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Enhancement of diffraction efficiency via higher-order operation of a multilayer blazed grating D. L. Voronov,* E. M. Gullikson, F. Salmassi, T. Warwick, and H. A. Padmore Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA *Corresponding author: [email protected] Received March 3, 2014; revised April 23, 2014; accepted April 24, 2014; posted April 25, 2014 (Doc. ID 207230); published May 20, 2014 Imperfections in the multilayer stack deposited on a saw-tooth substrate are the main factor limiting the diffraction efficiency of extreme ultraviolet and soft x-ray multilayer-coated blazed gratings (MBGs). Since the multilayer perturbations occur in the vicinity of antiblazed facets of the substrates, reduction of the groove density of MBGs is expected to enlarge the area of unperturbed multilayer and result in higher diffraction efficiency. At the same time the grating should be optimized for higher-order operation in order to keep high dispersion and spectral resolution. In this work we show the validity of this approach and demonstrate significant enhancement of diffraction efficiency of MBGs using higher-order diffraction. A new record for diffraction efficiency of 52% in the second diffraction order was achieved for an optimized MBG with groove density of 2525 lines∕mm at the wavelength of 13.4 nm. © 2014 Optical Society of America OCIS codes: (050.1950) Diffraction gratings; (120.6660) Surface measurements, roughness; (340.7480) X-rays, soft x-rays, extreme ultraviolet (EUV); (230.4170) Multilayers; (310.1860) Deposition and fabrication. http://dx.doi.org/10.1364/OL.39.003157

Multilayer-coated blazed gratings (MBGs) [1] are required for many scientific and technological x-ray applications where high spectral resolution and high throughput are of great importance. For example, resonant inelastic x-ray scattering is a new technique that aims to probe the soft low-energy excitations that underlie the complex behavior of correlated electronic materials [2]. The energy scale of these excitations is in the meV to tens of meV range and are accessed at energies up to 1 keV, and so extreme resolving power is required. This is an area where optimum design requires high-order MBGs. In the technological area, high-resolution and high-efficiency diffractive elements are required as spectral filters for extreme ultraviolet (EUV) lithography [3,4]. In both cases the throughput of the optical system is defined to a large extent by grating efficiency. Although there has been great progress in MBG efficiency since the first proof-of-principle experiments [1,5–7] there is still room for significant improvements, especially in fully utilizing the potential for high-order operation of MBGs. While the EUV reflectance of Mo/Sicoated mirrors exceeds 70% and approaches the theoretical limit due to extensive research in recent decades, diffraction efficiency of Mo/Si-coated blazed gratings is still far below the theoretical efficiency. The main source of the efficiency losses is the complicated growth of a multilayer (ML) on the highly corrugated surface of a saw-tooth substrate. This corrugation causes shadowing of the deposition flux, which leads to discontinuity of the layers. At the same time surface relaxation processes such as surface diffusion tend to smooth out sharp corners, which results in bending of ML interfaces and eventually can result in a sinusoidal-like profile of the grooves [8]. Both effects cause perturbation of the triangular shape of ideal grooves and reduction of diffraction efficiency. It is extremely challenging to avoid shadowing, excessive smoothing, and interface roughening simultaneously due to the interplay of the controls available for each effect. For example, one should provide high mobility of surface atoms to obtain smooth interfaces and to 0146-9592/14/113157-04$15.00/0

withstand to shadowing, but this can result in excessive smoothing and groove profile degradation. On the other hand, suppression of the surface diffusion increases the risk of shadowing and interface roughening. As we found in our recent work, such ML stack distortions limited the absolute efficiency of a 30 bilayer, Mo/Si-coated 5250 lines∕mm MBG to 39.6% for the first diffraction order at a wavelength of 13.1 nm [7]. The relative efficiency of the grating, i.e., the ratio of the absolute efficiency to the reflectance of the ML witness, was 0.62, while it approaches 1 for an ideal MBG. In principle, significantly higher performance could be achieved if the ML coating could be more conformal. Balancing of the smoothing and roughening processes by tuning the energy of the deposited particle via the sputtering gas pressure in magnetron sputter deposition allows one to slightly improve efficiency, but the intrinsic interplay of these effects limits efficiency so far to around 44% [7,9]. Beyond this point, either new more orthogonal controls on growth parameters need to be found, or a new strategy needs to be adopted. Although ML distortions can be minimized, the residual perturbed areas still will occupy a significant portion of a dense MBG period. Moreover, reduced surface mobility leads to interface roughness built up and affects ML reflection efficiency. Both kinds of ML imperfections increase with the coating thickness and prevent deposition of thicker ML coatings required for saturated reflectance. The impact of the perturbed areas on diffraction efficiency increases with groove density since the ML stack distortions occur mostly in the vicinity of the antiblazed facets. Therefore, diffraction efficiency can benefit from reduction of the groove density and hence result in a lowering in the fractional area of the perturbed ML stack in a grating period. This reduction in fractional perturbed area should therefore increase the overall diffraction efficiency. However, in this case the grating dispersion will be reduced by the decrease in groove density. To keep high dispersion and resolving power, the blazed grating should therefore be optimized for high-order operation in © 2014 Optical Society of America

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order to compensate for the lower line density. One should increase the blaze angle and groove depth to provide the blaze condition for a higher diffraction order. Following this approach we fabricated and investigated the structure and properties of a MBG with a groove density of 2525 lines∕mm optimized for the second-order blazing, and compared the performance of the grating with the 5250 lines∕mm first-order grating reported in our previous work [7]. Saw-tooth grating substrates with a period of 396 nm and a blaze angle of 2.0  0.2° were fabricated by the KOH wet anisotropic etching of Si single crystals as reported earlier [7]. The post KOH treatment of the gratings that is used to remove Si nubs left by the KOH process was modified. Previously we used Piranha (H2 SO4  H2 O2 ) and HF oxidation/oxide etch steps to remove the nubs. This method provides highly controllable removal of a surface layer of Si in an isotropic manner with almost no roughening of the surface of the blazed facets [7]. However, the method is somewhat awkward due to the dangerous nature of the chemicals used. In this work we used an RCA SC-1 (NH4 OH  H2 O2 ) process [10], which combines oxidation and oxide etch in a single step. The RCA process is known to result in some roughening of the Si surface. However, our atomic force microscopy (AFM) measurements showed that the roughening occurs in a very high spatial frequency domain and hence is expected to be smoothed out easily during the ML growth [11]. Mo/Si multilayers with a target d-spacing of 6.74 nm were deposited on a saw-tooth substrate and a flat Si wafer simultaneously using a magnetron DC-sputtering setup at the Center for X-Ray Optics, LBNL [12]. The ML d-spacing was half the groove depth of the saw-tooth substrates. This is required to provide blazing condition for the second diffraction order of the MBG according to [1]. Due to the high-order optimization the gratings are expected to have about same spectral resolution as the 5250 lines∕mm first-order MBGs reported earlier [7]. The number of bilayers was increased to 40, which was sufficient to provide saturation of the ML reflectance. The surface of the saw-tooth substrate is relatively coarse since the grooves are twice as deep and so poses a higher risk of shadowing. To compensate possible shadowing, the grating was deposited at the pressure of 1 mTorr of Ar gas, i.e., in the highly smoothing regime. The same regime was used previously for the 5250 lines∕mm MBG [7]. An AFM image of a top surface of the saw-tooth substrate prior the ML deposition is shown in Fig. 1(a). Numerous tiny features (bumps) on the grating facets are a signature of the RCA SC-1 treatment of a Si surface. The RCA provides slow isotropic removal of surface atoms and causes stochastic roughening of the surface, contributing to the overall roughness of 0.36 nm RMS of the anisotropically etched facets measured over a 2 μm × 2 μm AFM scan. A power spectral density (PSD) spectrum [black curve in Fig. 1(c)] shows that this roughening due to nub removal occurs in a high frequency range of 0.02–0.05 nm−1 causing a broad frequency increase in the PSD on the top of the fractal slope. These high-frequency imperfections can be eliminated during ML deposition due to smoothing ability of the MLs [11], as was directly

Fig. 1. (a) AFM images of a saw-tooth substrate after RCA SC-1 treatment and (b) the same after Mo/Si-40 deposition. The images were flattened by applying first-order polynomial flattening to each line of a scan. The height scale bar is 5 nm. (c) PSD spectra of the substrate before (black curve) and after (red curve) ML deposition.

demonstrated earlier [13]. Indeed after deposition of a Mo/Si ML, both grating surface morphology [Fig. 1(b)] and the PSD spectrum [red curve in Fig. 1(c)] are free

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of high-frequency features, and surface roughness reduces to 0.33 nm RMS. Cross-sectional transmission electron microscopy (TEM) reveals the profile of the saw-tooth grooves and details of the internal structure of the ML stack of the second-order 2525 lines∕mm MBG [Fig. 2(a)]. The image of the 5250 lines∕mm grating is shown in Fig. 2(b) for comparison. Signs of shadowing are clearly seen for the coarser grating. The strongest shadowing occurs in the beginning of deposition causing distortion of the layers. Following this, the relaxation processes overcome the shadowing gradually resulting in continuous layers in the upper part of the ML stack. The shadowing could be caused by both the relatively large antiblazed facets of the substrate and the slightly different angle of deposition. An average direction of the atomic flux is depicted by arrows for both the gratings. Some variation of the angle is not crucial for deposition of multilayers on flat substrates but can significantly affect growth of a multilayer on a jagged surface and can cause shadowing effects. Systematic investigation of the impact of the angle of deposition on the structure of a multilayer stack and optimization of the angle for highest diffraction efficiency of the MBGs should be performed in the future. Although the character of the layer distortions is

Fig. 2. Cross-sectional TEM images of the (a) 2525 lines∕mm second-order blazed grating coated with a Mo/Si-40 multilayer and (b) 5250 lines∕mm first-order blazed grating coated with a Mo/Si-30 multilayer [7].

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different for the different angles, the area affected by the distortions seems to be approximately the same in size for both the gratings while the overall fractional area of the distortions is much less for the lower-density grating as expected. Measurements of the diffraction efficiency of the gratings were performed at beamline 6.3.2 at the Advanced Light Source. The dependence of the absolute efficiency of the 2525 lines∕mm MBG as well as a flat ML witness reflectance as a function of wavelength are shown in Fig. 3(a) with solid and dashed red curves, respectively. The measurements were taken for the second negative order at a constant incidence angle of 3° by wavelength scanning with the beamline monochromator. The detector slit width of 5 mm provided separation of the diffraction orders and at the same time accommodated second-order diffraction for the whole range of the wavelength scans. A detector scan at a wavelength of 13.5 nm is shown as a red curve in Fig. 3(b). In this case a 0.5 mm wide detector slit was used. The efficiency of the first positive order of the 5250 lines∕mm grating for the incident angle of 7° and its flat ML witness reflectance are shown in Figs. 3(a) and 3(b) for comparison.

Fig. 3. (a) Dependence of diffraction efficiency of the 2525 lines∕mm second-order blazed grating and 5250 lines∕mm first-order blazed grating (red and blue solid curves, respectively), and reflectance of Mo/Si-40 and Mo/Si-30 flat multilayers (red and blue dash curves, respectively). (b) Detector scans for the 2525 lines∕mm and 5250 lines∕mm gratings (red and blue curves, respectively).

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Although both the gratings manifest a strong blazing effect and good suppression of nonblazed orders [Fig. 3(b)], the efficiency measurements demonstrate the superiority of the second-order blazed grating. Its absolute efficiency reaches 52.0% while the efficiency of the first-order grating did not exceeded 40%. Partially the higher efficiency comes from the thicker multilayer with enhanced reflectance (reflectances of 63.7% and 66.9% were measured for the 30 and 40-bilayer MLs, respectively). However, the additional layers cause only a small contribution to the increased diffraction efficiency. The main improvement of the diffraction efficiency comes from the reduction in the defective fractional area of the ML stack when optimized as a second-order grating. This is supported by the fact that the groove efficiency, i.e., the ratio of the absolute efficiency to the ML reflectance, which is a measure of perfection of a ML stack, increases from 0.62 to 0.78 for the higher-order grating. In summary, we fabricated a multilayer-coated blazed grating with reduced groove density of 2525 lines∕mm but optimized for the second blazed order operation. Comparing to a 5250 lines∕mm first-order grating, the second-order grating has much higher diffraction efficiency due to a reduction in the fractional perturbed area in a period around the antiblazed facet. Reduction of the fractional area of defects in the multilayer stack allows the possibility to deposit thicker ML stacks with saturated reflectivity. These two effects together resulted in our achievement of a record EUV diffraction efficiency of 52% without reduction of the resolving power of the grating. It is clear from this work that there are many advantages if we optimized MBG performance in higher orders compared to conventional first-order gratings,

and offers the prospect of highly efficient, high-resolution gratings for EUV science and technology. This work was supported by the US Department of Energy under contract number DE-AC02-05CH11231. References 1. J. C. Rife, T. W. Barbee, Jr., W. R. Hunter, and R. G. Cruddace, Phys. Scr. 41, 418 (1990). 2. A. Kotani and Sh. Shin, Rev. Mod. Phys. 73, 203 (2001). 3. P. P. Naulleau, W. C. Sweatt, and D. A. Tichenor, Opt. Commun. 214, 31 (2002). 4. V. V. Medvedev, A. J. R. van den Boogaard, R. van der Meer, A. E. Yakshin, E. Louis, V. M. Krivtsun, and F. Bijkerk, Opt. Express 21, 16964 (2013). 5. M. P. Kowalski, R. G. Cruddace, K. F. Heidemann, R. Lenke, H. Kierey, T. W. Barbee, Jr., and W. R. Hunter, Opt. Lett. 29, 2914 (2004). 6. J. H. Underwood, C. Kh. Malek, E. M. Gullikson, and M. Krumrey, Rev. Sci. Instrum. 66, 2147 (1995). 7. D. L. Voronov, E. H. Anderson, E. M. Gullikson, F. Salmassi, T. Warwick, V. V. Yashchuk, and H. A. Padmore, Opt. Lett. 37, 1628 (2012). 8. D. L. Voronov, E. H. Anderson, R. Cambie, S. Cabrini, S. D. Dhuey, L. I. Goray, E. M. Gullikson, F. Salmassi, T. Warwick, V. V. Yashchuk, and H. A. Padmore, Opt. Express 19, 6320 (2011). 9. D. L. Voronov, E. H. Anderson, E. M. Gullikson, F. Salmassi, T. Warwick, V. V. Yashchuk, and H. A. Padmore, Appl. Surf. Sci. 284, 575 (2013). 10. W. Kern, J. Electrochem. Soc. 137, 1887 (1990). 11. D. G. Stearns, D. P. Gaines, D. W. Sweeney, and E. M. Gullikson, J. Appl. Phys. 84, 1003 (1998). 12. http://www.cxro.lbl.gov/. 13. S. Bajt, D. G. Stearns, and P. A. Kearney, J. Appl. Phys. 90, 1017 (2001).

Enhancement of diffraction efficiency via higher-order operation of a multilayer blazed grating.

Imperfections in the multilayer stack deposited on a saw-tooth substrate are the main factor limiting the diffraction efficiency of extreme ultraviole...
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