A major challenge for extreme ultraviolet lithography (EUVL) is avoiding defects in the fabrication of multilayered (ML) mask blanks. Substrate defects and adders during ML coating are responsible for ML defects which causes changes on phase and amplitude of EUV light. ML defects must be identified by inspection prior to absorber patterning in order to reduce the effects of ML defects via covering them with patterns to permit the use of fewer ML defect blanks. Fiducial marks (FMs) on ML blanks can
be used for mask alignment and to accurately and precisely determine the locations of ML defects. In this study, we fabricated an FM mask by resist exposure using an e-beam writer and etching. Then, we inspected FMs and ML defects with an EUV actinic full-field mask blank inspection tool developed by EIDEC-LaserTec (LT ABI). Next, we evaluated the ML defect location accuracy on the mask based on FMs of several line depths by deriving center position of FMs and defects with Lorentz, Gaussian fitting and center-of-mass calculation. Here, we explain the estimation of defect location accuracy using FMs and the LT ABI, and discuss the defect numbers which can be covered by absorber patterns. Fewer than 19 defects per blank should be required for EUV blanks to cover ML defects with patterns.
A major challenge for extreme ultraviolet (EUV) lithography is avoiding defects in the fabrication of multilayered (ML) mask blanks. Substrate defects and adders during ML coating are responsible for ML defects, which cause changes on phase and amplitude of EUV light. The ML defects must be identified by inspection prior to absorber patterning in order to reduce the effects of ML defects via covering them with patterns to permit the use of fewer ML defect blanks. Fiducial marks (FMs) on ML blanks can be used for mask alignment and to accurately and precisely determine the locations of ML defects. In this study, we fabricated an FM mask by resist exposure using an e-beam writer and etching. Then, we inspected FMs and ML defects with an EUV actinic full-field mask blank inspection tool developed by EIDEC-LaserTec (LT ABI; EIDEC, Tsukuba, Japan and LaserTec, Yokohama, Japan). Next, we evaluated the ML defect location accuracy on the mask based on FMs of several line depths. Here, we explain the estimation of defect location accuracy using FMs and the LT ABI and discuss the defect numbers which can be covered by absorber patterns. Fewer than 19 defects per blank should be required for EUV blanks to cover the ML defects with patterns.
We directly extracted the phase-shift values of an EUV mask by measuring the reflectance of the mask. The mask had
gradient absorber thickness along vertical direction. We measured the reflectance of the open multilayer areas and the
absorber areas by using an EUV reflectometer at various absorber thicknesses. We also measured the diffracted 0th order
light intensities of grating patterns having several sizes of lines or holes. The phase-shift values were derived from these
data assuming a flat mask interference model of the diffracted lights. This model was corrected by including the
scattering amplitude from the pattern edges. We recalculated the phase-shift values which was free from the mask
topological effect. The extracted phase-shift value was close to 180 degrees at 67 nm and 71 nm absorber thicknesses.
The phase measurement error around 180 degree phase shift was 5 degrees (3σ).
For extreme ultraviolet lithography (EUVL), the fabrication of defect free multi-layered (ML) mask blanks is a challenge. ML defects are generated by substrate defects and adders during ML coating and are called phase defects (PDs). If we can accept ML blanks with a certain number of PDs, the blank yield will be drastically increased. We can use fewer PD blanks and reduce PD influence by covering them with an absorber layer. To do this, PDs should be located during ML blank defect inspection before absorber patterning. To locate PDs on blanks accurately and precisely, the fiducial marks (FMs) on ML blanks can be used for mask alignment. The defect location accuracy requirement is below 10 nm. We present the results of a feasibility study on the requirements of FMs on EUVL masking by simulations and experiments to establish a PD mitigation method with the EUV actinic blank inspection tool. Based on the results, the optimum ranges for FM lines etched into the ML are 3 to 5 μm in width and at least 100 nm in depth.
In order to mitigate phase defects on EUVL mask blanks, Fiducial Marks (FMs) should be located with high registration
accuracy by the EUV Actinic Blank Inspection tool, the E-Beam (EB) writer, and other inspection tools. The proposed registration accuracy is less than 10 nm for each tools [1]. In our previous studies [2] and [3] we fabricated FMs by FIB etching, or resist exposure by the EB writer and etching process, and inspected those FMs by the EUV Actinic full-field mask Blank Inspection (ABI) prototype developed at MIRAI-Selete. The registration accuracy of those FMs with the MIRAI EUV ABI prototype were not enough due to a pixel size of 500 nm on the mask in order to achieve total less than 10 nm defect location accuracy on the EUVL mask blank. We studied the registration accuracy improvement of FM on the blank by the experiment with the MIRAI EUV ABI prototype to establish the phase defect mitigation method.
In this experiment, we took some images of FMs by moving the mask stage of the MIRAI EUV ABI prototype by lengths of sub-pixels. And we analyzed registration accuracy of FM by stringing those images shifting sub-pixel. This method can achieve less than 2.5 nm FM registration accuracy by the conditions equal to or more than 4 times quasimagnification, trench detection, and 50% threshold level In this presentation, we will explain the experiment results of FM registration accuracy improvement on the EUVL mask with MIRAI EUV ABI prototype to establish the phase defect mitigation method.
For Extreme Ultra-Violet Lithography (EUVL), fabrication of defect free multi-layered (ML) mask blanks is one of the difficult challenges. ML defects come from substrate defects and adders during ML coating, cannot be removed, and are called as phase defect.
If we can accept ML blanks with certain number of phase defects, the blank yield will be drastically up. In order to use such blanks, the phase defects need to be identified and located during ML blank defect inspection before absorber patterning. To locate phase defects on the blanks accurately and precisely, Fiducial Marks (FM) on ML blanks are needed for mask alignment and defect location information. The proposed requirement of defect location accuracy is less than 10 nm [1].
In addition to the previous study for which FMs were etched by Focused Ion Beam (FIB) [2], we fabricated FMs by resist exposure by E-Beam (EB) writer and etching process, and inspected FMs with EUV Actinic full-field mask Blank Inspection (ABI) prototype developed at MIRAI-Selete, EB writer and other mask inspection tools. Then we estimated FM registration accuracy for several line widths and depths.
In this paper, we will present the result of feasibility study on the requirements of FM on EUVL mask by experiments to establish the phase defect mitigation method. And the optimum ranges of FM line width, depth, and fabrication method on EUVL mask based on above results are 3 - 5 m line width, not less than 100 nm depth FM etched into ML respectively.
The measurement and extraction method of phase-shift values for thin and thick absorber Extreme Ultra-Violet (EUV)
masks has been studied by both of experiments and simulations. We fabricated 4 EUV masks with different absorber
thicknesses. We first estimated the phase-shift values from the absorber thicknesses of each mask and the nandk values which were derived in advance by other experiments. This method is indirect and may contain plate-by-plate errors. In order to extract the phase-shift values directly, we developed a phase-shift measurement method based on scatterometry. We measured the reflectivity of the open and dark area of the 4 masks by using the EUV reflectometer at Lawrence Berkeley National Laboratory (LBNL). We also measured the diffracted light intensities of grating pattern. The phaseshift values were derived from these data assuming an interference of reflected and diffracted lights. We calibrated the method by including the shadowing effect of 6 degree incident angle, and adding the information on the measured mask patterns. The extraction results of phase-shift values by this method agreed well within the error bar. The absorber thickness having 180 degree phase-shift, which works as an embedded attenuated phase-shifting mask, could be somewhere between 66 nm and 76 nm. The measurement accuracy of this method depends on the phase-shift values. The error becomes the largest at 180 degree phase-shift, and the worst one in this experiment was much larger than the proposed phase-shift measurement accuracy of ± 2 degree [1]. Much effort will be required to achieve this target.
For Extreme Ultra-Violet Lithography (EUVL), fabrication of defect free multi-layered (ML) mask blanks is one of the
difficult challenges. ML defects come from substrate defects and adders during ML coating, cannot be removed, and are
called as phase defect.
If we can accept ML blanks with certain number of phase defects, the blank yield will be drastically up. In order to use
such blanks, the phase defects need to be identified and located during ML blank defect inspection before absorber
patterning. To locate phase defects on the blanks accurately and precisely, Fiducial Marks (FM) on ML blanks are
needed for mask alignment and defect location information. The proposed requirement of defect location accuracy is ≤20 nm [1].
In this paper, we will present the result of feasibility study on the requirements of FM on EUVL mask by simulations &
experiments to establish the phase defect mitigation method with EUV Actinic Blank Inspection (ABI) tool. And the
optimum ranges of FM line width, depth, and fabrication method on EUVL mask based on above results are ≥ 5 um line
width, ≥ 100 nm depth FM etched into ML respectively, and additional finer FMs for magnified optics.
A variety of phase defects (PDs) such as programmed bump and pit PDs, and native bump and pit PDs were detected by
a dark-field ABI (Actinic Blank Inspection) tool. Among the PDs, some of them seemed to grow and propagate in an
angular direction, away from substrate surface as was found by a TEM analysis. This presentation reports on the
influence of 3-D phase defect on wafer printability, and on defect detection signals of an ABI tool. The result shows that
the impact of the inclination angle on printing performance was quite significant when the PDs were not covered with
the absorber pattern. On the other hand, the defect detection signal intensity was negligibly small in the case where the
inclination angle was less than 9 degrees. However, ABI with its high magnification optics can pinpoint the PD's actual
location as defined by the EUV light, rather than the ones that are not so clearly define by the surface topography.
Phase-shifting effect of EUV masks with various absorber thicknesses has been studied both by simulations and
experiments. In EUV lithography, masks with 180 phase shifting absorber work like embedded attenuated phase-shifting
masks. At 66nm thickness of TaN/TaON absorber, 180 degree phase shifting can be achieved in theory. Based on the
experiments, we observed that the true180 degree phase shifting can be achieved with absorber thickness between 66 and
76 nm. In this paper, phase shifting impact of the various thickness absorbers has been characterized. Imaging
performance of masks with 51 nm, 66 nm and 76 nm thick absorber has been experimentally compared. The process
window of various thickness absorber masks are rigorously studied.
This paper describes the critical dimension (CD) accuracy of metal-layer patterns for the 15-nm logic node and beyond
replicated with model-based optical proximity correction, flare variation compensation, and shadowing effect correction.
The model fitting took resist shrinkage during CD measurements into account so as to reduce the modeling error. Since
sufficient accuracy was obtained for various patterns under the assumptions of device production, and since conventional
illumination could be used, it was possible to establish a design rule with few restrictions for the 15-nm node. For the 12-nm logic node, an SRAM pattern for a cell size of 0.0288 μm2 was fabricated using dipole illumination.
Advanced pre-production optics were used to assess the impact of flare on CD variation. Since chemical flare occurs in
SSR4, a top coating was used to prevent acid re-adsorption during the post-exposure bake. The flare due to the optics
was reduced to half that of conventional optics, and the CD variation due to flare was found to be predictable from the
point spread function of the projection optics. This means that the established concept of flare correction is usable with
advanced optics.
EUV lithography is considered one of the options for high volume manufacturing (HVM) of 16 nm MPU node devices
[1]. The benefits of high k1(~0.5) imaging enable EUVL to simplify the patterning process and ease design rule
restrictions. However, EUVL with its unique imaging process - reflective optics and masks, vacuum operation, and
lack of pellicle, has several challenges to overcome before being qualified for production. Thus, it is important to
demonstrate the capability to integrate EUVL into existing process flows and characterize issues which could hamper
yield. A patterning demonstration of Intel's 32 nm test chips using the ADT at IMEC [7] is presented, This test chip
was manufactured using processes initially developed with the Intel MET [2-4] as well as masks made by Intel's mask
shop [5,6]. The 32 nm node test chips which had a pitch of 112.5 nm at the trench layer, were patterned on the ADT
which resulted in a large k1 factor of 1 and consequently, the trench process window was iso-focal with MEEF = 1. It
was found that all mask defects detected by a mask pattern inspection tool printed on the wafer and that 90% of these
originated from the substrate. We concluded that improvements are needed in mask defects, photospeed of the resist,
overlay, and tool throughput of the tool to get better results to enable us to ultimately examine yield.
A multilayer coating mirror of Mo/Si is usually integrated into an EUV optics for space science, especially for He-II (30.4 nm) radiation, because it is highly stable under vacuum and atmosphere and achieves the fairly high reflectance of 15-20%. But space science community needs the coating of higher reflectance at 30.4 nm radiation for the future satellite missions. In this work, to develop a new multilayer mirror of He-II radiation, we report the design of a multilayer consisting of a pair of Mg and SiC, and its production, and aging change of the reflectance under the atmosphere and vacuum circumstance.
A multilayer coating of Mo/Si is usually used as an EUV optics for space science, especially for He-II (30.4nm) radiation, because it is highly stable under vacuum and atmosphere. The fairly high reflectivity of 15-20% was achieved. But space science community will need higher reflective coating at 30.4 nm radiation for the future satellite missions. In this work, for developing new multilayer mirror of He-II radiations, we report the design of a multilayer, consisting of a pair of Mg and SiC, and its fabrication, and result of the reflectance with the monthly degradation under the atmosphere circumstance.
Extreme and far ultraviolet imaging spectrometers will be boarded on the low-altitude satellite of the upcoming mercury msision (the BepiColombo mission) conducted by ISAS and ESA. The UV instrument, consisting of the two spectrometers with common electronics, aims at measuring, (1) emission lines from molecules, atoms and ions present in the Mercury's tenuous atmosphere, and (2) the reflectance spectrum of Mercury's surface. The instrument pursues a complete coverage in UV spectroscopy. The extreme UV spectrometer covers the spectral range of 30-150 nm with the field of view of 5.0 degree, and the spectrum from 130 nm to 430 nm is obtained by the far UV spectrometer. The extreme UV spectrometer employs a Mo/Si multi-layer coating to enhance its sensitivity at particular emission lines. This technology enables us to identify small ionospheric signals such as He II (30.4nm) and Na II (37.2nm), which the previous mission could not identify.
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