## Abstract

We report on the development of a hybrid mirror realized by integrating an EUV-reflecting multilayer coating with a lamellar grating substrate. This hybrid mirror acts as an efficient Bragg reflector for extreme ultraviolet (EUV) radiation at a given wavelength while simultaneously providing spectral-selective suppression of the specular reflectance for unwanted longer-wavelength radiation due to the grating phase-shift resonance. The test structures, designed to suppress infrared (IR) radiation, were fabricated by masked deposition of a Si grating substrate followed by coating of the grating with a Mo/Si multilayer. To give the proof of principle, we developed such a hybrid mirror for the specific case of reflecting 13.5 nm radiation while suppressing 10 μm light, resulting in 61% reflectance at the wavelength of 13.5 nm together with the 70 × suppression rate of the specular reflection at the wavelength of 10 μm, but the considered filtering principle can be used for a variety of applications that are based on utilization of broadband radiation sources.

© 2013 OSA

## 1. Introduction

The use of diffractive optical elements plays an important role in advanced optical systems that are based on the utilization of radiation emitted by broadband sources. Various spectral-selective diffractive elements are applied either for spectral analysis or for spectral filtering. In the past 20 years various combinations of grating structures with multilayer Bragg-reflector coatings were intensively explored as optical elements with improved spectral characteristics specifically for the EUV and soft X-ray ranges of the electromagnetic spectrum [1–7]. Particularly, such multilayer gratings are attractive for optical applications that require high reflection for radiation in the EUV wavelength range and simultaneously demand suppression of radiation outside this prime range. A most obvious need for radiation with high “spectral purity” can be found in extreme ultraviolet lithography (EUVL) [8]. In order to fulfill demands of high productivity, EUVL requires high power radiation sources of EUV. A promising technology for such sources is based on the emission from dense tin plasmas produced by pulsed CO_{2} laser radiation with 10.6 μm wavelength [9–12]. These plasmas emit, apart from useful in-band EUV (13.5 ± 0.1 nm) light, also unwanted, so called out-of-band (OoB) radiation, e.g. longer/shorter wavelength EUV, ultraviolet, visible and scattered infrared light from the laser [11,12]. All sorts of OoB radiation are efficiently reflected by the mirrors in the optical system. To prevent problems such as mirror heating, wafer heating, resist flare and imaging contrast loss, strong mitigation of OoB radiation is highly demanded.

Along with a variety of different types of spectral purity filters [13–20], multilayer grating designs have been proposed earlier for suppression of OoB radiation in [6,7]. As an advantage application of multilayer grating mirrors for the radiation filtering allows straightforward cooling unlike free-standing grids and foils, and these ideally could replace a standard multilayer optical element that allows to avoid introduction of additional elements in the existing optical system. These filter designs use a filtering principle based on the diffractive deflection of OoB radiation from the direction of EUV beam propagation. For instance, van den Boogaard et al. showed the potential of achieving full spectral separation of EUV and IR radiation by application of a Mo/Si multilayer coated blazed grating [6]. However fabrication of such blazed multilayer grating possessing high EUV reflectance requires an application grating substrate with roughness adequate for deposition of Mo/Si coating [21], which is still a big technical challenge. Suppression of ultraviolet OoB radiation has been demonstrated recently along with high EUV reflection efficiency with a diffractive filter based on Mo/Si multilayer integrated in rectangular-groove grating [7].

In this paper we focus on the further development of a diffractive optical element to suppress the IR radiation using a rectangular grating rather than a blazed grating. Rigorous calculations of the optical response of metallic rectangular grating were used in order to determine optimal geometrical parameters of such a diffractive filter. Furthermore, we applied an alternative manufacturing procedure allowing a significant reduction of the grating period with respect to the previously reported method [7] thus improving the angular separation of “useful” and “parasitic” radiation.

## 2. Working principle

The design of the diffractive EUV filter is given schematically in Fig. 1. This hybrid structure consists of the rectangular-groove grating with an EUV-reflecting multilayer coating deposited on top. Mo/Si coating is intended to be used because of its high reflectance at the 13.5 nm wavelength. High lateral electrical conductivity of the Mo/Si coating causes efficient reflection of IR radiation from the multilayer. The lateral periodicity of the structure results in diffraction of the reflected radiation: besides the specularly reflected radiation, additional intensity is observed in the diffraction maxima at off specular angles. These diffraction angles are determined by the grating equation:

where*p*is the grating period,

*λ*is the wavelength of incident radiation,

*θ*is the angle of incidence,

*m*is an integer representing the

*m*

^{th}diffraction order, and

*ϑ*is the diffraction angle corresponding to the

*m*

^{th}order. As can be calculated with Eq. (1) characteristic values of the diffraction angles corresponding to 13.5 nm and 10.6 μm wavelength’s values differ by about three orders of magnitude. Therefore, when reflecting from the described diffraction filter the diffracted IR radiation will be significantly deflected from the direction of propagation of the reflected EUV radiation. Only the specularly reflected beam of IR radiation, corresponding to the 0th diffraction order, remains undeflected. By tuning the geometrical parameters of the grating (

*p*,

*d*,

*h*) the 0th order can be suppressed for the IR light.

The 0th order suppression principle can be clearly demonstrated when considering the optical response of the grating structure within the scalar theory of diffraction. Consider a 2-dimensional infinite rectangular-groove grating illuminated with a normal-incident plane wave with wavelength *λ*. The grating is characterized by its period *p*, width *d* and depth *h* of the grooves. Analogous to the situation of a transmission grating, which is given in [22], one can derive the following expression for the n^{th} order diffraction efficiency (the ratio of diffracted intensity to incident intensity):

*R*is the total reflected intensity, Γ =

_{tot}*d*/

*p*and

*A*is the phase shift factorFor the particular case of the 0th order Eq. (1) results in

*h*=

*λ*/4 +

*mλ*/2, where

*m*= 0,1,2,… When the 0th order reflection is suppressed, the total reflected radiation is distributed between the remaining diffraction orders: the off-specularly reflected waves. Note that for the specific case of Γ = 0.5 Eq. (2) results in zero intensity of even diffraction orders as well. The main part, about 80%, of the intensity of the reflected light is distributed between the 1st and the −1st diffraction orders (see Fig. 2). The actual value of the diffraction angle of these most intense diffraction orders is determined by the value of the grating period, as can be seen in Eq. (1). The choice of the period is discussed in the following section. Here we conclude that the smallest resonant groove depth

*h*=

*λ*/4 is the most convenient for filtering applications from the practical point of view.

## 3. Calculations

In this paragraph we present the results of rigorous calculations of the plane wave reflection from a metallic rectangular grating representing the above described diffraction filter. These calculations allow to accurately estimate the suppression rate of the specular reflectance in the infrared range. We also discuss illumination of the grating structure with finite size source in order to evaluate the impact of a limited spatial coherence on IR and EUV diffraction. Based on the results of these calculations we estimate the range of grating period values providing the required more than 100 × suppression of the reflectance at 10.6 μm wavelength.

The calculations were performed by using the approach described in [23]. The perfect-conductor (PC) approximation works well for the long-wavelength infrared range for calculations of the electromagnetic wave reflection from metal gratings [24]. This perfect-conductor approximation also allows to consider reflection from the grating as a function of the dimensionless parameters *p*/λ and *h*/λ that is convenient for scaling procedures. All calculations were performed for both TE and TM polarization states of the incident plane wave, but the results below are presented in terms of half-sum for TE and TM reflectance, *R* = 0.5(*R*^{TE} + *R*^{TM}), in order to simulate reflection of unpolarized light which is relevant for our applications.

First we consider the illumination of rectangular PC grating with a normal incident wave with wavelength λ. Grating parameters *h* and Γ are fixed at λ /4 and 0.5 respectively, while the *p*/λ ratio is varied in order to determine the optimal range of period values. Figure 3 shows the dependence of the 0th order reflectance (*R*_{0}) on the ratio *p*/λ. One can see periodical modulation of reflectance as a function of *p*/λ, which is mainly due to energy redistribution between waveguide modes in the groove cavities. However, the average reflectance gradually decreases with increasing of *p*/λ. When the ratio *p*/λ passes the value of 5.1, specular reflectance does not exceed 1%, thus meeting the requirement for the applications.

However, practical applications require suppression of the reflectance not only for normal incidence, but within a finite range of angles. For the oblique incidence we calculated *R*_{0} as a function of the angle of incidence (*θ*) for a set of the *p*/λ ratio values (see Fig. 4). From Fig. 4 one can see that specular reflectance can be suppressed more than 100 times for the angles up to 20°.

Note that the above given calculated results correspond to the case of an infinite grating with plane wave illumination. While in practice limited spatial and temporal coherence of the radiation source will affect diffraction from the considered grating structure. Obviously, a finite coherence size of the illumination spot on the grating will abridge the number of orders involved in the diffraction pattern. The resulting redistribution of the reflected intensity between the limited number of diffraction orders will determine the suppression efficiency of the IR specular reflectance. The characteristic coherence length in the illumination spot can be estimated via the relation for the radius of spatial coherence

where*b*stands for the characteristic radiation source size and

*z*is the distance between the grating and the source. For the scattered CO

_{2}laser radiation in LPP sources the plasma volume can be considered as a secondary source of IR radiation with a characteristic source size

*b*= 100-300 μm [25]. Figure 5 (red lines) shows how the coherence radius at 10.6 μm wavelength, ${\rho}_{IR}$, varies with the distance

*z*within the length-scale relevant to the typical sizes of EUV source-collector systems [26]. We have also used Eq. (5) for the estimation of the coherence radius values at 13.5 nm wavelength, ${\rho}_{EUV}$, in order to evaluate the impact of the source size on the diffraction of EUV radiation (Fig. 5, blue lines). It is seen from Fig. 5 that the ${\rho}_{IR}$ values are in the centimeter range while ${\rho}_{EUV}$ is in the order of tens of micrometers.

Now based on the calculations it is possible to outline the range of the geometrical parameters of the grating structure which would be suitable for the application purposes. As it was stated above the step-height *h* determines via the *h* = *λ*/4 requirement the resonant wavelength of the specular reflectance suppression in the long-wavelength range (IR in our particular case). The *p*/λ ratio governs the intensity of the specular reflectance *R*_{0} at the resonance, i.e. the suppression of the 0th order intensity improves with the increase of the *p*/λ ratio. At the same time the angular deviation of the ± 1st orders of diffraction (which are the most intense) from the 0th order decreases as $\mathrm{arc}\mathrm{sin}\left(\lambda /p\right)$. In practice, diffraction angles should be large enough to provide deflection of the ± 1st orders from the radiation acceptance aperture of a particular EUV optical system. At the same time the ${\rho}_{IR}/p$ ratio should be maximized to guarantee near-zero specular reflection at the given wavelength. These are the main limiting factors for the *p*/λ ratio. In practice, the choice of the *p*/λ ratio in the filter design will be a trade-off between the suppression of the specularly reflected wave intensity and the angular separation of the off-specularly diffracted waves. In our particular case of the 10.6 μm wavelength, *p* ~100 μm provides a satisfactory angular separation (≈5.7°) and a predicted suppression of the specular reflectance by more than two orders of magnitude for *θ* values up to 20° (see Fig. 4). Period values in this range are 2 orders of magnitude smaller than the IR coherence length scale (Fig. 5) thus providing efficient diffraction. Furthermore, the relatively small EUV coherence length (Fig. 5) destroys the EUV diffraction pattern preventing possible EUV diffraction losses when *p* ~100 μm .

## 4. Experimental details

#### 4.1 Fabrication method

In this section, we describe the fabrication of the considered above EUV reflecting/IR-suppressing multilayer grating structures. We used a lift-off contact lithography process, as schematically shown in Fig. 6, to fabricate a silicon substrate with a lamellar grating design on top of which a ML was deposited. First, a silicon wafer is spincoated with a layer of UV-sensitive lift-off photoresist of 3μm thick, which is slightly thicker than the aimed step height of *h* = *λ*/4. This resist is pre-baked at 95°C for 2 mins and subsequently exposed with UV light while in contact with a mask consisting of a 50 μm half-pitch grating. The period value of the resist mask, 100 μm, was chosen from the above described theoretical considerations. The substrate was then allowed to stabilize at room temperature for 10 mins. After stabilization, a reversal step consisting of a bake at 120°C for 2 mins and a UV flood exposure was performed. This reversal step gives rise to a trapezoidal mask profile, which is required for the resist strip chemicals to easily access the resist in the lift-off step [27]. The resist was then developed, where the area is initially protected by the mask, i.e. not exposed to UV light in the first exposure step, were removed from the substrate.

After the preparation of the lift-off resist, a Si spacer layer was deposited (step 3 of Fig. 4). Magnetron sputtering deposition was employed, enabling layer growth with sub-nanometer roughness levels over the thickness of the spacer layer. This is a critical requirement in order to obtain high EUV reflectance from the Mo/Si multilayer deposited onto the spacer layer [28]. The experimental geometry and working pressure (≈10^{−3} mbar) resulted in local angle of incidence variations of the deposited atoms at the sample up to 10° from normal. In order to obtain a sharp projection of the mask structure into the deposited layer under these conditions, i.e. to prevent half-shadow regions, the mask thickness should be much smaller than the mask apertures. In the presented experiments the thickness/aperture ratio was 3/50.

After silicon deposition, the actual lift-off is performed by stripping the remaining resist from the substrate using ultrasonic baths of acetone and IPA. The initial silicon substrate and the deposited silicon structures thus form a grating pattern defined by the mask. Finally, on top of this silicon grating structure, a high reflecting multilayer was deposited using electron beam evaporation in combination with ion beam smoothing [21].

The fabricated PsR structures were characterized with atomic force microscopy. The average step height of the grating was measured to be 2.35 ± 0.05 μm corresponding to 9.4 μm wavelength of the zero-order suppression spectral minimum. In spite of the mismatch with the targeted 10.6 μm wavelength this test multilayer grating structure can be used for the validation of the filtering principle.

#### 4.2 Spectral characterization of samples

EUV reflectometry of the samples was performed at the EUV beam line of the Physikalisch-Technische Bundesanstalt (PTB) in Berlin. The measurements were performed at the angle of 1.5° from normal incidence and with a detector aperture of 0.5°.

Infrared reflection spectra of the sample were recorded with a Bruker Vertex 70v Fourier-transform infrared (FTIR) spectrometer. The measurements were performed in the 400 – 2000 cm^{−1} spectral range. A pyroelectric detector was used in order to provide a broad dynamic range (four orders of magnitude) in the measurements. The spectra were recorded with a resolution of 4 cm^{−1} and in each case 64 spectra were averaged. An aluminium mirror was used as the reference sample for the reflectivity measurement.

Accurate measurements of the specular reflectance (0th order diffraction efficiency) from the grating structure are complicated due to relatively small diffraction angle (*θ* ≈5.7°) at the wavelength of interest. Typical numerical apertures of mirrors in commercially available reflection accessories for the used spectrometer exceeds this diffraction angle value resulting in collection of undesired off-specularly reflected radiation. Note that in our measurements we separated the off-specular from specular signal by decreasing the aperture of the mirrors with slit diaphragms oriented perpendicularly to the plane of diffraction.

## 5. Results and discussions

The results of the EUV reflectance measurements are shown in Fig. 7. The peak reflectance of a reference Mo/Si multilayer on a polished substrate, co-deposited during multilayer deposition on the Si grating substrate, is 69%. The reflectance of a Mo/Si multilayer deposited on the spacer layer outside the masked area, is 68%, the 1% reduction of the reflectance probably caused by roughness of the spacer layer. However, 61% has been measured on the final multilayer grating samples, representing a loss of 7%. The influence of structural imperfections of the multilayer are considered the major cause of the reflectance loss. Edge rounding effects lead to non perfect periodicity of the multilayer near the edges of the grating facets. As it was studied with transmission electron microscopy in detail in our previous work, the width of this distorted area scales with the thickness/aperture ratio of the deposition contact mask, and with the angular anisotropy of the deposition flux [29]. Therefore the EUV reflectance loss could be minimized by employing lower mask thickness/aperture ratios, but in practice this requires increased pitch lengths [7]. Alternatively, further optimization of the deposition anisotropy by collimated deposition techniques is considered promising [30].

Figure 8 shows reflectance spectra of the sample measured with the FT-IR spectrometer. According to a polynomial fit of the measured data the spectral position of the reflectance minimum corresponds to 1030 cm^{−1} wavenumber (9.7 μm wavelength). The absolute value of the zero-order reflectance at the minimum is 1.4% corresponding to 71 times suppression of the IR light. The 9.7 μm wavelength value allows to estimate the grating groove depth as 2.43 μm via the *h* = *λ*/4 criterion, which is close to the AFM measured *h* value of 2.35 ± 0.05 μm.

The measured minimum reflectance value of *R*_{min} = 1.4% exceeds our theoretical expectations of *R*_{0} = 0.3% for the *p*/λ ≈10 ratio (Fig. 3), which limits the suppression rate of the 0th order intensity. We suggest that the discrepancy is caused by our measurement method. The calculations are made for the case of the infinite-size grating illuminated with a monochrome plane wave, which has infinite spatial coherence length by definition. Illumination of the grating structure in the FTIR spectrometer with the finite spatial coherency radiation limits the number of periods of the grating that interfere destructively and, consequently, results in the reduction of the interference contrast. For our measurement scheme this has led to the increase of *R*_{min}. The application of diaphragms for angular filtering required in our specific case also results in the reduction of the illumination spot size and consequently reduction of the number of the grating periods involved in the formation of the diffraction pattern. In the case of illumination of the described diffractive filter with the scattered CO_{2} laser radiation in EUV sources we expect the suppression rate of the 0th order reflectance to be closer to our theoretical estimations due to the larger coherence length. If stronger suppression is needed, a possible way to meet this requirement is to increase the period of the structure.

## Conclusions

We have developed a special combination of a high-reflectance multilayer mirror for EUV radiation and a lamellar grating substrate that provides spectrally selective suppression of longer-wavelength radiation. This suppression of the long-wavelength specular reflectance occurs due to quarter-wavelength phase-shift resonance. It is not affecting the high reflectance of the multilayer structure for EUV light. This system allows for example the suppression of unwanted radiation from laser plasma-based EUV and soft X-ray sources, having an undesired component of scattered IR laser radiation. Such spectral filtering is of high importance for emerging lithographic applications, high harmonic generation systems and water window microscopy.

A test mirror has been fabricated using a contact mask deposition of the grating substrate followed by the deposition of a Mo/Si multilayer designed for normal incidence reflectance at λ = 13.5 nm. The grating has been optimized for mid-IR suppression. A factor of 70 reduction has been experimentally demonstrated at 9.7 μm wavelength together with 61% EUV peak reflectance at 13.5 nm wavelength at near normal incidence.

## Acknowledgments

This work is part of the research program “Controlling photon and plasma induced processes at EUV optical surfaces (CP3E)” of the “Stichting voor Fundamenteel Onderzoek der Materie (FOM)” which is financially supported by the Nederlandse Organisatie voorWetenschappelijk Onderzoek (NWO). The CP3E programme is cofinanced by Carl Zeiss SMT GmbH (Oberkochen), ASML (Veldhoven), and the AgentschapNL through the Catrene EXEPT program.

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