Microtexture measurement of copper damascene line with EBSD

Material Science Forum Vols. 408-412(2002) pp. 529-534 2002 Trans Tech Publications, Switzerland Microtexture measurement of copper damascene line with EBSD Dong-Ik Kim 1*, Jong-Min Paik 1, Young-Chang Joo 1, Kyu Hwan Oh 1, Hu-Chul Lee 1, Keith Dicks 2 1 School of Materials Science & Eng, Seoul National Univ. ENG445, San 56-1, Shilim-dong, Kwanak-ku, Seoul 151-744, Korea 2 Oxford Instruments Analytical, Halifax Road, High Wycombe, Bucks, HP12 3SE, UK Keywords: EBSD, Copper, damascene, microtexture, coating Abstract. Microtexture of 0.18µm, 0.25µm, 0.70µm, and 2µm widths Cu damascene line is measured by EBSD technique. The image drift could be reduced by the shortening of measurement time, and could be completely removed by selected area mapping technique. Using carbon and gold dual layer coating method, image drift and pattern indexing fraction is improved but the EBSD pattern quality is decreased. By increasing the accelerating voltage from 15kV to 20kV, pattern quality loss could be compensated without resolution loss. Strong {111} texture is obtained in 0.18 and 0.25µm line, and weak {111} texture is observed in 2µm line. And every line was observed to have more than 60% CSL boundaries. Introduction Aluminium has been used for several decades for IC interconnects, and many interconnecting fabrication methods such as metal deposition, photolithographic patterning, subtractive line definition through reactive ion etching and dielectric deposition have been developed. 1 As the faster IC chip be developed, the more transistors are to be integrated in the same area, and the smaller width in interconnect lines is demanded. Copper which has lower bulk resistivity and high melting point compared to aluminium was actively studied to substitute aluminium for interconnect materials, and the damascene-processing method has been developed for interconnects with line width less than 0.25µm. 1 It is well known that the aluminium interconnects with a bamboo grain structure and strong {111} texture are good for the withstanding of electromigration (EM) under high current densities, and that high special boundary fractions also increase EM life time. 2-3 By EBSD, both texture and microstructural information can be obtained simultaneously and grain boundary characteristics can be analyzed at the same time. Several EBSD investigations have been performed on copper interconnects. 2,4,5 But there are few EBSD studies reporting copper interconnects with line widths less than 0.25µm. An EBSD system equipped Field Emission Gun (FEG) SEM has the resolution under 20nm, 6 easily capable of measuring the texture of the interconnect lines of the width previously mentioned. However, image drift becomes significant at the high magnification required and further, the proportion of indexed patterns decreases appreciably when the line width is less than 0.25µm. This paper focuses on the EBSD data variations according to experimental conditions such as sample preparation technique, sample coating method, probe current, and accelerating voltage, and briefly refers the texture and grain boundary characteristics variations according to interconnects line width change. * Corresponding author. Tel : 82-2-880-8785, Fax : 82-2-872-8307 E-mail address : ggam@plaza1.snu.ac.kr

530 Texture of Materials Experimental Details Copper lines for this study were fabricated by damascene process in PETEOS oxide having various widths ranging from 0.18 to 2µm and trench depth of 0.5 µm. 400Å thick TaN film was used as the Cu barrier layer. Overlayers consist of 7000Å thick SiN film and the same thickness of silicon oxide film, which was removed for EBSD analysis using dilute HF solution. Some samples are coated by gold and carbon dual layer coating technique, which is described in Fig. 1. At first, the sample was coated by gold with ion sputter coater to prevent charge build up at the specimen surface. In this case the thickness of coated layer usually exceeds 500Å, so the interested area for the EBSD experiment should be protected by cover. Then the specimen is coated by thin layer of carbon to prevent inhomogeneity at the gold coated and uncoated interface. EIKO IB-3 ion sputter coater is used for the gold coating for 10 minutes, and Cressington 108 carbon coater is used for carbon coating at 4V for 1sec. It is important that the thickness of carbon coating should not exceed 10nm, to minimize the decrease of back scattered electron intensity. Fig. 1. Gold and carbon dual layer coating to prevent image drift and improve pattern indexing fraction a) Copper damascene line specimen, b) Thick Au coating with disk cover, c) Thin carbon coating for whole specimen In this study, JEOL 6500F Schottky type FEG-SEM equipped with Oxford INCA Crystal EBSD system was used. EBSD experiments were carried out at 15kV accelerating voltage and 5nA prove current and the accelerating voltage was raised up to 20kV to compensate the pattern quality degradation by carbon coating, and Kikuchi patterns were integrated for 80ms in each analysis point. Grain boundary characteristics and grain size of copper grains were also investigated by INCA crystal software. The coating technique used greatly enhanced charge dissipation. However, despite the coating, charge accumulation during examination still lead to image drift at the high magnification required for this examination. The charging was brought under control by using a feature of the INCA Crystal software controlling EBSD acquisition and beam positioning. Selected areas were defined to produce crystal orientation maps (COMs). Multiple fields were defined such that the areas chosen to map only covered the metallic tracks, and not the substrate between tracks. In this manner the beam was prevented from dwelling on the substrate and confined to the areas of interest only. Thus charge build up in the vicinity of the copper tracks was minimized. This technique was combined with high speed acquisition to effectively overcome image drift.

Materials Science Forum Vols. 408-412 531 Results and Discussion Controlling image drift during EBSD acquisition at high magnification with multiple phase specimens is critical. Each phase may have different electrical conductivity. Because of this, different levels of charge can accumulate at the interface between phases. The amount of charge built up is dependant on the difference in conductivity of each phase. In turn, the amount of charge built up controls the extent of image drift. The copper damascene line surface consists of the three phases of copper, tantalum nitride barrier, and silicon oxide. The conductivity of each is quite different. Consequently, it is common to encounter image drift at magnifications in excess of X10,000 on these samples. The charging, and drift, is exacerbated if higher beam current is used. It may be desirable to use higher beam current in the interest of enhancing pattern quality. Experimentation was required to find the optimum conditions. Figure 2a shows the image drift during EBSD acquisition on 700nm wide copper tracks acquired under typical conditions i.e. 15kV accelerating voltage, 5nA probe current, 256 steps per horizontal scan. Significant image drift is evident and it is difficult to compare the SE image and the crystal orientation mapping (COM) results. Figure 2b shows a comparative reduction in image drift achieved by reducing the total mapping time, and thus exposure of the sample to the electron beam. However, despite an increase of magnification from 20,000 to 30,000 and a reduction of the number of mapped points (128 steps per horizontal scan), image drift, although reduced, remains. The observed (reduced) degree of image drift is most probably a product of the reduced acquisition time rather than due to any reduction in the drift rate. Fig 2c shows the complete prevention of image drift by using selected area mapping. Confining the electron beam to irradiate only the copper lines prevents charge build-up at the phase interfaces and thus completely eliminates image drift. In the case of crystal orientation mapping narrower copper interconnects, i.e. lines of 0.18µm and 0.25µm width, the charge build-up problem became worse. Because the widths of the Si oxide substrate and copper lines are similar, charge build-up become very significant. Further, it is challenging to obtain good SE images and EBSD patterns at such high resolution. The EBSD indexing fraction i.e. the proportion of patterns that can be solved, is directly linked to pattern quality which decreases with worsening pattern quality. Fig 3a shows the crystal orientation map data of the entire area of interest on 0.18 µm wide copper lines, acquired at 15kV accelerating voltage, 5nA prove current, and 128 steps per horizontal scan. The SE image quality is too poor to observe the surface topography of the specimen, and there is significant image drift. The EBSD pattern quality was degraded such that the reliability of the orientation information and texture calculated from COM data is in question. However, by applying the selected area mapping technique, the pattern indexing fraction is improved, but there remains a degree of image drift. The image drift in the narrowest width copper interconnects was sufficiently severe that the gold and carbon dual layer coating technique, previously described, was introduced. To prevent the charge build up on the sample surface, the incident electron should flow off readily. To increase conductivity of the specimen surface, it was gold coated for 10 minutes using an ion sputter coater. The area of interest was protected by a cover to prevent gold deposition, because the thickness of the coating layer exceeds the depth of EBSD pattern formation. By gold coating, the total amount of charge built up on the specimen was reduced, but the addition of one more phase (gold) to the specimen induced the problem of inhomogeneity at the interface of the coated and uncoated area. The whole area of the specimen was lightly carbon coated to make the whole specimen surface homogeneous. An alternative preparation method was to coat the specimen with carbon only, but light carbon coating did not prevent charge build-up completely. Thus the gold and carbon dual layer coating technique was adopted as a sample preparation method for multiple phase specimens. Figure 3b shows the orientation map acquired by selected area mapping of the 0.18 µm width copper lines with gold and carbon dual layer coating. The image drift problem is completely eliminated and pattern indexing fraction is improved by up to 70%, which is sufficient to analyze the microstructure, texture, and boundary characteristics of the specimen.

532 Texture of Materials Fig. 2. Image drift variation with the EBSD measurement condition in 700nm width copper damascene line. a) 24nm step mapping at x20,000 (256 step per horizontal scan), b) 32nm step mapping at x30,000 (128 step per horizontal scan), c) 32nm step mapping at x30,000 and selected area mapping Fig. 3. Orientation mapping of 0.18µm width copper interconnect line. a) Mapping of whole interested area without coating. b) Selected area mapping with gold and carbon dual layer coating

Materials Science Forum Vols. 408-412 533 The EBSD pattern quality is predictably decreased by carbon coating, because some backscattered electrons are captured or deflected by the coated carbon layer. If the electron accelerating voltage is increased to 20kV to raise the backscattered electron energy, and the pattern quality is restored to the level of uncoated copper lines. Resolution loss by the penetration volume extension was not observed in these experiments. There is an another important factor relating to the image drift phenomena observed during high magnification EBSD acquisition. Image drift can be caused by mechanical and electromagnetic optical instability, and time is required to stabilize the specimen stage and electromagnetic lens system for EBSD experiments over 20,000X magnification. The specimen stage is stabilized in relatively short time, 1-2 minutes, but 15-20 minutes are required for the stabilization of the electromagnetic lens system. Figure 3b was acquired after 20minutes waiting time, after selecting the area of interest and setting up the experimental parameters. Grain boundaries were defined for adjacent points as having a misorientation greater than 5degrees. In 0.18µm wide lines, 7,000 crystal lattice orientations from 380 grains were investigated. Bamboo grain structure is evident with an average grain size of 193nm. In 0.25µm lines, 5800 orientations from 213 grains were collected. Again, bamboo grain structure was evident and the average grain size was 265nm. In 2µm lines, 17,000 points from 2,300 grains were investigated. In this case a polycrystalline grain structure was observed in the cross sectional direction, with an average grain size of 923nm. Texture development in the copper lines was calculated using the WIMV method. Figures 4a, b, c show texture development of interconnect lines. 0.18µm, 0.25µm lines have strong {111} texture and weak minor texture as {114} in 0.18µm and {112} in 0.25µm. 2 µm lines show relatively weak {111} texture only. This paper concentrates on the experimental technique, and consequently the number of grains analyzed does not provide a statistically significant texture evaluation. In order to achieve statistical reliability to the f(g) level calculated from an EBSD measurement, orientations from more than 600 individual grains are required. However, confirmation of the texture development characteristics requires data from only 100 grains, and this number can be decreased in the case of obtaining several orientations from each grain. 7 In this experiment, orientation information is Fig. 4. Texture and CSL boundaries fraction change of copper interconnects according to line width variation a) ODF of 0.18µm width copper interconnect, b) ODF of 0.25µm width copper interconnect, c) ODF of 2.0 µm width copper interconnect, d) CSL boundary fractions of three interconnects.

534 Texture of Materials collected from a minimum of 200 grains and usually 10-20 orientations are measured in each grain. Therefore the texture development tendency can be considered reasonably reliable and the f(g) value is also credible in the 2µm line. The boundary characteristics of each line were investigated using the INCA Crystal software. Each copper track exhibited high fractions of special boundaries. The sigma 3 boundary fraction of 0.18µm line is 50%, which increases to 60% in 0.25 µm, and 55% in 2µm wide lines. The total special boundary fraction reaches 60% in 0.18µm, and 70% in both 0.25µm, and 2.0µm lines. Previous research on copper interconnects fabricated by the Damascene process show no close relationships between {111} texture development and the electromigration (EM) life time observed in aluminium interconnects. The fraction of special boundaries is still implicated in the EM life time of copper interconnects. 2,4,5 Samples analyzed in this experiment have different level of {111} texture, but they have a very high fraction of special boundary, and consequently good EM performance is expected. Conclusion Various EBSD experimental conditions to reduce image drift of copper interconnects fabricated by the Damascene process were investigated in this study. Shortening of measurement time and use of a selected area mapping technique reduced the image drift problem. Using carbon and gold dual layer coating, image drift was prevented and pattern indexing fraction was improved. Increasing the accelerating voltage from 15kV to 20kV compensated for the EBSD pattern quality loss caused by coating, without sacrificing resolution. Strong {111} and weak minor texture was observed in 0.18 and 0.25µm lines, and only weak {111} texture was observed in 2µm lines. All lines showed more than 60% CSL boundaries. References [1] L.Vanasupa, Y.C.Joo, P.R.Besser, and S.Pramanick : J. Appl. Phys. Vol.85(1999), p.2583 [2] D.P.Field, D.Dornisch, and H.H.Tong : Scripta Mater. Vol.45(2001), p.1069 [3] D.B.Knorr, K.P.Rodbell : J. Appl. Phys. Vol.79(1996), p.2409 [4] T.G.Koetter, H.Wendrock, H.Schuehrer, C.Wenzel, and K.Wetzig : Microelectronics Reliability Vol.40(2000), p.1295 [5] S.Baunack, T.G.Kötter, H.Wendrock, K.Wetzig : Appl. Surf. Sci. Vol.179(2001), p.245 [6] F. J. Humphreys : Proc. ICOTOM12, (1999) Montreal, ed. J.A.Szpunar, p.74 [7] O.Engler, J.Jura, S.Matthies : Proc. ICOTOM12, (1999) Montreal, ed. J.A.Szpunar, p.68