An x-ray scattering based metrology was conceived over 20 years ago as part of a collaboration between National Institute of Standards and Technology (NIST) and International Business Machines Corporation (IBM) to evaluate the performance of lithographic patterning materials for the semiconductor industry. This methodology treated a periodic array of lithographic structures as a diffraction grating and focused on extracting the physical dimensions of the structures in the grating by analyzing the diffraction patterns. In the early stages of the work the focus was on developing the transmission small-angle x-ray scattering (tSAXS) as a metrology tool to measure the critical dimensions (CD) of the lithographic features vital to the integrated circuit chip fabrication. Later, the focus shifted to include grazing incident small-angle x-ray scattering and x-ray reflectivity as parts of the CD metrology tool due to their unique capabilities. Frequently the term critical dimension small-angle x-ray scattering (CDSAXS) has been used as a synonym for the metrology of using tSAXS for CD measurements without mentioning transmission. Various milestones in the CDSAXS development are reviewed in this article together with some prospects regarding the future growth of x-ray-based metrology for complex three-dimensional nanostructures important to semiconductor industries.
Non-planar transistor architectures, such as tri-gates or "FinFETs", have evolved into important solutions to the severe
challenges emerging in thermal and power efficiency requirements at the sub-32 nm technology nodes. These
architectures strain traditional dimensional metrology solutions due to their complex topology, small dimensions, and
number of materials. In this study, measurements of the average dielectric layer thickness are reported for a series of
structures that mimic non-planar architectures. The structures are line/space patterns (≈ 20 nm linewidth) with a
conformal layer of sub-15 nm thick high-k dielectric. Dimensions are measured using a transmission X-ray scattering
technique, critical dimension small angle X-ray scattering (CD-SAXS). Our test results indicate that CD-SAXS can
provide high precision dimensional data on average CD, pitch, and high-k dielectric layer thickness. CD-SAXS results
are compared with analogous data from both top-down scanning electron microscopy and cross-sectional transmission
electron microscopy. In addition, we demonstrate the capability of CD-SAXS to quantify a periodic deviation in pitch
induced by an imperfection in the phase shift mask.
Direct patterning of low-dielectric constant (low-k) materials via nanoimprint lithography (NIL) has the potential to
simplify fabrication processes and significantly reduce the manufacturing costs for semiconductor devices. We report
direct imprinting of sub-100 nm features into a high modulus methylsilsesquioxane-based organosilicate glass (OSG)
material. An excellent fidelity of the pattern transfer process is quantified with nm precision using critical dimension
small angle X-ray scattering (CD-SAXS) and specular X-ray reflectivity (SXR). X-ray porosimetry (XRP) and positron
annihilation lifetime spectroscopy (PALS) measurements indicate that imprinting increases the inherent microporosity of
the methylsilsequioxane-based OSG material. When a porogen (pore generating material) is added, imprinting decreases
the population of mesopores associated with the porogen while retaining the enhanced microporosity. The net effect is a
decrease the pore interconnectivity. There is also evidence for a sealing effect that is interpreted as an imprint induced
dense skin at the surface of the porous pattern.
Critical dimension small angle X-ray scattering (CD-SAXS) is a metrology platform capable of measuring the average
cross section and line width roughness (LWR) with a sub-nm precision in test patterns with line widths ranging from 10
to 500 nm. The X-ray diffraction intensities from a collimated X-ray beam of sub-Angstrom wavelength were collected
and analyzed to determine line width, pitch, sidewall angle, LWR, and others structural parameters. The capabilities of
lab-scale and synchrotron-based CD-SAXS tools for LWR characterization were tested by measuring a set of identical
patterns with designed roughness amplitude and frequency. These test patterns were fabricated using EUV lithography
with sub-50 nm linewidths. To compensate for the limited photon flux from the lab-based X-ray source, the incident
beam of the lab system was collimated to a less extent than the synchrotron beam-based tool. Consequently, additional
desmearing is needed to extract information from data obtained from lab-based equipment. We report the weighted
nonlinear least-squares algorithm developed for this purpose, in addiiton to a comparison between the results obtained
from our lab system and the synchrotron beam-based tool.
Critical dimension small angle X-ray scattering (CD-SAXS) is a measurement platform that is capable of measuring the
average cross section and sidewall roughness in patterns ranging from (10 to 500) nm in pitch with sub-nm precision.
These capabilities are obtained by measuring and modeling the scattering intensities of a collimated X-ray beam with
sub-nanometer wavelength from a periodic pattern, such as those found in optical scatterometry targets. In this work, we
evaluated the capability a synchrotron-based CD-SAXS measurements to characterize linewidth roughness (LWR) by
measuring periodic line/space patterns fabricated with extreme ultraviolet (EUV) lithography with sub-50 nm linewidths
and designed with programmed roughness amplitude and frequency. For these patterns, CD-SAXS can provide high
precision data on cross-section dimensions, including sidewall angle, line height, line width, and pitch, as well as the
LWR amplitude. We also discuss the status of ongoing efforts to compare quantitatively the CD-SAXS data with topdown
critical dimension scanning electron microscopy (CD-SEM) measurements.
The need to characterize line edge and line width roughness in patterns with sub-50 nm critical dimension challenges
existing platforms based on electron microscopy and optical scatterometry. The development of x-ray based metrology
platforms provides a potential route to characterize a variety of parameters related to line edge roughness by analyzing
the diffracted intensity from a periodic array of test patterns. In this study, data from a series of photoresist line/space
patterns featuring programmed line width roughness measured by critical dimension small angle x-ray scattering (CDSAXS)
is presented. For samples with periodic roughness, CD-SAXS provides the wavelength and amplitude of the
periodic roughness through satellite diffraction peaks. In addition, the rate of decay of intensity, termed an effective
"Debye-Waller" factor, as a function of scattering vector provides a measure of the fluctuation in line volume. CDSAXS
data are compared to analogous values obtained from critical dimension scanning electron microscopy (CDSEM).
Correlations between the techniques exist, however significant differences are observed for the current samples.
Calibrated atomic force microscopy (C-AFM) data reveal large fluctuations in both line height and line width, providing
a potential explanation for the observed disparity between CD-SEM and CD-SAXS.
KEYWORDS: Line edge roughness, Diffraction, Line width roughness, Satellites, Data modeling, X-rays, Scattering, Laser scattering, Scanning electron microscopy, Sensors
We are developing a transmission X-ray scattering platform capable of measuring the average cross section and line edge roughness in patterns ranging from 10 nm to 500 nm in width with sub-nm precision. Critical Dimension Small Angle X-ray Scattering (CD-SAXS) measures the diffraction of a collimated X-ray beam with sub-Angstrom wavelength from a repeating pattern, such as those in light scatterometry targets, to determine the pattern periodicity, line width, line height, and sidewall angle. Here, we present results from CD-SAXS with an emphasis on line edge roughness characterization. Line edge roughness measurements from CD-SAXS are compared with top-down scanning electron microscopy values and comparative definitions are discussed.
To address several of the challenges associated with nanoimprint lithography, new measurement techniques that can correlate the physical structure of an imprinted nanostructure with the materials used and the imprinting conditions are critical for optimizing imprint processes. Specular X-ray reflectivity (SXR) is a widely used technique to quantify the thickness, density, and roughness of the non-patterned films. Here we extend the applicability of SXR to imprinted nanostructures by characterizing the pattern height, the line-to-space ratio as a function of pattern height, the residual layer thickness, and the fidelity of pattern transfer.
The thermal embossing form of nanoimprint lithography is used to pattern arrays of nanostructures into several different polymer films. The shape of the imprinted patterns is characterized with nm precision using both X-ray scattering and reflectivity techniques. By studying the time dependent response of the pattern shape at temperatures near the glass transition temperature, we are able to perceive large levels of residual stress induced by the imprinting process. The large shear fields that result as the viscous polymer flows into the mold leads to residual stresses. At elevated temperatures in the freestanding structures (once the mold has been separated from the imprint), there is an accelerated reduction in pattern height in the reverse direction from which the material originally flowed into the mold. Two factors that influence this residual stress include the molecular mass of the polymer resist and the amount of time the pattern is annealed at high temperature in the presence of the mold.
The primary measure of process quality in nanoimprint lithography (NIL) is the fidelity of pattern transfer, comparing the dimensions of the imprinted pattern to those of the mold. Routine production of nanoscale patterns will require new metrologies capable of nondestructive dimensional measurements of both the mold and the pattern with subnanometer precision. In this work, a rapid, nondestructive technique termed critical dimension small angle x-ray scattering (CD-SAXS) is used to measure the cross sectional shape of both a pattern master, or mold, and the resulting imprinted films. CD-SAXS data are used to extract periodicity as well as pattern height, width, and sidewall angles. Films of varying materials are molded by thermal embossed NIL at temperatures both near and far from the bulk glass transition (TG). The polymer systems include a photoresist and two homopolymers. Our results indicate that molding at low temperatures (T-TG<40°C) produces small-aspect-ratio patterns that maintain periodicity to within a single nanometer, but feature large sidewall angles. While the observed pattern height does not reach that of the mold until very large imprinting temperatures (T-TG70°C), the pattern width of the mold is accurately transferred for T-TG>30°C.
Developing non-destructive, high precision characterization methods of pattern cross section is critical to the maturation of sub-65 nm technologies. In this study, Criticial Dimension Small Angle X-ray Scattering (CD-SAXS) data are used to non-destructively characterize the dimensions of dense, high aspect ratio patterns with sub-nm precision. Data are presented for a series of photoresist patterns with varying degrees of footer and sidewall angle. The scattering intensity, I, is measured as a function of the scattering vector, q, and the sample rotation angle, w. The resulting data are transformed to an effective Qx-Qz plane, representing the Fourier transform of the average pattern cross section. From this data, complex features of cross sectional shape can in principle be obtained through systematic analysis of appropriate intensity slices. Here, we demonstrate the protocol for high precision measurements using CD-SAXS. Data are analyzed to extract average values of pitch, line width, line height, sidewall angle, and the size of a foot or undercut. In addition, the ability of the technique to analyze more complex features, such as lines with curved sidewalls and corner rounding, is discussed. The CD-SAXS technique is found to provide dimensions with sub-nm precision and sidewall angles with sub-degree precision in a non-destructive manner, providing an option for characterization of next generation microelectronic and nanofabricated structures.
Controlling the thickness and uniformity of the unpatterned, residual layer is a critical challenge to sub-50 nm patterning with nanoimprint lithography (NIL). While nanometer level uniformity is essential, there is currently a lack of metrological capability for residual layer characterization. Specular X-ray reflectivity (SXR) is a versatile and widely used metrology to quantify the thickness, density, and roughness of thin smooth films. Here we extend specular X-ray reflectivity (SXR) to measure the thickness of the residual layer with sub-nm resolution. In addition to the residual layer thickness, X-ray reflectivity also reveals detailed information about the pattern height, the line to space ratio, and the relative line width variations of the pattern as a function of the pattern height.
The primary measure of process quality in nanoimprint lithography (NIL) is the fidelity of pattern transfer, comparing the dimensions of the imprinted pattern to those of the mold. As a potential next generation lithography, NIL is capable of true nanofabrication, producing patterns of sub-10 nm dimensions. Routine production of nanoscale patterns will require new metrologies capable of non-destructive dimensional measurements of both the mold and the pattern with sub-nm precision. In this article, a rapid, non-destructive technique termed Critical Dimension Small Angle X-ray Scattering (CD-SAXS) is used to measure the cross sectional shape of both a pattern master, or mold, and the resulting imprinted films. CD-SAXS data are used to extract periodicity as well as pattern height, width, and sidewall angles. Films of varying materials are molded by thermal embossed NIL at temperatures both near and far from the bulk glass transition (TG). The polymer systems include a photoresist, representing a mixture of a polymer and small molecular components, and two pure homopolymers. Molding at low temperatures (T-TG < 40°C) produces small aspect ratio patterns that maintain periodicity to within a single nanometer, but feature large sidewall angles. While the pattern height does not reach that of the mold until very large imprinting temperatures (T-TG ≈ 70°C), the pattern width of the mold is accurately transferred for T-TG > 30°C. In addition to obtaining basic dimensions, CD-SAXS data are used to assess the origin of loss in pattern fidelity.
Line edge roughness (LER) remains a predominant measure of pattern quality used to evaluate processing parameters throughout the many steps of fabricating microelectronics. In the effort to minimize LER, a critical component is a metrology capable of rapid and non-destructive characterization of fluctuations in the position of the pattern, or line, edge. Previously, we have demonstrated a non-destructive metrology capable of sub-nm precision in the measurement of pitch and linewidth termed Critical Dimension Small Angle X-ray Scattering (CD-SAXS). Here, we explore the capability of CD-SAXS to measure line edge fluctuations using the diffuse scattering from diffraction peaks. Models of varying forms of line edge roughness are used to explore the effects of different types of line edge roughness on CD-SAXS results. It is found that the frequency and the degree of correlation of the roughness between patterns greatly influences the scattering pattern predicted. Model predictions are then compared to CD-SAXS results from a photoresist grating.
Neutron and x-ray reflectivity measurements quantify the non-uniform distribution of water within poly(4-tert-butoxycarbonyloxystyrene) (PBOCSt) and poly(4-hydroxystyrene) (PHOSt) thin films on silicon wafer substrates. Two contrasting surface treatments were examined, silicon oxide, representing a hydrophilic interface and a trimethylsilane primed surface, representing a hydrophobic interface. The distribution of water in the films was sensitive to the surface preparation and photoresist relative hydrophilicity. Depending upon the water contact angle of the substrate in comparison to the polymer film, an excess of water near the interface occurs when the substrate is more hydrophilic than the photoresist. Likewise, interfacial depletion results when the photoresist is more hydrophilic than the substrate. These non-uniform water distributions occurs within (50 ± 10)Å of the photoresist/substrate interface. The water concentration in this interfacial region appears to be independent of the photoresist properties, but is strongly dependent upon the substrate surface energy.
The form and magnitude of line edge roughness (LER) is increasingly important in semiconductor processing due to continued reductions in feature sizes. While a large body of work connects processing factors to LER magnitude, the spatial dependence of LER is needed to provide a more complete description. The distribution of deprotection within the resist is represented as a collection deprotection paths created by individual photoacid generators (PAGs). In the limit of dilute PAG concentration, the form and size of the average deprotection path is measured using Small Angle Neutron Scattering (SANS) for a model photoresist polymer and PAG mixture. The heterogeneity of the deprotection volume produces “fuzzy blobs”. The shape of these blobs is compared to the form of LER at a idealized sidewall. The sidewall morphology is consistent with models of spatially random etching up to a cutoff length scale. The cutoff length scale is ≈ 5 times the size of a single deprotection volume, suggesting that collective phenomena are responsible for observed LER.
We demonstrate that poly(4-hydroxystyrene) and (5, 15, and 20) % tert-butoxycarboxy protected copolymers are polyelectrolytes when dissolved in aqueous base solutions. The polyelectrolyte effect is quantified through the observation of a correlation peak, measured with small-angle neutron scattering. Polyelectrolyte effects are weakened with added salts and excess base. These studies emphasize that salt additives screen the electrostatic interactions, while pH leads to the ionization of the chain. Solvent quality is quantified and the chain configurations are measured in the limit of high ionic strength. It is speculated that the developer-resist interactions will play an important role in development-induced roughness, hence these equilibrium solution measurements can serve a predictive function for future photoresists dissolution models incorporating solvent quality as a parameter.
A series of experiments are presented to demonstrate thin film confinement effects on the diffusive properties in poly(tert-butoxycarboxystyrene) (PBOCSt). Bilayer diffusion couple measurements reveal that as the thickness of a PBOCSt film is decreased, the kinetics of the deprotection reaction-front propagation (a process involving both the diffusion and reaction of photochemically activated acidic protons) are dramatically hindered. Incoherent neutron scattering measurements suggest that this retardation can be traced to a suppression of local fast relaxations (200 MHz or faster) native to the PBOCSt polymer. The reduced mobility in the thin PBOCSt films is further confirmed with moisture vapor uptake studies performed on a quartz crystal microbalance (QCM). As the film thickness drops below 500 Angstrom there is a strong reduction in the diffusivity of water in the film. In total, these are the first evidences suggesting that the deviations in lithographic performance with decreasing film thickness observed with the bilayer experiments can are due to changes in mobility, not reactivity, within a chemically amplified resist.
We utilize near edge X-ray absorption fine structure spectroscopy (NEXASFS) to provide detailed chemical insight into two interfacial problems facing sub-100 nm patterning. First, chemically amplified photo-resists are sensitive to surface phenomenon, which causes deviations in the pattern profile near the interface. Striking examples include T-topping, closure, footing, and undercutting. NEXAFS was used to examine surface segregation of a photo-acid generator at the resist/air interface and to illustrate that the surface extent of deprotection in a model resist film can be different than the bulk extent of deprotection. Second, line edge roughness becomes increasingly critical with shrinking patterns, and may be intimately related to the line edge deprotection profile. A NEXAFS technique to surface depth profile for compositional gradients is described with the potential to provide chemical information about the resist line edge.
The challenges facing current dimensional metrologies based on scanning electron microscopy (SEM), atomic force microscopy (AFM), and light scatterometry for technology nodes of 157 nm imaging and beyond may require the development of new metrologies. We provide results of initial tests of a measurement technique based on Small Angle X-ray Scattering (SAXS) capable of rapid measurements of test samples produced using conventional test masks without significant sample preparation. Using a sample photoresist grating, the technique is shown to apply to both organic, including photoresist, and inorganic patterns, including metal and oxide. The sub-Angstrom wavelength provides nanometer level resolution, with significant room for increased resolution. SAXS provides a dramatic improvement over the use of small angle neutron scattering (SANS) in measurement resolution. An additional advantage is the potential of developing a SAXS-based metrology tool on a laboratory scale.
The use of chemically amplified photoresists for the fabrication of sub-100 nm features will require spatial control with nanometer level resolution. To reach this goal, a detailed understanding of the complex reaction-diffusion mechanisms at these length scales is needed and will require high spatial resolution measurements. In particular, few experimental methods can directly measure the spatial evolution of the deprotection reaction front and correlate it with the developed structure. In this work, we demonstrate the complementary use of neutron (NR) and x-ray (XR) reflectometry to measure the reaction front profile with nanometer resolution. Using a bilayer geometry with a lower deuterium-substituted poly(tert-butoxycarboxystyrene) (d-PBOCSt) layer and an upper poly(hydroxystyrene) (PHOSt) layer loaded with a photoacid generator (PAG), we directly measure the spatial evolution of the reaction front. We show that the reaction front profile is broader than the initial interface after a post-exposure bake and the compositional profile changes upon development in an aqueous base solution. We also directly correlate the final developed structure with the reaction front profile. The spatial detail enabled by this general methodology can be used to differentiate between and evaluate quantitatively reaction-diffusion models.
Using Small Angle Neutron Scattering (SANS), we present the first quantitative measurements of the 3-dimensional conformation of macromolecules in thin polymer films of D < RG,Bulk, where D is the film thickness and RG,Bulk is the bulk radius of gyration. For D approximately equals 0.5 RG,Bulk, the molecular size along the film normal is less than 0.5 RG,Bulk, while relatively small changes are observed parallel to the surface. The observed changes in molecular size agree with predictions of molecular dynamics simulations of polymers confined in thin films, resulting from increased molecular orientation rather than chain distortion. Segregation of molecular centers of mass to the film center facilitates molecular packing while minimizing chain distortion. Estimates of length scales relating to the onset of chain distortion and chain orientation in thin film resists are D approximately equals 0.1 RG,Bulk and D approximately equals N(sigma ) respectively, where D is the film thickness, N is the number of chain segments, and (sigma) is the persistence length.
As critical dimensions continue to decrease with each technology node, the precise characterization of line width and profile becomes an increasingly challenging task. Small angle neutron scattering (SANS) offers several advantages for the characterization of sub-100 nm structures, particularly as a calibrating measurement method. In this work, SANS is used to characterize three samples prepared with the same mask and focus conditions, but different photoresist formulations. The mask pattern consists of parallel lines with a nominal line width of 180 nm and a 1:2 line to space ratio. Scattering data are taken using both a focused neutron beam instrument (two-dimensional data) and a perfect crystal diffraction ultra-high resolution small angle neutron scattering (USANS) instrument. From the location and intensity of observed diffraction peaks, both the periodicity of each grating pattern and the average line widths are determined from simple analytic expressions with nanometer resolution.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.