One of the key challenges in critical dimension (CD) metrology is finding suitable dimensional calibration standards. The transmission electron microscope (TEM), which produces lattice-resolved images having scale traceability to the SI (International System of Units) definition of length through an atomic lattice constant, has gained wide usage in different areas of CD calibration. One such area is critical dimension atomic force microscope (CD-AFM) tip width calibration. To properly calibrate CD-AFM tip widths, errors in the calibration process must be quantified. Although the use of TEM for CD-AFM tip width calibration has been around for about a decade, there is still confusion on what should be considered in the uncertainty analysis. We characterized CD-AFM tip-width samples using high-resolution TEM and high angle annular dark field scanning TEM and two CD-AFMs that are implemented as reference measurement systems. The results are used to outline how to develop a rigorous uncertainty estimate for TEM/CD-AFM calibration, and to compare how information from the two electron microscopy modes are applied to practical CD-AFM measurements. The results also represent a separate validation of previous TEM/CD-AFM calibration. Excellent agreement was observed.
Lattice-selective etching of silicon is used in a number of applications, but it is particularly valuable in those for which the lattice-defined sidewall angle can be beneficial to the functional goals. A relatively small but important niche application is the fabrication of tip characterization standards for critical dimension atomic force microscopes (CD-AFMs). CD-AFMs are commonly used as reference tools for linewidth metrology in semiconductor manufacturing. Accurate linewidth metrology using CD-AFM, however, is critically dependent upon calibration of the tip width. Two national metrology institutes and at least two commercial vendors have explored the development of tip calibration standards using lattice-selective etching of crystalline silicon. The National Institute of Standards and Technology standard of this type is called the single crystal critical dimension reference material. These specimens, which are fabricated using a lattice-plane-selective etch on (110) silicon, exhibit near vertical sidewalls and high uniformity and can be used to calibrate CD-AFM tip width to a standard uncertainty of less than 1 nm. During the different generations of this project, we evaluated variations of the starting material and process conditions. Some of our starting materials required a large etch bias to achieve the desired linewidths. During the optimization experiment described in this paper, we found that for potassium hydroxide etching of the silicon features, it was possible to independently tune the target linewidth and minimize the linewidth nonuniformity. Consequently, this process is particularly well suited for small-batch fabrication of CD-AFM linewidth standards.
A robust technique is presented for auto-aligning nanostructures to slow-etching planes during crystallographic etching of silicon. Lithographic mask patterns are modified from the intended dimensions of the nanostructures to compensate for uncertainty in crystal axis orientation. The technique was employed in fabricating silicon nanolines having lengths of 600 nm and widths less than 5 nm, subjected to intentional misalignment of up to ±1 deg. After anisotropic etching, the auto-aligned structures exhibited as little as 1 nm of width variation, as measured by a critical dimension atomic force microscope, across 2 deg of variation in orientation. By contrast, the widths of control structures fabricated without auto-alignment showed 8 nm of variation. Use of the auto-alignment technique can eliminate the need for fiducial-based alignment methods in a variety of applications.
The extraction of nanoscale dimensions and feature geometry of grating targets using signature-based optical
techniques is an area of continued interest in semiconductor manufacturing. In the current work, we have
performed angle-resolved scatterometry measurements on grating targets of 180 nm pitch fabricated by electron
beam lithography and anisotropic wet etching of (110)-oriented silicon. The use of oriented silicon results
in grating lines with nominally vertical sidewalls, with linewidths estimated by scanning electron microscopy
(SEM) to be in the sub-50 nm range. The targets were designed to be suitable for both optical scatterometry
and small-angle x-ray scattering (SAXS) measurement. As a consequence of the lattice-plane selective etch
used for fabrication, the target trenches do not have a flat bottom, but rather have a wide vee shape. We demonstrate
extraction of linewidth, line height, and trench profile using scatterometry, with an emphasis on modeling
the trench angle, which is well decoupled from other grating parameters in the scatterometry model and
is driven by the crystalline orientation of the Si lattice planes. Issues such as the cross-correlation of grating
height and linewidth in the scatterometry model, the limits of resolution for angle-resolved scatterometry at the
wavelength used in this study (532 nm), and prospects for improving the height and linewidth resolution obtained
from scatterometry of the targets, are discussed.
The interpretation of scatterometry measurements generally assumes that the grating extends over an area
large enough to intercept all the illumination provided by an incident beam. However, in practice, the gratings
used in scatterometry are relatively small. Thus, the detected light also includes both that scattered by
the grating as well as that from a region surrounding the grating because, generally, the incident beam illuminates
both the grating and the surrounding region. To model the effects of such real structures, simulations of
the effective reflectance were performed whereby the reflection from the grating was considered to be the
sum of the diffraction by the grating and the diffraction of the surrounding region, taking into account the
beam profile. To demonstrate the model, the illumination field was assumed to be Gaussian. Results are
shown for a specific target design consisting of a 50 μm square measured by normal incidence reflectometry.
Significant errors occur when the incident profile has wings that fall outside of the profile and when the scattered
light is partially apertured.
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 (k = 2) expanded uncertainty, and describe advances that bring
the step height uncertainty of recent measurements to an estimated 0.6 nm (k = 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.
Critical dimension atomic force microscopes (CD-AFMs) are rapidly gaining acceptance in semiconductor
manufacturing metrology. These instruments offer non-destructive three dimensional imaging of structures and can
provide a valuable complement to critical dimension scanning electron microscope (CD-SEM) and optical metrology.
Accurate CD-AFM metrology, however, is critically dependent upon calibration of the tip width. In response to this
need, NIST has developed prototype single crystal critical dimension reference materials (SCCDRMs).
In 2004, a new generation of SCCDRMs was released to the Member Companies of SEMATECH - a result of the
fruitful partnership between several organizations. These specimens, which are fabricated using a lattice-plane-selective
etch on (110) silicon, exhibit near vertical sidewalls and high uniformity and can be used to calibrate CD-AFM tip
width to a standard uncertainty of about ± 1 nm.
Following the 2004 release, NIST began work on the "next generation" of SCCDRM standards. A major goal of this
thrust was to improve upon the SCCDRM characteristics that impact user-friendliness: the linewidth uniformity and
cleanliness. Toward this end, an experiment was designed to further optimize the process conditions. The first round of
this experiment was recently completed, and the results show great promise for further improvement of the SCCDRM
manufacturing process.
Among other observations, we found that the minimum linewidth and linewidth uniformity were primarily sensitive to
different factors - and can thus be independently tuned to meet our future goals - which include linewidths as small as
20 nm and a standard uncertainty due to non-uniformity at the ± 0.5 nm level. Our future work will include a new
refining experiment to further optimize the important factors that we have identified, and extension of the methodology
to a monolithic 200 mm implementation.
KEYWORDS: Transmission electron microscopy, Calibration, Standards development, Crystals, Electron microscopes, Scanning electron microscopy, Metrology, Silicon, Atomic force microscope, Image resolution
One of the key challenges in critical dimension (CD) metrology is finding suitable calibration standards. Over the last
few years there has been some interest in using features measured with the transmission electron microscope (TEM) as
primary standards for linewidth measurements. This is because some modes of TEM can produce lattice-resolved
images having scale traceability to the SI (Systeme International d'Unites or International System of Units) definition of
length through an atomic lattice constant. As interest in using calibration samples that are closer to the length scales
being measured increases, so will the use of these TEM techniques.
An area where lattice-traceable images produced by TEM has been used as a primary standard is in critical dimension
atomic force microscope (CD-AFM) tip width calibration. Two modes of TEM that produce crystal lattice-traceable
images are high resolution transmission electron microscope (HR-TEM) and high angle annular dark field scanning
transmission electron microscope (HAADF-STEM). HR-TEM produces lattice-traceable images by interference
patterns of the diffracted and transmitted beams rather than the actual atomic columns, while HAADF-STEM produces
direct images of the crystal lattice. The difference in how both of these techniques work could cause subtle variations in
the way feature edges are defined.
In this paper, we present results from width samples measured using HR-TEM and HAADF-STEM. Next we compare
the results with measurements taken from the same location by two different CD-AFMs.
Both of the CD-AFM instruments used for this work have been calibrated using a single crystal critical dimension
reference material (SCCDRM). These standards, developed by the National Institute of Standards and Technology
(NIST) and SEMATECH, used HR-TEM for traceable tip-width calibration. Consequently, the present work and the
previous SCCDRM work provide a mutual cross-check on the traceability of the width calibration. Excellent agreement
was observed.
The technical objective of the work reported here is to assess whether radio-frequency (RF) measurements made on coplanar
waveguide (CPW) test structures, which are replicated in conducting material on insulating substrates, could be
employed to extract the critical dimension (CD) of the signal line using its center-to-center separation from the groundlines
as a reference. The specific near-term objective is to assess whether this CPW-based CD-metrology has sensitivity
and repeatability competitive with the other metrology techniques that are now used for chrome-on-glass (COG)
photomasks. An affirmative answer is encouraging because advancing to a non-contact and non-vacuum
implementation would then seem possible for this application. Our modeling of specific cases shows that, when the
pitch of the replicated lines of the CPW is maintained constant, the sensitivity of its characteristic impedance to the CDs
of the signal and ground lines is approximately 60 Ω/μm. This is a potentially useful result. For the same
implementation, the quantity ∂C/∂w has a value of approximately 45 (pF/m)/μm, which appears to be large enough to
provide acceptable accuracy.
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 10-3 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 implementation of a new test structure for HRTEM (High-Resolution Transmission Electron Microscopy) imaging, and the use of CD AFM (CD Atomic Force Microscopy) to serve as the transfer metrology, have resulted in reductions in the uncertainties attributed to critical dimension (CD) reference-material features, having calibrated CDs less than 100 nm. The previous generation of reference materials, which was field-tested in 2001, used electrical CD as the transfer metrology. Calibrated CD values were in the range 80 nm to 150 nm and expanded uncertainties were approximately ± 14 nm. The second-generation units, which have now been distributed to selected industry users for evaluation, have uncertainties as low as ±1.5 nm and calibrated CDs as low as 43 nm.
The National Institute of Standards and Technology (NIST) and SEMATECH have been working together to improve the traceability of critical dimension atomic force microscope (CD-AFM) dimensional metrology in semiconductor manufacturing. A major component of this collaboration has been the implementation of a Reference Measurement System (RMS) at SEMATECH using a current generation CD-AFM. An earlier tool, originally used at SEMATECH, has now been installed at NIST. Uncertainty budgets were developed for pitch, height, and CD measurements using both tools. At present, the standard uncertainties are approximately 0.2 % for pitch measurements and 0.4% for step height measurements. Prior to the current work, CD AFM linewidth measurements were limited to a standard uncertainty of about 5 nm. However, this limit can now be significantly reduced. This reduction results from the completion of the NIST/SEMATECH collaboration on the development of single crystal critical dimension reference materials (SCDDRM). A new generation of these reference materials was released to SEMATECH Member Companies during late 2004. The SEMATECH RMS was used to measure the linewidths of selected features on the distributed specimens. To reduce the uncertainty in tip width calibration, a separate transfer experiment was performed in which samples were measured by CD-AFM and then sent for high resolution transmission electron microscopy (HRTEM). In this manner, CD-AFM could be used to transfer the HRTEM width information to the distributed samples. Consequently, we are now able to reduce the limit on the standard uncertainty (k = 1) of CD-AFM width measurements to 1 nm.
Rudy Schlaf, Yusuf Emirov, Jay Bieber, Arun Sikder, J. Kohlscheen, Deron Walters, Mohammed Islam, Bhavik Metha, Z. Ren, Terri Shofner, Benjamin Rossie, Michael Cresswell
Carbon nanotubes (CNT) are among the candidates for atomic force microscopy probes for use in high aspect ratio critical dimension metrology (CDM). Their mechanical strength at small diameters makes them ideal probes for narrow and deep features. The synthesis of CNT has been making great progress in recent years. The use of CNT in scanning probe microscopy, however, has been limited due to a number of problems. While the CNT probes generally appear to be long lasting, the manufacture of precisely aligned CNT of defined length, diameter and number of walls poses a number of challenges. Yet, such precisely defined CNT probes seem to be required if the cantilevers are to be used for CDM. Our result demonstrate, for example, that the attachment angle of CNT with respect to the cantilever beam is crucial for their application in CDM. We report about our efforts to overcome these problems by growing well-defined CNT on standard Si cantilevers using chemical vapor deposition in combination with focused ion-beam machining techniques.
Prototype linewidth reference materials with Critical Dimensions (CDs) as narrow as 70 nm have been patterned in silicon-on-insulator films. The sidewalls of the reference features are parallel, normal to the substrate surface, and have almost atomically smooth surfaces. Linewidth calibration begins with the measurement of the electrical CDs of multiple reference features located at a selection of die sites on a wafer. The absolute widths of the cross sections of a sub-set of reference features on several chips that are diced from the wafer are then subjected to high resolution transmission electron microscopy (HRTEM) imaging to determine their physical CDs by lattice-plane counting. Sample preparation for lattice-plane counting by HRTEM Is destructive, and other reference features on the same chip become unusable for reference-material purposes. However, a calibration curve for converting the measured electrical CDs of reference features on other chips on the wafer, known as 'product reference features', to their physical values is obtained. The uncertainty attributed to the physical CD values of the product reference features generally varies inversely with the linear correlation between the cross- section lattice-plane counts and the corresponding electrical CD measurements of the sub-set of reference features that were selected for HRTEM imaging. A linear correlation value of approximately 0.97 has been obtained from a sub-set of 12 HRTEM measurements. In this case, the uncertainty attributed to the physical CD values of the product reference features is believed to be responsible for most of the product reference feature uncertainty. However, it has now been found that a forming-gas annealing treatment appears to prevent the referenced time dependence and thus has the potential for reducing the uncertainty level.
This paper describes a novel non-contact capacitive-sensor metrology tool developed for chrome photomasks. This work further describes suitable types of test structures printed on photomasks appropriate for linewidth metrology. The Critical Dimension (CD) metrology sensor is developed using a Low Temperature Co-Fired Ceramic (LTCC) technology to reduce the effects of parasitic capacitances. The sensor is based on non-contact micro-capacitance measurements of features on chrome-on-glass reticles. The CD-extraction algorithms based on the capacitance measurements are formulated from extensive evaluation with a Maxwell simulator. The purpose of the non-contact micro-capacitance sensor is to measure chrome-feature linewidths in the range of 0.4 (mu) M - 0.5 micrometers . These dimensions correspond to a 0.18 micrometers process on a 4X mask.
The effect of the instrument on the measurement must be known in order to generate an accurate linewidth measurement. Although instrument models exist for a variety of techniques, how does one assess the accuracy of these models? Intercomparisons between techniques which rely upon fundamentally different measurement physics can play an important role in model verification. We report here such an intercomparison. The average linewidth of a test pattern on a BESOI (Bonded and Etched-back Silicon on Insulator) sample, which is single crystal silicon with a buried insulating oxide, was measured using scanning electron microscopy (SEM) and electrical critical dimension (ECD) techniques. The higher conductivity of the BESOI sample compared to a previously measured SIMOX (another silicon on insulator technology) sample reduced the ECD uncertainty. Unexpected features in the SEM image were fit by modeling the cross sectional geometry of the line as a skewed trapezoid with deviations of a few tenths of a degree from the expected 90 degree angles. The SEM and ECD results differed by 0.67% of the electrical tap spacing, a nominal difference of 55 nm. This is larger than can be accounted for by known sources of measurement uncertainty.
Electrical test structures of the type known as cross-bridge resistors have been patterned in (100) epitaxial silicon material that was grown on bonded and etched-back silicon-on- insulator (BESOI) substrates. The critical dimensions (CDs) of a selection of their reference segments have been measured electrically and by lattice-plane counting and they have been inspected by scanning-electron microscopy (SEM) cross-section imaging. The lattice-plane counting is performed on phase- contrast images made by high-resolution transmission-electron microscopy (HRTEM). The reference-segment features were aligned with <110> directions in the BESOI surface material. They were defined by a silicon micro-machining process which results in their sidewalls being nearly atomically planar and smooth and inclined at 54.737 degree(s) to the surface (100) plane of the substrate. This (100) implementation may usefully complement the attributes of the previously reported vertical-sidewall implementation for selected reference-material applications. The HRTEM, and electrical CD (ECD) linewidth measurements that are made on BESOI features of various drawn dimensions on the same substrate are being investigated to determine the feasibility of a CD traceability path that combines the low cost, robustness, and repeatability of the ECD technique and the absolute measurement of the HRTEM lattice-plane counting technique. Other novel aspects of the (100) Silicon-On- Insulator (SOI) implementation that are reported here are the ECD test-structure architecture and the making of HRTEM lattice-plane counts from both cross-sectional, as well as top-down, imaging of the reference features. This paper describes the design details and the fabrication of the cross- bridge resistor test structure. The long-term goal is to develop a technique for the determination of the absolute dimensions of the trapezoidal cross sections of the cross- bridge resistors's reference segments, as a prelude to making them available for dimensional reference applications.
Uncertainty in the locations of line edges dominates the uncertainty budget for high quality sub-micrometer linewidth measurements. For microscopic techniques like scanning electron microscopy (SEM) and atomic force microscopy (AFM), the image of the sharp edge is broadened due to the instrument's non-ideal response. Localizing the true edge position within its broadened image requires a model for the instrument-sample interaction. Ideal left and right edges are mirror images of one another, so any modeling error in the position assignment will have opposite signs for the two types of edges. Linewidth measurements inherently involves such opposite edges and consequent doubling of model errors. Similar considerations apply to electrical critical dimension (ECD) measurement. Although ECD is a non-imaging technique, one must still model the offset between the position of the physical edge and the effective edge of the conducting part of the line. One approach to estimating the reliability of existing models is to compare result when fundamentally different instruments measure the same line. We have begun a project to perform such an intercomparison, and we report here initial results for SEM, AFM, and ECD measurements of sub-micrometer lines in single crystal Si. Edge positions are determined from SEM images using Monte Carlo tracing of electron trajectories to predict the edge shape.In the AFM, we estimate and correct for tip geometry using tools from mathematical morphology. ECD measurements are corrected for band bending in the neighborhood of the edges.
The National Institute of Standards and Technology (NIST) is exploring the feasibility of using artifacts fabricated on silicon-on-insulator (SOI) materials to quantify methods divergence, for critical dimension (CD) metrology applications. Test structures, patterned on two types of (110) SOI materials, SIMOX (Separation by IMplantation of OXygen) and BESOI (Bonded-and-Etched-back Silicon-on-Insulator), have been compared. In this paper, we describe results of electrical critical dimension (ECD) measurements and the relative performance of the test structures fabricated on the two SOI materials.
Measurements of the linewidths of submicrometer features made by different metrology techniques have frequently been characterized by differences of up to 90 nm. The purpose of the work reported here is to address the special difficulties that this phenomenon presents to the certification of reference materials for the calibration of linewidth-measurement instruments. Accordingly, a new test structure has been designed and fabricated, and has undergone preliminary tests. Its distinguishing characteristics are assured cross-sectional profile geometries with known side-wall slopes, surface planarity, and compositional uniformity when it is formed in mono-crystalline material at selected orientations to the crystal lattice. To allow the extraction of electrical linewidth, the structure is replicated in a silicon film of uniform conductivity which is separated from the silicon substrate by a buried oxide layer. The utilization of a silicon-on-insulator (SOI) substrate further allows the selective removal of substrate material from local regions below the reference features, thus facilitating measurements by optical and electron-beam transmission microcopy. The combination of planar feature surfaces having known side-wall slopes is anticipated to eliminate factors which are believed to be responsible for methods divergence in linewidth measurements, a capability which is a prerequisite for reliable certification of the linewidths of features on reference materials.
This paper presents a critical review of electrical test methods for determining feature placement with total measurement uncertainties below 10 nm and electrical linewidth for sub-half-micrometer design linewidths with measurement precision below 1 nm. Control of feature placement and control of linewidth have been and are expected to continue to be two of the most important challenges required in the manufacturing of advanced microelectronic devices. Traditional methods of measuring these parameters suffer from both measurement speed and equipment expense. Microelectronic test structures are electrical devices that are used to determine selected tool, process, device, material, or circuit parameters by means of electrical tests. They are supported by a variety of commercial test equipment often found in semiconductor manufacturing facilities. They provide low-cost, post-patterning metrology for determining both feature placement and electrical linewidth. Properly characterized test structures and measurement methods provide an economic means of determining the critical parameters needed to develop, control, and operate the next generation of patterning tools.
This test structure is based on the voltage-dividing potentiometer principle and was originally replicated in a single lithography cycle to evaluate feature placement by a primary pattern generator. A new test structure has now been developed from the single-cycle version and has been used for measuring the overlay of features defined by two different exposures with a stepping projection aligner. The as-measured overlay values are processed by an algorithm that minimizes the effects of nominal random pattern imperfections. The algorithm further partitions measurements of overlay into contributions that derive, respectively, from misregistration of the image fields projected by the two masks and from the drawn misplacement of features on the masks. The numerical estimates of these contributions so obtained from the electrical measurements were compared with those extracted from the same features by the NIST line scale interferometer, providing traceability to absolute length standards. The two sets of measurements were found to agree to within the several-nanometer uncertainty cited for the line scale interferometer's readings alone.
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