Influence of barrier interlayers on the performance of Mo/Be multilayer mirrors for next-generation EUV lithography

M. V. Svechnikov; N. I. Chkhalo; S. A. Gusev; A. N. Nechay; D. E. Pariev; A. E. Pestov; V. N. Polkovnikov; D. A. Tatarskiy; N. N. Salashchenko; F. Schäfers; M. G. Sertsu; A. Sokolov; Y. A. Vainer; M. V. Zorina

doi:10.1364/OE.26.033718

1. Introduction

For 25 years, one of the most important applications of multilayer reflective nanostructures has been photolithography in the extreme ultraviolet range (EUVL) [1,2]. The choice of operational wavelength for a particular technology is dependent on the possibility of creating optical elements with the required performance and spatial and spectral resolution, and the availability of effective radiation sources in this spectral range. In fact, only the two most promising spectral ranges of EUVL are considered to have industrial potential: a “strontium-beryllium window” (10.5 to 12.4 nm) and a “silicon window” (12.5 to 14 nm) [3–5]. The maximum reflectance achieved by a Mo/Be multilayer mirror (MLM) was 70.2% at a wavelength of 11.34 nm, and the spectral width was 0.27 nm [3,6]. The maximum reflectance of an Mo/Si-based MLM is 70.15% at 13.5 nm and 70.5% at 13.3 nm, with a spectral width of 0.53–0.56 nm [7]. In recent years, studies of Mo/Si have dealt primarily with growth processes and the formation of transition regions [8–10], the engineering of interfaces to reduce the interdiffusion of materials [11], and methods of increasing resistance to heat and oxidation [12–14], and these have not led to an increase in the reflection coefficient. Progress made in recent years in terms of increasing EUVL productivity is associated with an increase in the conversion factor of laser radiation energy to EUV radiation and an increase in the power of laser systems [15].

Since the beginning of this century, when the final choice between the “beryllium” and “silicon” ranges of EUVL technologies was made in favor of silicon, the synthesis and study of MLMs based on Be has virtually ceased. The strict sanitary requirements for the equipment used when working with Be have also played a significant role in this stagnation. However, Be is a unique, transparent material in the EUV region at wavelengths >11.1 nm, and is even more transparent than Si in the vicinity of 13.5 nm, according to [16]. Hence, the potential of this material as a component of highly reflective coatings for lithography is still far from being exhausted [17]. Mo/Be reflective coatings are also demanded in space telescopes for solar astronomy, since wavelength region 11.1–12.4 nm, containing emission lines of solar corona, is still uncovered by other multilayers. Mo/Be is also a good alternative to Mo/Si coatings for imaging at solar lines near 13.2 nm due to narrower reflective band and, consequently, higher spectral resolution [18].

In [19], an analysis is carried out of the prospects of technologies with operating wavelengths of 6.7, 10.8 and 11.2 nm. The authors conclude that 11.2 nm technology with Be-based mirrors still shows promise as branch of modern EUVL technology, as it has a much higher performance than the 6.7 nm technology, and is a “cleaner” source than the 13.5 nm with comparable performance.

Prospects for the use of the 11.2 nm EUVL wavelength have increased further in connection with work on maskless X-ray lithography [20,21], since the cost of the equipment and its use is a more important factor in small-lot production than the productivity of the lithographic process. A source based on xenon in the 11 nm region has a maximum conversion efficiency that is close to that of a tin source [19], and is much cheaper and easier to operate. Thus, the development, application and optimization of reflective multilayer coatings in the vicinity of the K-edge of Be absorption is currently of considerable interest to researchers.

In earlier work [22] it was found that introducing the barrier layer to Mo-on-Be boundary of Mo/Be MLM can change the interface quality. In this paper, we demonstrate the effect of barrier layers on the reflection coefficient and the quality of the interfaces in Mo/Be MLM. For the materials of the barrier layers, we chose C and B₄C, which have been shown to be efficient in Mo/Si mirrors at 13.5 nm [7,11], and also Si, which has been previously used as an amorphization layer amorphization layer in Be/Al structures designed for a wavelength of 17.1 nm [23]. We note that C and B₄C are relatively transparent, while Si shows strong absorption at a wavelength of 11.2 nm. We therefore expect a negative impact in terms of performance from the use of an Si interlayer at this working wavelength. Nevertheless, this still supports the current interest in studying the influence of thin Si layers on the quality of the interfaces in Mo/Be-based MLMs.

2. Experimental overview

Be-containing MLMs were deposited in a specially certified laboratory, since beryllium is a highly toxic material. Beryllium dust constitutes a threat to human health, and its effects have an accumulative nature. As far as deposited MLMs do not produce small Be particles that can be breathed in, according to health and safety standards, the storage, research and long-term operation of Be-containing MLMs are not harmful to human health, and these activities do not require special precautions [24].

For this study, we synthesized four types of MLM, as shown schematically in Fig. 1: Mo/Be, Mo/Be/B₄C, Mo/Be/C and Mo/Be/Si. The names of the structures reflect the sequence of materials in the order of deposition, i.e., from the substrate to the surface. All barrier layers were deposited on top of the Be layer. Films were deposited on a Si substrate with a microroughness of less than 0.2 nm (roughness was integrated over a frame of 2 × 2 μm, and was captured with an atomic-force microscope). The sample size was approximately 20 × 20 mm, and the substrate thickness was 0.5 mm. The number of periods was 110. The approximate layer thicknesses were 2–2.5 nm for Mo and 3–3.5 nm for Be, and the thickness of the interlayers was 0.4–0.5 nm. The thickness of the period for all structures was about 5.8 nm. More precise values for the parameters of each sample were found via combined X-ray and EUV data reconstruction, and the results are given below. No special protective capping layer was deposited on the top of the MLM.

Fig. 1 Types of Mo/Be-based structures investigated; the number of periods in each MLM was 110.

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MLM deposition was carried out in a DC magnetron sputtering facility, pumped out to a residual pressure of 4 × 10⁻⁵ Pa. The working pressure of argon during the deposition was 0.11 Pa, and the chemical purity of Ar was about 99.99%. The target materials were disks of diameter 150 mm and thickness 5 mm. The process utilized 270 W of power for the Be target, 160 W for the Mo target and 150 W for the B₄C, C and Si targets.

Characterization of the layered structure at a wavelength of 0.154 nm was carried out using a PANalytical X’Pert PRO diffractometer, equipped with a four-crystal Ge (220) monochromator. The dynamic range achieved in these reflectivity measurements extended down to 10⁻⁶. Measurements near the target wavelengths within the EUV range (11.2 nm) were made using two types of equipment. The first of these was a laboratory reflectometer with a Be-X-ray tube as a radiation source; monochromatization of the radiation was carried out using a grazing incidence grating monochromator, which provided a spectral width of the probing beam of δλ = 0.03 nm (λ/δλ≈380). The reflectometer is described in detail in [25]. Precise reflectivity measurements in the EUV region were carried out using at-wavelength metrology equipment, with an 11-axis reflectometer end station on the optics beamline of a BESSY-II synchrotron radiation source [26,27]. The accuracies of the measurements were ± 0.02% on the wavelength scale and 0.02° on an angular scale, and the spectral impurity of the incident radiation was smaller than 0.01%. The size of the incident beam in this experiment was about 0.6 mm × 0.25 mm (width × height). The active area of the detector of 4 × 4 mm² was sufficient to receive specular reflection and most of the scattered part of reflected beam.

The roughness of the hidden interfaces was compared using measurements of the diffuse scattering of CuK_α radiation on the same laboratory diffractometer. Scans were obtained using a detector (2θ scans) at θ₀ = θ_bragg and rocking curves were obtained under conditions of quasi-Bragg amplification of the scattered radiation, $d (\sin θ_{0} + \sin θ) = m λ$ , where θ₀ and θ are the grazing angles of the probing and scattered radiation respectively.

The surface roughness of the samples and substrates was studied using an atomic-force microscope (AFM) stand on the base of an NTEGRA Prima NT-MDT [28].

Finally, TEM images were obtained for the overall characterization of the samples. Cross sections were prepared as lamellas using double-beam Quanta 3D FEG equipment with a focused ion beam (FIB) source. This method is described, for example, in [29]. FIB-fabricated lamellas were also polished using a low-energy (~300 eV) Ar⁺ source to reduce the thickness of the damaged layers. TEM measurements were made with a LIBRA 200 MC high-resolution transmission electron microscope. These images have become the main source of information on the relative quality of various interfaces within the same structure, since although the reflection and scattering of X-ray radiation allows for the quantitative characterization of interlayer areas, it does not give information on which of the interfaces is better or worse [30,31].

3. Brief theoretical background

3.1 Radiation in layered structures

Use Calculation of the field of a plane wave in a one-dimensional piecewise layered medium with sharp boundaries is an exactly solvable problem. This field can be recalculated from one boundary to another within the framework of recurrence relations [32,33]:

\begin{array}{l} r (z_{j}) = \frac{r_{j}^{F} + r (z_{j + 1}) \exp (2 i χ_{j + 1} l_{j + 1})}{1 + r_{j}^{F} r (z_{j + 1}) \exp (2 i χ_{j + 1} l_{j + 1})}, j = 0.. N, \\ r (z_{N + 1}) = 0, \\ t (z_{j}) = \frac{t (z_{j + 1}) t_{j}^{F} \exp (i χ_{j + 1} l_{j + 1})}{1 + r_{j}^{F} r (z_{j + 1}) \exp (2 i χ_{j + 1} l_{j + 1})}, j = 0.. N, \\ t (z_{N + 1}) = 1, \end{array}

where

r (z_{j})

and

t (z_{j})

are the complex reflection and transmission coefficients of a structure lying below the j-th boundary. Likewise,

r (z_{0})

and

t (z_{0})

are the reflection and transmission of radiation for the structure as a whole. The z-components of the wave number and the thickness of the layers appear in the exponent.

r_{j}^{F}

and

t_{j}^{F}

are the Fresnel coefficients of reflection and transmission of a wave through the material interface.

When calculating the specular reflection, we took into account the presence of roughness and mixing of materials at the boundaries in two ways: for faster initial fitting of the model to the experimental data, we used damping factors for the Fresnel coefficients; to refine the result, the structure was divided into smaller layers with thickness ~1 Å, and the transition layers were taken into account in the form of smooth transition functions. The form of the transition layer was not fixed, and was sought as a linear combination of a predetermined set of dependencies. This extended model is described in detail in [31].

3.2 Scattered radiation

The scattering of probing radiation due to the roughness of the interface can be calculated within the framework of perturbation theory. The most widely used approach is the distorted-wave Born approximation (DWBA), in which the scattered radiation is represented as an expansion in plane waves, and the scattering amplitude is viewed as a series of perturbations of the permittivity (or “scattering order”). A small parameter in this case is the relative perturbation of the permittivity $| \frac{Δ ε}{ε} |$ . In most papers, the DWBA is further simplified: the unperturbed wave field in a vacuum is replaced by the analytical continuation of the unperturbed field inside the sample [34–36]. An alternative approximation is the application of so-called perturbation theory to the height of the roughness [37]. Both approaches are valid for small values of roughness, but the specific conditions necessary for their applicability differ. A comparison between the above approximations and the exact DWBA is also given in [37]. When calculating the scattering from MLM, it is also possible to choose between these approaches [38,39].

In [39], strong differences are illustrated between the interference effects of scattering in MLMs with total, partial and zero in-depth interlayer correlation of roughness. For the case of a semi-infinite structure with two layers in a period and an identical roughness spectrum for all boundaries, the author gives the following formula for the scattering intensity in analytical form:

Φ_{c} (θ, φ) \approx \frac{k^{4} {| ε_{1} - ε_{2} |}^{2}}{2 \sin θ_{0}} {| ψ (0, θ_{0}) ψ (0, θ) |}^{2} P S D_{2 D} (\vec{ν}) \times {| \frac{ψ (0, θ_{0}) ψ (0, θ) - ψ (Γ d, θ_{0}) ψ (Γ d, θ)}{ψ (0, θ_{0}) ψ (0, θ) - ψ (d, θ_{0}) ψ (d, θ)} |}^{2}

Φ_{u} (θ, φ) \approx \frac{k^{4} {| ε_{1} - ε_{2} |}^{2}}{2 \sin θ_{0}} {| ψ (0, θ_{0}) ψ (0, θ) |}^{2} P S D_{2 D} (\vec{ν}) \times \frac{{| ψ (0, θ_{0}) ψ (0, θ) |}^{2} + {| ψ (Γ d, θ_{0}) ψ (Γ d, θ) |}^{2}}{{| ψ (0, θ_{0}) ψ (0, θ) |}^{2} - {| ψ (d, θ_{0}) ψ (d, θ) |}^{2}}

where

Φ_{c} (θ, φ)

and

Φ_{u} (θ, φ)

are two-dimensional scattering indicatrices for completely correlated and completely uncorrelated roughness;

ψ (z, θ_{0})

is the field amplitude at coordinate z where the probing radiation falls at angle θ₀;

P S D_{2 D} (\vec{ν})

is the power spectral density of the roughness at all interfaces; d is the MLM period; and Γ is the thickness factor, i.e. the ratio of the thickness of the first layer to the period. The coefficient of proportionality between

Φ (θ, φ)

and

P S D_{2 D} (\vec{ν})

is the electrodynamic factor, which depends on the structure, angles of incidence and scattering radiation.

In this paper, we did not model the scattering of X-rays by the MLM; however, we did use the interference enhancement effect of the scattered radiation when the quasi-Bragg condition $d (\sin θ_{0} + \sin θ) = m λ$ was satisfied, where θ₀ and θ are the grazing angles of the probing and scattered radiation, respectively. While measuring the rocking curve, the sum of the angles θ₀ + θ = 2θ_bragg was fixed, and corresponded to the first Bragg peak. Scans were also carried out with the detector (2θ scans) at a fixed angle of θ₀ = θ_bragg.

Under conditions of quasi-Bragg resonance (which is possible in the presence of in-depth correlation of roughness), spatial frequencies that are completely inherited throughout the entire structure will give much stronger scattering than in the non-resonant case. Since the period of the MLM studied was about 5.8 nm, and the lateral size of the observed roughness was 0.22–3 μm, it can be assumed that all of the “long” features of the relief are repeated in each layer. A higher scattering intensity also allows further advances into the region of high spatial frequencies, which is especially important when using a laboratory diffractometer.

4. Experimental results

4.1 Reflectometry

The main sources of information about the MLM were grazing incidence X-ray reflectometry (GIXR) at the CuK_α wavelength and extreme ultraviolet reflectometry (EUVR) at a wavelength near 11.4 nm. The reflectivity measurements were done less than in 2 weeks after MLMs synthesis. Numerical fitting of the model was performed on three curves simultaneously: angular at 0.154 nm, angular in the vicinity of 11.4 nm and spectral at a grazing angle of 88°. The results of fitting are given in Table 1, and graphs illustrating the correspondence of the calculations to the experimental data are shown in Fig. 2. For fitting purposes in MLM model we added ~1 nm of adhesive and oxidized layer to top of each structure; the thickness and density of this layer was also fitted.

Table 1. Main characteristics of the samples after fitting, where <d> is the average period, and <h(M)> is the film thickness averaged over all periods

View Table

Fig. 2 Experimental (red) and fitted (blue) reflectometric curves for the samples under study. Two angular dependences and one spectral dependence are presented for each sample. The main parameters of the models are given in Table 1.

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Be-on-Mo boundary of is of the greatest interest due to it presents in all the samples under study and has high optical contrast in both hard X-ray and EUV ranges. Due to its narrowness, Be-on-Mo boundary largely determines EUV reflectance. From many fits we can estimate the error bars for the σ_Be-on-Mo restored value as ± 0.02 nm. From Table 1 we can establish the ratio of effective Be-on-Mo interface widths: σ(Mo/Be/Si): σ(Mo/Be): σ(Mo/Be/C): σ(Mo/Be/B₄C) = 1: 1.18: 1.4: 1.78.

The maximum reflectance achieved for Mo/Be MLM was 70.25% at a wavelength of 11.28 nm, with a grazing angle of 84° (see the angular curve in Fig. 2(b)); thus, the highest value recorded 20 years ago [3,6] was reached.

4.2 X-ray scattering

The widths of the transition layers, which were found by numerical modeling of the reflectometric curves in the previous section, are “effective” quantities, in the sense that they are not separated into diffuse and scattering components. The diffuse part of the transition layer is due to the mutual penetration of the materials of the layers as a result of mechanical mixing during sputtering; this mixing occurs as a result of thermal diffusion and chemical interaction. The scattering part is the interface roughness, inherited from the underlying layers and acquired as a result of the crystallization of materials and non-uniform surface diffusion of the deposited atoms. The scattering of radiation can be used to distinguish these components, at least for a sufficiently large lateral scale of roughness.

Figure 3(a) shows the rocking curves for all investigated MLM. The scattering is normalized to the intensity of the probing beam. The opening angle between the source and the detector is fixed, and corresponds to the first Bragg peak; the sample is tilted from the specular position by {−0.5°, 0.5°}. The range of rocking angle, from −0.1° to 0.1°, should be excluded from consideration, since strong hardware distortions arise due to the proximity to the specular direction and the imperfections of the samples (and in particular, the arbitrary drift of the period in depth in the Mo/Be and Mo/Be/B₄C samples).

Fig. 3 (a) Rocking curves near the specular direction (the first Bragg peak); and (b) the same curves, normalized to the Mo/Be rocking curve. The mutual position of the source and the detector is fixed and corresponds to the first Bragg peak for each sample (θ_bragg = 0.8°–0.815°).

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Figure 3(b) shows the rocking curves for samples with interlayers divided by the curve of the Mo/Be structure. For |θ_sample|>0.1°, the Mo/Be/Si sample shows scattering that is 10%–20% lower than for the pure Mo/Be MLM. Mo/Be/C gives 25% higher scattering at small angles, but at |θ_sample|>0.4°, the scattering intensity is comparable to that in Mo/Be. Mo/Be/B₄C gives scattering that is equal to or greater than that of Mo/Be/C. The rocking curve for the sample within the range {−0.5°, 0.5°} allows us to observe scattering by roughness with a spatial frequency ν<1.5 μm⁻¹. The lower bound |θ_sample|>0.1° means that the minimum observable spatial frequency of the relief is ν≈0.3 μm⁻¹.

Thus, the main conclusion that can be drawn from the rocking curves is that when adding Si interlayers, a decrease is shown in the roughness of Mo/Be-based mirrors over the entire frequency range 0.3<ν<1.5 μm⁻¹. When adding C and B₄C interlayers, the roughness is increased, at least in the range 0.3<ν<1.3 μm⁻¹.

In addition to rocking curves, we obtained detector scans at θ₀ = θ_bragg. The results are shown in Figs. 4(a) and 4(b). In accordance with the condition $d (\sin θ_{0} + \sin θ) = m λ$ , if the probing radiation falls within the first Bragg peak (i.e. $d (\sin θ_{0} + \sin θ_{0}) = λ$ , θ = θ₀), then the nearest quasi-Bragg resonance will be in the direction θ = 3θ₀ (as $d (\sin θ_{0} + \sin θ_{0}) = λ$ ⇒ $d (\sin θ_{0} + \sin 3 θ_{0}) = 2 λ$ ). A path difference of 2λ means that the scattering amplification in this case is due to the presence of a second spatial harmonic in the direction of the depth of the structure.

Fig. 4 (a) Detector scan in the vicinity of the first Bragg peak; and (b) the region of quasi-Bragg resonance on a linear scale.

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Figure 4(a) represents scans of all samples with an origin of coordinates in the direction of specular reflection. The scattering is normalized to the intensity of the probing beam. The region of quasi-Bragg amplification of the scattered radiation is presented in more detail on a linear scale in Fig. 4(b). The difference in the position of the peaks is due to a slight difference in the periods of the samples: the Mo/Be/Si period is maximal, and the Mo/Be period is minimal (see Table 1). The shift of the detector from the mirror direction of 1.5° corresponds to the observation of the spatial frequency, ν≈4.6 μm⁻¹. From Fig. 4(b), it can also clearly be seen that at this roughness frequency, the scattering intensities are still different for different samples, with the qualitative sequence remaining the same: Mo/Be/Si shows least scattering, Mo/Be gives an intermediate result, and Mo/Be/C and Mo/Be/B₄C scatter most strongly. The scattering peaks for Mo/Be/C and Mo/Be/B₄C are approximately equal in width and magnitude. Thus, the conclusion about the roughness drawn on the basis of the curves in Fig. 3 is confirmed.

From the form of Eqs. (2) and (3), we can estimate the ratio of the “effective” roughness of the samples: $I_{s c a t t e r e d} \propto Φ (θ, φ) \propto P S D (\vec{ν}) \propto σ^{2}$ . According to Fig. 4(b), σ(Mo/Be/Si): σ(Mo/Be): σ(Mo/Be/C): σ(Mo/Be/B₄C) = 1: 1.1: 1.25: 1.25.

4.3 Atomic-force microscopy

The surface roughness of the samples was also studied using AFM. The material of the upper layer MLM is Be, B₄C, C or Si, as shown in Fig. 1. The dimensions of the surface frames were 2 × 2 and 1 × 1 μm. The resulting surface maps are shown in Fig. 5. The height maps were pre-processed, which involved subtraction of the second-order surface and sifting of the outliers. Subtraction of the surface is necessary to eliminate distortions associated with the trajectory and hysteresis of the probe, and the strong localized spikes are due to surface contamination. These spikes were filtered using the following algorithm: if the height of the point differs from the average by more than three standard deviations, then this height is replaced by the median height of its neighbors within a circle with a radius of five points.

Fig. 5 AFM surface maps of the studied samples, with frames of 2 × 2 and 1 × 1 μm; the number of points in each frame is 256 × 256, and the root-mean-square roughness σ is given after subtracting the noise value.

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This filter can be sequentially applied an unlimited number of times; the result will quickly approach saturation, and the map ceases to change. To correct the roughness values, the roughness of the acoustic noise and vibrations was subtracted from the initial values. To do this, maps of free probe deflections were taken in idle mode between measurements.

Graphs comparing the values of roughness are shown in Fig. 6. A frame of each size was obtained at two points on the sample surface, and the greatest difference in roughness between frames was ~20% for one sample (Mo/Be/B₄C).

Fig. 6 Surface roughness of samples under study: (a) for a frame of 2 × 2 μm; (b) for a frame of 1 × 1 μm; the two points correspond to two surface areas for each sample.

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The data in Fig. 6(a) match the pattern found in the previous sections, in which σ(Mo/Be/Si) < σ(Mo/Be) < σ(Mo/Be/C) < σ(Mo/Be/B₄C). However, in Fig. 6(b), the observed roughness for Mo/Be/Si exceeds the roughness of the Mo/Be sample. This seems to show that differences of less than 10%–20% in the AFM data may be the result of nonuniformity in the surface roughness, and are not meaningful. According to average values of roughness for each MLM type σ(Mo/Be/Si): σ(Mo/Be): σ(Mo/Be/C): σ(Mo/Be/B₄C) = 1: 1: 1.3: 1.6.

4.4 TEM microscopy

To obtain TEM images, lamellae were cut from samples using a focused Ga⁺ ion beam with energy 30 keV. Etching with a low-energy Ar⁺ beam at 300 eV was then used to refine the resulting sections. A description of the low-energy ion beam equipment can be found in [40]. High-resolution TEM images are given in Fig. 7.

Fig. 7 HRTEM images for the cross sections of the samples; bright areas correspond to the transparent material (Be), while the dark areas correspond to the opaque material (Mo). For clarity, images are scaled to an equal 5.75 nm period.

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From Fig. 7, we can see the different degree of curvature for layers of different structures. The most curved layers are shown by the Mo/Be/B₄C structure, followed by Mo/Be/C, and then Mo/Be and Mo/Be/Si. This is in agreement with the data on surface roughness and sample interfaces described in the previous sections. For a more detailed comparison, Fig. 8 shows profiles of micrograph contrast from Fig. 7, averaged over 290 parallel lines to eliminate pixel noise and heterogeneity of brightness.

Fig. 8 Averaged brightness profiles for the HRTEM images in Fig. 7; bright areas correspond to the transparent material (Be), while the dark areas correspond to the opaque material (Mo). For clarity, the profiles are scaled to the same period of 5.75 nm.

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All samples contain a Be-on-Mo boundary, the quality of which can be compared using the profiles obtained. A typical Be-on-Mo boundary is shown in close-up in Fig. 9(a). In Fig. 9(b), Mo-on-Be and Be-on-Mo boundaries are compared within a Mo/Be binary structure.

Fig. 9 (a) HRTEM profiles of Be-on-Mo transition regions; (b) comparison of the Mo-on-Be and Be-on-Mo transition regions in the pure Mo/Be multilayer (sample).

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From Fig. 9(a), it can be seen that the Mo/Be/Si structure exhibits the sharpest transition, followed by Mo/Be/C, and that Mo/Be and Mo/Be/B₄C have more extended transitions that are approximately identical. These data do not correspond to the trend identified by the AFM, X-ray scattering, and X-ray reflectometry methods; however, only the diffusion and mixing of materials is taken into account here, rather than the geometric roughness. The width of the observed transition regions is also largely determined by the thickness of the prepared lamellae and the quality of the HRTEM image. The exact shape of HRTEM profiles of different samples is conditioned by complicated mixture of real physical processes of MLM growth, influenced by barrier layer material, and sample preparation and cannot be analyzed here.

Figure 9(b) contains information on the asymmetry of boundaries in a binary structure. It follows clearly from the profile that the Mo-on-Be boundary is much wider than the Be-on-Mo boundary, by at least 1.7 times, taking into account the quality of the original image and the final resolution of the electron microscope. This information is very important for the reflectometric reconstruction of the structure, since the X-ray reflection curves are very insensitive to the sequence of sharp and diffuse boundaries. In the reconstruction in Section 4.1, this knowledge was explicitly used as a basis for choosing the inverse problem solution.

5. Summary and conclusions

We carried out a comparative study of the structure of multilayer periodic Mo/Be mirrors and mirrors with thin (~0.5 nm) interlayers of Mo/Be/B₄C, Mo/Be/C and Mo/Be/Si. Reflectometric measurements show that the reflectance of the binary structure reaches a value of >70% at 11.28 nm, and decreases with the introduction of a barrier layer from any of the materials considered. In this case, the introduction of the interlayer increases the spectral width of the reflection curve, Δλ_1/2, by approximately 0.02 nm, due to the increase in absorption within the structure. The material with the worst reflective efficiency at the target wavelength was (as expected) Si (a drop of 3.5% compared to the reflectance of Mo/Be), followed by B₄C (a 2% drop) and then C (a 1.5% drop). Based on the results of reflectometric reconstruction, it was established that the effective width of the sharpest of the two high-contrast interfaces in the structures, the Be-on-Mo boundary, also depends on the interlayer material. The broadest effective (i.e. roughness plus mixing) Be-on-Mo boundary was Mo/Be/B₄C, followed by Mo/Be/C, Mo/Be and finally Mo/Be/Si. AFM and GIXS measurements were performed to separate the contributions from material interdiffusion and geometric roughness. X-ray scattering showed that Mo/Be/B₄C and Mo/Be/C have similar roughness, Mo/Be scatters more weakly, and Mo/Be/Si has the lowest roughness. AFM measurements confirmed these data; however, it was not possible to reliably distinguish Mo/Be and Mo/Be/Si from the point of view of surface roughness. Finally, HRTEM images made it possible to compare the Mo-on-Be and Be-on-Mo interfaces in the Mo/Be structure, and thus to choose a model for reflectometric reconstruction. We also compared the Be-on-Mo interfaces in all samples, in terms of material mixing. Using TEM, the narrowest Be-on-Mo transition region was shown by Mo/Be/Si, followed by Mo/Be/C, and the widest were found within Mo/Be and Mo/Be/B₄C. However, these results depend strongly on the preparation of the samples and the quality of the HRTEM images, and are ambiguous.

Hence, for the MLMs considered using EUVL at a wavelength in the vicinity of 11.2 nm, the pure Mo/Be structure was the most highly reflective (up to 70.25%). However, the high perfection of the Mo/Be/Si MLM allows us to state that the introduction of the Si layer at the Mo-on-Be boundary decreases the width of the opposite Be-on-Mo interface from 0.33 nm to 0.28 nm. Mo/Be/Si structures therefore have strong potential for EUVL at wavelengths greater than the silicon absorption edge, for example in modern lithography at 13.5 nm [17] and in other areas in which the advantages of the structural perfection of an MLM outweigh the disadvantages associated with the optical absorption of materials.

Funding

State Project 0035-2014-0204; Russian Foundation for Basic Research (RFBR) (Grants: 18-32-00173 for the fitting, 18-02-00588 and 17-52-150006 for deposition of the MLM, 17-02-00640 for AFM measurements, 18-07-00633 for the laboratory X-ray diffractometry), Russian Science Foundation (RSF) 17–12–01227 for the laboratory X-ray reflectometry; RSF 16-42-01034) and Deutsche Forschungsgemeinschaft (DFG) SO 1497/1-1 for measurements, using the BESSY-II facility and the processing of experimental data.

Acknowledgments

We thank HZB for the allocation of synchrotron radiation beam time. Equipment at the Center for Collective Use “Physics and Technology of Micro- and Nanostructures” was also used to carry out this work.

References

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Sample	Composition	<d>, nm	<h(M)>, nm	Interface width, nm	Δλ_1/2, nm	λ_peak 88°, nm	R, %
D364	Mo/Be	5.67	h(Be) = 3.44 h(Mo) = 2.23	Mo-on-Be = 0.67 Be-on-Mo = 0.33	0.26	11.31	69.7
D366	Mo/Be/B₄C	5.75	h(B₄C) = 0.36 h(Be) = 3.34 h(Mo) = 2.05	Mo-on-B₄C = 0.74 B₄C-on-Be = 0.77 Be-on-Mo = 0.5	0.29	11.45	67.6
D381	Mo/Be/C	5.78	h(C) = 0.39 h(Be) = 2.95 h(Mo) = 2.44	Mo-on-C = 0.6 C-on-Be = 0.87 Be-on-Mo = 0.39	0.29	11.45	68.2
D383	Mo/Be/Si	5.85	h(Si) = 0.48 h(Be) = 3.04 h(Mo) = 2.33	Mo-on-Si = 0.72 Si-on-Be = 0.16 Be-on-Mo = 0.28	0.29	11.59	66.5

Influence of barrier interlayers on the performance of Mo/Be multilayer mirrors for next-generation EUV lithography

Abstract

1. Introduction

2. Experimental overview

3. Brief theoretical background

3.1 Radiation in layered structures

3.2 Scattered radiation

4. Experimental results

4.1 Reflectometry

4.2 X-ray scattering

4.3 Atomic-force microscopy

4.4 TEM microscopy

5. Summary and conclusions

Funding

Acknowledgments

References

Cited By

Figures (9)

Tables (1)

Equations (3)

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