IMPROVED GRADED MULTILAYER MIRRORS FOR XRD APPLICATIONS

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1 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol IMPROVED GRADED MULTILAYER MIRRORS FOR XRD APPLICATIONS C. Michaelsen, P. Ricardo and D. Anders Institute of Materials Research, GKSS Research Center, Geesthacht, Germrmy M. Schuster, J. Schilling and H. Giibel Siemens AG, ZT MF 7, Otto-Hahn-Ring 6, Mtinchen, Germany ABSTRACT We report on the design, fabrication and application of improved curved graded multilayer mirrors for laboratory x-ray diffractometry. The use of high-precision prefigured optical substrates led to a new generation of curved graded multilayer mirrors in which the geometrical aberrations connected with the previous substrate bending technique are eliminated. The use of prefigured substrates was made possible by a dedicated sputtering process that fulfills the stringent precision requirements imposed by the technique of direct deposition onto prefigured substrates. In addition. the WlSi and W/B& multilayers previously used in these mirrors have been replaced by WSi,/Si, Ni/Mg and Ni/B,C, improving the spectral resolution and/or the peak reflectivity. An important quality criterion is the intensity gain factor obtained with the mirror when the x-ray beam is coupled into a channel-cut monochromator. This gain factor was improved from 6 to 16, a value which is close to the theoretical maximum. INTRODUCTION The application of laterally graded multilayers on figured reflectors for beam conditioning in x-ray diffractometry has become very successful since its introduction by H. Gobel [ 1,2]. Graded multilayers ( Gobel Mirrors, Fig. 1) serve as monochromators, which, as a result of a layer thickness gradient, fulfill Bragg s law for the x-ray wavelength of interest across the entire mirror length. The purpose of these graded multilayer mirrors is to modify the divergent x-ray beam emerging from a point or line focus source in that way that it can optimally be used in x-ray diffraction systems with Guinier focussing (convergent beam), Bragg-Brentano focussing (divergent beam) or parallel beam geometries. Most important is the parabolically curved graded multilayer mirror, which allows the conversion of a divergent beam into a parallel beam. The use of a parallel beam in reflectometry, grazing incidence diffractometry, protein and microbeam diffractometry resulted in remarkable intensity improvements [3-S]. Combined with channel-cut monochromator crystals, parallel beams provided a significant step towards higher dynamics in HRXRD [3].

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

3 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol By coupling x-rays from a fine focus tube with a parabolically curved graded multilayer mirror into a Ge(022) ( ) Cu Ka, monochromator and comparing the intensity with that of the same configuration without the multilayer mirror, a considerable intensity gain factor of 6 was obtained [3]. Theoretical considerations, however, suggest a factor in the range for this configuration [9]. The missing intensity gain could be attributed to non-uniformities in the beam profile, which were caused by imperfect optical figures of the mirrors [3,4]. source Fig. 1: The principle of a parabolic Gobel Mirror. curved graded multilayer The first goal of the present paper is to demonstrate the advantage of improved figured optical substrates which eliminate the geometrical aberrations of the reflected beam. The use of these substrates was made possible by an optimized sputtering process that fulfills the more stringent precision requirements imposed by the technique of direct deposition onto prefigured substrates. The second goal of this paper is to demonstrate the improvements that have been made by changing the multilayer materials. The current generation of Gobel Mirrors is based on multilayers using the strongly scattering, but also highly absorbing, material W as the reflector (B&Z or Si as the spacer). However, higher peak reflectivities can be achieved by an optimized relation of dispersion/absorption of the reflector material and a higher spectral resolution (smaller line width) can be obtained by multilayers in which a larger number of layer pairs contribute to the diffracted beam. OPTICAL FIGURE AND SUBSTRATE SURFACE For the preparation of the optical figure of the mirrors, a bending-and-gluing technique is widely being used. The multilayer is first deposited on a silicon wafer and the wafer with the multilayer is then bent to the desired figure. A parabolic figure, for example, is used to convert a divergent beam into a parallel beam. Let us define a parabola by y2 = 2px, where p is the parabola parameter and x the coordinate of the incidence point measured from the vertex along the parabola symmetry axis. The x

4 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol coordinate is connected with the focal lengthfby by O(x) = arccot y. P f = x + p/2. The angle of incidence is then given A typical choice for p is 0.1 mm, andfis in the range mm, so that 0 is in the range A typical radius of curvature is 10 m, corresponding to a bow of 45 pm for a 60 mm long mirror. For the present x-ray optics, Bragg reflections of multilayers are used for which the reflection widths A@,,,, are between 0.02 and 0.05 for Cu Ka radiation [lo]. As shown later, WSi,/Si and Ni/B,C multilayers have Bragg peak widths of about In order not to loose reflectivity, the waviness (angle error) of the figure must fulfill Ay/dx I l/2 A@,,,, (1) and the figure deviation of the parabolic mirror must fulfill Ap I.&i. A@,,,,. (31 For Cu Ka radiation and a parabolic W/Si multilayer mirror with a focal distancef= 150 mm and a parabola parameter p = 0.1 mm, the maximum acceptable angle error is = 133 arcsec. For WSi,/Si and Ni/B,C, we obtain = 54 arcsec. These figure errors correspond to a 50 % intensity loss. A consequence of this is the necessity of a true parabola. A spherical approximation of the parabolic figure would be insufficient. Figured mirrors have been produced previously, for example, by pressing the coated Si wafer onto a negative form which has a parabolically machined surface, and then gluing a backing plate onto the back side of the wafer. After curing, the wafer is removed from the negative form, and the glue has to maintain the curvature of the mirror. Many variations of this bending and gluing technique exist. However, some problems generally occur in the gluing techniques. Firstly, a negative form of a high quality optical figure is required. Secondly, dust particles between wafer and negative with sizes > 1 pm alter the curvature significantly so that particle-free surfaces are essential for the preparation. Thirdly, the glue tends to relax and is not completely long-term stable. Lastly, a degradation has been observed at the front side of the mirrors when the glue was irradiated by x-rays. Because of these problems, the quality and long-term stability of mirrors fabricated in this way are limited. Bent and glued mirrors have typical figure errors of 50 arcsec peak to valley, and arcsec r.m.s. [ 111. Therefore, they contain tilted facets which result in a reduced reflectivity. To overcome these limitations, we have used prefigured quartz substrates. The surfaces of these substrates were processed by mechanical and ion-beam polishing in order to produce the desired parabolic figure. The r.m.s. figure errors of these substrates were below 1.65 arcsec = , about a factor of 10 better than in the bent and glued mirrors. Furthermore, the quality of a multilayer film strongly depends on the surface roughness of the substrate. As a rule, the substrate roughness should not exceed 10 % of the multilayer period.

5 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Roughness causes x-rays to be scattered at different height positions and to interfere destructively. The limit for a l/e intensity loss is given by Ay = i1/(4nsin 0) = d/(2~) where h is the x-ray wavelength and d is the multilayer period. For Cu Ka radiation and a parabolic W/Si multilayer mirror with a focal distance f = 150mm and a parabola parameter p = 0.1 mm, the maximum acceptable height deviation (roughness) is 0.67 nm. The present prefigured quartz substrates have surface roughnesses below 0.25 nm and, therefore, fulfill the requirement. REQUIREMENTS IN THE DEPOSITION PRECISION Let us consider the period requirements of a parabolically figured multilayer mirror that reflects radiation emitted from its focus into a parallel beam. In order to establish Bragg reflection on the parabola, the multilayer period has to fulfill the equation d= (4) where 6 is the mean decrement of the refractive index of the multilayer, and 0 is given by Eq. (1). In most cases, the period grading of the multilayer mirror can be approximated by a linear relationship. According to the above equations, the multilayer films used in parabolically curved mirrors have periods in the range 3-6 nm. The choice of the lower period limit of 3 nm (which in turn confines the choice of p) is also based on the increasing difficulties in producing thinner multilayers of good quality. The thickness ratio I- = d(rejzector)/d has been chosen to r= 0.5. At this r ratio, the unwanted second order as well as higher even order Bragg reflections are suppressed. A more detailed treatment, which includes absorption and refraction, leads to an optimum rratio typically 1% smaller. Previous multilayer mirrors were deposited on planar silicon wafer substrates and bent to the desired figure after the deposition process. So, in a first step, the multilayers can be deposited over a wide period range on the silicon wafer. It is then sufficient to establish only the correct period grading Ad/Af. The lateral position of the starting period is not important. The exact period characterization by x-ray reflectometry after the deposition process allows cutting out the desired period range. This cut out wafer piece with the correct period range is then bent to the desired figure and glued to a backing. Such a subsequential positioning of the period grading relative to the optical figure is not possible if the multilayer is deposited directly on the prefigured substrate. The period precision imposed by direct deposition can be calculated by differentiating Bragg s law and inserting Eq. ( I), leading to Ad -=-cot@.a@=d With A0 = 1/2AO,,,,, we obtain IAd/dl < 1.6 % for a W/Si multilayer, Cu Ka radiation, p = 0.1 mm, andf = 70 mm [lo]. This is a conservative limit because 70 mm is our shortest focal (5)

6 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol distance and lad/dl increases withf according to Eq. (5). For the narrower Bragg peaks of the WSi,/Si and Ni/B,C multilayers, we obtain IAd/dl < 0.8 %. Therefore, we have a very stringent period precision requirement of about 1 % in the case of direct deposition onto prefigured substrates that puts high demands on the deposition control. OPTIMIZED MATERIAL COMBINATIONS The first generation of Gijbel Mirrors is based on multilayers with the strongly scattering, but also highly absorbing, element W as the reflector, and C, B,C or Si as the spacer. These high-z low-z material combinations are widely being used in multilayer optics because they are uncritical in the fabrication, and provide high reflectivities of about 75 % in the first order Bragg reflection for the wavelengths of most x-ray tubes. However, higher peak reflectivities can be achieved by an optimized relation of dispersion/absorption of the reflector material. Furthermore, for a fixed r of 0.5, the spectral resolution (line width) is dominated by the multilayer period and the number of layer pairs contributing to the diffracted beam. In W-based multilayer mirrors, only a few layer pairs contribute to the reflectivity due to the dynamical effect of high extinction. Multilayers with W as the reflector produce relatively broad Bragg peaks resulting in a limited spectral resolution and, therefore, in a relatively poor KP suppression. An improvement in the spectral resolution can be obtained by replacing W by a material of lower absorption. Figure 2 shows the absorption of W as a function of the x-ray energy compared with the alternative reflector materials WSi, and Ni. The idea to use WSi, rather than W as a reflector is based on the lower density (9.5 g/cm ) and, therefore, lower absorption of WSi,. The motivation to use Ni as reflector is because of the favorable position of the Ni-absorption edge with respect to most x-ray wavelengths. MO Ka Fig. 2: Absorption of W, WSi,, and Ni, as a function of x-ray energy [ 121. The characteristic lines of different x-ray tubes are indicated by arrows. Energy (kev)

7 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol The calculated curves displayed in Fig. 3 show that WSiJSi multilayers should have similar peak reflectivities as WEi multilayers, but more than twice the spectral resolution. More than 90 % peak reflectivities could be obtained in a multilayer with Ni as the reflector. Since Si is known to react with Ni and to form silicides at low temperatures, B,C rather than Si has been chosen in the calculation of Fig. 3 as a possible spacer in the combination with Ni. However, based on x-ray optical properties, several low-z materials could be used as spacer materials to provide similar performance. The choice of the best spacer material is dictated by the chemical interaction between the two materials rather than by x-ray optical properties, i.e., by the requirement to have sharp, smooth and stable multilayer interfaces. / WlSi Rpea,, = 73.6 % FWHM = Fig. 3: Calculated Cu Ka reflectivity versus incidence angle for WlSi, WSi,/Si, and Ni/B,C multilayers (100 layer pairs, 4 nm period, thickness ratio r = 0.5, layer densities equal to bulk densities, 0.3 nm interface roughness). NilB.,C Incidence angle 8 Weg) DEPOSITION TECHNIQUE The sputtering system used in the present work is sketched in Fig. 4. The system comprises a ultra-high vacuum system of 60 cm diameter with a versatile sample holder, which allows rotation, heating and cooling of the substrates, positioned in the center of the chamber. Two magnetron sputtering sources were used for the deposition. The fabrication of multilayer films was done using a computer-controlled shutter positioned in front of the sources. Figure 4 shows that the deposition was performed in an off-normal geometry between sources and substrates. As a result of this geometry, the distance between sources and substrates varies across the substrate holder, leading to a layer thickness gradient across the sample. This effect has been used to fabricate multilayers with laterally graded periods. The sputtering gas was Ar of purity 7.0, and the pressures were in the range Pa. In the present work, no in situ monitoring of the film quality was conducted. However, efforts were made to maximize the stability and reproducibility of

8 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol the deposition rates. A previous basic research on the thermodynamics and kinetics of thin film reactions using the sputtering system of Fig. 4 is summarized in Ref. [ 131. Fig. 4: Setup of the magnetron sputtering system used for the deposition of multilayer films. PERIOD GRADIENT AND PRECISION One feature of the sputtering system is the high precision and reproducibility of the sputtering rates. Figure 5 shows a typical period variation across a graded multilayer, in comparison to the ideal curve. The sputtering process used for the multilayer deposition allows us to fabricate multilayers with periods that fit well within a 1 % tolerance band around the desired values, in a controlled and reproducible way. This precision could be maintained and reproduced during subsequent depositions. Note that the tolerance band in Fig. 5 is only l/5 of a typical atomic diameter ~~~-~ I, - _,. A..* 1 _*.-x- * 4.6 t I _,I. -.(c _A,.*.A 4.4,,@-5= Focal distance f (mm) Fig. 5: Period d versus focal distance f across a parabolically curved graded WEi multilayer measured by x-ray reflectometry (dots). The solid line gives the ideal period. As a guide to the eye, the dashed lines indicate a tolerance band of f 1 % around the ideal curve.

9 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol PEAK REFLECTIVITIES As an example, Fig. 6 shows the reflectivity curve of a WSiJSi multilayer film with a period of 3.5 nm. The WSi, layers were sputtered from an alloy target. From the comparison with a simulated reflectivity curve in Fig. 6, it can be seen that the WSi,/Si multilayer interfaces have outstanding smoothness and conformity in periodicity. The interface roughness is only 0.25 nm. For the 60-period multilayer in Fig. 6, thickness oscillations between the Bragg peaks can still be resolved. Also, the second order suppression is excellent, indicating equal layer thicknesses and interfaces. For 100 layer pairs, we measured Cu Ka peak reflectivities between 60 and 70 % for periods in the range 3-5 nm. These values are close to the theoretical maximum of this material system, and they are only slightly smaller than those obtained for comparable W/Si multilayers. It can be concluded that WSi,/Si is a good material combination for multilayer mirrors with improved spectral resolution. Furthermore, WSi,/Si multilayers have a thermal stability superior to W&i multilayers [ nm W.Si;! I nm Si 0.25 nm roughness Fig. 6: Reflectivity of Cu Ka radiation versus diffraction angle measured on a WSi,fSi multilayer. For comparison, the calculated reflectivity is given (shifted by 2 decades). I n-6 I Diffraction angle 20 (deg) In contrast to the WSi,/Si system, we found that it is more difficult to achieve good multilayer films with Ni as the reflector material. As shown in Fig. 7, we deposited various multilayers with Ni in combination with different spacer materials under various sputtering conditions. For periods larger than 4.5 nm, it can be seen that different spacers may be used in combination with Ni to obtain reflectivities of about 90 %. However, all the multilayers with periods less than 4.5 nm exhibited a loss of reflectivity. In particular, our Ni/C multilayers showed a steep decrease of reflectivity for periods smaller than 4.5 nm. Cross-sectional transmission electron microscopy studies [ 151 performed on these Ni/C multilayers revealed that the Ni layers are crystalline, and that smooth and sharp interfaces are present at periods larger than 4.5 nm. However, multilayers with shorter periods exhibited non-contiguous Ni layers, indicating that the percolation threshold [ 161 at which Ni forms coalescent layers in Ni/C multilayers is at 2.25 nm for our deposition conditions.

10 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Fig. 7: Peak reflectivities for Cu Ka radiation versus the period d measured on Ni/C, Ni/Mg, and Ni/B,C multilayers. All multilayers have 100 layer pairs and a thickness ratio r= 0.5. Period (nm) As can be seen in Fig. 7, Ni/Mg multilayers showed better reflectivities than Ni/C at periods below 4 nm. However, Ni/Mg multilayers are not very stable against aging, since interdiffusion and corrosion are limiting factors in this system. Despite of this, our Ni/Mg prototypes still provide good performance even two years after fabrication, so that they may be considered for future commercial applications. The best reflectivities were obtained with Ni/B,C multilayers. However, these multilayers were more difficult to fabricate. Slight drifts observed in the B,C sputtering rates tend to broaden the Bragg peaks. More critically, we noticed large compressive stresses in these Ni/B,C multilayers that limit their substrate adhesion. In our investigation, Ni/B,C turned out as the most promising, but also the most challenging material combination. Kf3 SUPPRESSION Monochromatic radiation is desired in most XRD applications. Using x-ray tubes, at least, the KP radiation and the Bremsstvahlung shall be suppressed. The suppression of unwanted x-ray wavelengths was examined by measuring a quartz powder sample in Bragg-Brentano geometry with Cu radiation and different parallel beam multilayer mirrors in the incident beam path. As can be seen in Fig. 8, WEi multilayer mirrors have a relatively poor suppression of Cu KP and W La lines. The use of WSi,/Si multilayer mirrors leads to an improvement of the spectral purity. Compared to the Cu Ka SiO,(lOl) reflection (normalized to 100 %), the Cu KP reflection is about 0.3 % for the WEi multilayer, 0.1 % for the WSi,/Si multilayer, and less than 0.05 % for the Ni/B,C multilayer. Apparently, Ni/B,C multilayer mirrors are best. This is due to the favorable position of the Ni K edge, which is located just between Cu Ka and Cu KP wavelengths. Therefore, the Ni layers in the multilayer serve as an additional KP filter when used with Cu radiation.

11 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol %.!Y z E i >.Z % E 2 I 1 % 4 4 Cu KP w La Cu Ku e (de91 28 mirror: W/S i NSi#i NilB4C an Fig. 8: Diffraction patterns measured on quartz powder using WlSi, WSi,/Si, and Ni/B,C curved graded multilayer mirrors to quantify the suppression of Cu KP radiation by different mirrors. The mirrors were 40 mm long, and the center of each mirror was positioned at f = 90 mm. The Cu Ka SiO,( 101) peaks were normalized to 100 %. BEAM UNIFORMITY Tests of beam uniformity and divergence were performed by coupling the parallel beam into Ge(220) ( ) channel-cut monochomators, in the same way as in Refs. [3,4]. Due to the extremely small acceptance angle of these channel-cut monochromators, imperfections of the parallel beam are readily revealed in this high-resolution configuration. Therefore, it represents a crucial test for the quantification of the overall mirror quality. (a> T 3 -E z L E E 20- ii m 80/ ' ' ' ' 0: -1.o o Lateral Coordinate [mm] (b) Fig. 9: Beam profile obtained from a prefigured Ni/Mg multilayer after coupling into a channel-cut monochromator: (a) photo image and (b) detector scan. The beam uniformity was a problem in previous multilayer mirrors, where coated Si wafers were bent and glued to a backing plate [3,4,9]. In these bent multilayer mirrors, the reflected beam

12 . Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol exhibited striations over its width of 1 mm that indicate imperfect, slightly faceted curvatures of the mirrors. Figure 9 shows the beam profile of a Ni/Mg multilayer deposited on a prefigured substrate. In spite of the smaller peak width of a Ni/Mg multilayer compared to a W/Si or a W/B,C multilayer, Fig. 9 reveals that these striations are no longer present in the multilayer mirrors which were deposited directly onto prefigured substrates. The remaining intensity variation (Fig. 9b) is likely the result of period imperfections. The maximum intensity variation of 20 % would correspond to a relative period error Ad/d of only 0.26 %. Based on the improved beam profiles, we can expect a significantly higher intensity with the prefigured multilayer mirrors than was possible with bent and glued mirrors. INTENSITY GAIN Table 1 summarizes the intensity gain factors obtained by coupling the parallel beam from various parabolically curved graded multilayer mirrors into a channel-cut monochromator. The parallel nature of the beam leads to a considerable increase in the throughput when coupled into these low-acceptance channel-cut monochromators. Compared to a configuration without a multilayer mirror, an intensity gain factor of 6 has been achieved previously using a bent and glued multilayer mirror [3]. As can be seen in Tab. 1, the use of high-quality quartz substrates improved the intensity gain factor from 6 to 11 when WEi multilayer mirrors were used. WSi,/Si multilayer mirrors showed similar gain factors. A further intensity improvement was obtained applying Ni-based multilayer mirrors. The resulting gain factor of 16 for Ni/B,C has significantly approached the theoretical limit of about Table 1: Intensity gain factors obtained by parallel-beam coupling of Cu Kcx radiation from a fine focus tube into a Ge(220) ( ) monochromator. The mirrors were 60 mm long, and the center of each mirror was positioned atf= 150 mm. Details of the experimental setup are the same as in Ref. [3]. The multilayer mirrors had a thickness ratio of r= 0.5, and consisted of 50 layer pairs for W/Si, and 100 layer pairs for the other material combinations. optical figure multilayer intensity gain factor bent and glued Si wafer [3] WlSi 6 prefigured quartz WfSi 11 prefigured quartz WSi,/Si 12 prefigured quartz NUMg 15 prefigured quartz Ni/B,C 16 theoretical limit [9] 20-25

13 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol CONCLUSIONS It can be concluded that significant improvements were obtained in the design, fabrication and application of curved graded multilayer mirrors for laboratory x-ray diffractometry. Previous limitations could be overcome by the use of prefigured substrates of higher quality in conjunction with optimized multilayer materials. This was made possible by a dedicated deposition process that satisfies the more stringent precision requirements imposed by direct deposition on prefigured substrates. The new prefigured multilayer mirrors improve beam uniformity and provide higher intensities than bent and glued mirrors, as well as eliminate long-term stability problems connected with the glue. In the optimization of the multilayer materials, WSi,/Si multilayers showed promising results such as excellent smoothness and layer conformity, even for short periods and more than 100 layer pairs. Therefore, compared to WEi multilayer mirrors, almost identical peak reflectivities, but improved spectral purities, were obtained for this material combination. Therefore, WSi,/Si is currently being introduced into the market of commercially available Giibel Mirrors. A further improvement of intensity gain and spectral purity was obtained using Ni/Mg and Ni/B,C multilayer mirrors. Therefore, Ni-based multilayers will undoubtedly be used in the next generation of commercially available Giibel Mirrors. As an important result of our efforts, an intensity gain factor of 16 was obtained in HRXRD applications. This value has to be compared to the factor of 6 reported in earlier work, and approximates closely to the theoretical limit of ACKNOWLEDGEMENTS We would like to thank K. Wulf (GKSS) for assistance in multilayer deposition, and Ch. Borchers (Institute of Materials Physics, University Gijttingen) for electron microscopy investigations. The help of H. Domann (GKSS) and M. Dahms (GKSS) in the preparation and characterization of figured substrates is gratefully acknowledged. We are also grateful to R. Bormann (GKSS), L. Briigemann (Bruker-AXS) and R. Stiimmer (Siemens AG, ZT MF 7) for experimental support and valuable discussions, and to U. Scheithauer (Siemens AG, ZT MF 7) for Auger analyses on the multilayers. REFERENCES [l] H. Gijbel, Paper 101, ACA Meeting, Pittsburgh 1992 [2] H. Giibel, Abstracts 38th Annual Denver Conference on Applications of X-ray Analysis (August l-5, 1994, Steamboat Springs, Colorado, USA) [3] M. Schuster and H. GBbel, J. Phys. D: Appl. Phys., 28 (1995) A270 [4] M. Schuster and H. Gijbel, J. Phys. D: Appl. Phys., 29 (1996) 1677 [5] M. Schuster and H. Gijbel, Adv. X-ray Anal., 39 (1997) 57 [6] R. Stiimmer, T. Metzger, M. Schuster, and H. Gabel, II Nuovo Cimento, 19 (1997) 465 [7] M. Schuster, H. Giibel, and F. Burgzzy, Bruker Report, 145 (1998) 9

14 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol [8] R. Stammer, H. GBbel, A. R. Martin, W. Hub, and U. Pietsch, Semiconductor International, (May 1998) 81 [9] G. Gutman and B. Verman, J. Phys. D: Appl. Phys., 29 (1996) 1675 [lo] J.H. Underwood and T.W. Barbee Jr., Proc. Topical on Low Energy X-Ray Diagnostics (Monterey, California, USA) June 8-10, 1981, in AIP Conf. Proc. (USA) 75 (198 1) 170 [ 1 l] H. Domann, unpublished [ 121 B.L. Henke, E.M. Gullikson, and J.C. Davis, Atomic Data and Nuclear Datu Tables, 54 (2) (1993) 181 [ 131 C. Michaelsen, K. Barmak, and T. P. Weihs, J. Phys. D: Appl. Phys., 30 ( 1997) [14] R. Senderak, M. Jergel, S. Luby, E. Majkova, V. Holy, G. Haindl, F. Hamelmann, U. Kleineberg, and U. Heinzmann, J. Appl. Phys., 81 (1997) 2229 [15] Ch. Borchers, P. Ricardo, and C. Michaelsen, unpublished [16] Ch. Morawe and H. Zabel, J. Appl. Phys., 80 (1996) 3639