrology tools still play an important role in the nanofabrication research conducted here.
Some of the metrology tools available to users of the NanoFab include stylus profiling, scanning electron microscopy
(SEM), and atomic force microscopy (AFM).
Since 2001, NIST has collaborated with SEMATECH to implement a reference measurement system (RMS) using
critical dimension atomic force microscopy (CD-AFM). NIST brought metrology expertise to the table and
SEMATECH provided access to leading edge metrology tools in their clean room facility in Austin. Now, in the newly
launched "clean calibrations" thrust at NIST, we are implementing the reference metrology paradigm on several tools in
the CNST/NanoFab. Initially, we have focused on calibration, monitoring, and uncertainty analysis for a three-tool set
consisting of a stylus profiler, an SEM, and an AFM.
Our larger goal is the development of new and supplemental calibrations and standards that will benefit from the
Class 100 environment available in the NanoFab and offering our customers calibration options that do not require
exposing their samples to less clean environments. Toward this end, we have completed a preliminary evaluation of the
performance of these instruments. The results of these evaluations suggest that the achievable uncertainties are
generally consistent with our measurement goals.
The National Institute of Standards and Technology (NIST) has a multifaceted program in atomic force microscope
(AFM) dimensional metrology. Three major instruments are being used for traceable measurements. The first is a
custom in-house metrology AFM, called the calibrated AFM (C-AFM), the second is the first generation of
commercially available critical dimension AFM (CD-AFM), and the third is a current generation CD-AFM at
SEMATECH - for which NIST has established the calibration and uncertainties. All of these instruments have useful
applications in photomask metrology.
Linewidth reference metrology is an important application of CD-AFM. We have performed a preliminary comparison
of linewidths measured by CD-AFM and by electrical resistance metrology on a binary mask. For the ten selected test
structures with on-mask linewidths between 350 nm and 600 nm, most of the observed differences were less than 5 nm,
and all of them were less than 10 nm. The offsets were often within the estimated uncertainties of the AFM
measurements, without accounting for the effect of linewidth roughness or the uncertainties of electrical measurements.
The most recent release of the NIST photomask standard - which is Standard Reference Material (SRM) 2059 - was also
supported by CD-AFM reference measurements. We review the recent advances in AFM linewidth metrology that will
reduce the uncertainty of AFM measurements on this and future generations of the NIST photomask standard.
The NIST C-AFM has displacement metrology for all three axes traceable to the 633 nm wavelength of the iodine-stabilized
He-Ne laser. One of the important applications of the C-AFM is step height metrology, which has some
relevance to phase shift calibration. In the current generation of the system, the approximate level of relative standard
uncertainty for step height measurements at the 100 nm scale is 0.1 %. We discuss the monitor history of a 290 nm step
height, originally measured on the C-AFM with a 1.9 nm (<i>k</i> = 2) expanded uncertainty, and describe advances that bring
the step height uncertainty of recent measurements to an estimated 0.6 nm (<i>k</i> = 2). Based on this work, we expect to be
able to reduce the topographic component of phase uncertainty in alternating aperture phase shift masks (AAPSM) by a
factor of three compared to current calibrations based on earlier generation step height references.
The ever decreasing size of semiconductor features demands the advancement of critical dimension atomic force microscope (CD-AFM) technology, for which the fabrication and use of more ideal probes like carbon nanotubes (CNT) is of considerable interest. The recent progress in the precise control of CNT orientation, length, and end modification, using manipulation and focused ion beam processes, allowed us to implement ball-capped CNT tips and bent CNT tips for CD-AFM. Such CNT tips have been tested for the first time in a commercial CD-AFM to image a grating and line edge roughness samples. We found out that CNT tips can reasonably scan the pattern profiles including re-entrant sidewalls with the CNT tip geometries we used and with the available range of scan parameters. There still remain important issues to address - including tighter control of tip geometry and optimization of scan parameters and algorithms for using CNT tips.
The National Institute of Standards and Technology (NIST) has a multifaceted program in atomic force microscope
(AFM) dimensional metrology. There are two major instruments being used for traceable AFM measurements at NIST.
The first is a custom in-house metrology AFM, called the calibrated AFM (C-AFM), and the second instrument is a
commercial critical dimension AFM (CD-AFM). The C-AFM has displacement metrology for all three axes traceable
to the 633 nm wavelength of the Iodine-stabilized He-Ne laser. In the current generation of this system, the relative
standard uncertainty of pitch and step height measurements is approximately 1.0 x 10<sup>-3</sup> for pitches at the micrometer
scale and step heights at the 100 nm scale, as supported by several international comparisons. We expect to surpass this
performance level soon. Since the CD-AFM has the capability of measuring vertical sidewalls, it complements the
C-AFM. Although it does not have intrinsic traceability, it can be calibrated using standards measured on other
instruments - such as the C-AFM, and we have developed uncertainty budgets for pitch, height, and linewidth
measurements using this instrument. We use the CD-AFM primarily for linewidth measurements of near-vertical
structures. At present, the relative standard uncertainties are approximately 0.2% for pitch measurements and 0.4% for
step height measurements. As a result of the NIST single crystal critical dimension reference material (SCCDRM)
project, it is possible to calibrate CD-AFM tip width with a 1 nm standard uncertainty. We are now using the CD-AFM
to support the next generation of the SCCDRM project. In prototypes, we have observed features with widths as low as
20 nm and having uniformity at the 1 nm level.
The interaction of probe and sample is a well known factor affecting the measurement accuracy of atomic force microscopy (AFM). The emergence of ultra-sharp carbon nanotube tips provides a good approach to minimizing the distortion of the measured profile caused by interaction with the finite probe tip. However, there is nearly always a significant tilt angle resulting when the nanotube is attached to an ordinary probe. As a result, we can obtain an accurate sidewall image of only one side of the linewidth sample rather than two sides. This somewhat reduces the advantage of using nanotube probes. To solve this problem, a dual image stitching method based on image registration is proposed in this article. After the first image which provides an accurate profile of one side of the measured line is obtained, we rotate the sample 180° to obtain the second image, which provides an accurate profile of the other side of the line. We keep the sidewall data for the better side of each image and neglect the data for the other side of each image. Then, we combine these better two sides to yield a new image for which the linewidth can be calculated. The sample is inevitably located at slightly different spatial positions in the two measurements. Image registration based on an improved iterative closest point (ICP) method was applied to remove the position difference between these two images. We are working to demonstrate that the calculated sidewall angle and linewidth value after registration and stitching is more accurate than obtained from only one image.
Nano-scale linewidth measurements are performed in semiconductor manufacturing, the data storage industry, and micro-mechanical engineering. It is well known that the interaction of probe and sample affects the measurement accuracy of linewidth measurements performed with atomic force microscopy (AFM). The emergent ultra-sharp carbon nanotube tips provide a new approach to minimizing the distortion of the measured profile caused by interaction with the finite probe tip. However, there is nearly always a significant tilt angle resulting when the nanotube is attached to an ordinary probe. As a result, we can obtain an accurate sidewall image of only one side of the linewidth sample rather than two sides. This somewhat reduces the advantage of using nanotube probes. To solve this problem, a dual image stitching method based on image registration is proposed in this article. After the first image is obtained, which provides an accurate profile of one side of the measured line, we rotate the sample 180° to obtain the second image, which provides an accurate profile of the other side of the line. We keep the sidewall data for the better side of each image and neglect the data taken for the other side of each image. Then, we combine these better two sides to yield a new image for which the linewidth can be calculated.
A basic scheme of direct, highly accurate dimensional measurements of nanostructures is presented. We have constructed a scanning tunneling microscope (STM) unit combined with a diode laser-based Michelson interferometer module. The compact size of the STM allows it to be installed in an ultra high vacuum (UHV) chamber and is capable of measuring atomic spacings on a reconstructed single crystal surface. This method aims at direct dimensional calibration of microelectronic structures such as linewidths and line/space features. The calibrated dimensions of these features will be traceable to the international unit of length through the He-Ne laser wavelength and be a reliable standard for next generation nanostructures and nanofabrication.
Critical dimension metrology of silicon integrated circuit features at the sub-micrometer scale is an essential task in state-of-the-art semiconductor manufacturing. Determining the width of a feature or the scale in a pitch measurement with appropriate accuracy is consistently one of the most challenging elements of semiconductor metrology and manufacturing.
Atomic force microscopes (AFMs) are used in the semiconductor industry for a variety of metrology purposes. Step height measurements at the nanometer level and roughness measurements at sub-nanometer levels are often of interest. To perform accurate measurements, the scales of an AFM must be calibrated. We have been exploring the use of silicon single atomic steps as height standards for AFMs in the sub-nanometer regime. We have also designed and developed the calibrated AFM (C-AFM) to calibrate standards for other AFMs. Previously, we measured the step height of silicon single atomic steps on Si (111) (with native oxide) using the C-AFM. The value we obtained was 304 +/- 8 pm (k=2). From three independent measurement techniques, including our C-AFM result, we estimate an accepted value for the silicon step height of 312 pm +/- 12 pm (k=2), which corresponds to an expanded uncertainty of about 4 %. We have also completed a NIST led comparison of AFM measurements of silicon step samples to further evaluate their suitability as standards in industrial applications. If the reproducibility of the participants' measurements is sufficient, the accepted value could be used to calibrate the scale of the measuring tools in this sub-nanometer regime. The participants sent the data to NIST for analysis. This was done so that all of the data would be analyzed in a uniform manner. The results of our analysis indicate that these samples can be used effectively as standards. The average standard deviation of all of the participants results was 6 pm. Hence, it should be possible to use these specimens as sub-nanometer z-axis calibration standards with an expanded uncertainty of about 6 %.
Atomic force microscopes (AFMs) generate three dimensional images with nanometer level resolution and, consequently, are used in the semiconductor industry as tools for sub-micrometer dimensional metrology. Measurements commonly performed with AFMs are feature spacing (pitch), feature height (or depth), feature width (critical dimension), and surface roughness. To perform accurate measurements, the scales of an AFM must be calibrated. We have designed and developed the calibrated AFM (C-AFM) to calibrate physical standards for other AFMs. The C- AFM has displacement metrology for all three axes traceable to the 633 nm wavelength of the Iodine-stabilized He-Ne laser. This is accomplished through the integration of a flexure x-y translation stage, heterodyne laser interferometers, and a z- axis piezoelectric actuator with an integrated capacitance sensor. This capacitance sensor is calibrated with a third interferometer. We have performed both pitch and height measurements for external customers. Recently, we performed pitch measurements on holographic gratings as part of an ongoing international comparison driven by BIPM (Bureau International des Poids et Measures). We have also completed a preliminary design of a prototype pitch/height standard and are evaluating preliminary test samples. Additionally, we are working toward the development of linewidth standards through the comparison of C-AFM width measurements with values obtained from other methods. Our step height and linewidth measurements are in good agreement with those obtained by other methods, and we are working to improve the lateral resolution and hence the uncertainty of our probe-based linewidth measurements by studying the use of nanotubes and other types of sharp tips as linewidth probes.
The measurement of bump heights and pit depth on compact discs (CD) with atomic force microscopes (AFMs) is quite different from the measurement of step heights on step height calibration standards. Both the bumps and the pits show much larger transition regimes and more structural irregularities. The irregularities disqualify the effective use of profile based algorithms, which minimize the influence of any remaining motion deviations of the scan apparatus, to determine the height. Therefore a histogram height algorithm has to be used. The results of the bump height and pit depth measurements varied about 20 nm over the different sample regions. The remaining approximately 30 nm difference between the average of the bump height and pit depth is believed to result from the sample preparation procedure. By itself, the large sample variation observed will result in rather large measurement uncertainties for the measurement of the average height and depth of these features, if the averaging does not include a large amount of data taken at many different sample positions.
AFMs are increasingly used in the semiconductor industry as tools for sub-micrometer dimensional metrology. The scale of an AFM must be calibrated in order to perform accurate measurements. We have designed and developed the calibrated AFM (C-AFM) at the NIST to calibrate standards. Specifically, our primary calibrations are expected to be of combined pitch/height, or 3D magnification standards for AFM. THe C-AFM has metrology traceable to the International System of Units meter for all three axes. This is accomplished through the integration of a flexure x-y translation stage, heterodyne laser interferometers, and a z-axis piezoelectric actuator with an integrated capacitance sensor. Our first pitch measurements for an outside customer were recently compete, in which we were able to report relative expanded uncertainties as low as 1 percent on sub- micrometer pitches. The uncertainty budget for these measurements includes the effect of sample non-uniformity, which is the dominant contribution for some of the reported uncertainties. Four samples were measured - two with grid patterns and two with grid recently made considerable improvements in our uncertainty budget for step height measurements. For example, we recently achieved 0.2 nm expanded uncertainty on a 20 nm step, and achieved 0.008 nm expanded uncertainty in the measurement of the approximately 0.3 nm single atom step on Si. We also participated in the recently competed first round of the NIST linewidth correlation project, in which linewidht measurements obtained from different methods are compared. In this paper, we will report on the current status of the C-AFM, and on our plans for further development.
We are developing the instrumentation and prototype samples at NIST to enable the counting of atom-spacings across linewidth features etched in silicon. This is an effort to allow the accurate counting of atom-spacings across a feature in a controlled environment and to subsequently transfer that dimensionally stabilized artifact to other measuring instruments. In this paper we will describe the sample preparation techniques, sample configurations and imaging instrumentation used in this project. We have constructed a multi-chamber ultra-high vacuum (UHV) system with silicon processing capabilities which include the high temperature removal of native oxides and the appropriate temperature control and vacuum environment for preparing long range atomically ordered silicon surfaces. We can also passivate the silicon surfaces by oxidation in a temperature and pressure controlled environment or simply allow a native oxide to grow in an air ambient. This facility has a scanning tunneling microscope (STM) with atomic lateral imaging capabilities and a 0.2 angstrom vertical noise floor. The loadlock chamber allows rapid transfer of multiple tips and samples into the UHV environment. The facility is additionally equipped with a field-ion/field-electron microscope (FIFEM) which can atomically image, measure, and prepare the STM tips. The FIFEM enables the use of STM tips of known dimensions and cleanliness on a regular basis.
Because atomic force microscopes (AFMs) are capable of generating three dimensional images with nanometer level resolution, these instruments are being increasingly used in many industries as tools for dimensional metrology at sub- micrometer length scales. To achieve high accuracy, the scales of an AFM must be calibrated. Presently available standards for this purpose are commonly calibrated using stylus instruments and optical techniques. We have developed the calibrated AFM (C-AFM) in order to calibrate pitch and height standards using an AFM. Our instrument has metrology traceable to the wavelength of light for all three axes. This is accomplished through the integration of a flexure x-y translation stage, heterodyne laser interferometers, and a digital-signal-processor based closed-loop feedback system to control the x-y scan motion. The z-axis translation is accomplished using a piezoelectric actuator with an integrated capacitance sensor, which is calibrated using a heterodyne laser interferometer. When fully developed, this instrument will be a calibration tool for pitch and height standards for scanning probe microscopes. We have recently completed a reevaluation of the titling motions of the C-AFM scanner. This has allowed a refinement in our estimate of the Abbe error contribution to our measurement uncertainty. Our most recent pitch measurements are consistent with this new estimate and thus support our refined uncertainty budget. We have recently completed measurements of pitch on several samples, including both grid type and linear scale patterns, for an industrial user. We are also working toward the development of linewidth standards through the comparison of C-AFM width measurements with values obtained from other methods, including an electrical resistance techniques. In this paper, we will describe the current status of the C-AFM, discuss the use of the instrument for measurements of pitch and width, and describe our plans for future measurements.
A survey of nanonewton force calibration techniques suitable for micro-electromechanical systems (MEMS) is presented. The reviewed techniques include: mass-derived force, pendulums, calibrated master springs, resonance and electromagnetic techniques. Considerable background material is provided to support the hypothesis that virtual power methods, such as those employed on the NIST watt balance are applicable to the MEMS force calibration problem. A review of progress at NIST on two prototype nanowatt balances designed for MEMS calibrations is given.
Standard Reference Material (SRM) 484 is an artifact for calibrating the magnification scale of a scanning electron microscope. Since 1977 the National Institute ofStandards and Technology (MST) has produced seven issues of SRM484 amounting to approximately 1 150 samples in all. The standards are fabricated by electroplating alternate layers ofnickel and gold onto a substrate of a Monel sheet metal. The plate is then diced, and the individual pieces are mounted on edge in a holder. Each sample is metallographically polished to obtain a smooth surface and to reveal gold lines. The samples are calibrated using a scanniig electron microscope incorporated with a laser interferometer. A piezo flexure stage carries the sample across the stationary electron beam. A backscattered electron detector detects an intensity peak at each location where the electron beam interacts with a gold line. The displacement ofthe traveled stage between lines is monitored by the interferometer. A computer program records the intensity peaks and displacement information and determines the distance (spacing) between any two peaks. The spacing is measured from a peak-to-peak algorithm rather than an edge-to-edge algorithm in order to avoid the determination ofline edge positions. Properties ofthe SRM484 and the measurement system result in recent expanded uncertainties (at the level oftwo standard deviations) of approximately 4% for 0.5 jim spacings and 0.5% for 50 tm spacings.
Key words: Interferometer, Measurement uncertainty, Scanning Electron Microscope, SEM magnification, Standard Reference Material.