In this article the recent progress in the elements of EUV lithography is presented. Source power around 205W was demonstrated and further scaling up is going on, which is expected to be implemented in the field within 2017. Source availability keeps improving especially due to the introduction of new droplet generator but collector lifetime needs to be verified at each power level. Mask blank defect satisfied the HVM goal. Resist meets the requirements of development purposes and dose needs to be reduced further to satisfy the productivity demand. Pellicle, where both the high transmittance and long lifetime are demanded, needs improvements especially in pellicle membrane. Potential issues in high-NA EUV are discussed including resist, small DOF, stitching, mask infrastructure, whose solutions need to be prepared timely in addition to high-NA exposure tool to enable this technology.
Extreme ultraviolet (EUV) lithography has received much attention in the semiconductor industry as a promising candidate to extend dimensional scaling beyond 10nm. Recently EUV pellicle introduction is required to improve particle level inside scanner for EUV mass production. We demonstrate that a new pellicle material, nanometer-thick graphite film (NGF), is one of the best candidates of EUV pellicle membrane. A NGF pellicle with excellent thermal (ε≥0.4 @R.T, <100nm), mechanical (415MPa @~100nm), chemical and optical (24hrs durability under exposure of EUV/H2 at 4W/cm2 with pH2~5Pa) properties can be a promising and superb candidate for EUV pellicle membrane compared to Si pellicles with capping layers.
In EUVL, major impacts on determining critical dimension (CD) are resist process, scanner finger print, and mask characteristics. Especially, reflective optics and its oblique incidence of light bring a number of restrictions in mask aspect. In this paper, we will present one of the main contributors for wafer CD performance, such as center wavelength (CW) of multilayer (ML) in EUVL mask. We evaluate wafer CDs in 27.5nmHP L/S, 30nmHP L/S, and 30nmHP contact patterns with NXE3100 by using masks with purposely off-targeted CW ranging from 13.4 to 13.7nm. Based on the results from the exposure experiments, we verify that the CW specification for NXE3100 is regarded as 13.53 ± 0.015nm at CWU=0.03nm to satisfy the wafer CD requirements. According to verified simulations, however, we suggest a new CW specification for NXE3300 with higher values considering wide illumination cone angle from larger numerical aperture (0.33NA). Moreover, simulations in different exposure conditions of NXE3300 with various patterns below 20nm node show that customized CW specification might be required depending on target layers and illumination conditions. We note that it is also important to adjust CW and CWU in final mask product considering realistic difficulties of fabrcation, resulting in universal CW specification.
As feature size continuously decreasing new techniques to improve quality of wafer are developed. Hence a lot of new
problems in semiconductor industry arise. Strict control of quality of wafer during production process is very important
as many factors can influence on it, but the main contribution gives scanner error and mask. Thus at least impact of mask
should be reduced.
In this work we apply rigorous model to predict impact of microstructures to pattern fidelity on wafer. Such
microstructures are commonly generated in quartz layer to control transmittance distribution on photomask. It is shown
that effect from microstructures is not only changing of mask transmittance but also distortion of the pattern fidelity on
wafer. Rigorous modeling gives us possibility to calculate aerial image and CD on wafer in case of presence of microstructures
in the quartz. We vary optical parameters, such as refractive indexes, number, size and location of these
elements in order to reduce the distortion of pattern fidelity on wafer.
Our result allows prediction of the impact of microstructures in photomask on wafer pattern fidelity instead of doing set
of experiments. Moreover, the best conditions for experiment are found and discussed.
EUV lithography is one of the most developing and promising lithography techniques. Recently many papers are
focused on defect control of EUV mask multilayer blank, but development of profile metrology is also very important.
2D scatterometry becomes insufficient in conditions of further shrinking of feature size and complication of mask
patterns. To overcome these limitations 3D scatterometry should be used.
In this paper we study the precision of 3D scatterometry measurements of two-dimensional EUV mask features with
variety of geometrical shapes. As in reflectometry of EUV mask we can use only one or a few wavelengths, we have to
take into account intensities of many reflected orders to extract profile precisely. We calculate the library of diffraction
efficiencies for periodic circular, elliptical, and rectangular shaped with rounded corners features using 3D RCWA
method. Then we find the amplitudes of reflected diffraction orders from feature with random arbitrary shape, compare
them with each set of data in the library, and extract the most appropriate shape. After that we analyze whether the
extracted shape is really close to initial arbitrary shape or not.
In some cases extracted shape is not the closest one to the real. It is demonstrated that non-zero value of azimuth angle
of incident light influence on precision of feature shape determination and lead to deterioration of results. Using of
polarized light helps to improve precision of results, but unlike 2D scatterometry the optimal polarization can not be
determined unambiguously. According to received data we provide recommendations for optimal 3D EUV
scatterometry measurements and determine the necessary steps of varying of geometrical parameters for library features.
Simulation and experimental study results are reported to solve align/overlay problem in dark hard mask
process in lithography. For simulation part, an in-house simulator, which is based on rigorous coupled wave analysis and
Fourier optics method of high NA imaging, is used. According to the simulation and experiment study, image quality of
alignment and overlay marks can be optimized by choosing hard mask and sub-film thickness carefully for a given
process condition. In addition, it is important to keep the specification of film thickness uniformity within a certain limit.
Simulation results are confirmed by experiment using the state of art memory process in Samsung semiconductor R&D
facility.
Decreasing of node size significantly increases requirement to overlay precision. Complex structure of target demands
using of compound structures of overlay mark, which usually contain features with acute sidewall angles, coated by
several layers.
In this paper the possibility and limitations of image-based overlay with compound structures with overlay mark and
coating layers are analyzed in detail. Dependence of overlay signal shape on overlay offset is considered. Structures with
asymmetric sidewall angle, non-uniform thicknesses of layers and curved shape of layer borders are examined. Influence
of thickness variation, difference between left and right sidewall angles of asymmetric shape and curvature of layer
borders are investigated. For the simulation of such complex structures of overlay marks, our in-house simulator based
on rigorous coupled-wave analysis (RCWA) module is used. Maximum allowed values of these parameters are studied
in order to determine the limitations of image-based overlay.
Results of this consideration can be used for improvement of overlay precision and elaboration of optimal overlay
strategy in conditions of node shrinking in the semiconductor industry.
Control of critical dimension (CD) of 65nm and beyond nodes is the hot issue now. As feature size reduces it becomes
difficult to measure CD precisely. A lot of factors can influence on accuracy of measurement. Scatterometry method is
applicable for both production and development purpose, and can be used for in-situ or ex-situ control.
In this work we study influence of CD non-uniformity and sidewall angle as well as influence of parameters of
measurement system on precision of result. TE, TM and unpolarized light with different angle of incidence on grating
structure is considered to find the best conditions for CD measurements of 65 and 45nm nodes. Rigorous coupled-wave
analysis (RCWA) is used for theoretical spectra calculation and least square method for results extraction. Reflected
spectrum from structures containing non-uniform or uniform CDs with variation of sidewall angle is compared with the
set of theoretical spectra, and CD value with layer thickness is extracted in the same way as in the real experiment. It is
shown that CD non-uniformity and sidewall angle can be estimated through comparison of results obtained with different
polarization state of light. Best choice of polarization, angle of light incidence, range of wavelength for spectrum
measurement and parameters of library for spectrum analysis are obtained in order to provide precise and fast
scatterometry measurement for 65 and 45nm nodes mask structures.
As the on-wafer transistor sizes shrink, and gate nodes reduce below 90 nm, it is becoming very important to precisely
measure and control the critical dimension (CD) on the mask. Phase shift technology for masks is essential for
decreasing of the feature size, therefore CD and profile metrology on the phase shifting materials becomes critical.
Scatterometry provides fast and nondestructive method of profile and CD measurements.
In this paper the conditions of determining of profile and CD measurement are analyzed. In the real experiment scattered
spectrum from structure with unknown profile is measured. Before experiment the library of spectra is generated. Spectra
in the library correspond to structures with various parameters (such us thickness, CD, sidewall angle, etc.). For
calculation of this library rigorous coupled-wave analysis (RCWA) was used. This method allows us to get precise
solution of Maxwell equations and find directly amplitude of zero diffraction order which is measured in the experiment.
To determine the possibility of measurement of sidewall angle various spectra with different sidewall angle value were
calculated. Calculated spectrum is changed by adding or deduction of random value. The randomly changed spectrum is
compared with spectra in the library in order to find spectrum with best fit. Therefore sidewall angle and CD can be
determined. Precision, possibility and maximum allowed error in the spectra measurements is obtained. Moreover,
influence of polarization of incident light on precision of extracted results was found.
A theoretical model of confocal microscopy of phase defect in EUV mask blank is developed using Fourier optics and rigorous coupled-wave analysis(RCWA). This model is verified by comparison with experimental data and then applied to calculating confocal microscopy signal of phase defect with conformal and flat surface for wavelength of 488 nm and 266 nm, respectively. From this simulation, it is shown that phase defect with flat surface is undetectable even by 266 nm-wavelength confocal microscope, while it is printable at 13.5 nm-wavelength. Subsequent simulation of energy flow rate through Mo/Si multilayer shows that this is because incident optical wave is absorbed within 4 pairs of Mo/Si multilayer, but 40 pair of Mo/Si multilayer is semi-transparent at 13.5 nm-wavelength. Based on this result, a deposition scheme of Mo/Si multilayer is suggested.
Simulation works are reported for the optimization of alignment mark geometries to enhance the magnitude of alignment signal and to improve precision of alignment and overlay process. Modeling of diffraction of electromagnetic wave on alignment mark is based on rigorous coupled-wave analysis (RCWA). This simulation allows calculating of overlay signal and optimizing of alignment mark. Using of RCWA approach significantly decreases computational time and required memory size comparing with FDTD. Overlay signal is usually measured using one of the diffraction orders. Thus the great advantage of using RCWA instead of FDTD is possibility to find amplitude of this diffracted order directly.
Rigorous coupled-wave analysis(RCWA) is applied to computing near-field of mask scattered by patterned absorber and defects buried in Mo/Si multilayer. Especially, a method of modeling phase defect for application of RCWA is provided, which transforms the multilayer structure deformed by defect into straight multilayer structure with inhomogeneous dielectric constant. This mask near-field is used to get the aerial image as well as mask inspection image of confocal microscope. Using these simulation methods, printability of both phase and amplitude defect are investigated over various size of defect. This study shows that the change in critical dimension(CD) of line and space pattern increases linearly with defect height of phase defect, while increases nonlinearly with that of amplitude defect. A modeling of confocal microscopy is also shown with an example of actinic inspection simulation.
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