Electromigration of Plasma Etched Copper Lines of Various Widths and Lengths
Authors: Li Mingqian , Kuo Yue 
Published: February 2020

Copper (Cu) interconnect lines are widely used in advanced, high-density ICs. Compared with aluminum (Al), Cu has many advantages, such as the higher conductivity and longer lifetime. However, Cu is difficult to etch into fine lines using the plasma etching method because the reaction product is nonvolatile. Another problem of Cu interconnect is that it has poor adhesion to the dielectric film unless an adhesion layer is used. Recently, Kuo’s group solved the etching problem with a novel room-temperature process that consumes the patterned Cu thin film with a plasma reaction and then removes the reaction product with a liquid solution [1,2]. This method has been used in the fabrication of ICs and TFT LCDs [3]. One of the most critical issues in applying Cu lines in products is the reliability - lifetime prediction, which is usually done by the electromigration (EM) measurement [4]. Previously, the Kuo’s group has studied temperature and mechanical bending effects on the Cu fine line’s lifetime [5,6]. However, there are few studies on the geometry effects of the Cu line on the lifetime. In the paper, authors investigated the relationship between the width or length of the Cu line and the EM lifetime.
The TiW (10 nm)/Cu (200 nm) thin film stack was sputter deposited on a Corning glass. The TiW film was deposition at 5 mTorr in Ar at 75 W for 15 minutes. The Cu film was deposited at 10 mTorr in Ar at 80W for 60 minutes. The Cu film was patterned with a lithography process using a line-and-space mask. Then, the sample was exposed to a Cl2 /CF4 plasma in the PlasmaTherm 700C system operated in the RIE mode at room temperature. The condition was Cl2/CF4 10/5 sccm at 50 mTorr, 600 W for 3 minutes, corresponding to -VDC
= 237 V [7]. After the RIE reaction, the sample was dipped in the 10:1 diluted HCl solution to dissolve the CuClx reaction product. Then, the underneath TiW was RIE etched with CF4 5 sccm at 40 mTorr, 600W for 2 minutes corresponding to -VDC
= 273V. The failure time was determined by stressing a sample with the constant current density at room temperature until the current suddenly jumped by several orders of magnitude.
Relationships between the line width or length and the EM failure time at different current densities were measured, as shown in Figures 1 and 2, where “L” means the line length and “W” means the line width. Several conclusions can be summarized from these figures. First, the line failure time is shortened with the increase of the current density. Second, under the same current density, the narrow line has a longer lifetime than the wide line. Third, the influence of the line length to the failure time is highly dependent on the line width. In Fig. 1, for 30 µ wide lines, the 800 µ long line has a slightly longer lifetime than that of the 400 µ long line. However, in Fig. 2, for 10 µ wide lines, the 20 µ long line has a lifetime slightly longer than that of the 200 µ long line at the current density of 2.5x106 A/cm2. Separately, for the same 20 µ line length, the lifetime of the 3 µ wide line is longer than that of the 10 µ wide line. For all 10 µ wide lines, the lifetime appears to be longer with the shortening of the line length.
The further study on the Cu line width and length effect for narrow Cu lines, i.e., less than 3 µ, is in progress of which the result will be reported. In addition, the breakdown of the Cu line may be related to the edge roughness or the TiW/Cu interface quality, which will also be clarified and reported.
Authors acknowledge to support of this work through the NSF CMMI 1633580 project.
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5. G. Liu and Y. Kuo, Electrochem. Soc., 156(7), H579-H584. (2009).
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Figure 1