Layer Thickness Effect on Lifetime of Copper Oxide Passivated Plasma Etched Copper Line
Authors: Su Jia Quan , Kuo Yue 
Published: July 2021

Cu is a widely used interconnect material in high density ICs, large area TFT LCDs, etc. Due to the difficulty in etching the Cu thin film using the conventional plasma etching method [1], the CMP method was developed to achieve this goal [2]. Kuo’s group developed a plasma-based Cu etch process that had a high rate even at the room temperature. [3,4]. This process has been applied to the fabrication of BiCMOS chips and TFTs [5,6]. Recently, the authors reported that CuOx could be an effective passivation material for Cu [7]. In this study, the CuOx passivation layer thickness effect on the reliability of the Cu line is studied using the electromigration (EM) method [8,9].
The TiW/Cu stack was sputter deposited on the DHF cleaned Corning 1737 glass and etched into a 4-point test pattern. The Cu layer was etched by a 2-step plasma-based process [3,4]. The first step was to convert the patterned Cu film into a CuClx layer using the CF4/HCl (5/20 sccm) plasma at 70 mTorr and 600W in a parallel-plate reactor (PlasmaTherm 700C) operated at the RIE mode. The second step was to dissolve the CuClx layer in a dilute HCl solution (HCl:H2O=1:8 v/v). the TiW barrier film was etched in the CF4 (20 sccm) plasma at 60 mTorr and 600 W in the same RIE reactor. After stripping off the photoresist, a CuOx passivation layer was formed on the Cu surface by exposing the sample to the O2 plasma using the same reactor operated at the plasma etching (PE) mode. The EM stress was carried out under the constant current densities (J) condition at room temperature using the Agilent E3645A meter on a probe station (Signatone S-1160).
Figure 1 shows the optical emission spectrum (OES) of the O2 plasma oxidation condition, i.e., 40 sccm, 200 mTorr, 300W at the PE mode. The two large peaks at 777 nm and 845 nm belong to the O radicals that are generated from the dissociative excitation. They correspond to transition states of 3p5P 3s5S and 3p3P 3s3S, respectively [10,11]. The O2
+ vibrational state oxygens should show as peaks between 500 and 600 nm, which are very weak in Fig. 1. Therefore, the ion bombardment effect in this Cu oxidation process is much weaker than that of the oxygen radicals. Since the Cu layer was thin, the CuOx passivation layer was probably uniform over the Cu surface.
Figure 2 shows the EM lifetimes of the TiW/Cu and TiW/Cu/CuOx lines with various CuOx passivation layer thicknesses. The general trend is that the lifetime is shortened with the increase of the current density. The breakage of the line can be contributed by the mechanism of voids formation and merging, which is accelerated by the increase of the current density [9]. In addition, the unpassivated line has a longer lifetime than the CuOx passivated line, i.e., red (12.5 nm thick CuOx layer) and blue (40 nm thick CuOx layer) lines. The thicker the passivation layer is, the shorter the EM lifetime is, which may be related to the heat dissipation effect. Since CuOx has a low thermal conductivity [12], the Cu line with a thick passivation layer is hotter than that with a thin passivation layer. The high temperature accelerates the voids formation and merging process [9].
More detailed characterization of 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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Figure 1