AES analysis of failures in Cu based electromigration test samples

Applied Surface Science 179 (2001) 245±250

AES analysis of failures in Cu based electromigration test samples S. Baunack*, T.G. KoÈtter, H. Wendrock, K. Wetzig Institut fuÈr FestkoÈrper- und Werkstofforschung Dresden, Postfach 270016, D-01171 Dresden, Germany

Abstract Failures occurring in electromigration test of copper interconnects have been characterized by electron backscatter diffraction (EBSD) and scanning Auger microscopy (SAM). The Cu interconnects were 2 mm wide and 500 nm thick stripes on a Ta/TaN barrier. They are imbedded in trenches in a SiO2 layer on Si. The failure manifests as the appearance of voids with lateral dimension of some micrometers. By EBSD mapping, it could be veri®ed that no sidewall texture in the interconnect exist. Auger analysis clearly showed that the Ta/TaN barrier layer has not been destroyed at the site of electromigration failure. The interaction of the electron beam with small particles (0.5 mm) was modelled to understand the contribution of electron scattering in the voids to the lateral resolution. # 2001 Elsevier Science B.V. All rights reserved. PACS: 85.40.Ls; 66.30.Qa; 61.16.M; 61.14.Lj Keywords: Cu metallization; Electromigration; Scanning Auger microscopy; Electron backscattering diffraction; Monte Carlo simulation

1. Introduction We are in the midst of technology transition from Al based to Cu based ULSI interconnects. The need for a high conductive metallization to reduce the RC-delay and the need for a more reliable conductor material, because of the further increasing current density, has forced the change from Al to Cu as the conductor material of choice for ULSI technology. Electroplating has been selected as a preferred deposition method for Cu metallization for its superior ability to ®ll the submicron trenches [1]. Diffusion along surfaces, grain boundaries (GB) and interfaces are found to be mechanisms to explain the failures caused by electromigration [2]. It is the aim of this work to analyze the lower barrier and the microstructure of interconnect failures after electromigration testing. Therefore, Auger electron

microanalysis of the barrier has been performed inside and electron backscatter diffraction (EBSD) analysis around the voids which caused the failure. Scanning Auger microscopy (SAM) is a powerful tool to investigate the composition of surfaces, interfaces and thin ®lms. Using Schottky type ®eld emitters, a beam diameter of 10 nm can be achieved. On rough surfaces, in voids and trenches the lateral resolution is not determined by the beam diameter alone, but by scattering of the primary electrons. Further, the composition of the test samples is chemically heterogeneous: Cu interconnect (Z ˆ 29), SiO2 insulator (Z ˆ 10), diffusion barrier composed of Ta (Z ˆ 73) and TaN (Z  40). The large differences in Z in¯uence both the electron backscattering and the background of the Auger spectra. 2. Experimental

*

Corresponding author. Tel.: ‡49-351-4659-387; Fax: ‡49-351-4659-452. E-mail address: [email protected] (S. Baunack).

An 800 mm long and 2 mm wide NIST structure has been used for the electromigration experiments.

0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 2 8 7 - 2

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S. Baunack et al. / Applied Surface Science 179 (2001) 245±250

Damascene trenches (depth 600 nm) were patterned into 1 mm thick thermal oxide on a 525 mm thick 6 in. single crystal Si wafer. The barrier of 30 nm Ta and 50 nm TaN and a 50 nm Cu seed layer were deposited by physical vapour deposition (PVD) into the trenches using a STS clustertool. A layer of 1500 nm copper has been plated into the trenches by electrodeposition in a non commercial plating tool using a commercial plating bath. Then the excess Cu was removed by chemical±mechanical polishing using a commercial slurry, and the barrier has been removed by reactive ion etching. The ®nal thickness of the Cu interconnect is about 500 nm. The microstructure of the copper interconnects has been characterized by analysis of EBSD patterns. EBSD detects the backscattered electrons inside a scanning electron microscope (SEM) from the top 10±50 nm. A certain area is stepwise irradiated by

the primary electron beam and the system obtains the crystallographic orientation of each point within the area by detecting the Kikuchi pattern projected on a ¯uorescent screen. Kikuchi patterns of high quality allow to identify individual grains and also the GB and their GB angles. For this study, a LEO DSM 962 SEM with LaB6 cathode and an EBSD-tool of HKL-Technology, with an on chip integration CCD-camera, have been used. EBSD mappings in the SEM were performed at an acceleration voltage of 30 kV. A beam size <100 nm could be realized and the step size amounted to 100 nm. After pointwise recording of the orientation, the GB can be evaluated as the set of all points where orientation changes. The electromigration testing has been carried out on package level on a heating stage as described in previous work [3] inside a commercial vacuum chamber under high vacuum conditions of less than

Fig. 1. SEM image of failure (a), hillocks (b), voids (c) on the 2 mm wide interconnect. The direction of electron ¯ux is from the top to the bottom. The EBSD mapping (d) of position (c) shows the deviation from h1 1 1i orientation in surface normal direction in degrees coded in grey values and additionally the net of grain boundaries (black: high angle boundary, white: low angle boundary).

S. Baunack et al. / Applied Surface Science 179 (2001) 245±250

1:5  10 4 Pa to prevent oxidation. Accelerated electromigration (EM) tests were performed under constant voltage conditions, with current densities of 3 mA/cm2 at a medium line temperature of 3508C. For Auger measurements, a scanning Auger microprobe PHI660 (Physical Electronics, USA) equipped with a LaB6 cathode was used. The beam diameter at 10 keV was about 200 nm at 7 nA and 80 nm at 0.5 nA as determined by using the 20/80% criterion when the electron beam is scanned over a grid. The samples were cleaned by sputtering with argon ions impinging at 608 to the normal with energies 1.5 keV. For sputtering, the sample was aligned so that the ions were impinging along the axis of the interconnect. 3. Results and discussion Fig. 1 shows electromigration damages of a 2 mm wide interconnect, a failure (a), hillocks (b), voids (c),

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and an EBSD mapping (d) of the sites around the damages of (c). The direction of electron ¯ux is from the top to the bottom. The SEM image of the failure shows a white line on the lower barrier. Corresponding to this line, the hillocks in Fig. 1b and the voids in Fig. 1c have been formed in the middle of the interconnect. This points to a fast diffusion path in the middle of the line. Two causes be taken into consideration for this: defects in the barrier or a side wall texture with a series of high angle GB in the middle of the line. The latter may be the consequence of grain growth during the EM test at accelerated temperature [1,4]. The EBSD mapping did not con®rm this presumption as Fig. 1d shows. The examination of the lower barrier has been performed by AES inside the failure shown in Fig. 1a. Auger survey spectra were recorded at three points (Fig. 2a) on the bottom of the failure (Fig. 2b). Due to the low primary electron current, the signal-to-noiseratio is worse and the spectra were smoothed. No Cu peaks were identi®ed in the spectra. Conventionally, derivative spectra are used for quanti®cation yielding

Fig. 2. Investigation of the failure in the SAM. The SEM images were obtained at 0.5 nA (a) and 7 nA (c). The dark spot in the upper Cu strip is a SiO2 test pattern. Survey spectra (b) were recorded at the three points in (a) and show no Cu. The bars mark the positions of the main peaks. The elemental composition shown in (d) was measured along the line in (c). The graphics is rotated to allow assignment of the position to the features visible in (c).

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Fig. 3. Auger maps recorded around the failure at a resolution of 64  64 points. In the bars, the number of counts above background is given. The upper images show the original data, in the lower part the result of adaptive ®ltering the images is shown.

an increased noise level. From the derivative spectra an upper limit for the Cu concentration of 3% was estimated. The bright line in the middle of the line exhibits the same spectrum but on a higher background indicating a larger atomic number, presumably from the tantalum bottom layer. The dark area in the upper Cu strip is a SiO2 test pattern. It was used as a reference structure for compensating sample shift during spectrum registration. Due to the small beam current of 0.5 nA the measurement time was about 40 min yielding a high level of carbon contamination. Using a higher current of 7 nA, the elemental distribution in the defect was investigated by Auger line pro®les and maps. The elemental composition in Fig. 2d was determined from a line pro®le along the line shown in Fig. 2c. The spectra obtained at 32 points were processed by linear least squares ®t and factor analysis to analyze peak shapes and to reduce noise. In the region of the SiO2 test pattern, no Cu signal was found indicating that the lateral resolution is better then half the width of structure i.e. about 0.5 mm. Inside the failure, also no Cu was detected.

Tantalum and nitrogen signals show that the interconnect is completely removed. The C(KLL) transition displays different peak shapes on Cu and SiO2, which can be attributed to a carbide-like and graphitelike state, respectively. Auger maps (Fig. 3) were recorded for the following signals: Ta (1674 eV), Cu (920 eV), Si (1615 eV), O (505 and 511 eV), and C (266 eV) over the area in Fig. 2c and show that Cu is completely removed inside the failure. The maps show the intensity in the peak over a background obtained by linear extrapolation of two points below and above the peak, respectively. This reduces the in¯uence of topography but remains sensitive to changes as well in peak position as in the background of inelastically scattered Auger electrons at higher energy. This is visible in the two O(KLL) signals obtained at different energy. The high intensity for the C(KLL) transition at the edges is obviously caused by changes in peak shape discussed above. High lateral resolution requires low beam currents leading to a bad signal-to-noise-ratio. An increase in measurement time would be limited by stability

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Fig. 4. Results of Monte Carlo modelling with a 10 keV electron beam striking the test structure at different locations. Different scales for depth and lateral extension are in (d), the number of trajectories is given in the brackets. (a) Silica particle embedded in Cu (N ˆ 500); (b) Cu particle embedded in silica (N ˆ 600); (c) silica particle on barrier (N ˆ 500); (d) Cu particle on barrier (N ˆ 900).

problems. To improve interpretation, the maps were smoothed by the adaptive smoothing ®lter (ASF) [5]. This ®lter convolutes the image with an averaging mask whose coef®cients depend on the magnitude of the gradient at the spatial position [6]. It results in a sharpening of edges and in smoothing inside homogenous regions. Auger measurements are generally considered to have a superior lateral resolution compared to electron probe microanalysis because Auger electrons are emitted from a very thin surface layer where the primary beam has not broadened and the interaction volume should not concern. If a sample is excited by a delta function like primary beam, an additional Auger emission due to backscattered electrons emerges from a broad region surrounding this d-function. For a

smooth sample, the effective diameter of Auger excitation is therefore given by the convolution of the beam pro®le with the backscattering broadening. The spatial extent of this background region is in the range of 0.2±0.5 mm as found by Monte Carlo simulation [7]. For a very narrow incident beam, the ¯ux from the disc of backscattered electrons is low. If the beam size is comparable to the backscattering radius, then the Auger electrons come from a region that is broadened and enhanced by backscattering [7]. For rough surfaces, the scattering of the primary electrons and the emission of Auger electrons by scattered primaries has to be taken into account. Due to the complexity of the problem, this has only done scarcely [8,9] and often only scattering of the primary electrons is considered [10].

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The modelling of the primary beam striking the electromigration test structure was performed using Electron Flight Simulator.1 This program incorporates Monte Carlo modelling work of David Joy. Within the capabilities of the software, the sample was assumed to consist of a bulk Si substrate with a continuous diffusion barrier of 30 nm Ta and 50 nm TaN. The test structures are 500 nm thick Cu imbedded in silicon oxide of the same thickness. For the initial structure, we considered perpendicular incidence of the electron beam (Fig. 4) either (a) on the insulator (in the center of an 500 nm silica particle imbedded in a Cu layer) or (b) on the Cu interconnect (500 nm Cu particle in silica). The conditions in the failure were simulated by (c) a free standing 500 nm silica particle on the intact barrier, and (d) Cu particle on a barrier, respectively. For a 10 keV electron beam striking Cu, a nearly spherical primary excitation volume with a diameter in the order of the particle dimension is found. In the lighter silica, either the lateral and the in-depth scattering of the electrons are more pronounced and excitation of Auger electrons from the neighboring walls is likely. For 20 keV primary electrons, the electron range is signi®cantly larger than the considered structure size and scattered electron can excite Auger electrons from distant regions. Therefore, for lateral resolved analysis on rough or heterogeneous surfaces, the optimum beam energy must be determined between an utmost small beam diameter calling for increasing beam energy and the accompanying increase of interaction volume and scatter range of primary electrons. 4. Conclusion Failures occurring in electromigration tests of copper interconnects have been characterized by EBSD and SAM. By EBSD mapping, it could be veri®ed that

1 Electron Flight Simulator V3.1, Small World Inc., Vienna, USA (http://members.aol.com/smworld100/index.htm).

no sidewall texture in the interconnect exist. The Auger analysis clearly showed that the Ta/TaN barrier layer has not been destroyed at the site of electromigration failure. The relief line in the middle of the interconnect must therefore be an effect of non-correct etching of the trench into SiO2. Due to the copper technology, the failure manifests as a hole surrounded by walls of Cu and SiO2. To achieve a high lateral resolution, scattering of primary electrons should be minimized by low primary beam voltage. Acknowledgements The authors are grateful to C. Wenzel and H. SchuÈhrer, Technical University Dresden, for preparing the interconnect test samples. This work was supported by the Saxon Ministry of Science and Art. References [1] C. Lingk, M.E. Gross, W.L. Brown, J. Appl. Phys. 87 (2000) 2232. [2] C.K. Hu, R. Rosenberg, K.Y. Lee, Appl. Phys. Lett. 74 (1999) 2945. [3] K. Wetzig, H. Wendrock, A. Buerke, Th. KoÈtter, in: Proceedings of the 5th International Workshop on Stress Induced Phenomena in Metallization, Stuttgart, Vol. 491, 1999, p. 89. [4] C. Lingk, M.E. Gross, W.L. Brown, Appl. Phys. Lett. 74 (1999) 682. [5] S.D. BoÈhmig, H. Beilschmidt, B.M. Reichl, Fres. J. Anal. Chem. 346 (1993) 196. [6] P. Saint-Marc, J.-S. Chen, G. Medioni, IEEE Trans. PAMI 13 (1991) 514. [7] M.M. El-Gomati, M. Prutton, Surf. Sci. 72 (1978) 485. [8] R.R. Olson, L.A. La Vanier, D.H. Narum, Appl. Surf. Sci. 70 (1993) 266. [9] A.M.D. Assad, M.M. El-Gomati, J. Dell, Ultramicroscopy 79 (1999) 141. [10] J.R. Kingsley, D.W. Harris, Proc. SPIE 3332 (1998) 501.