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Titanium films prepared by standard direct-current (DC) magnetron physical vapor deposition (PVD) and ionized metal plasma PVD (I-PVD), with Al (0.5wt.%Cu) films on them, were studied. The surface roughness, reflectivity, and crystalline texture of Ti on SiO^sub 2^/Si and Al on TiN/Ti/SiO^sub 2^/Si were investigated with the same thickness of Al, TiN, and Ti. The surface roughness of Al films with Ti/TiN underlayers was found to be capable of monitoring Al(111) texture. So, the reflectivity of Al/TiN/Ti film stack can be used as a quick monitor for the electromigration (EM) lifetime.
Key words: Roughness, texture, electromigration
INTRODUCTION
Many efforts have been made to improve the Al(IIl) texture to get better EM resistance. Park et al. pointed out that the standard direct-current (DC) magnetron sputtering Ti is better than collimated Ti (c-Ti) film.2 Tsutomu and Hitoshi3 and Knorr et al.4 found that Al(IIl) texture is better with a better Ti(0002) texture or TiN(Ul) texture. Later, it was found that ionized physical vapor deposition (I-PVD) can improve Al(IIl) texture furthermore.5,6,8 The higher energy and density of Ti plasma in the I-PVD method bring denser Ti films with fewer gas atoms and voids.5
In this work, we studied single Ti films and Al films on TiN and Ti underlayers. The Ti films deposited by the I-PVD method are compared with Ti films prepared by conventional PVD methods. The effect of different Ti underlayers on the texture of Al films was confirmed. To find a quick monitor scheme of Al and Ti texture, the relationship of the surface roughness, reflectivity, and crystalline textures of Al films and Ti films were investigated in detail. The surface roughness of Al films with Ti/TiN underlayers was found to be capable of monitoring Al(IIl) texture. The reflectivity of Al/TiN/Ti film stack can be used as a quick monitor for the EM lifetime.
EXPERIMENTS
At first, 200-nm SiO^sub 2^ was deposited on 8-in. silicon wafers by plasma-enhanced chemical vapor deposition. Then, three kinds of Ti films were prepared: standard Ti (s-Ti) film processed by standard DC magnetron sputtering with a nitrogen-contaminated Ti target, pure Ti (p-Ti) processed by the same method but with a pure target, and i-Ti film by the I-PVD method. All of the Ti films were kept at the same thickness of 30 nm. In succession, 20-nm TiN film and 500-nm Al (0.5wt.%Cu) film were sputtered in the same system by the standard PVD process without vacuum break. Finally, single 30-nm Ti films and stack of 500-nm Al/20-nm TiN/30-nm Ti were comparatively investigated with the following metrological techniques: reflectivity measurement at the wavelength of 463 nm, surface roughness measurement, and texture analysis by x-ray diffraction (XRD). The textures of the Al and Ti films were examined by the rocking curve of the Al(IIl) and Ti(0002) planes. For acquiring the rocking curves of Al(IIl) or Ti (0002) planes, 2θ was fixed at the Bragg angle of the Al(IIl) or Ti(0002) planes, and the diffraction intensity was measured while tilting the sample. Therefore, the diffraction intensity at a tilt angle indicated the population of the Al(IIl) grains tilted from the surface by the same tiling angle. The FWHM of the rocking curve were compared as the index of texture. The scanning area (50 µm × 50 µm) of the surface roughness was a little larger than the reflectivity measurement area (a round spot with diameter = 40 µm). The surface roughness was evaluated by the root-mean-square of the surface height.
RESULTS AND DISCUSSION Al Texture with Different Ti Underlayer
Figure 1 shows the rocking curves of Al(IIl) plane on different TiN/Ti underlayers. Figure 2 shows the FWHM of Ti(0002) and Al(111) planes with the different TiN/Ti underlayers. From the two figures, we can find a strong correlation between the texture of Al(IIl) and Ti(0002). The i-Ti improved Al(IIl) texture formation because of the better Ti(0002) texture compared to s-Ti and p-Ti. This means that the Ti film texture impacts and controls the Al film texture formation, which is consistent with the work of others.5,6,8
Roughness, Reflectivity, and Texture of Al Films
Interestingly, the roughness of the Al surface shows a strong correlation with the Al(111) texture and surface reflectivity (Fig. 3). The roughness and reflectivity correlation is apparent. The correlation between the roughness and texture can be explained by the structure evolution during processing polycrystalline films. During Al deposition, the grain boundaries are mobile and the grain structure evolves during the coalescence process and continues to evolve during film thickening.7 Furthermore, the process temperature is relatively high and the film is thick enough. So, different crystalline planes with different growth rates are adequately exhibited. If more Al grains have (111) texture, more Al atoms will be deposited with the same growth rate parallel to the surface. Thus, a smoother surface will be shown. Figure 4 shows the Al surface roughness using a high-resolution probe with i-Ti and p-Ti underlayers.
Correlation of Al Reflectivity and EM Lifetime
To confirm the texture impact on EM lifetime, package level data were collected. The Al interconnects were processed with different stacks: s-Ti/ TiN/Al/TiN, p-Ti/TiN/Al/TiN, and i-Ti/TiN/Al/TiN. Two 8-in. wafers were used for each condition. All six wafers were synchronously processed. The EM testing was carried with the temperatures of 210°C and 250°C and the current density of 1 mA and 15 mA. The EM failure criterion is 20% resistance change. Ten samples were randomly chosen from every wafer for the testing. In Fig. 5, the EM lifetime, which was deducted from the mean time to failure of the samples, shows a strong correlation to the reflectivity of Al films. This finding indicates that the reflectivity of Al films with Ti/TiN underlayers can be used as a quick monitor scheme for EM lifetime evaluation.
CONCLUSIONS
A parameter set of different Ti and Al/TiN/Ti films was studied. The following results were found.
* The Al interconnect texture is controlled by the substrate texture; I-Ti is the best for excellent Al(IIl) texture compared with s-Ti and p-Ti.
* The Al film texture is correlated to its surface roughness. The reflectivity can be used to monitor the Al film texture.
* The Al interconnect EM lifetime correlates to the reflectivity of Al films on Ti/TiN underlayers. The reflectivity can be used as a quick monitor to predict the lifetime.
ACKNOWLEDGEMENTS
The authors thank Professor Shi, Fudan University, for her help with the XRD analysis, and the Quality & Reliability Assurance Department of the Grace Semiconductor Manufactory Company, for their support in this work.
REFERENCES
1. S. Vaidya and A.K. Sinha, Thin Solid Films 75, 253 (1981).
2. YB. Park, D.W. Lee, H.H. Ryu, and W. Lee, J. Electron. Mater. 30, 1569 (2001).
3. Tsutomu Sasaki and Hitoshi Dohnomae, Jpn. J. Appl. Phys. 37, 6544 (1998).
4. D.B. Knorr, S.M. Merchant, and M.A. Biberger, J. Vac. Sci. Technol. B 16, 2734 (1998).
5. S. Li, YK Lee, W. Gao, T. White, Z.L. Dong, and K. Maung Latt, J. Vac. Sci. Technol. B 19, 388 (2001).
6. Won-Jun Lee, Sun-Jae Kim, and Wonhee Lee, J. Mater. Sci.-Mater. E 15, 9 (2004).
7. C.V. Thompson, Annu. Rev. Mater. Sci. 30, 159 (2000).
8. R. Jaiswal et al., Proc. Adv. Met. Conf. 735 (1999).
WENJIE ZHANG,1,2,4 LEEWARD YI,1,2 JUINENG TU,3 PINGYI CHANG,3 DULI MAO,3 and JIN WU3
1.-Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China. 2.-Graduate School of Chinese Academy of Sciences, Beijing 100049, China. 3.-Grace Semiconductor Manufacturing Corporation, Shanghai 201203, China. 4.-E-mail: wenjiezhang@gsmcthw.com
Copyright Minerals, Metals & Materials Society Oct 2005
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