MICROSTRUCTURAL CHARACTERIZATION OF THIN FILMS AND SURFACES BY A NEW GRAZING INCIDENT X-RAY DIFFRACTOMETER

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1 The Rigaku Journal Vol. 17/ No. 2/ 2000 CONTRIBUTED PAPERS MICROSTRUCTURAL CHARACTERIZATION OF THIN FILMS AND SURFACES BY A NEW GRAZING INCIDENT X-RAY DIFFRACTOMETER SHIN-YA MATSUNO*, MASAYUKI KUBA, TAKESHI NAYUKI, SUKETAKA SOGA AND P. W. T. YUEN a) Analytical and Computational Science Laboratories, Asahi Chemical Industry CO. LTD.,2-1, Sameiima, Fuji, , Japan a) Present address: DERA, Winfrith Technology Center, Winfrith, Newburgh, Dorchester, Dorset DT2 8XJ, U. K, 1. Introduction Modern devices require the production and characterization of state-of-the-art-ultra-thin films under 10-nanometers thick. Examples can be found in integrated circuits, magnetic pickup heads and high effi-ciency solid state lasers. In thin films with thickness greater than 10 nanometers, the crystallographic properties (crystalline quality, orientation relationship between films and substrates, etc.) were mainly investigated by rocking curve measurements and reciprocal lattice mapping using a high resolution X-ray diffractometer [1-4]. However, the characterization of ultra thin films with thickness less than 10 nanometers has been a scientific challenge. To analyze crystallographic and physical properties such as film uniformity, interfacial morphology and density of the films etc., X-ray reflectivity at grazing incidence has been a powerful [5, 6]. The heteroepitaxial growth of thin films by sputtering normally grows along the uni-axial crystallographic orientation of the substrate. Conventional X-ray diffraction measurement however only probes lattice planes parallel to the surface with a characteristic single diffraction peak normally obtained from these heteroepitaxial films. Unfortunately, in-plane information is severely lacking by using this technique. Due to the weak nature of signals as obtained from grazing incidence X-ray diffraction, the efficiency of the method for characterization has been a major concern for researchers in this field. There is a desperate need to design, and to develop an efficient high intensity and high resolution structural *Author to whom correspondence should be addressed; matsuno.sb@om.asahi-kasei.co.jp probe that enables one to characterize both the inplane and out-of-planeproperties of thin films. The purpose of this work is to describe the design, the functionality and the performance of a highly efficient high resolution X-ray diffractometer, the ATX-G, for both X-ray reflectivity measurement and in-plane X-ray diffraction analysis. This system is the result of collaborative work with the research team at Rigaku Corp. in Japan. This machine is now being used our daily work in our laboratory at Asahi Chemical for the evaluation of film inhomogeneity and anisotropy. 2. Overview-The Method of Characterization of Thin Films by Grazing Incident X-ray 2-1. Evaluation of Structural Morphology on a Nanometer Scale Using X-ray Reflectivity Measurements X-ray reflectivity measurements have been one of the most powerful structural probes for characterizing morphological properties of thin films on nano-meter scales such as film thickness, density, and interface roughness [5, 6]. The basic principles and the effectiveness of X-ray reflectivity measurements has already been introduced in the Rigaku Journal [7-9]. In short, it is available to understand the depth profile of density in materials using X-ray reflectivity measurements. The effectiveness of X-ray reflectivity for depth profile information of density stems from the momentum transfer of the X-ray in the direction perpendicular to the surface of the sample (see Fig. 1). Synchrotron radiation is usually used for the characterization of ultra thin SiO 2 films of a few nanometers thickness [10]. X-ray reflectivity measure- 36 The Rigaku Journal

2 ments cover a dynamic intensity range of more than six orders of magnitude, and in general they are shown in logarithmic scales. In the sense, synchrotron radiation is useful for evaluation of ultra thin films, which show long periods of oscillations of X-ray reflectivity. A dynamic intensity range varied over high orders of magnitude is advantageous to evaluate the interface roughness, because X-ray reflectivity curves change larger and larger in the high angle region. Analysis of the X-ray diffuse scattering around the specular reflection also gives information about in-plane density-density correlation. Although we can not distinguish the interface roughness from diffusion of each layer by X-ray reflectivity measurements, we Fig. 1. Momentum transfer vector in X-ray reflectivity measurement. can distinguish between the two phenomena by X-ray diffuse scattering measurements, because X-ray diffuse scattering arises from only physical roughness of interfaces. In the case of measurements of diffuse scattering intensity, the angle between incident X-ray and sample surface should be changed while the scintillation counter is held at a fixed position and it is called a 'rocking scan'. Thinking of reciprocal space, momentum transfer vector moves parallel to the sample surface, so in-plane density-density profiles can be evaluated by analyzing rocking scan profile Structural Characterization of Surfaces by Grazing Incidence X-ray Diffraction The grazing incidence X-ray diffraction (GIXD) method utilizing X-ray total external reflection is one of the most useful analytical methods for surface structures. For this method, a very wellcollimated high intensity X-ray beam with precisely control incident and exit angle is needed. Generally, this method can be implemented using the three arrangements as shown in Fig. 2. In Seemann-Bohlin's arrangement where the grazing angle between the incident X-ray and the sample surface is fixed and detector is rocked in the plane perpendicular to the incident beam. In this configuration, the Bragg angle can be shifted as the result of X-ray refraction and causing the direction of diffracting plane to vary as the detector is scanned. Fig. 2. In-plane arrangement of XRD with two kinds of conventional XRD arrangement. Vol. 17 No

3 Fig. 3. Calculated penetration depth and reflectivity for Cu-Kα1 radiation as a function of the incident angle. Fig. 4. X-ray diffration patterns of TiSi2 thin films on Si substrates. These problems, however, rarely occur in the conventional X-ray diffraction experiment. Up until now, the measurement of the properties of films with thickness in the range nm such as Al on GaAs substrates [11], or 30 nm TiSi 2 thin films on Si substrates [12] were done exclusively using grazing incident X-ray diffraction with synchrotron radiation. The sensitivity of these experiments were the result of suppressing the X-ray penetration depth using grazing incident beam. The relation between the angle of the incident X-ray beam to the Si surface and 1/e intensity penetration depth of the X-ray beam with Cu-K α1 radiation is shown in Fig. 3. It is seen from Fig. 3 that the X-ray penetrates only a few nanometer into the Si surface for incident angles below the critical angle of total external reflection. Most of the oriented grown films showing a single diffraction peak in the conventional X-ray diffraction experiment, due to the uni-axial orientations of the films with respect to the substrates. For example, the X-ray diffraction pattern of Ti thin films grown on Si substrates by sputtering and subsequent annealing at 650EC in N 2 gas atmosphere as obtained from the conventional diffraction experiment is shown in Fig. 4. In this case, it is difficult to identify the phase of the thin films, in addition to the complications caused by stress induced shift of the Bragg angles. 3. Conception and Features of a New Diffractometer As mentioned earlier, it is crucially important to characterize the morphology of the films in nanometer scale in addition to determining the structural information on an atomic scale. Additional information, as obtained from in-plane X-ray diffraction measurements are also important for characterization of thin films that are uni-axially oriented. Bearing all these considerations in mind, we have successfully produced an inovative high performance grazing incidence X-ray diffractometer. Distinct features of this new design are as follows: (1) Extremely high intensity X-ray beam (2) Multiple-axis goniometer for performing inplane X-ray diffraction measurements These features are the result of completely designed X-ray optics using a parabolic multilayer mirror with graded d-spacings and an asymmetrically cut Ge(220) channel monochrometer, and a specially designed four-axis goniometer. The apparatus is schematically shown in Fig. 5 consisting of an 18-kW rotating anode X-ray generator, a parabolic multilayer mirror, Ge(220) monochrometer, and a four-axis goniometer [13]. Divergent Cu K X-rays from the rotating anode X-ray generator are first collimated and monochromatized by the graded multilayer mirror to a broad Cu Kα X-ray beam with 0.045E divergence. This Cu Kα X-ray beam is further collimated to a narrow and parallel X-ray beam of 0.015E divergence by the asymmetrical-cut Ge(220) channel monochrometer. The goniometer has two conventional T/22 axes and two in-plane N/22 P axes. As shown in Fig. 6, the N/22 P axes are mounted horizontally on the vertical T 38 The Rigaku Journal

4 Fig. 5. Schematic illustrations of the apparatus. Fig. 6. Schematic illustration of the goniometer. axis. The T/22 axes are used for measurements of outof-plane diffraction, and the N/22 P axes for in-plane diffraction. The specimen stage has two rotation axes, Rx and Ry, for tilting the specimen surface so that the surface normal is properly aligned parallel to the N axis and centered at the intersection of the N and the T axes. Detector is a scintillation counter, and an Al plate with pre-calibrated attenuation factors can be inserted in front of the detector when a very high intensity reflected beam is encountered. 4. Field Tests of the Apparatus on Thin Solid Films 4-1. Evaluation of Thickness and Interface Roughness of Ultra Thin SiO 2 Films on Si Wafers The ability to control and to characterize the thickness and the interface roughness of the ultra-thin SiO 2 gate of 10nm thickness is becoming more routine in the ultra-large-scale integrated (ULSI) devices fabrication process. Fig. 7. X-ray specular reflectivity curves for Sample 1 and 2. In the following section, we would like to demonstrate the use of this newly designed system to determhine the layer thickness, surface and interface roughness of two SiO 2 films grown by the thermal oxidation process on Si wafers. Sample 1 was grown under under N 2 /O 2, and Sample 2 under pure O 2. The GIXR technique was used to determine the layer thickness, surface and interface roughness of the films. The experimental reflectivity curves for Sample 1 and 2 are shown in Fig. 7. Sample preparation condition and obtained results are shown in Table 1. The interference fringes were weak because of the small difference between the electron density between SiO 2 and Si. Reflectivity intensities, which varied over eight orders of magnitude, were collected so that a reliable analysis of the reflectivity data could be obtained. Fourier transform analysis [14] of both reflectivity curves shows a strong peak at a thickness of 6.5 nm (see Fig. 8 for Sample 1). The experimental reflectivity curves were also analyzed by fitting with calculated reflectivity curves derived from Parratt's recursive formula modified Vol. 17 No

5 with the distorted wave Born approximation (DWBA). [15, 16] The modified Parratt's recursive formula is as follows. R R R, + + F, F n n n n n n = a , n 1 n, n n, n a i d R a E n π exp n n, E a n n n, +, λ φ ( 2 2 ) φ θ δ i β n n n 1 2 R n n 0 1 (1) Here, F n-l,n is the Fresnel reflectivity coefficient of the interface between n-l-th layer and n-th layer, dn the thickness of n-th layer, and R n-l,n the reflectivity amplitude of the interface between n-l-th and n-th layer. The Lenvenburg-Marquardt method was used in this study to match the calculated reflectivity curves with the experimental reflectivity data. In this case, analysis of interface roughness was done by the following procedure. First, the surface roughness was determined prior to the least squares fitting analysis, because of its strong effect on the mean reflectivity. This situation is shown in Fig. 9(a). The results for Sample 1 are shown graphically in Fig. 9(b), and the results with respect to layer thickness and roughness for both samples are given in Table 1. The results as shown in Table 1 indicate both Samples 1 and 2 have the same SiO 2 layer thickness of 6.5nm, in good agreement with the results obtained by the Fourier transform method. The results also show that the surface roughness for the both specimens were approximately the same within 0.04nm. The Table 1. Sample preparation conditions and obtained results. Sample 1 Sample 2 Atmosphere N 2 /O 2 -flow O 2 -flow Temperature (EC) Time (min) 8 10 SiO 2 thickness (nm) Surface roughness (nm) Interface roughness (nm) Fig. 8. The result of Fourier transform of the reflectivity curves for Sample 1. Fig. 9. a) The mean reflectivity curves dependence of surface roughnesses b) Experimental reflectivity data (open circles) and a fitteed curve (solid line) for Sample 1 40 The Rigaku Journal

6 interface roughness between the SiO 2 layers and the Si substrate for the two samples are, however, significantly different from each other with 0.1 nm for sample 1 and 0.25 nm for sample 2. The results indicate that the buried interface between the SiO 2 layer and its Si substrate were significantly smoother for the layer grown under N 2 /O 2 than under O 2, 4-2. Density of Porous Silica Thin Films [17, 18, 19] Propagation decay, cross-talk noise and power dissipation due to resistance capacitance (RC) coupling becomes significant in the modern ULSI technology when the device dimension shrinks to less than 0.18 nm (transistor gate length). It has been reported that the dielectric constant (k) of the separating insulator has been identified as the crucial factor for causing all these problems. Recent advancements to cure these problems have replaced standard SiO 2 with new interconnect dielectrics (ILD) possessing k values less than 2.5. One possible candidate is the nano-porous silica like ALCAP -S material made by ASAHI KASEI Corp. in Japan. Research data indicate that the density of these porous materials has paramount effect on the k value and it is, therefor essential to evaluate density of these porous materials during the ULSI fabrication process. By using GIXR such as the system described in this paper, the control and monitoring of the fabrication of small k insulators for ULSI applications becomes a simple and straight forward process. Fig. 10. X-ray reflectivity measurements of organic/ inorganic hybrids before and after curing. The porous silica (ALCAPT -S) thin films that were grown from organic/inorganic hybrids were fabricated on SiN (about 1.2 µm thickness)/si substrate by spin-coating method. One of the specimens was cured at 400EC in an N 2 atmosphere. The GIXR results for these films are shown in Fig. 10. Due to the presence of less dense materials on the surface, two critical angles, one at 2Theta 0.3 deg and the other at 2Theta 0.45deg, can be seen clearly at very small incident angles. The former corresponds to the critical angle of the porous silica, and the latter due to the SiN layer. The former critical angle of cured sample is seen to reduce when comparing with the non-cured sample. This indicates that the density of the cured sample is smaller than that of the non-cured sample. This result also suggests that the nano-porous silica thin films were formed predominantly by decomposing and vaporization of organic materials. This experiment demonstrates yet another way to investigate the properties of the porous thin film nondestructively by using grazing incident X-ray technique Depth Profile Information Using In-Plane X-ray Diffraction Measurements on TiSi 2 Thin Films The film used in this study was deposited from a Ti target onto a Si substrate by sputtering and was subsequently annealed at 650EC for 1 minute under a nitrogen atmosphere. A conventional X-ray diffraction pattern for the film obtained by a ω/2θ (or θ/2θ) scan is shown in Fig. 11(a). A strong and sharp TiSi 2 (150) diffraction peak together with two relatively weaker and broad TiN(111) and TiN(200) peaks were observed. The Ti thin film appeared to be reacted completely with the Si and N 2 forming TiSi 2 and TiN during the hightemperature annealing process, as suggested by the absence of Ti diffraction peaks. The in-plane diffraction technique can be used to identify the surface and the interface layers unambiguously as illustrated in this sample. The in-plane diffraction patterns obtained using incident angles (ω) fixed at 0.20, 0.25 and 0.30 are plotted in Fig. 11(b). For the pattern obtained at ω=0.20e, only two TiN peaks, TiN(111) and TiN(200), were detected. At ω=0.20e, the 1/e penetration depth for a Cu-K α X-ray beam was calculated to be 4 nm. When the incident angle increases, the xray beam penetrates deeper into the film, and TiSi 2 - C49(060) and C49(131) peaks begin to appear when ω is 0.25 and 0.30E (see Fig. 1 1 (b)). This result concludes that the Ti film is transformed into two layers: a TiN layer with an estimated surface thickness of about 10nm at the surface, and a TiSi 2 layer at the interface between the film and the Si substrate. Vol. 17 No

7 Fig. 11. (A) Conventional X-ray diffraction patterns of TiSi 2 thin films. (b) In-plane x-ray diffraction patterns for three kinds of X- ray incidant angle; ω=0.20, 0.25, 0.30 deg. Fig. 12. In-plane X-ray diffraction patterns of the same sample ( ω=0.30 deg). Next, in-plane X-ray diffraction measurements were also performed on the same sample after stripping the TiN surface layer by alkaline aqueous solution. The in-plane XRD result was shown in Fig. 12. The plot shows several distinct diffraction peaks in additions to those observed by the conventional X- ray diffraction measurements. These peaks are identified as the Bragg peaks of TiSi 2 -C49 phase. This experiment demonstrates the importance of obtaining film properties in the plane perpendicular to the sample surface using an in-plane arrangement in addition to information regarding to the plane parallel to the sample surface in a conventional XRD arrangement. In this example, we can easily identify the phases of uni-axial oriented thin films from the distinct diffraction peaks as recorded by the in-plane XRD measurement. It is noted that the devalue as observed by conventional XRD appears to be larger than that registered in ICDD-JCPDS data base, but an opposite was observed by the inplane XRD data! This result clearly shows that the TiSi 2 -C49 layer in question is under compressive strain. Hence, a combined experiment using conventional and in-plane XRD measurements is extremely useful for stress analysis of oriented thin films as described in more details in the next section Stress Analysis and Depth Profiles of Crystalline Quality of Si Thin Films Grown on Sapphire Substrates [20] The procedure for growing epitaxial Si films on Sapphire substrate (SOS) is shown in Fig. 13. Two SOS films were used in this study: Film A & B were prepared with and without substrate cooling, respectively during Si ion implantation (see Fig. 13). Conventional XRD measurements and inplane XRD measurements were performed on both Film A and Film B. The devalue of the parallel plane Si(004) and the perpendicular plane Si(040) were deduced from the conventional and in-plane XRD data, respectively. The diffraction results as obtained from Films A & B are shown in Fig. 14. It is intriguing to note that the Bragg peaks as obtained from both methods are not the same, indicating a significant distortion from the cubic symmetry. By comparing the present data with the bulk Si(004) value from the ICDD (JCPDS) data base, a 0.3% larger and smaller d-spacing for the parallel and perpendicular planes respectively of the SOS films are found. This means that the Si thin films were compressed on the order of 100 Mpa. Next, in-plane X-ray rocking curve (XRC) measurements were done by rotating the specimen about the φ axis, while keeping the detector at a fixed angle 2θ χ recording the Si(040) peak. The in-plane 42 The Rigaku Journal

8 Fig. 13. Preparation procedure for a Si film epitaxially grown on a sapphire substrate. Fig. 14. Si(004) and Si(040) profiles measured by conventional XRD and in-plane XRD, respectively. XRC obtained with an incident angle of ω=0.25e is plotted in Fig. 15(a). It shows that the full width at half maximum (FWHM) for Film A is narrower than that for Film B. This indicates that the epitaxial Si film obtained with substrate cooling had a smaller in-plane Si(040) axis dispersion and, hence, better crystallinity than the film that was grown without substrate cooling. Similar results with smaller FWHMs for Film A and larger FWHMs for Film B were also obtained at ω=0.15, 0.20, 0.25, 0.30 and 0.35E (see Fig. 15(b)). Values of FWHM are found to be more uniform for Film A than for Film B. By making use of the variation of X-ray penetration depth with the incident angle, we can evaluate the quality of the film as function of thickness. As revealed from the experiment, it is discovered that the crystalline quality of the Si thin film B (not cooled) varies from surface to substrate. In contrary, film A (cooled substrate) shows a uniform good quality film from surface to substrate. This is a versatile way to evaluate the depth profile of the crystalline quality of the over growth in nano-meter scale. 5. Summary In order to characterize the surfaces and thin films, precisely and timely, in laboratories, a new inhouse X-ray diffractometer was developed. This system features an extremely high intensity, parallel and monochromatic X-ray beam, which provides over eight orders of magnitude of X-ray reflectivity measurements and in-plane XRD measurements. This is achieved by a high-intensity rotating anode X- ray generator, a parabolic multilayer mirror, and an asymmetric-cut Ge(220) channel monochrometer. The in-plane XRD measurements, which provides structural depth-profile information is met by a new designed goniometer that enables a precise control for both of the incident and exit angles of the X-ray. To demonstrate the efficiency of the new apparatus, XRR measurements of ultra thin SiO 2 layers and in-plane XRD of TiSi 2 thin films were performed. The film thickness and interfacial roughness of SiO 2 layers that were grown under various conditions were deduced from the XRR data taken by this new system. The SiO 2 /Si interface roughness had been determined as 0.1 nm for the specimen grown under N 2 /O 2 (specimen 1) and 0.25 nm for the one grown under O 2 (specimen 2). The depth profiles of TiSi 2 layers on Si substrates as revealed by XRD data suggested that the Ti film transformed into two layers: a TiN layer with an estimated surface thickness of about 10nm at the surface, and a TiSi 2 layer at the interface between the film and the Si substrate. Finally, using the newly developed apparatus, microstructural characterization of silicone thin layers on sapphires were performed using this newly developed X-ray apparatus for profiling information with respect to crystalline quality and stress. In-plane X-ray rocking curve (XRC) measurements were performed on two samples of which one of them Vol. 17 No

9 Fig. 15. In-plane XRD of Si(040) at ω=0.25 deg (a) and FWHMs of Si(040) profiles (b). employed a cooled substrate during Si ion implantation. Results indicated that the crystalline quality of the cooled sample showed a much uniform film with better quality than the one without cooling. The uncooled one also exhibited a high degree of variation in crystalline quality from the surface to the substrate. Acknowledgments We are grateful to the heads of Analytical and Computational Science Laboratories; Mr. Sadao lbe, Dr. Kazuya Neki and Mr. Osamu Mitsui and Mr. Yasuhiro Ueshima for their support and encouragement. Also we would like to thank all the members of our laboratories for their interest to this study. References [1] P. F. Fewster, Semicond. Sci. Technol., 8, 1915 (1993). [2] X. Wang et al., J. of Crystal Growth, 171, 401 (1997). [3] T. Hirai, K. Nagashima, H. Koike, S. Matsuno and Y. Tarui, Jpn. J. of Appl. Phys., 35, 5150 (1996). [4] T. Fukunaka, T. Matsui, and S. -Y Matsuno, J. of Materials Research, 14, 39 (1999). [5] D. K. Bowen and M. Wormington, Adv. X-Ray Anal., 36,171 (1993). [6] T. C. Huang, Adv. X-Ray Anal., 38,139 (1995). [7] K. Sakurai, THE RIGAKU JOURNAL, 26, 8 (1995) (in Japanese). [8]. Kojima and Boquan Li, THE RIGAKU JOURNAL, 16(2), 31 (1999). [9] K. Kago, H. Matsuoka, H. Yamaoka, THE RIGAKU JOURNAL, 30(2), 14 (1999) (in Japanese). [10] N. Awaji, S. Ohkubo, T. Nakanishi, Y. Sugita, K. Takahashi, and S. Komiya, Jpn. J. of Appl. Phys., 35, L67 (1996). [11] W. C. Marra, P. Eisenberger, and A. Y. Cho, J. of Appl. Phys., 50, 6927 (1979). [12] H. Tomita, S. Komiya, Y. Horii, and T. Nakamura, Jpn. J. of Appl. Phys., 34, L876 (1995). [13] S. Matsuno, M. Kuba, K. Omote, M. Sakata, Advances in X-ray Chemical Anal., 30,189 (1999) (in Japanese). [14] K. Sakurai and A. lida, Jpn. J. of Appl. Phys., 31, L113 (1992). [15] G. H. Vineyard, F4iys. Rev., B26, 4146 (1982). [16] K. Sinha, E. B. Sirota, S. Garoff, and H. B. Stanley, Phys. Rev. B, 38, 2297 (1988). [17] H. Hanahata et al., IEEE 200 International Interconnect Technology Conference proceedings, p (June 5-7, 2000). [18] C. Jin et al., MRS BULLETIN, 22, No.10, 39 (1997). [19] T. loka et al., SEMATECH ULTRA LOW K WORKSHOP, 95 (1999). [20] S.-Y. Matsuno et. al., Advances in X-ray Anal., 43 (2000), in press. 44 The Rigaku Journal