Multilayer Roughness Research Articles

Effects of extreme ultraviolet multilayer roughness on 2D patterning

Effects of extreme ultraviolet multilayer roughness on 2D patterning

Experimental study and modeling of extreme ultraviolet 4000 lines/mm diffraction gratings coated with periodic and aperiodic Al/Mo/SiC multilayers.

Multilayer coated diffraction gratings are crucial components for extreme ultraviolet (EUV) applications such as spectroscopy or spectro-imaging. However, for high groove density, the smoothening of the grating surface profile with multilayer deposition remains a limitation that requires further investigation. In this paper, we report on the design, characterization, and modeling of 4000 lines/mm diffraction gratings coated with periodic and aperiodic Al/Mo/SiC multilayers for EUV radiation. Two types of gratings with different groove depths are compared. Multilayer coatings were designed using a genetic algorithm to maximize the first-order diffraction efficiency in the 17-21 and 19-23nm wavelength ranges at normal incidence. Periodic and aperiodic multilayers with different numbers of layers were deposited by magnetron sputtering on the two types of fused silica gratings, and the grating groove profile evolution was measured by atomic force microscopy and cross-section transmission electron microscopy. The first-order diffraction efficiency was measured in the EUV at 5° incidence using monochromatic synchrotron radiation and modeled using the rigorous coupled-wave analysis method. The simulation models refined by using the Debye-Waller factor to account for the multilayer interfacial roughness show good agreement with experimental data. The results reported in this study will allow for designing efficient EUV multilayer gratings for high-resolution spectro-imaging instruments.

Improving superamphiphobicity by mimicking tree-branch topography

when a droplet impacts on a superhydrophobic structured surface below a certain impact velocity, the droplet can bounce off completely from the surface. However, above such velocity a fraction of the droplet will pin on the surface. Surfaces capable of repelling water droplets are ubiquitous in nature or have been artificially fabricated. However, as the surface tension of the liquid is reduced, the capability of the surface to remain non-wetting gets hindered. Despite progress in previous research, the understanding and development of superamphiphobic surface to impacting low surface tension droplets remains elusive. It is proposed that multi-layer re-entrant like roughness can further enhance the anti-wetting properties also for low surface tension fluids.In this work, we produce patterned conical micro-structures with lateral nano-sized roughness. Furthermore, the droplet impact experiments are conducted on various surfaces with variable surface tensions (27 mN/m - 72 mN/m) by using droplets with different Weber numbers (2–170).We show that conical microstructures with lateral roughness mimicking tree-branches provides a surface topology capable of absorbing the force exerted by the droplet during the impact which prevents the droplet from pinning on the surface at higher impact velocity even for low surface tension droplets. Our study has significance for understanding the liquid interaction mechanism with the surface during the impact process and for the associated surface design considerations.

Applying Membrane Distillation for the Recovery of Nitrate from Saline Water Using PVDF Membranes Modified as Superhydrophobic Membranes.

In this study, a flat sheet direct contact membrane distillation (DCMD) module was designed to eliminate nitrate from water. A polyvinylidene fluoride (PVDF) membrane was used in a DCMD process at an ambient pressure and at a temperature lower than the boiling point of water. The electrical conductivity of the feed containing nitrate increased, while the electrical conductivity of the permeate remained constant during the entire process. The results indicated that the nitrate ions failed to pass through the membrane and their concentration in the feed increased as pure water passed through the membrane. Consequently, the membrane was modified using TiO2 nanoparticles to make a hierarchical surface with multi-layer roughness on the micro/nanoscales. Furthermore, 1H,1H,2H,2H-Perfluorododecyltrichlorosilane (FTCS) was added to the modified surface to change its hydrophobic properties into superhydrophobic properties and to improve its performance. The results for both membranes were compared and reported on a pilot scale using MATLAB. In the experimental scale (a membrane surface area of 0.0014 m2, temperature of 77 °C, nitrate concentration of 0.9 g/Kg, and flow rate of 0.0032 Kg/s), the flux was 2.3 Kgm−2h−1. The simulation results of MATLAB using these data showed that for the removal of nitrate (with a concentration of 35 g/Kg) from the intake feed with a flow rate of 1 Kg/s and flux of 0.96 Kgm−2h−1, a membrane surface area of 0.5 m2 was needed.

Effects of bilayer number and thickness ratio on structure and properties of (Cr, N)-DLC/DLC multilayer films

(Cr, N)-DLC/DLC multilayer films with different bilayer numbers and thickness ratios were prepared on cemented carbides and silicon wafers by cathodic vacuum arc technology. Surface and cross-section morphologies of multilayer films were observed by scanning electron microscope. Roughness, hardness, adhesion strength, friction coefficient and wear rates of the deposited films were investigated by surface profiler, nanoindentor, scratch tester, and ball-on-disk tribometer, respectively. Alternating stress field from adjacent sublayers benefited the mechanical properties while macroparticles piercing through interfaces between sublayers provided passages for cracks propagation and deteriorated films' mechanical properties. As bilayer numbers varied from 10 to 20 average friction coefficient and roughness of multilayer films had the same changing trend and wear rates increased linearly from 2.45 × 10−7 mm3/Nm to 3.81 × 10−7 mm3/Nm. Improving proportion of (Cr, N)-DLC layer in multilayer films increased quantity of macroparticles, leading to the rise of multilayer films' roughness, average friction coefficient and wear rates.

Origin of perpendicular magnetic anisotropy in Co/Ni multilayers

We studied the variation in perpendicular magnetic anisotropy of (111) textured $\mathrm{Au}/N\ifmmode\times\else\texttimes\fi{}[\mathrm{Co}/\mathrm{Ni}]/\mathrm{Au}$ films as a function of the number of bilayer repeats $N$. The ferromagnetic resonance and superconducting quantum interference device magnetometer measurements show that the perpendicular magnetic anisotropy of Co/Ni multilayers first increases with $N$ for $N\ensuremath{\le}10$ and then moderately decreases for $N>10$. The model we propose reveals that the decrease of the anisotropy for $N<10$ is predominantly due to the reduction in the magnetoelastic and magnetocrystalline anisotropies. A moderate decrease in the perpendicular magnetic anisotropy for $N>10$ is due to the reduction in the magnetocrystalline and the surface anisotropies. To calculate the contribution of magnetoelastic anisotropy in the Co/Ni multilayers, in-plane and out-of-plane x-ray diffraction measurements are performed to determine the spacing between Co/Ni (111) and (220) planes. The magnetocrystalline bulk anisotropy is estimated from the difference in the perpendicular and parallel $g$ factors of Co/Ni multilayers that are measured using the in-plane and out-of-plane ferromagnetic resonance measurements. Transmission electron microscopy has been used to estimate the multilayer film roughness. These values are used to calculate the roughness-induced surface and magnetocrystalline anisotropy coefficients as a function of $N$.

Amplitude versus phase effects in extreme ultraviolet lithography mask scattering and imaging.

It is now well established that extreme ultraviolet (EUV) mask multilayer roughness leads to wafer-plane line-width roughness (LWR) in the lithography process. Analysis and modeling done to date has assumed, however, that the roughness leading to scatter is primarily a phase effect and that the amplitude can be ignored. Under this assumption, simple scattering measurements can be used to characterize the statistical properties of the mask roughness. Here, we explore the implications of this simplifying assumption by modeling the imaging impacts of the roughness amplitude component as a function of the balance between amplitude and phase induced scatter. In addition to model-based analysis, we also use an EUV microscope to compare experimental through focus data to modeling in order to assess the actual amount of amplitude roughness on a typical EUV multilayer mask. The results indicate that amplitude roughness accounts for less than 1% of the total scatter for typical EUV masks.

Background level in extreme-UV dark-field image for mask blank inspection

The background level (BGL) in an extreme-UV (EUV) dark-field image for EUV mask blank inspection was analyzed. The background refers to the light scattered by the multilayer roughness of the mask blank, and the variation in the BGL is attributed to random and fixed factors. We found that the BGL variation due to the fixed factor could be successfully reproduced by simulation using surface information obtained by atomic force microscope measurement with the assumption of a multilayer roughness structure. This means that the fixed factor contributing to the BGL variation was found to be speckle noise due to the multilayer roughness. Moreover, this reproduction procedure was found to be valuable for estimating the multilayer roughness structure up to several layers below the surface.

Broader view on extreme ultraviolet masks: adding complementary imaging modes to the SHARP microscope

The authors are expanding the capabilities of the SHARP microscope by implementing complementary imaging modes. SHARP (the SEMATECH High-NA Actinic Reticle Review Project) is an actinic, synchrotron-based microscope dedicated to extreme ultraviolet photomask research. SHARP’s programmable Fourier synthesis illuminator and its use of Fresnel zoneplate lenses as imaging optics provide a versatile framework, facilitating the implementation of diverse modes beyond conventional imaging. In addition to SHARP’s set of standard zoneplates, we have created more than 100 zoneplates for complementary imaging modes, all designed to extract additional information from photomasks, to improve navigation, and to enhance defect detection. More than 50 new zoneplates are installed in the tool; the remaining lenses are currently in production. We discuss the design and fabrication of zoneplates for complementary imaging modes and present image data, obtained using Zernike phase contrast and different implementations of differential interference contrast (DIC). First results show that Zernike phase contrast can significantly increase the signal from phase defects in SHARP image data, thus improving the sensitivity of the microscope. DIC is effective on a variety of features, including phase defects and intensity speckle from substrate and multilayer roughness. The additional imaging modes are now available to users of the SHARP microscope.

Chemical control of polyelectrolyte film properties for an effective cardiovascular implants endothelialization

The aim of this study was to improve properties of blood contacting materials such as polyurethane, in a form of intelligent, self-organizing and self-controlling coatings, which allow the selective mobilization and colonization of the endothelial cells on their surface. The prepared multilayer polyelectrolyte scaffolds were cross-linked chemically by EDC/NHS reagents in order to control their physicochemical properties and thus improving potential to endothelialization. Four types of coatings, i.e. non-cross-linked, cross-linked by 260mM, 400mM and 800mM EDC reagent, were investigated. Their comparison was performed based on the results of the surface topography measurements using Atomic Force Microscopy (AFM), cellular morphology and proliferation analysis using Confocal Laser Scanning Microscopy (CLSM) and the mechanical properties examinations.The optimal multilayer rigidity and surface roughness parameters were found for an effective control of the endothelial cells growth. Surface topography analysis indicated an increase in the coating’s roughness due to application of higher EDC cross-linker concentrations. Mechanical studies revealed that cross-linking caused a significant increase in the hardness and elastic modulus. The results from the cellular experiments allowed the conformation that 400mM cross-linked PLL/HA films possess desired properties.

Modeling of LbL multilayers with controlled thickness, roughness, and specific surface area

We present computer simulation results of the layer by layer self-assembling process of colloidal particles. We have generated five multilayer structures of monodisperse spherical particles according to a generalized model of random sequential adsorption of hard spheres. The multilayers, each created at a different single-layer surface coverage, are of similar thickness. We have compared the transparency of the five multilayers and the structure of their outer layers in terms of the two-dimensional pair-correlation function. We have analyzed the variation of multilayer thickness with the number of adsorbed layers. We have also calculated the root-mean-square roughness of the multilayers as a function of the number of adsorption cycles. Finally, we have determined the specific surface area of the porous films as a function of the distance from the solid substrate. Our results suggest that in the limit of low porosity the multilayer transparency decreases exponentially with its porosity. The multilayer thickness is directly proportional to the number of adsorption cycles. The average single-layer thickness grows asymptotically with the single-layer coverage. We have also found that with the number of adsorbed layers the multilayer roughness increases to an asymptotic value. We have observed oscillatory variations of the multilayer specific surface area, decaying exponentially with the distance from the substrate. The decay length of the oscillation increases exponentially with the surface coverage. We have also determined the particle layer interpenetration for each multilayer and we have found that it decreases exponentially with the increase of the coverage. Our results suggest that all the film characteristics strongly depend on the method of its preparation and can be controlled by manipulating the single-layer surface coverage or deposition time. The results can be useful for efficient designing multilayers with desired properties.

Simulation study of cleaning induced extreme ultraviolet reflectivity loss mechanisms on mask blanks

It is widely recognized in the semiconductor industry that getting to defect-free extreme ultraviolet (EUV) mask blanks is critical in achieving high volume chip manufacturing yield beyond the 22 nm half-pitch node. Finished mask blanks are normally subjected to a cleaning process to get rid of the loosely adhered particles on the top. It is important that this cleaning process does not degrade the properties of the multilayer blank or introduce additional particles or pits during the process. However, standard cleaning processes used to clean multilayer blanks can result in EUV reflectivity loss, loss of uniformity in reflectivity, increased roughness, and add pits and particles on mask blanks. The standard cleaning process consists of multiple steps, each of which may cause the oxidation of the ruthenium capping layer, as well as the underlying bilayers, etching of the multilayer stack, and increased roughness of the bilayers, thus leading to a loss in EUV reflectivity. It is a challenging task to experimentally correlate the processing steps to the resulting damage and to quantify the reflectivity loss. Further, due to the high cost of materials we have not been able to perform extensive experiments to determine the root cause of problems. In this work, we have combined mask blank cleaning using standard processes, x-ray photoelectron spectroscopy, transmission electron microscope cross section, and atomic force microscope studies with simulations to quantify the impact of the multilayer oxidation, etching, and roughness on the EUV reflectivity loss and mask blank degradation.

Mask roughness challenges in extreme ultraviolet mask development

Despite significant progress in the commercialization of extreme ultraviolet (EUV) lithography, many challenges remain. Although availability of a reliable high power source is arguably the most daunting of these challenges, important mask issues are also of major concern. The issue of EUV phase roughness that can arise from either multilayer or capping layer roughness has recently become of increasing concern. The problem with mask phase roughness is that it couples to image plane speckle and thus line-edge roughness (LER). The coupling; however, depends on many factors including roughness magnitude, roughness correlation length, illumination partial coherence, aberrations and defocus, and numerical aperture. Analysis shows that only on the order of 50 pm multilayer roughness may be tolerable at the 22 nm half-pitch node. The analysis; however, also shows that the difficulty does not scale with future node reductions. Moreover, it is found that ruthenium is a particularly bad choice for capping layer from the perspective of phase roughness and that cleaning damage in such a multilayer could lead to unacceptable image-plane LER.

Evolution of the roughness of amorphous quartz surfaces and Cr/Sc multilayer structures upon exposure to ion-beam etching

The effect of ion-beam etching on surfaces of Cr/Sc multilayer structures and amorphous quartz roughness is studied. The dependence of roughness on the depth of etching, incidence angle, and ion energy is considered. Optimal parameters of etching which provide minimal evolution of surface roughness are presented.

Polymer/Colloid Surface Micromachining: Micropatterning of Hybrid Multilayers

Fabrication of multicomponent patterned films comprising polymer/nanoparticle multilayers using conventional lithography and bottom-up layer-by-layer nanofabrication techniques is described. The work is motivated by the potential to extend polymer surface micromachining capabilities toward construction of integrated systems by connecting discrete domains of active materials containing functional nanoparticles. Modified surfaces illustrate tunability of the physical (thickness, roughness, 3D structures) and chemical (inorganic/organic material combinations) properties of the nanocomposite micropatterns. Intriguing nanoscale phenomena were observed for the structures when the order of material deposition was changed; the final multilayer thickness and surface roughness and mechanical integrity of the patterns were found to be interdependent and related to the roughness of layers deposited earlier in the process.

Thermally evaporated Cu–Co top spin valve with random exchange bias

A cobalt-copper top spin valve was prepared by thermal evaporation of a stack of ferromagnetic thin films separated by thin layers of the diamagnetic metal, with a cap layer containing an antiferromagnetic (AFM) exchange-biasing material. A nonconventional top AFM layer was used, in order to optimize the multilayer roughness and to avoid electrical interference with metallic layers; it consists of a composite material easily processed by means of optical lithography, basically a polymeric matrix composite with a dispersion of nickel oxide microparticles. Magnetization and magnetoresistance measurements were performed from 4to300K. The measurements of both quantities indicate random pinning action of the top AFM layer, resulting in a small exchange-bias field and in asymmetric magnetization and magnetoresistance curves. A simple model explains the observed physical effects.

Modelling self-assembling of colloid particles in multilayered structures

Simulations of particle multilayer build-up in the layer by layer (LbL) self-assembling processes have been performed according to the generalized random sequential adsorption (RSA) scheme. The first (precursor) layer having an arbitrary coverage of adsorption centers was generated using the standard RSA scheme pertinent to homogeneous surface. Formation of the consecutive layers (up to 20) was simulated by assuming short-range interaction potentials for two kinds of particles of equal size. Interaction of two particles of different kind resulted in irreversible and localized adsorption upon their contact, whereas particles of the same kind were assumed to interact via the hard potential (no adsorption possible). Using this algorithm theoretical simulations were performed aimed at determining the particle volume fraction as a function of the distance from the interface, as well as the multilayer film roughness and thickness as a function of the number of layers. The simulations revealed that particle concentration distribution in the film was more uniform for low precursor layer density than for higher density, where well-defined layers of closely packed particles appeared. On the other hand, the roughness of the film was the lowest at the highest precursor layer density. It was also predicted theoretically that for low precursor layer density the film thickness increased with the number of layers in a non-linear way. However, for high precursor layer density, the film thickness increased linearly with the number of layers and the average layer thickness was equal to 1.58 of the particle radius, which is close to the closely packed hexagonal layer thickness equal to 1.73. It was concluded by analysing the existing data for colloid particles and polyelectrolytes that the theoretical results can be effectively exploited for interpretation of the LbL processes involving colloid particles and molecular species like polymers or proteins.

Interface roughness in Mo/Si multilayers

In this work we present a study of surface roughness development at the molybdenum-on-silicon and silicon-on-molybdenum interfaces in Mo/Si multilayers as employed in Extreme UV lithography. Thin Mo/Si multilayers, with layer thicknesses of 3–5 nm, were deposited using electron beam evaporation. The effect of ion treatment on the surface roughness was studied by X-ray reflectometry and transmission electron microscopy. Without ion treatment we observed build up of correlated roughness. The roughness development is shown here to depend strongly on the thickness of the crystalline Mo layer. Independent of the Mo ratio in a period, we show that a minimal amount of ion treatment is required to smoothen the multilayer roughness, which is also confirmed by EUV reflectivity measurements. At high ion energies the layers become smoother due to a larger ion penetration depth. The higher penetration depth is also shown to initiate additional interdiffusion and structural changes at buried interfaces.

Effects of multilayer mask roughness on extreme ultraviolet lithography

Effects of multilayer roughness are explored based on rigorous electromagnetic simulation with TEMPEST and aerial image calculation with SPLAT. The TEMPEST simulation on small roughness features using an average material properties technique is validated by comparison with the simulation using finer gridding and approximate analytical methods. Simulations for rms roughness from 0 to 1.4 nm show a reflectivity reduction, a phase shift, and a polarization dependence. Midsize Gaussian defects are shown to be slightly less printable in the presence of roughness. For extreme ultraviolet lithography systems with roughness rms of 0.15 nm, the above effects are very second order.

Extreme-ultraviolet multilayer mirrors deposited usingradio-frequency-magnetron sputtering: the influence of self-biasvoltage on reflectivity and roughness

Mirrors for the imaging optics of extreme-ultraviolet (EUV)lithography systems require optimal reflectivity. In this paper, results on Mo/Si multilayers for EUV radiation are reported. Themultilayers were deposited using radio-frequency-magnetronsputtering; during deposition the self-bias voltages were measured, and the influence of their variation on the multilayer reflectivityand roughness was tested. The samples were characterized usingsmall-grazing-angle x-ray reflectivity and their performance wasevaluated by measuring the EUV reflectivity at near-normalincidence. The results show that high reflectivity (>67%) andlow roughness values (<2.5 Å) can be obtained, depending on thestability of the self-bias voltage during deposition.