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ispartof: JOURNAL OF MICRO-NANOPATTERNING MATERIALS AND METROLOGY-JM3 vol:21 issue:4 status: published

ispartof: JOURNAL OF MICRO-NANOPATTERNING MATERIALS AND METROLOGY-JM3 vol:21 issue:4 status: published

How many ways can we explore the Sun? We have images in many wavelengths and squiggly lines of many parameters that we can use to characterize the Sun. We know that while the Sun is blindingly bright to the naked eye, it also has regions that are dark in some wavelengths of light. All of those classifications are based on vision. Hearing is another sense that can be used to explore solar data. Some data, such as the sunspot number or the extreme ultraviolet spectral irradiance, can be readily sonified by converting the data values to musical pitches. Images are more difficult. Using a raster scan algorithm to convert a full-disk image of the Sun to a stream of pixel values creates variations that are dominated by the pattern of moving on and off the limb of the Sun. A sonification of such a raster scan will contain discontinuities at the limbs that mask the information contained in the image. As an alternative, Hilbert curves are continuous space-filling curves that map a linear variable onto the two-dimensional coordinates of an image. We have investigated using Hilbert curves as a way to sample and analyze solar images. Reading the image along a Hilbert curve keeps most neighborhoods close together as the resolution (i.e., the order of the Hilbert curve) increases. It also removes most of the detector size periodicities and may reveal larger-scale features. We present several examples of sonified solar data, including sunspot number, extreme ultraviolet (EUV) spectral irradiances, an EUV image, and a sequence of EUV images during a filament eruption.

The extreme ultraviolet (EUV) phase retarder is an important optical element for polarization analysis and conversion of EUV synchrotron radiation. In this paper, a linearly chirped Mo/Si multilayer mirror is used to design an EUV phase retarder. With increasing thickness variation of the chirped multilayer, the reflective phase retardation between s- and p-polarized light increases at first and then reaches its maximum value. When the bilayer number increases from 2 to 20, the maximum phase retardation for an EUV source with a photon energy of 90 eV increases from 5.97° to 245.10° for a linearly chirped Mo/Si multilayer with 14.7 nm central thickness. In addition, the phase retardations of chirped mirrors at different photon energies (80 eV, 85 eV and 90 eV) are also investigated and compared. Furthermore, the physical mechanism of reflective phase retardation improvement is also studied by investigating the field intensity distributions inside chirped mirrors.

EUV lithography enters the high-volume manufacturing stage, and the semiconductor industry considers a lithography-wavelength- matched actinic patterned mask inspection (APMI) tool to be a crucial infrastructure for EUV mask qualification. ACTISTM, the world’s first high-sensitivity actinic patterned mask inspection system, was released in 2019. ACTIS detects lithographic impact defects that cannot be seen with the existing DUV inspection tools. The actual results of production mask inspection show that only an actinic EUV inspection system can visualize small surface topography and phase changes that propagate through multilayer stacks. In this paper, we present the progress of ACTIS inspection technology, defect sensitivity, die-to-database inspection and through pellicle inspection. For technology nodes beyond N3, a high-NA EUV anamorphic lithography system will be used. The mask structure in the high-NA era will be different from the current configuration. For inspection tool design, it is necessary to adopt a different magnification of mask-to-wafer projection in the vertical and horizontal directions. ACTIS has the extendibility to a high-NA system since its projection NA area has room for extension in one direction. The high-NA EUV inspection tool will be discussed in this paper as well.

The mutual optical intensity (MOI) model is extended to the simulation of the interference pattern produced by extreme ultraviolet lithography with partially coherent light. The partially coherent X-ray propagation through the BL08U1B beamline at Shanghai Synchrotron Radiation Facility is analysed using the MOI model and SRW (Synchrotron Radiation Workshop) method. The fringe intensity at the exposure area is not uniform but has similar envelope lines to Fresnel diffraction, which is explained by the diffraction from the finite grating modelled as a single aperture. By balancing the slit size and photon stop size, the fringe visibility, photon flux and intensity slope can be optimized. Further analysis shows that the effect of pink light on the aerial images is negligible, whereas the third-harmonic light should be considered to obtain a balance between high fringe visibility and high flux. Two grating interference exposure experiments were performed in the BL08U1B beamline. The aerial image depth showed that the polymethyl methacrylate photoresist depth was determined by the X-ray coherence properties.

Thin mask model has been conventionally used in optical lithography simulation. In this model the diffracted waves from the mask are assumed to be Fourier transform of the mask pattern. In EUV (Extreme UltraViolet) lithography thin mask model is not valid because the absorber thickness is comparable to the mask pattern size. Fourier transformation is not suitable for calculating the diffracted waves from thick masks. Rigorous electromagnetic simulations such as finitedifference time-domain method, rigorous coupled wave analysis and 3D waveguide model are used to calculate the diffracted waves from EUV masks. However, these simulations are highly time consuming. We reduce the calculation time by adapting a CNN (Convolutional Neural Network). We calculate the far-field diffraction amplitudes from an EUV mask by using the 3D waveguide model. We divide the diffraction amplitudes into the thin mask amplitudes (Fourier transform of the mask pattern) and the residual mask 3D amplitudes. The incident angle dependence of the mask 3D amplitude for each diffraction order is fitted by using three parameters which represent the on-axis and the off-axis mask 3D effects. We train a CNN where the inputs are 2D mask patterns and the targets are the mask 3D parameters of all diffraction orders. After the training, the CNN successfully predict the mask 3D parameters. The CNN prediction is 5,000 times faster than the electromagnetic simulation. We extend the transmission cross coefficient formula to include the off-axis mask 3D effects. Our formula is applicable to arbitrary source shapes and defocus. We can use the eigen value decomposition method to accelerate the calculation.

We have studied the laser pulse shape dependence of the conversion efficiency of λ = 1.03 μm laser pulse energy into 13.5 nm extreme ultraviolet (EUV) emission from a Sn laser-produced plasma. Laser pulses of arbitrary temporal shape ranging from hundreds of picoseconds to several nanoseconds were generated using a programmable pulse synthesizer based on a diode-pumped chirped pulse amplification Yb: YAG laser. Measurements show that the conversion efficiency favors the use of nearly square pulses of duration longer than 2 ns, in agreement with hydrodynamic/atomic physics simulations. A 35% increase in conversion efficiency was obtained when Q-switched pulses were substituted by square pulses of a similar duration. Experiments conducted irradiating a Sn target with a sequence of two time-delayed 250 ps pulses showed a 30 percent increase in the EUV yield respect to a single pulse of the same total energy when the pulse separation was optimum at 2.1 ns. This suggests that re-heating of the plasma with delayed laser pulses could be used to improve the EUV yield. The spectroscopic characterization of EUV emission and in-band EUV images that characterize the source size are also presented.

Extreme-ultraviolet (XUV) sources including high-harmonic generation (HHG), free-electron lasers (FELs), soft-X-ray lasers and laser-driven plasmas are widely used for applications ranging from femtochemistry and attosecond science to coherent diffractive imaging and EUV (or XUV) lithography. The bandwidth of the XUV light emitted by these sources reflects the XUV generation process used. Whereas light from soft-X-ray lasers1 and seeded XUV FELs2 typically has a relatively narrow bandwidth, plasma sources and HHG sources often emit broadband XUV pulses3. Since these characteristic properties of a given source impose limitations on applications, techniques enabling modification of the bandwidth are highly desirable. Here we introduce a concept for efficient spectral compression by four-wave mixing (FWM), exploiting a phase-matching scheme based on closely-spaced resonances. We demonstrate the compression of broadband radiation in the 145–130 nm wavelength range into a narrow-bandwidth XUV pulse at 100.3 nm wavelength in the presence of a broadband near-infrared (NIR) pulse in a krypton gas jet. Our concept provides new possibilities for tailoring the spectral bandwidth of XUV beams. Through a dense krypton gas jet in the presence of a broadband near-infrared pulse, spectral compression of broadband XUV radiation between 145 and 130 nm wavelengths into a narrow-bandwidth XUV pulse at 100.3 nm wavelength by four-wave mixing is demonstrated.

Recent significant progress in the development of multilayer normal-incidence EUV mirrors is primarily related to an increase in the reflection coefficient at the working wavelength, a decrease in the spectral width of the reflection curve, and application of high-efficiency multilayer coatings for short-wavelength (3–9 nm) and long wavelength (greater than 50 nm) spectral ranges. Such mirrors allow astrophysical study of the Sun, since relatively narrow spectral widths of the mirrors and high reflection coefficients make it possible to image corona in monochromatic lines. Telescopes based on the mirrors are promising for dynamic spectral diagnostics of the solar disk with the aid of imaging spectroscopy. The method is based on the detection of monochromatic images of the Sun using the EUV spectral lines with relatively high spatial and temporal resolution. Possible progress in the solar study related to application of specified optical elements is discussed.

ispartof: JOURNAL OF MICRO-NANOPATTERNING MATERIALS AND METROLOGY-JM3 vol:21 issue:2 status: published

Thin mask model has been conventionally used in optical lithography simulation. In this model the diffracted waves from the mask are assumed to be Fourier transform of the mask pattern. This assumption is the basis of Hopkins' method and sum of coherent system model. In EUV (Extreme UltraViolet) lithography thin mask model is not valid because the absorber thickness is comparable to the mask pattern size. Fourier transformation cannot be applied to calculate the diffracted waves from thick masks. Rigorous electromagnetic simulations such as finite-difference time-domain method, rigorous coupled wave analysis and 3D waveguide method are used to calculate the diffracted waves from EUV masks. However, these simulations are highly time consuming. We reduce the calculation time by adapting a convolutional neural network. We construct a convolutional network which can predict the diffracted waves from 1D EUV mask patterns. We extend the TCC method to include the off-axis mask 3D effects. Our model is applicable to arbitrary source shapes and defocus.

Optics express 28(18), 25664-25681 (2020). doi:10.1364/OE.396755, Proper diagnostics of intense free-electron laser (FEL) X-ray pulses is indisputably important for experimental data analysis as well as for the protection of beamline optical elements. New challenges for beam diagnostic methods are introduced by modern FEL facilities capable of delivering powerful pulses at megahertz (MHz) repetition rates. In this paper, we report the first characterization of a defocused MHz 13.5-nm beam generated by the free-electron laser in Hamburg (FLASH) using the method of multi-pulse desorption imprints in poly(methyl methacrylate)(PMMA). The beam fluence profile is reconstructed in a novel and highly accurate way that takes into account the nonlinear response of material removal to total dose delivered by multiple pulses. The algorithm is applied to experimental data of single-shot ablation imprints and multi-shot desorption imprints at both low (10 Hz) and high (1 MHz) repetition rates. Reconstructed response functions show a great agreement with the theoretical desorption response function model., Published by Soc., Washington, DC

We present the results of an experimental study of soft X-ray (SXR) and extreme ultraviolet emission spectra of the plasma produced by exposing a plane solid beryllium target to laser radiation with broad frequency and angular spectra. SXR lines up to 1s – 9p of Be IV as well as the plasma continuum are recorded for a laser focal-spot intensity of 5.3 × 1013 W cm−2. To model the SXR beryllium plasma spectra, simulations are carried out using the INDHAUS programme and the FLYCHK code in the framework of local thermodynamic equilibrium model, which agree nicely with experimentally obtained results.

This paper presents the required structure and function of a ring-FEL as a radiation source for extreme ultraviolet radiation lithography (EUVL). A 100 m-long straight section that conducts an extremely low emittance beam from a fourth-generation storage ring can increase the average power at 13.5 nm wavelength to up to 1 kW without degrading the beam in the rest of the ring. Here, simulation results for a ring-FEL as a EUVL source are described.

A two-mirror projection Schwarzschild objective (reduction ratio ×5), formed by two aspheric mirrors and developed as part of upgrading a lithographic workbench with the operating wavelength of 13.4 nm, is proposed. The measurement procedure and results of correction of the shape of the aspheric mirrors and aberrations of the objective lens in the assembled version are described in detail. As a result of several corrections of local errors by an ion beam, a root-mean-square deviation of the shape of the primary mirror of 0.8 nm is achieved. The objective-lens aberrations were equal to 5.4 nm. Simulation of the resolving power of the objective lens, taking into account the measured aberrations, shows that 200-nm-wide strips are resolved with a contrast of 32%, 100 nm—23%, and 30 nm—15%. The reflectivities of the multilayer Mo/Si mirrors with an anti-stress Cr/Y sublayer were 63–65% depending on the coordinate on the mirror. The reasons why it was not able to correct the aberration of the objective lens to the level of 1 nm necessary for achievement of the diffraction limit of resolution are discussed.

We present a general method for designing XUV aperiodic multilayer mirrors that can mimic a given target spectrum, specifically, the spectral transmission of an XUV optical system. The method is based on minimizing a merit function and using fidelity parameters that quantify the matching of the multilayer reflectivity spectrum with that of the target spectrum. To assess the feasibility of fabricating such a system, we show how to reduce the layer-to-layer thickness variations throughout the aperiodic layer stack. We demonstrate the design method using an example of an EUV optical system composed of 12 identical Mo/Si multilayer mirrors having a reflectivity peak at 13.5 nm. We found that the target spectrum can be mimicked with high fidelity either with a single reflection at an aperiodic multilayer mirror combined with standard absorbing filters or, if required, with two subsequent reflections at a mimic mirror. These examples demonstrate the applicability for metrology at XUV sources, including spectrally proper source imaging. Because our approach is of general applicability, the process can be used to mimic any other narrowband, single-peaked target spectrum in the XUV region.

As extreme ultraviolet (EUV) lithography enters high volume manufacturing (HVM) to enable the sub-7nm scaling roadmap, characterizing and monitoring defects that print at wafer level are of critical importance to yield. This is especially true for defects coming from the EUV mask, due to multi-layer defects or added particles or growth on mask during its use, and for defects coming from the pattern formation process itself, which are known as stochastic printing defects or simply stochastic defects. In this paper we focus on developing inspection solutions for stochastic defects, which essentially are random local variations that occur between structures that should, in principle, print identically. These defects occur at significant frequencies with current state-of-the-art processes. Specifically, we discuss the importance of detecting these defects using KLA broadband plasma (BBP) optical inspection systems and the development of useful analysis methodologies. In addition, we describe use of traditional focus-dose process window discovery methodology developed on these optical inspection systems, for a foundry N5 equivalent M2 layer with both hybrid EUV and direct EUV patterning schemes. We highlight the impact of EUV stochastics to the overall process window.

High-harmonic generation (HHG) from a compact solid-state medium is extremely desirable for apps such as consistent attosecond pulse generation and extreme ultraviolet (EUV) spectroscopy1, but the typically small transformation of pump light to HHG can mainly hamper its applications.Here we use a material working in its epsilon-near-zero (ENZ) region2-11, where the real part of its permittivity disappears, to significantly enhance the microscopic effectiveness of the HHG method. In tests, we report high-harmonic emissions up to 9th order straight from a low-loss, solid-state ENZ medium: indium-doped cadmium oxide with a GW cm-2 level of excitation intensity. In addition, the observed HHG signal shows a pronounced red-shift spectrum as well as an expansion of linewidth arising from the photo-induced electron heating and the resulting time-dependent resonant frequency of the ENZ film. Our findings provide a fresh nano-photonic platform for powerful field physics, show fresh degrees of liberty for spectral and temporal HHG control and open up opportunities for compact solid-state attosecond light sources.

Mask multilayer defect and optical proximity effect bring harmful impact on the imaging quality of extreme ultraviolet lithography. The undesirable impact could be compensated for by modification of the mask patterns. Based on machine learning and quasi-rigorous electromagnetic method, two fast simulation methods are utilized to accelerate the optimization of the defect compensation patterns. Defect compensation considering simultaneous optical proximity correction is also studied for further improvement of pattern fidelity. Combined with a well-performed evolutionary searching algorithm, the two fast simulation methods are adopted in the defect-compensation optimization for different mask patterns. Mask modification optimization considering both defect compensation and optical proximity correction is also demonstrated by a simulation study. Simulation results demonstrated the acceleration due to the adoption of the two fast simulation methods. Simulation results also validated that both the defect impact and the optical proximity effect can be compensated for under the same mask modification frame. The mask modification strategy combining the two fast simulation methods and the evolutionary algorithm provides a potential and flexible compensation approach for EUVL masks that influenced by multilayer defect and proximity effect.

Increasing miniaturization and complexity of nanostructures require innovative metrology solutions with high throughput that can assess complex 3D structures in a non-destructive manner. EUV scatterometry is investigated for the characterization of nanostructured surfaces. The reconstruction is based on a rigorous simulation using a Maxwell solver based on finite-elements and is statistically validated with a Markov-Chain Monte Carlo sampling method. Here it is shown that this method is suitable for the dimensional characterization of the nanostructures and the investigation of oxide or contamination layers. In comparison to grazing-incidence small-angle X-rayscattering (GISAXS) EUV allows to probe smaller areas. The influence of the divergence on the diffracted intensities in EUV is much lower than in GISAXS, which also reduces the computational effort of the reconstruction.

Extreme ultraviolet lithography (EUV) materials and processes are constantly evolving and maturing, making sub-30 nm pitch single-exposure yield feasible. Characterization methodologies for stochastic variations are well-documented at this stage, with most employing critical dimension scanning electron microscopy (CDSEM) and a form of defect inspection to quantify line edge roughness (LER), local critical dimension uniformity and patterning failures. In this paper, the link between LER and stochastic defectivity is explored at a deeper level, as we probe the possibility of nucleating stochastic failures with programmed LER structures at 30 nm pitch using CDSEM, optical inspection and electrical yield data. First, we characterize LER post-develop and post-etch using unbiased LER measurements. Second, we study the defectivity and optical characteristics of the programmed LER pattern post hard mask open using optical inspection. Specifically, we measure the optical noise induced by programmed LER, the inspection performance in terms of defect capture and nuisance rate, and the predictive capability of optical inspection through a comparison of defectivity with electrical yield data.

The mask 3D effect is important even for deep ultraviolet lithography. After the wavelength becomes shorter in extreme ultraviolet (EUV) regime, it becomes even more important. We also need to consider the asymmetric effect as well as the shadow effects now. To model these effects correctly, it is critical to compute the electromagnetic near field around the EUV absorbers correctly. Though FDTD, FEM, and RCWA methods have been applied to do so, we are here trying to combine the FEM method with deep learning techniques to achieve a better computational competence in the speed and accuracy. We only compute the one-dimensional (1D) situation with TE type incident wave. With parts of the near field signal just below the absorber computed by the FEM method, 1D patch generative adversarial network (GAN) technique is used to learn the paired mapping between the distribution of the near field below the absorber and the geometry of the mask absorber. The scattering model of the EUV absorbers obtained this way can be combined with the reflector model afterward to form the whole EUV mask model.

A compact damping ring with limited circumference of about 160 m is proposed for producing kilowatt-level coherent EUV radiation. The electron bunch in the ring is modulated by a 257nm wavelength laser with the help of the angular dispersion induced micro-bunching method [C. Feng and Z. Zhao, Sci. Rep. 7, 4724 (2017)]. Coherent radiation at 13.5 nm with an average power of about 2.5 kW can be achieved with the state-of-the-art accelerator and laser technologies.

We have conducted a series of experiments to study how defectivity depends on NILS, aerial image intensity and depth-of-focus. Three sets of illuminations were designed to vary only one of these parameters while keeping the others constant for a 48 nm pitch orthogonal contact hole array. We focus on the after etch defectivity, for which two different etch processes were studied. We correlate on-wafer LCDU and defectivity for both etch processes to the different illumination settings. In summary, we found that defect levels correlate best with the maximum intensity for missing holes and minimum intensity for merging holes. Stochastic LCDU is however better predicted by NILS. By improving NILS by 35 percent and the maximum intensity by 32 percent the defect levels could be reduced by 2 orders of magnitude. Depth-of-focus appears to have little or no impact on defectivity for the nominal focus condition.

Experimental results demonstrated that the EUV low-n mask improves both LCDU and dose for dense features. For semi-isolated and isolated features best-focus shifts through pitch can occur with the low-n mask as well as with the Tabased mask. In this paper, we show how mask-3D induced phase offsets between diffraction orders lead to best focus shifts for dense and (semi-) isolated features. Subsequently we investigate what are the options to mitigate best focus shifts. First, when the pitch is large enough, sub-resolution assist features (SRAFs) can significantly improve contrast and reduce best-focus shifts. Second, optimizing the target and mask bias, is very effective for focus-shift mitigation. Especially the low-n mask shows a strong response to target and mask bias. Third, using bright field imaging instead of dark-field imaging generally also results in smaller best focus shifts through pitch. Using source-mask optimization (SMO) examples through pitch, we show how these mitigation strategies are effectively applied to through pitch use cases. We find NILS DOF improvements up to 65% with the low-n mask and optimized retargeting.

Contact holes are traditionally designed as squares or rectangles on the mask. The lithography tool acts as a low-pass filter of the spatial frequencies and is not able to image the corners. Therefore, the contact holes will appear as circles or ellipses on the wafer, even if they are square on the mask. Nevertheless, the presence of the designed corners imposes a limit to the center-to-center distance that can be obtained in certain contact hole configurations. Indeed, as the distance between contact holes is decreased, these corners are the first features to create violations in the Mask Rule Check (MRC). Rounded contact hole corners therefore give more design freedom. The recent introduction of multi-beam mask writing makes writing curvilinear shapes on actual masks practical, including contact holes with corner rounding. While the benefits to design freedom are obvious, there is a need for studies identifying the impact of this additional corner rounding parameter on the imaging properties. In this work, we investigate the imaging impact of corner rounding on pitch-40nm contact holes, exposed with an EUV scanner (ASML NXE:3400B at imec). The study aims to experimentally establish the difference in imaging quality introduced by the corner rounding. More specifically, we first characterize the pattern fidelity on the mask by means of mask CD-SEM images. Next, we assess the impact of corner rounding on imaging properties such as Local Critical Dimension Uniformity (LCDU), Mask Error Enhancement Factor (MEEF) and Exposure Latitude.

This paper extends the 2019 and 2020 symposium contributions clearly showing that (local) mask defects and non-local mask defects (NLMDs) act as triggers for increased stochastic failure probability on the EUV printed wafer. The present work focuses on anamorphic imaging at 0.55 NA, including horizontal and vertical pattern orientations, and comprises defocus conditions and line breaking as a second failure mechanism. Two roughness type NLMDs are studied: multilayer (ML) ripple relates to a non-fully planar coating of the ML mask stack. Mask absorber line-edge roughness is addressed as a second roughness type NLMD. The longer-term intent is to inspire defining limits to their impact, from the perspective that an increased mask contribution to stochastics of high-NA EUV lithography must be avoided.

State-of-the-art EUV exposure systems utilize EUV radiation around 13.52 nm with a full band spectrum extend- ing from 13.2 nm to 13.8 nm. The variation of the wavelength in this range modifies the diffraction angles with an impact on the image blur and non-telecentricity effects. Dispersion of the materials on the EUV mirrors and on the 3D mask introduce an additional sensitivity of the imaging characteristics to the exposure wavelength. We employed the simulation models of the Fraunhofer IISB lithography simulator Dr.LiTHO in combination with measured optical material data from PTB to quantify the resulting image impact and to differentiate between the identified contributors to polychromatic imaging effects.

With the transition of EUV lithography to high volume manufacturing, EUV mask metrology has become a critical requirement. At PSI, we are developing RESCAN, a lensless actinic microscope dedicated to EUV mask inspection. RESCAN is based on coherent diffraction imaging (CDI), a method that reconstructs the complex image of the sample through its diffraction spectrum measured with a CCD detector. While this approach can overcome the cons and limitations of traditional optical imaging systems, in CDI, the quality of the recorded diffraction data is crucial for the reliable reconstruction of a high-resolution image. Ultimately, the signal-to-noise ratio of the recorded diffraction data depends on several parameters, such as the reflectance of the sample, the quantum efficiency of the detector, its full well capacity, and the intensity of the illumination. This paper investigates the optimal photon flux for RESCAN and analyzes the relation between the image quality and the EUV illumination intensity for a CDI-based imaging tool dedicated to EUV mask inspection and review.

Extending 0.33NA EUV single patterning to pitch 28nm will enable metal layers cost-efficiency and significantly shorter process flow for N2 node. At the same time, EUV single patterning becomes very challenging in terms of stochastic defectivity and process window. In this paper, the lithographic performance of the M1 layer of an imec N3 (foundry N2 equivalent) random logic layout is evaluated by means of source mask optimization on these three mask candidates: a standard binary Ta-based absorber mask, a high extinction (high-k) absorber mask and a low-n attenuated phase-shift mask. The impact of mask tonality (bright field vs. dark field) and insertion of sub-resolution assist features on pattern fidelity and process window is evaluated.

As semiconductor technology became more integrated, the numerical aperture (NA) of extreme ultraviolet (EUV) exposure technology has increased from 0.33 to 0.55 to realize finer patterning. High NA EUVL aims to enable 8 nm half pitch resolution. As the NA increases and a smaller linewidth needs to be implemented, the mask 3D effects on patterning will become more severe. For mass production of EUV, it is essential to find an optimal absorber structure to alleviate serious problems such as mask 3D effects and shadowing effects. Among the various factors constituting the EUV photomask, phase shift and reflectance of the absorber are the most important factors that determine the image quality. Through computational simulation, the optimal structure was selected in consideration of phase shift and reflectance, and imaging performance and process latitude were calculated.

Alternatives to Ta-based absorbers are being considered for next generation lithography nodes to reduce 3D mask effects and to improve image modulation through phase interference. Low complex refractive index (n – ik) materials can provide phase shifting behavior at thicknesses less than those needed for conventional absorbers, essentially acting as attenuated phase shift mask (attPSM) films. Identifying attPSM absorber thickness and consequent phase requires determining optimum phase shift mask reflectance. Imaging with absorbers at high reflectance show better imaging performance. The absorber thickness is determined where the interference effects lead to high absorber reflectivity. Low refractive index (n) materials are therefore desired as candidate attPSM absorbers. Low – n material combinations identified using Wiener bounds and Effective media approximation (EMA) modelling are optimized for NILS and MEEF using absorber reflectivity on line-space and contact-holes patterns. Absorber candidates at optimum thickness for contact holes are compared with conventional Ta-based absorber using reflected nearfield intensity imaging.

Mask blank defect is one of the most important factors that degrades the image quality of extreme ultraviolet (EUV) lithography system, and further leads to a yield lose. In order to compensate the amplitude and phase distortions caused by the EUV mask blank defects, this paper proposes an advanced algorithm to optimize the mask absorber pattern based on genetic algorithm. First, a successive approximation correction method is used to roughly compensate the effect of mask blank defect. Then, an advanced genetic algorithm is proposed to obtain higher efficiency and compensation accuracy, which uses an adaptive coding strategy and a fitness function considering normalized image log slope of lithography image. For illustration, the proposed method is verified based on rectangular contact patterns and complex pattern with different defects. The aerial images of optimized masks are evaluated by a commercial lithography simulator. It will show that the proposed method can mitigate the impact of mask defects, and improve the fidelity of lithography print image. The simulation results also demonstrate the higher convergence efficiency and mask manufacturability can be guaranteed by the proposed method.