The photoacid diffusion length is a critical issue for extreme ultraviolet (EUV) lithography because it governs the critical
dimension (CD), line-edge-roughness (LER), and line-width-roughness (LWR) of photoresist materials. Laboratorybased
experimental methods that complement full lithographic testing would enable a rapid screening of materials and
process conditions. This paper provides an approach to characterize the photoacid diffusion length by applying a bilayer
stack technique. The method involves quantitative measurements of the deprotection kinetics as well as film thickness at
each process step: radiation exposure, post-exposure bake, and development. Analogous to a contrast curve, by
comparing the film thickness of the bilayer before and after development, the photoacid diffusion length was deduced in
a commercial EUV photoresist and compared to EUV lithography. Further, by combining the experiments with kinetics
modeling, the measured photoacid diffusion length was predicted. Lastly, based upon the measured kinetics parameters,
a criterion was developed that next-generation resists must meet to achieve a 16 nm photoacid diffusion length. These
guidelines are discussed in terms of correlations and contributions from the photoacid and resist properties. In particular,
the trapping kinetics of the photoacid provides a route to reduce LER and the CD at low dose.
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.
It has been recently postulated that sub-22 nm photolithography with polymeric photoresists has reached a materials design barrier due to its large molecular mass and distribution. In this argument, the "pixel" size, which is related to the smallest molecular unit, determines the feature fidelity and resolution of the lithographic process. This hypothesis remains unproven, but molecular glass photoresists can provide a test because they can share similar chemical functionality to polymer resists, but with low molecular mass and a monodisperse molecular mass distribution. The low molecular mass leads to the smaller pixel size compared to the radius of gyration of the polymer photoresist. In this work, we compare the deprotection reaction-diffusion kinetics of a common photoacid generator in a polymer and molecular glass resist with similar resist chemistry to elucidate effects of molecular architecture on photoresist performance. We determine the mechanism of reaction, photoacid trapping behavior, and diffusivity by measuring and comparing the reaction kinetics parameters as a function of temperature and exposure dose. These results permit an analysis of the latent image formation which is a crucial factor in resolution and line-edge roughness. Further, knowledge of the reaction-diffusion parameters of each type of resist provides a quantitative approach to predict line-space features, crucial for design for resolution-enhancement features.
Current extreme ultraviolet (EUV) photoresist materials do not yet meet performance requirements on exposure-dose sensitivity, line-width roughness, and resolution. In order to quantify how these trade-offs are related to the materials properties of the resist and processing conditions, advanced measurements and fundamental studies are required that consider EUV-resist specific problems. In this paper, we focus on the correlations between the latent image and developed image in EUV exposed line/space features. The latent images of isolated lines produced by EUV lithography are characterized by atomic force microscopy through the change in topology caused by change in film thickness that occurs upon deprotection. The resulting latent-image deprotection gradient (DGL), based on line cross-sections, and latent-image line-width roughness (LWRL) provide metrics and insight into ways to optimize the lithographic process. The results from a model poly(hydroxystyrene-co-tert-butylacrylate) resist and a model calix[4]resorcinarene molecular glass type resist show the general applicability of the metric before development.
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.
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.
The photon-stimulated emission of organic molecules from the photoresist during exposure is a serious problem for
extreme-ultraviolet lithography (EUVL) because the adsorption of the outgassing products on the EUV optics can lead
to carbonization and subsequent reflectivity loss. In order to accurately quantify the total amount of outgassing for a
given resist during an exposure, we have constructed a compact, portable chamber that is instrumented with a spinning
rotor gauge and a capacitance diaphragm gauge that, unlike the more commonly used ionization gauge or quadrupole
mass spectrometer, provides a direct and accurate measurement of the total pressure that is largely independent of the
composition of the outgas products. We have also developed a method to perform compositional analysis on the outgas
products and, more generally, on any contaminants that might be present in the stepper vacuum. The method involves
collecting the vacuum contaminants in a trap cooled to liquid-nitrogen temperature. Once collected, the products from
the trap are transferred to a system for analysis with gas chromatography with mass spectrometry. We will describe the
workings of the instruments in detail as well as results of initial tests.
Current extreme ultraviolet (EUV) photoresist materials do not yet meet requirements on exposure-dose sensitivity,
line-width roughness (LWR), and resolution. Fundamental studies are required to quantify the trade-offs in materials
properties and processing steps for EUV photoresist specific problems such as high photoacid generator (PAG) loadings
and the use of very thin films. Furthermore, new processing strategies such as changes in the developer strength and
composition may enable increased resolution. In this work, model photoresists are used to investigate the influence of
photoacid generator loading and developer strength on EUV lithographically printed images. Measurements of line
width roughness and developed line-space patterns were performed and highlight a combined PAG loading and
developer strength dependence that reduce LWR in a non-optimized photoresist.
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.
More demanding requirements are being made of photoresist materials for fabrication of nanostructures as the feature critical dimensions (CD) decrease. For extreme ultraviolet (EUV) resists, control of line width roughness (LWR) and high resist sensitivity are key requirements for their success. The observed LWR and CD values result from many factors in interdependent processing steps. One of these factors is the deprotection interface formed during the post-exposure bake (PEB) step. We use model EUV photoresist polymers to systematically address the influence of exposure-dose on the spatial evolution of the deprotection reaction at a model line edge for fixed PEB time using neutron reflectivity. The bilayer consists of an acid feeder layer containing photoacid generator (PAG) and a model photoresist polymer, poly(hydroxystyrene-co-tert-butylacrylate) with perdeuterated t-butyl protecting group. The deuterium labeling allows the protection profile to be measured with nanometer resolution. The evolution of two length scales that contribute to the compositional profile is discussed.
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.
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