Mo/Si multilayers for EUV lithography by ion beam sputter deposition

Mo/Si multilayers for applications in extreme ultraviolet (EUV) lithography have been prepared on Si wafer substrates using ion beam deposition. The multilayers were characterised by transmission electron microscopy, secondary ion mass spectroscopy, atomic force microscopy, photoelectron spectroscopy, X-ray reflectometry at grazing incidence, and EUV-reflectivity measurements at nearly normal incidence. The surface and the interfaces of the multilayers are rather smooth with only small roughness. The material properties of the layers are characterised by some intermixing and silicide formation at the Mo-Si interfaces and a polycrystalline grain structure of the Mo layers, which is in agreement with prior studies. Appearance of multilayer diffraction spots, well-resolved Kiessig fringes and other diffraction evidence indicate very good coherence of the wave fields and in this manner a good reproducibility of the multilayer period of 6.7 nm. Normal incidence peak reflectivities of 64-65% in the EUV spectral range were routinely obtained at 13.4 nm wavelength. This reflectivity value and the formation of an EUV standing wave field are confirmed using photoelectron spectroscopy, and an application for defect particle analysis is proposed. The obtained results are discussed in comparison to literature data of multilayers prepared by other deposition techniques and considering new attempts of interface engineering.     

Interface engineering of short-period Ni/V multilayer X-ray mirrors

Low-energy ion-assisted magnetron sputter deposition has been used for the synthesis of highly reflective Ni/V multilayer soft X-ray mirrors. A low ion energy and a high ion-to-metal flux ratio were employed in order to stimulate the adatom mobility while minimizing ion-induced intermixing at the interfaces. An analytic model, based on the binary collision approximation, was used in order to gain insight into low-energy ion–surface interactions as a function of ion energy and ion-to-metal flux ratio. The model predicted a favorable region in the ion energy-flux parameter space where only surface atomic displacements are stimulated during growth of Ni and V for multilayers. For a series of Ni/V multilayer mirrors with multilayer periods about Λ = 1.2 nm, grown with a continuous ion assistance using energies in the range 7–36 eV and with ion-to-metal flux ratios Φ_Ni = 4.7 and Φ_V=20.9, specular and diffuse X-ray scattering analyses revealed that ion energies of 27–31 eV produced the best trade-off between reduced interfacial roughness and intermixing. However, it was also concluded that an interface mixing of about ± 1 atomic distance is unavoidable when a continuous flux of assisting ions is used.

To overcome this limitation, a sophisticated interface engineering technique was employed, where the first 0.3 nm of each layer was grown with a high-flux low-energy ion assistance and the remaining part was grown with a slightly higher ion energy. This method was demonstrated to largely eliminate the intermixing while maintaining the smoothening effect of ion assistance. Two Ni/V multilayer soft X-ray mirror structures, one with 500 periods designed for near-normal incidence and one 150 periods reflecting polarizer at the Brewster angle, were grown utilizing the interface engineering concept. Both the near-normal incidence reflectivity as well as polarizability were improved by a factor of 2 as compared to previously reported data for an X-ray energy of E = 511 eV.     

Effects of ion-assisted growth on the layer definition in Cr/Sc multilayers

Nano-structural evolution of layer morphology and interfacial roughness in Cr/Sc metal multilayers grown with ion assistance during magnetron sputter deposition has been investigated by high resolution transmission electron microscopy and hard X-ray reflectivity. Calculations based on a binary collision model predict an ion-assisted growth window for optimized Cr/Sc multilayer interface sharpness, within the ion energy range of 21 eV to 37 eV and an ion flux of ∼ 10 ions per deposited atom. Multilayers with nominal modulation periods in the range of 1.6 nm to 10.2 nm, grown with these conditions, exhibit a well-defined layer structure with an improved flattening and abruptness of the interfaces. It is shown that multilayers with a modulation period smaller than 3.4 nm have clear benefit from the reduced intermixing obtained by utilizing a two-stage ion energy modulation for each individual layer. The amorphization of Sc and Cr layers, below certain thicknesses, is found to be independent of the low energy ion-assistance. It is also shown that the Cr/Sc multilayers, containing periods less than ∼ 2 nm are ‘self healing’ i.e. they re-gain abrupt interfaces and flat layers after morphological disturbances during ion assisted growth. In comparison, multilayers grown without ion-assistance exhibited severe roughness and layer distortions.     

Grenzflächen‐optimierte Mo/Si Multischichten als Reflektoren für den EUV Spektralbereich. Interface‐optimized Mo/Si multilayers as reflectors for the EUV spectral range

EUV lithography is the most promising technique for the fabrication of semiconductor structures below 50 nm. This requires the use of reflecting multilayers as optical elements. These multilayers must have reflectances as high as possible since it determines the efficiency of the technique and therefore the throughput of a future chip fab. In this work we present investigations on the interface quality of Mo/Si multilayers which are prepared by magnetron sputter deposition. Starting from the two‐component Mo/Si system, that has mainly been optimized with respect to interface roughness, we show that interface interdiffusion can also be reduced by the introduction of tiny barrier layers. In pure Mo/Si multilayers particularly a low Ar sputter gas pressure is important to get smooth layers, whereas the interdiffusion can be reduced by the deposition of C and B₄C barrier layers on the individual interfaces. As result of our work, we have prepared Mo/Si multilayers with outstanding high reflectances: R_EUV = 70.1 % (λ = 13.3 nm, α = 1.5°), R_EUV = 71.4 % (λ = 12.5 nm, α = 22.5°).     

Layer-by-layer design method for multilayers with barrier layers: application to Si/Mo multilayers for extreme-ultraviolet lithography

A previous layer-by-layer multilayer design method [J. Opt. Soc. Am. A 19, 385 (2002)] is completed by adding the possibility of alternating layers with fixed thicknesses along with layers whose thicknesses are optimized for the largest possible reflectance at a desired wavelength. The previous algorithm did not allow for layers with fixed thicknesses. The current formalism is particularly suited for a multilayer design in which barrier layers of given thicknesses are used to prevent diffusion and/or reaction between the multilayer constituents. The design method is also useful both when intermixing zones develop at multilayer interfaces and when capping layers are used. The algorithm allows the design of multilayers with complex barrier layers with any number of layers of any optical constants. The optimization can be performed either for normal incidence or for nonnormal incidence with either s- or p-polarized radiation. The completed method provides a fast and accurate procedure for multilayer optimization regardless of the number of different materials used in the multilayer. The optimum layer thickness is determined by means of functions suitable for implementation in a computer code. The performance of the current algorithm is exemplified through the design of Si/Mo multilayers with intermixing layers or with barrier layers that are optimized for the largest reflectance at 13.4 nm. The use of specific barrier layers on each multilayer interface is also discussed.     

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.     

Improved temperature stability of Mo/Si multilayers by carbide based diffusion barriers through implantation of low energy CHx ions

To improve the thermal stability of Mo/Si multilayers, a novel method to form carbide based diffusion barriers, produced by the implantation of Si with CH_x⁺ ions, has been developed. The multilayers were grown by e-beam evaporation, while CH_x⁺ ions were implanted at the Mo/Si interfaces, using a Kaufman ion source with a Ne / CH₄ gas mixture. Energies were varied from 300 to 1000 eV. The growth as well as the implantation procedure were monitored by in situ X-ray reflectometry. Auger Electron Spectroscopy was used to characterize the surface composition before and after CH_x⁺ ion implantation. The shift of the Si LVV Auger peak revealed the formation of SiC. Ex situ X-ray reflectometry showed a thermal stability of both the reflectivity and the multilayer period up to 430 K.     

氢离子束对Mo-Si多层膜界面改性的研究

为了研究氢离子束轰击Mo-Si多层膜界面的情况，采用氢离子束（能量150eV）轰击Si表面，即Si与Mo之界面。再用Kr离子束溅射刻蚀，并用俄歇电子能谱（AES）分析。实验结果说明氢离子束对Si表面轰击能有效防止界面混杂效应（intermixingeffect）。进而说明这是制备软X射线多层膜反射镜过程中解决界面混杂问题的有效途径。     

High reflectivity multilayer for He-II radiation at 30.4 nm

SiC/Mg and B(4)C/Mo/Si multilayers were designed for He-II radiation at 30.4 nm. These multilayers were prepared by use of a direct current magnetron sputtering system and measured at the National Synchrotron Radiation Laboratory, China. The measured reflectivities were 38.0% for the SiC/Mg multilayer at an incident angle of 12 deg and 32.5% for the B(4)C/Mo/Si multilayer at 5 deg, respectively. A dual-function multilayer mirror was also designed by use of the aperiodic SiC/Mg multilayer. Annealing experiments were performed to investigate the thermal stability of the SiC/Mg multilayer. The interface of the SiC/Mg multilayer before and after annealing was studied by electron-induced x-ray emission spectra, which evidences the absence of thermal reaction products at the interfaces after annealing.     

磁控溅射法制备Mo/B4C软X射线多层膜

用DC和RF磁控溅射法制备出了波长小于 10nm波段的Mo/B4 C软X射线多层膜反射镜。掠入射X射线衍射仪的测量结果表明 ,磁控溅射法有很高的控制精度 ,制备出的Mo/B4 C软X射线多层膜周期结构非常好 ,表 (界 )面粗糙度非常小 ,约为 0 4nm。     

Performance and quality analysis of Mo-Si multilayers formed by ion-beam and magnetron sputtering for extreme ultraviolet lithography

Mo-Si multilayer structures were grown by ion-beam and magnetron sputtering to make high-performance mask blanks for practical use in extreme-ultraviolet (EUV) lithography. For ion-beam sputtering, the effect of using Ar or Xe as the sputtering gas, and the impact of the acceleration voltage of Ar or Xe ions on the EUV reflectivity of multilayers were evaluated. In the wavelength range of 12.5-14.5 nm, the peak EUV reflectivity was 60-63% for 40 Mo-Si bilayers grown by ion-beam sputtering, and 62-65% for those grown by magnetron sputtering. Transmission electron microscopy images of Mo-Si multilayers revealed interface layers between the Mo and Si layers. They were found to be composed of a mixture of Mo and Si and to be formed during sputtering. They had a thickness of 1.5-2.0 nm for the deposition sequence Mo-on-Si, and 0.5-1.0 nm for Si-on-Mo. In addition, they were 20-30% thicker for ion-beam sputtering than for magnetron sputtering. Calculations of the EUV reflectivity spectrum for 40 Mo-Si bilayers indicate that interface layer thickness plays a crucial role in determining the EUV performance of multilayers. Finally, an ion-implantation model was found to provide a better explanation of the mechanism of interface layer formation than a thermal-interdiffusion model.     

In-situ ellipsometric monitor with layer-by-layer analysis for precise thickness control of EUV multilayer optics

Ion beam sputtering fabrications of Mo/Si multilayers for soft X-ray mirrors were studied using an automatic null ellipsometer. The ellipsometric growth curves plotted on the complex plane showed island structure formation for every Mo layer grown on Si when deposition was performed with a 1400 V Ar ion beam. The ellipsometric growth curves indicated that the multilayers fabricated with 900 V ions had sharper and smoother interfaces compared to those fabricated with 1400 V ions. Quantitative layer-by-layer analysis showed that the Si layers deposited at 1400 V became optically isotropic as thin as 1 nm. These data depict the usefulness of our in-situ ellipsometer in controlling layer thickness and also optimizing the deposition condition to form homogeneous and optically isotropic layer structures.     

The structure, diffusion and phase formation in Mo/Si multilayers with stressed Mo layers

Processes of diffusion and silicide formation in stressed multilayers of Mo/Si, as a result of their isothermal annealing, were studied in this paper by methods of cross-sectional transmission electron microscopy and X-ray diffraction techniques. It was found that reaction with growth of molybdenum disilicide of reduced density takes place at Mo-on-MoSi₂ interfaces up to formation of ∼ 7 nm thick layers, due to annealing treatment within 350-400 °C temperature range. Silicon atoms were found to be the dominating diffusive components. As a result of Si atoms' diffusion from Si layer, sublayers of a somewhat lower density are being formed at MoSi₂-on-Si interfaces. Growth of molybdenum disilicide is accompanied by reduction of multilayer period. Activation energy of diffusion process (phase formation) makes up ∼ 2.2 eV. Influence of compressive stresses (that exist in Mo layers) on process of phase formation, both in as-deposited and in annealed samples, is discussed in this paper.     

Silicide formation by Ar+ ion bombardment of Pd/Si

Palladium films, 45 nm thick, evaporated on to Si(111) were irradiated to various doses with 78 keV Ar⁺ ions to promote silicide formation. Rutherford backscattering spectroscopy (RBS) shows that intermixing has occurred across the Pd/Si interface at room temperature. The mixing behaviour is increased with dose which coincides well with the theoretical model of cascade mixing. The absence of deep RBS tails for palladium and the small area of this for silicon spectra indicate that short-range mixing occurs. From the calculated damage profiles computed with TRIM code, the dominant diffusion species is found to be silicon atoms in the Pd/Si system. It is also found that the initial compound formed by Ar⁺ irradiation is Pd₂Si which increases with dose. At a dose of 1×10¹⁶ Ar⁺ cm^–2, a 48 nm thickness of Pd₂Si was formed by ion-beam mixing at room temperature.     

Characterization of sputtered nickel/carbon multilayers with soft-x-ray reflectivity measurements

Several sputtered Ni/C multilayer mirrors with periods between 3.8 and 6.0 nm were investigated with the HASYLAB reflectometer to determine the peak reflectivity and the internal structure of the multilayers. The enhanced reflectivity in the Bragg maximum, especially below 284 eV (carbon K edge), is interesting for practical applications. For both materials (Ni and C), the optical constants were previously obtained from angular-dependent reflectivity measurements. The layer thicknesses, interface roughnesses, and mean thickness errors of multilayer mirrors are obtained from least-squares fits of theoretical reflectivity curves. A model is presented to describe the influence of interface roughness on other parameters obtained from the analysis. All Ni/C multilayers, with Ar as the sputter gas, were produced in the low-pressure triode-assisted sputtering facility of Sincrotrone Trieste.     

Energy dependence of aluminum diffusion in α-Fe under ion irradiation

The process of diffusion in the film-substrate system irradiated by Ar⁺ ions of various energies is considered. Data on the ion-bombardment-induced variation of the composition of surface layers at the film-substrate interface are presented. Possible mechanisms of the mutual diffusion and anomalously deep intermixing of components caused by the ion implantation are analyzed.     

Structure and normal incidence soft-x-ray reflectivity of Ni-Nb/C amorphous multilayers

Multilayers for the water window region of the soft x rays have been prepared by pulsed laser ablation with amorphous Ni(50) Nb(50) and amorphous C. The structural characterization of the multilayers, period d = 2.41 nm, shows that the interfaces are sharp with a roughness of only 0.4 nm that is chemical, not morphological, in origin. The interface roughness was found to be uncorrelated in the direction normal to the plane of the film. The normal incidence soft-x-ray reflectivity of the multilayer at 4.85-nm wavelength is 0.06%, 1 order of magnitude lower than the theoretically predicted value. However, the resolution limit lambda/Dlambda of the multilayer was found to be 16.7, close to the theoretically predicted value.     

Determination of crystallization as a function of Mo layer thickness in Mo/Si multilayers

Mo/Si multilayer samples with different Mo layer thickness were deposited by electron beam evaporation, while Kr⁺ ions (300 eV) were used for polishing the Si layers. Crystallization as a function of the Mo layer thickness deposited was investigated by grazing incidence X-ray diffraction, giving information on the crystalline phases, average size and crystallite formation. Comparison of these parameters for the samples examined provided novel results, especially regarding the in-plane and in-depth average sizes of the crystallites. The most important result is that crystallization takes place already when a 1 nm thick Mo layer has been deposited. Moreover, the average in-plane size of the crystallites was found to be independent of the layer thickness, while the average in-depth size corresponded to the thickness of the Mo layer. Depositions consist of polished Si layers were found to give a larger amount of crystalline material compared to those consist of unpolished Si layers.     

Ar+ ion beam induced silicide formation mechanism at the Pd-Si interface

Evaporated palladium films of 45 nm thickness on Si(1 1 1) were irradiated using 78 keV Ar⁺ ions with doses in the range of 1×10¹⁵ to 1.5×10¹⁶ cm^–2 for the purpose of studying silicide formation. Rutherford backscattering analysis shows that intermixing has occurred across the Pd-Si interface at room temperature. The mixing behaviour increases with increasing dose of the bombarding ions, which agrees well with a theoretical model of isotropic cascade mixing for palladium, and radiation-enhanced diffusion associated with an interstitial mechanism for silicon.     

Characterization of thermally stable W/Ni multilayers by X-rays and cross-sectional transmission electron microscopy

W/Ni multilayer structures (MLS) composed of 5 and 10 bilayers, with composition W(15 Å)/Ni(55 Å), have been deposited on float glass substrate using ion-beam sputtering. X-ray reflectivity and wide-angle X-ray diffraction techniques have been used to study their interface characteristics, such as layer thickness, interface roughness and change in structural parameters. The fabricated MLS were found to be oriented along (111) of Ni having superlattice modulation perpendicular to the film plane. Thermal annealing studies on these multilayers showed that these were stable up to 500 °C. Cross-sectional transmission electron microscopy and selected area electron diffraction studies on as-deposited W/Ni MLS of 10 bilayers revealed well formed interfaces without any correlated roughness. The thicknesses of different layers were found to vary along the film thickness.