Copyright (C) JCPDS International Centre for Diffraction Data 1999 346 Pulsed Laser Deposition of laterally graded NE-multilayers and their application in parallel beam X-ray optics T. Holz, R. Dietsch, H. Mai, FhG-IWS, Winterbergstrafle 28, D-01277 Dresden, Germany L. Briigemann, Analytical X-Ray Systems GmbH, Siemensallee 84, D-761 87 Karlsruhe, Germany S. Hopfe, R. Scholz, MPI fir Mikrostrukturphysik, Am Weinberg 2, D-06120 Halle, Germany R. Krawietz, B. Wehner, TU Dresden, Momtnsenstrafie 13, D-01069 Dresden, Germany Abstract Since the end of the eighties Pulsed Laser Deposition (PLD) has been successfully used for the preparation of multilayers having X-ray optical quality. Outstanding features of the PLD-process are high thickness uniformity, precision of deposition process, formation of smooth interfaces and suppression of columnar thin film growth regime. In order to gain high quality layer stacks involving uniform thickness or graded thickness distributions across 4 -wafers the conventional thin film PLDequipment has been modified. It provides a precise spatial control of the plasma plume orientation in the UHV deposition chamber. Layer stack morphology, interface roughness and reproducibility of period thickness in the total layer stack were characterized by means of grazing incidence X-ray reflectometry (Cu Ka radiation) and HREM (High Resolution Electron Microscopy). PLD-NYC multilayers consisting of up to 75 double layers with period thicknesses between 3.0 nm and 5.5 nm show constant thickness gradients in the range of 0.1-1.O run/cm across the total substrate length. A high reflectivity of up to 85% and an excellent energy resolution are obtainable. For parallel beam X-ray optics these multilayers are bent to a parabolical shape. The performance of such PLDmanufactured Giibel-Mirrors is demonstrated by selected results. Introduction X-ray optical multilayers are widely used for spectroscopic analysis, monochromatization and beam handling of soft X-rays in spectrometers and at synchrotron beam lines. The performance of these multilayers for selected X-ray wavelengths is determined both by geometrical parameters (period number N, reproducibility od of period thickness d, ratio of single layer thicknesses r, interface roughness DR and their correlation) and by structural parameters (refractive index n= 1-6+@, mass density p, concentration gradients). The interference of the X-rays that are reflected at each of the interfaces of a layer stack yields to high intensity BRAGG-peaks only when X-ray wavelenght h, angle of grazing incidence 0 and period thickness d meet BRAGGs equation (1) II& = 2d sin(@)( 1-6 / sin2(@)) ; m-integer (1)
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Copyright (C) JCPDS International Centre for Diffraction Data 1999 347 As opposed to the lattice spacing of natural crystals the period thickness of layered synthetic microstructures can be chosen within a particular range. The laterally graded period thickness is the basic of the so-called,,gobel mirrors [l] which transform a polychromatic divergent beam into a monochromatic parallel beam. It is possible by graded period thickness to fulfill BRAGG s equation at each point of a parabolically shaped surface for the X-ray source located in the focal point of the substrate surface. Experimental / Characterization Graded Ni/C- multilayers were prepared by PLD in an UHV-environment. The typical features of PLD thin film growth are substantial kinetic energy particles (O.Ol... 1 kev), high deposition rates per pulse (=lnm/min, lohz), vapor phase condensation far from thermal equilibrium and pulsed ablation and deposition regime [2]. The experimental arrangement and the basic processes of the interaction between pulsed laser beam and target material were already described in detail [3,4]. Fig.1 shows the principle of large area PLD. A dual laser beam PLD-source (Q-switched Nd-YAG-laser, wavelength h= 1064nm, pulse energy Ep=l 85, pulse length r,,=7-9ns, divergency 0.16 mrad) was used to deposit multilayers onto silicon substrates at room temperature. A particular target and substrate handling regime allows the deposition of graded period thickness across 4 -wafers. Fig. 1 Principle of large area PLD Model specimens of a typical multilayer design but reduced period number have been effectively prepared for a wide spectrum of characterization experiments and for testing the synthesis technology itself. The Ni/C-multilayers were characterized by X-ray reflectometry (Cu Ka) and HREM. Grazing incidence X-ray reflectometry is a very sensitive tool to test the reproducibility of the deposition process. Measured characteristics and simulation of the reflectivity of a graded Ni/C-multilayer (Si-SiO,-29*(CNi) ) are shown in Fig. 2 and Fig. 3, respectively. Here the reflected intensities are normalized to the incident intensities. Despite of the low period number (N=29) the X-ray reflectivity of lst order BRAGG peaks reaches R=50-70% (Fig. 2a) and a constant gradient of Ad/A~=1.8*10-~ (Fig. 2b) is derived from period thicknesses at three different substrate locations x. The detection of all Kiessig fringes evidenced by
Copyright (C) JCPDS International Centre for Diffraction Data 1999 348 theory within the 2 d BRAGG order interval by high resolution measurement (Fig. 3b) proves the reproducibility of the period thickness in the total layer stack. Single layer thicknesses of carbon (dc=3.02nm) and nickel (dniz1.88nm) are deduced from simulation at substrate location x=0 (Fig. 3a) so that the thickness ratio r=dni/d amounts to 0.38. yea,,,,,,!: -30-20 -10 0 10 20 30 - x/mm 0.5 1.0 1.5 2.0 2.5 3.0 Oi Fig. 2 Measured rejlectivity (a) and period thickness (b) of a 29 period M/C multilayer at three d$erent substrate positions x lo4 IJ: IO 4.g 102 B.s measurement 10 LO0 simulation lo- I I I I I I 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 O/ Fig. 3 Survey (a) and high resolution measurement (b) of X-ray rejlectivity prove the reproducibility of the period thickness in the total layer stack of the 29 period multilayer
Copyright (C) JCPDS-International Centre for Diffraction Data 1999 349 Multilayers used in our experiments as GOBEL-mirrors consist of 75 Ni/C periods thus providing near optimum reflectivity and spectral resolution. High resolution measurements proved the equivalency of the basic morphology of model specimen (N=29, Fig. 3b) and real X-ray mirrors (N=75, Fig. 4) so that the analytical results can directly be applied for the general description of the optical components. The different resolution of the Kiessig fringes observed in Fig 3b and Fig. 4 results only from a finite divergence of the reflectometer used. Since its equivalence remains still below the separation of adjacent Kiessig fringes all fringes of the multilayer (d,,t=300nm) have been resolved. A photo of the arrangement of X-ray source, Si( 111) crystal monochromator, slits, sample and scintillation counter is to be seen in [5]. IO0 F 10 F I I I I I I,,, I I, I, I, I I I, I I I, I NE-multilayer h =O. I 54nm lo- : : high resolution - measurement 1o 3 1w3 " " " " " " " " " " ' "I' I 0.2 0.4 0.6 0.8 1.0 1.2 1.4 @I Fig. 4 High resolution measurement of reflected intensities of a 75period multilayer Results of HREM and X-ray characterization of a graded multilayer are shown in Fig. 5 and Fig. 6, respectively. The HREM micrograph of a PLD-Ni/C multilayer is to be seen in Fig. 5.Both nickel (dark) and carbon layers are amorphous. A period thickness of d=3.65nm was determined. Layer stack regularity, low interface roughness and the absence of columnar thin film growth are characteristics of PLD-Ni/C multilayers. The interface roughness (TR has the strongest influence on the multilayer reflectivity. Depending on the used simulation model Debye-Waller factors describe quantitatively the effect of reflectivity reduction resulting from interface roughnesses or intermixed layers. With decreasing period thickness d the Debye-Waller factor decreases rapidly too, if the interface roughness exceeds a level of o~>d/lo [6]. So (TR < 0.4nm is decisive for a high reflectivity in the range of period thickness 3.5nm < d < 5.5nm.
Copyright (C) JCPDS-International Centre for Diffraction Data 1999 Fig. 5 HREMmicrograph of a 75 period NYC multilayer prepared by cleavage technique 350
Copyright (C) JCPDS International Centre for Diffraction Data 1999 351 gradient of ueriod thickness idi A =2.%IO-*. _.~A-L-1 1.x CbL_L_~> ~_L. 1 0.5 1.0 1.5 2.0 2.5 3.0 Fig. 6 Measured reflectivity (a) and period thickness (b) of a 75 period Ni/C multilayer at three different substrate positions x Experimental determined reflectivities between 50% and 84% (Fig. 4 and Fig. 6) as well as simulations and HREM results support the estimate of or=0.4nm. An important advantage offered by Ni/C X-ray mirrors for application in X-ray diffi-actometry is the discrimination of Kj3 radiation in the reflected beam. Simulations of reflectivity R&O) of a multilayer with the following parameters: ~~ 2.4 gcme3, onic8.1 gcmt3, N=75, or=0.4nm, I=05 and d=3.5nm were carried out for 1=0.139nm and h=o. 154nm. At the angle of incidence O= 1.30 ( lst order BRAGG-peak of Cu KU) the reflectivity R(Cu Ka) is typically 500 times higher than R(Cu KP). This means that a Ni/C-GOBEL mirror should be able to change the Cu Kc&u KP intensity ratio so that in the reflected parallel beam a ratio of almost 5000 should be observable. Experimental ratios were found in the range between 1000-3000. Further evaluations of expected data reveal a remarkable low FWHM of the 1 St order Bragg reflexes. This additional advantage of the NYC combination results in an almost parallel X-ray beam of a divergence as low as AO=O.O35. Some applications are given to confirm these predictions and to demonstrate the explained advantages of PLD GOBEL-mirrors of the Ni/C type. Applications (A) X-ray reflectometry, standard and grazing incidence diffraction (GID) measurements were carried out using a SIEMENS AX S diffractometer D 5005. Fig 7a shows the experimental setup for standard diffraction measurements.
Copyright (C) JCPDS-International Centre for Diffraction Data 1999 352 The Si(O04) peak was used to measure the divergence of the parallel beam of a GbBEL mirror. A rocking scan measurement and the Gaussian tit profile are to be seen in Fig. 7b. Rocking scan Fit profile: Gaussian 0 = 34.5094 (+o.oool~) 40kV, 4OmA, Cu NX Mirror, lmm -..._ I - --. Fig. 7a Experimental setup for standard d&j?-action measurement / ii I o,.4, i/ \~yyyt, Fig. 7b Rocking scan of Si(O04) 34.5 34.6 @I0 AO=O.O3 1 (FWHM) and an intensity maximum of 6OOOOOcps were found. This FWHM is a typical value for 1 St order BRAGG peaks obtained by Ni/C multilayer reflection and indicates that the geometry of GOBEL mirror curvature does not introduce any additional effects from irregular figure. Standard diffraction measurements of a quartz powder sample are to be seen in Fig. 7c. The positions of the strongest Cu Ka and Cu KP reflexes are marked. The Cu KP peak is totally suppressed, i.e. its intensity is lower than noise intensity (lcps), while Cu Ka peak intensity reaches 500~~s. 500 intensity [cps] 400 Standard 0.4 w-b---s - 300 1 Quartz 4OkV/4OmA, Cu Ni/C Mirror, lmm ti 2 Soller, Quartz, 0.2mm, 2 Soller, 0. lmm 1 0 20 21 22 23 24 25 26 27 28 201 Fig. 7c Standard dlfiaction measurements of a quartz powder sample
Copyright (C) JCPDS-International Centre for Diffraction Data 1999 353 09 A corundum powder sample (supplier: NIST, USA) was characterized by an experimental setup after Fig. 8a. The results are given in Fig. Sb. Details of the GID measurement are: step time t=ls, step width A(20)=0.02 and fixed angle of grazing incidence ~3. Two low intensity Cu Kp peaks at 20~3 1.8O and 20~5 1.5 were detected. The intensity ratio between Cu Ka and Cu K/3 observed in the A1203( 104) reflection is larger than 2000. Fig. 8a Experimental setup for GID (grazing incidence dlfiaction) intensity [cps] GID Corundum Al203 40kV/40mA, Cu Ni/C Mirror, lmm 2 Soller, NIST Corundum, 0.15 gracing incidence soiler slit Fig. 8b GID measurement of corundum (AlzOJ It is typical for the parallel beam setups that sample displacements or changes of the irradiated area at different angles of incidence will not result in peak shifts or changes in resolution. Various GID measurements of a quartz standard sample at different angles of incidence between 2-14 in steps of 2 (Fig. 8c) confirm this claim.
Copyright (C) JCPDS-International Centre for Diffraction Data 1999 354 intensity 600 IcPsl 67.0 67.5 68.0 68.5 69.0 2-o I O Fig. SC Various GID measurements of a Quartz standard sample at different incident angles between 2-14 in steps of 2 (C) A high intensity gain can be achieved in X-ray reflectometry by using GGBEL mirrors. As an example Fig. 9b shows analytical measurements involving a dynamic range as high as 7 orders of magnitude. The determined layer thicknesses, densities and interface roughnesses are listed in Tab. 1. reflectivity X-ray reflectometry --- measurement!. -. d,a @I Fig. 9a Experimental setup for rejlectometry Fig. 9b Measurement and simulation of X-ray reflectivity of a layer system (thermal rejlector on jloat glass) layer thickness /nm roughness / nm density / gem layer type Biz03 32.4 0.5 7.3 uniform Au 7.9 0.4 11.0 uniform Bi203 32.7 0.5 7.3 uniform float glass - 0.4 2.5 substrate Tab. I Numerical results (according to Fig. 9b) of thin Jilm analysis by X-ray reflectometry (Fig. 9a) using a GC%EL mirror
Copyright (C) JCPDS-International Centre for Diffraction Data 1999 355 Discussion The results of HREM and X-ray characterization show the peculiarities of the NiIC multilayers prepared by PLD. The requirements of GOBEL mirrors such as reproducibility and homogeneity of deposition process, low interface roughness providing high reflectivity and energy resolution of the multilayers are met by the PLD technique. Advantages of the Ni/C GGBEL mirrors are: l l intensity gain of up to 50 depending on the application suppression of Cu KP radiation in the monochromatized parallel beam so that I CuK, / ICuKp loo0 l l low divergence of the parallel beam A(p<O.O35 improvement of signal/noise ratio and resolution by application in the diffracted beam side in XRD [7] Acknowledgements The authors are indebted to Dr. H. Gobel for helpful discussions. The work was supported by the Federal Ministry of Education and Research of Germany under Grant No. 13 N 6487. References VI M.Schuster, H.Gobel, J. Phys. D: Appl.Phys. 28, A270 (1995). VI H.Mai et al., Proc. SPIE 2256,268 (1994). [31 R.Dietsch et al., Optical and Quantum Electronics 27, 1385 (1995). [41 R.Dietsch et al., presented at COLA 97, July 21-25, Monterey, CA, submitted for publication in Appl. Surf. Sci. 151 http://www.phy.tu-dresden.delikfplgiflroe2.gif E.Spiller, Soft X-Ray Optics, SPIE, Bellingham, Washington 1994 [61 [71 R. Stammer et al. (poster D 98), to be published in the proceedings of the 46th Denver X-Ray Conference, Advances of X-Ray Analysis 41.