In situ X-ray Microscopy Studies of Electromigration in Copper Interconnects G. Schneider1^, M.-A. Meyer2, E. Zschech2, G. Denbeaux3, U. Neuhausler4, P. Guttmann5 1 BESSYm.b.H., Albert-Einstein-Str. 15, D-12489 Berlin, Germany e-mail: [email protected] 2 AMD Saxony LLC & Co. KG, Materials Analysis Department, P.O. Box, 11 01 10, D-01330 Dresden, Germany 3 Lawrence Berkeley National Laboratory, Center for X-ray Optics, One Cyclotron Rd, MS 2-400, Berkeley, CA 94720 4 European Synchrotron Radiation Facility (ESRF), X-ray microscopy beamline ID21, BP 220, F-38043 Grenoble Cedex, France 5 Institute for X-ray Physics, c/o BESSYm.b.H., Albert-Einstein-Str. 15, D-12489 Berlin, Germany Abstract. Real-time X-ray microscopy is applied for degradation studies to understand electromigration-induced transport processes in on-chip copper interconnects. The material transport in inlaid Cu line/via structures is observed with about 40 nm lateral resolution. The image sequences show void formation, migration and nucleation processes. Correlation of the real-time X-ray images with post-mortem SEM micrographs is used to discuss degradation mechanisms in inlaid copper interconnects. Due to the high penetration power of X-rays through matter and its high spatial resolution, X-ray microscopy (XRM) overcomes several limitations of conventional microscopic techniques. It utilizes the natural absorption contrast between the structures of interest, i.e. for on-chip copper interconnects embedded in dielectrics. Due to their different X-ray absorption characteristics at 0.52 keV, even different silicon compounds like Si, SiO2, and Si3N4 can be distinguished in X-ray images of thinned layers as demonstrated. For failure analysis of thicker layers, phase contrast microscopy in the multi-keV photon energy range is proposed. are not well understood so far, but they are probably exacerbated by normal stresses at the copper/barrier and copper/capping layer interfaces as well as hydrostatic stress components. Particularly, fast diffusion paths have to be identified and failure mechanisms based on directed transport of atoms have to be understood. This knowledge is the basis for process and materials changes which improve interconnect reliability. In passivated interconnects, the mass flow due to EM is constrained by the embedding dielectrics. This leads to a high mechanical stress in the interconnect which significantly influences the material transport. Therefore, EM phenomena should be studied in situ at fully embedded interconnect structures using an intact layer system including barriers and passivation layers. For this purpose, an imaging technique is required which maintains high spatial resolution when penetrating through several microns of dielectrics. INTRODUCTION Advanced process technologies and new combinations of materials bring new reliability challenges: Different microstructure of the metallic interconnects, other types of interfaces and new degradation phenomena. Electromigration (EM), stress-induced degradation and mechanical weakness in case of low-k materials are reliability concerns for inlaid copper interconnects. The current generation of highly integrated microprocessors, requiring dense interconnects and increased current densities, has highlighted the EM issue. Formation of voids in copper lines induced by EM during normal microprocessor operation will cause an interconnect open or high resistance resulting in malfunction or speed degradation. Void formation, growth and movement, and consequently degradation, depend on the interface bonding and on the copper microstructure of individual vias [1]« Stress-induced degradation phenomena and later catastrophic failures CP683, Characterization and Metrology for VLSI Technology: 2003 International Conference, edited by D. G. Seiler, A. C. Diebold, T. J. Shaffner, R. McDonald, S. Zollner, R. P. Khosla, and E. M. Secula © 2003 American Institute of Physics 0-7354-0152-7/03/$20.00 480 X-RAY IMAGING OF COPPER INTERCONNECTS AND DIELECTRICS E Fresnel zone plates are used as imaging lenses in X-ray microscopes. The numerical aperture NA of a zone plate depends on its outermost zone width drn and the used diffraction order m, i.e. NA=A, m/(2 dr^, where 1 is the wavelength of the radiation. At present, mainly the first diffraction order (m=l) is used as standard scalar diffraction theory predicts a decay in diffracted flux proportional to 1/m2. Therefore, the size of drn which determines the achievable resolution needs to be minimized. To generate the zone plate pattern, electron beam lithography is applied. The diffraction efficiency, i.e. the fraction of radiation that actually contributes to the diffraction order used for imaging should be as high as possible in order to improve the throughput of the experiment. The efficiency depends strongly on the height and the shape of the diffracting structures. In general, the required structure heights to obtain acceptable efficiencies are much larger than the outermost zone width drn resulting in the need to fabricate nanostructures with high aspect ratios. The necessary height strongly increases with the X-ray photon energy, which is the reason why smaller structure widths (and thus better spatial resolution) can be realized in the soft X-ray range. With current technologies for nanostructuring, zone structures with about 20 nm width have been manufactured [2,3]. In the soft X-ray range, spatial resolution values of below 30 nm have been obtained. In the multi-keV range, values of about 60 nm are the present limit [4,5]. X-rays are able to penetrate integrated circuits with layer stacks on top of the silicon substrate, that are many micrometers thick. Additionally, due to the unique interaction with matter, X-rays provide a natural image contrast between different elements which we exploit to image Cu interconnect lines embedded within isolating SiO2. Copper interconnects embedded in dielectrics which consist of low-Z elements provide a high absorption contrast in X-ray microscope images. However, as shown in Fig. 1, at the low-energy side of the K-absorption edge of oxygen at 0.52 keV, it is even possible to distinguish different dielectric materials which are all Si compounds like Si, SiO2, Si3N4 due to their different attenuation lengths. The interconnect structures used for the experiments are located within the scribelines of production wafers. Fig. 2(a) shows a schematic crosssection of the test structures used in the experiments with a two-level copper interconnect. The focused ion beam (FIB) technique was used to thin the region of -a 1 | 0.5 £ 1 ! 0.1 o 0.05 100 500 1000 photon energy / eV FIGURE 1. Attenuation length, the depth into the material measured along the surface normal where the intensity of Xrays drops to 1/e of its value at the surface for Si, SiO2, Si3N4 and Cu layers as a function of photon energy. Relevant layer thicknesses for interconnect studies are in the micrometer range. interest to a thickness of about 2 \im (see Figs. 2(b)) and 2(c)). Material was removed from both sides of the copper structures under test, so that all neighboring metal lines were removed from the microscope's field of view. A 50 \im wide trench was cut, leading to the area of interest. The X-ray beam is able to penetrate the sample through this trench (see Fig. 2(b)). Fig. 2(d) shows an X-ray micrograph of the prepared interconnect stack imaged at 1.8 keV photon energy using the X-ray microscope XM-1 at the ALS [6]. Fig. 3 shows an X-ray micrograph of a similarly prepared sample which was imaged at 0.52 keV photon energy with the novel BESSY II X-ray microscope [7]. It demonstrates the possibility to visualize clearly different dielectric layers by absorption contrast. REAL-TIME ELECTROMIGRATION STUDIES The mass flow due to electromigration in on-chip interconnects of ICs is constrained by their encapsulation. Depending on the dielectric material used to isolate inlaid copper interconnects, a certain mechanical stress is obtained in the interconnect. Since the interfaces are main transport paths for copper interconnect systems and since the stress influences the material transport significantly, degradation studies 481 to understand the mechanisms of electromigrationinduced material transport should be performed in situ with an intact layer system including all barrier, capping layers. The full- field X-ray microscope, XM1, installed at the ALS was used to study the dynamics of void development in passivated Gu line/via structures [6,8]. Figs. 4(a)- 4(f) show a sequence of Xray micrographs of the copper via/line interconnect structure, which were captured during an in situ EM experiment at 1.8 keV photon energy. The electron flow was from left to right in the image, and upwards through the via. Fig. 4(a) shows the initial state of the interconnect structure without any voids. During the experiment, void formation (Fig. 4(b)), movement (Figs. 4(c) - 4(d)) and agglomeration (Fig. 4(f)) were seen in the via. The image sequence indicates that initial voids are formed in the copper bulk structure, probably at grain boundaries or grain boundary triple points, additionally to void formation, growth and movement along interfaces. FIGURE 2. (a) Schematic of the cross-section of dual-inlaid Cu interconnects, (b) Fully prepared sample for electromigration test, (c) SEM micrograph of the cross-section of the copper metallization system which was thinned locally using FIB. Note that the passivation layer of about 500 nm thickness significantly reduces the obtainable resolution when imaging buried Cu line/via structures, (d) X-ray micrograph recorded at 1.8 keV photon energy showing the sample region as Fig. 2(c). FIGURE 4. Selected X-ray images ((a) - (f)) from an image sequence taken at successive times showing void formation, movement and agglomeration inside the passivated Cu via. To obtain detailed structural information of the via after the EM experiment, a FIB cross section of the via was prepared and imaged in a SEM, which shows a grain boundary leading to the large void (see Fig. 5). Small voids are visible along this grain boundary, giving evidence for the grain boundary diffusion mechanism. By correlating the information showing the material transport visualized by X-ray microscopy with the copper microstructure obtained from electron FIGURE 3. X-ray micrograph imaged at 0.52 keV photon energy with the new BESSY II transmission X-ray microscope. Note that different dielectric materials can be clearly distinguished at this photon energy. 482 micrographs, the dominant copper diffusion pathways can be identified. This study gives evidence for theoretical models postulating that voids grow at sites of flux divergences like grain-boundary triple points and interfaces [9]. Void formation, growth and movement, and consequently degradation, depend clearly on the microstructure of individual vias. To detect the exact location (bulk or interface) of voids in interconnects, real-time X-ray tomography will be used for future experiments. (see Fig. 6) creates an image field free of zero order radiation from the zone plate objective. At the same time, it ensures the required phase shift of the undiffracted light while leaving the phase of the light diffracted by sample structures nearly unaltered [10]. Thereby, the phase information of the object is transformed into an intensity modulation in the image plane. In the Zernike-type phase contrast setup, even the image contrast of weakly phase shifting samples can be tuned to almost any value by employing optimized phase rings which also attenuate the undiffracted light. m FIGURE 5. SEM image of a FIB cross-section of the sample shown in Figure 4(f). A grain boundary leading to the large void as well as small voids are visible along this grain boundary, giving evidence for the grain boundary diffusion mechanism. FIGURE 6. Schematic of the optical setup of the X-ray microscope (TXM) in Zernike phase contrast mode at the ESRF. A zone plate condenser focuses the monochromatized undulator beam onto the object and a micro zone plate objective forms a magnified image on a phosphor-coupled CCD camera. The phase ring in the back-focal plane of the objective selectively phase shifts the undiffracted light of the object [10]. PHASE-CONTRAST MICROSCOPY FOR PHYSICAL FAILURE ANALYSIS Physical failure analysis in copper interconnects using X-ray microscopy techniques requires several micrometer thick samples. To penetrate such samples, photon energies in the multi-keV range are necessary. However, the absorption contrast provided by copper interconnects embedded in dielectrics decreases with increasing photon energy and penetration power of the X-rays. In general, phase shift is dominating over absorption in the multi-keV photon energy range. According to Zernike, phase contrast images are obtained in a full-field microscope by inserting a phase plate in the back focal plane of the objective. In the novel Zernike-type phase contrast X-ray microscope installed at the insertion device ID21 at the ESRF, a hollow-cone illumination provided by a ring-shaped condenser in combination with a central beam stop The high resolution phase contrast imaging technique in Zernike mode was applied to investigate on-chip copper interconnects. Phase contrast X-ray micrographs of test structures with inlaid copper structures are shown in Fig. 7. As demonstrated with these images, phase contrast X-ray microscopy is a powerful new technique for interconnect process development and failure analysis. In addition, due to the large depth of field of X-ray objectives operating in the keV photon energy range, high resolution tomography based on X-ray microscope images is possible. 483 Energy, office of Basic Energy Science under Contract No. DE-AC03-76SF00098 and the Deutsche Forschungsgemeinschaft under Contract No. SCHN 529/1-1. REFERENCES 1. Zschech, E., Geisler, H., Zienert, I., Prinz, H., Langer, E., Meyer, A. M., Schneider, G., Proc. of the Advanced Metallization Conference, San Diego (2002) 2. Peuker, M., Appl Phys. Letters 78, 2208 (2001) 3. Anderson, E.H., Olynick, D.L., Harteneck, B., Veklerov, E., Denbeaux, G., Chao, W., Lucero, A., Johnson, L. and Attwood, D., J. Vac. Sci. Technol. B18 (6), 2970 (2000) 4. Schneider, G., Neuhausler, U., Ludwig, W., Hambach, D.: "Zernike phase contrast microscopy with 60 nm resolution in the multi-keV photon energy range", in preparation (2003) 5. Neuhausler, U., Schneider, G., Ludwig, W., Hambach D., Proceedings of the 7th International Conference on X-ray Microscopy, Journal de Physique IV, in press (2003) FIGURE 7. Different copper interconnect structures, imaged in positive phase-contrast mode at the ID21 transmission microscope at the ESRF. Irregularities in the square structures - as indicated by white circles - can be seen (a). Furthermore, an irregularity within the copper interconnect is indicated (b) [ 10]. 6. Schneider, G., Denbeaux, G., Anderson, E.H., Bates, B., Pearson, A, Meyer, M.A., Zschech, E., Stach, E.A., Appl. Phys. Lett. 81, 2535 (2002). 7. Guttmann, P., Niemann, B., Thieme, J., Hambach, D., Schneider, G., Wiesemann, U., Rudolph, D., Schmahl, G., Nucl. Instr. Meth. A, 467-468, 849 (2001). CONCLUSIONS 8. Schneider, G., Meyer, M.A., Denbeaux, G., Anderson, E., Bates, W., Pearson, A., Knochel, C., Hambach, D., Stach, E.A., Zschech, E., J. Vac. Sci.Tech. B 20(6), 3089 (2002). In situ X-ray microscopy is a powerful technique to study degradation mechanisms like electromigration and stress voiding in on-chip copper interconnect structures. It is a complementary technique to SEM for physical failure analysis at interconnect structures, particularly to study void formation, migration and agglomeration mechanisms in inlaid copper interconnects. Depending on the photon energy used at synchrotron radiation sources, a spatial resolution up to 30 nm has been achieved, and several material combinations and defects have been visualized. Laboratory tools using X-ray tubes can be used for structural investigations with a spatial resolution of about 80 nm [11]. 9. Spolenak, R.: PhD Thesis, Universitat Stuttgart (1999) 10. Neuhausler, U., Schneider, G., Ludwig, W., Meyer, M.A., Zschech, E., Hambach, D., ESRF Highlights 2002, 84 (2002). 11. www.xradia.com ACKNOWLEDGMENTS We greatfully acknowledge the staff of ALS, BESSY and ESRF for providing excellent working conditions. This work is funded in part by the U.S. Department of 484
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