Copper Oxide Passivation Effect on Electromigration Lifetime of Plasma Etched Copper Lines
Authors: Su Jia Quan , Kuo Yue 
Published: December 2020

Copper (Cu) is a widely used interconnection material in high density ICs, large area TFT LCDs etc. CMP is a production method used in etching Cu fine lines in ICs [1,2]. Although plasma etching is widely used in preparing small geometry aluminum and other metal lines [3], it cannot be used to etch Cu at room temperature due to the low volatility of copper halides [4]. A plasma-based high-rate, room-temperature Cu etch process was invented by Kuo’s group [5-7]. It includes the plasma/Cu reaction followed with a dilute HCl dipping step. This process has been successfully demonstrated on BiCMOS and TFT LCD products [8,9]. The electromigration (EM) method has been used in the evaluation the reliability of Cu lines [10-14]. Recently, the authors have reported that copper oxide (CuOx) is a potential useful passivation material for Cu line [15]. In this study, layer thickness effects of Cu and CuOx passivation films on the reliability are studied.

The TiW/Cu stack was sputter deposited on a pre-cleaned Corning glass and etched into a 4-point line pattern. The Cu layer was converted into a CuClx layer under the condition of CF4/HCl 5/20 sccm, 70 mTorr, 600W, and room temperature for 2 minutes in a parallel-plate reactor (PlasmaTherm 700C) operated at the RIE mode. The CuClx layer was subsequently dissolved in a H2O:HCl (8:1 v/v) solution for 1 minute. Then, the TiW barrier film was etched in CF4 20 sccm at 60 mTorr and 600 W for 2 minutes in the same RIE reactor. After the photoresist pattern was wet stripped off, the sample was exposed to the O2 plasma at 200 mTorr, 100 W in the same plasma reactor but under the plasma etching mode. The CuOx passivation layer was formed on the exposed Cu surface. The EM lifetime of two types of samples, i.e., TiW/Cu and TiW/Cu/CuOx lines, were measured under the constant current density (J) stress condition.

Figure 1 shows the line broken time as a function of the stress current density of TiW/Cu/CuOx lines of different Cu film thickness. The general trend is that the lifetime of the line decreases with the increase of the current density. This result is consistent with previous reports on the TiW or Mo passivated Cu lines [14]. The large stress current accelerated the voids formation and merging process, which shortened the line breakage time. Fig. 1 also shows the sample with the thinner Cu layer has a longer lifetime than with the thicker Cu layer. This can be explained by more resistant to thermal strain and the longer thermal fatigue failure time of the thin Cu film [16].

Figure 2 shows the top view of a TiW/Cu/CuOx line broken region observed under the dark-field (DF) of an optical microscope. It clearly shows the distribution of a large number of voids along the current flow direction in this region while the adjacent unbroken region has much fewer voids.

More detailed characterization of the EM stressed lines and the CuOx passivation layer thickness effect will be presented and discussed.

Authors acknowledge the financial support of this work through NSF CMMI project 1633580.

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