Current sustainability and electromigration of Pd, Sc and Y thin-films as potential interconnects

Key Laboratory for the Physics and Chemistry of Nanodevices, and Department of Electronics, Peking University, Beijing 100871, P. R. China Institute of Physics, Chinese Academy of Sciences, Beijing 100080, P. R. China *Corresponding author. E-mail: xusy@pku.edu.cn; lmpeng@pku.edu.cn Current sustainability and electromigration of Pd, Sc and Y thin-films as potential interconnects Yong Yang, Shengyong Xu*, Sishen Xie and Lian-Mao Peng*

The equally high mobility of both electrons and holes in CNTs make it possible to fabricate high-performance p-type and n-type CNT-FETs with symmetric device parameters which are superior to those of conventional Si-based FETs [1][2][3][4]. However, progress on novel interconnects for the CNT-based electronic circuit is by far behind the remarkable development of CNT-FETs. In CNT-based electronics circuits, the key element of each individual device is a short piece of semiconducting CNT with diameter of only 12 nm. Hence the interconnect material used in current integrated circuits, Cu, seems not applicable, as it requires electrochemical deposition of Cu layers followed by chemical-mechanical polishing [5], which may easily damage the fragile structures of CNT-based devices.
Theoretically it is predicted that multi-walled CNT (MWCNT) could be a good candidate as interconnect for novel nanoelectronics [6][7][8]. Because MWCNTs are chemically and thermally stable and can sustain an extremely high working current density. But in practice, conventional lithography and thin-film technologies are not applicable for the MWCNTs, and it is hard to manipulate and/or grow MWCNTs with desired length and diameter to the target positions in a circuit, therefore to date only a few demonstrations have been reported that MWCNTs can be used as part of the interconnects in a prototype electronic circuit [9][10][11]. Since the state-of-the-art p-type CNT-FETs require palladium (Pd) electrodes [1], and n-type CNT-FETs require scandium (Sc) or yttrium (Y) electrodes [2,4], it is a natural choice to use the same electrode materials as the interconnects for the novel CNT-FET-based circuits, if any of them fulfills the requirements of such interconnects. This allows the fabrication of electrodes (either for p-type or n-type FETs) and interconnects in one deposition step thus simplifying the processing, and also maximally reduces the risk of contamination and interface effect that a metal other than the electrode materials (i.e., Pd, Sc or Y) brings to the circuit, which may degrade the performance of CNT-FETs.
For an interconnect material, sustainability of current density, resistivity, chemical and thermal stability, as well as technical compatibility and cost are some of the key parameters to be considered. Electromigration, which may lead to opens or shorts in interconnects of an integrated circuit after hours of use, is another important factor [12][13][14][15][16][17][18][19]. Indeed, the change of interconnects from Al to Cu in the development of integrated circuit industry is partly because of the electromigration [20][21][22].
Here we report our experimental results on the current sustainability, resistivity and electromigration effects of patterned stripes of Pd, Sc and Y thin-films, as well as Au stripes for a comparison, regarding them as potential interconnects.
Thin-film stripes with thickness of 4080 nm, width of 14 μm and length of 100, 300, 500, 700 and 1000 μm are made into a   Fig. 1c and Fig. 1d). Though the values are sample dependent, clearly Pd stripes can sustain much higher working current density than Sc and Y stripes do.
The 4-lead structure of the stripe (see Fig. 1a Fig. 2b), until the stripe breaks. Y stripes, however, show a random trend. Figure 2c plots the measured data of one of the Y samples. The phenomenon of linear increasing resistance with time, similar to what is measured in our Pd stripes, has been observed previously in Cu and Al (4% Cu) [17,24]. Theoretically this is attributed to the dynamics of vacancy and defects in the electromigration process [15,[25][26][27]. On the other hand, the annihilation of vacancy and imperfection can lead to decrease of the resistance at the beginning of the electromigration in low current [15], as what is observed in our Sc stripes. DOI:10.5101/nml.v2i3.p184-189 http://www.nmletters.org [28][29][30][31]. It is predicted that when the current is not high enough to induce significant Joule heating, the stress is tensile at the cathode end and compressive at the anode end [28]. Because materials fracture more easily under tensile stress than under compressive stress, a thin film is expected to fail at the cathode under low current. As the current level increases and Joule heating becomes more dominant, the whole stress (electromigration and thermomechanical) turns to being compressive, so the film will fail at the anode end. In the third situation, the current is large enough so that the electromigration stress is smaller than the thermonechanical stress, then the film is expected to fail catastrophically toward the center. Figure 3 shows scanning electron microscope images of a Pd stripe before and after it breaks. The break region is melting-like, as shown in Fig. 3c and Fig. 3d, which might be caused by the combination of electromigration force and Joule heating effect. Figure 4 shows that, for the same Pd stripe, after the stripe is broken by running a high working current for a long time, the change of surface morphology is obvious at the anode region (see Fig. 4c and Fig. 4d), but not at the cathode region (see Fig. 4a and Fig. 4b). This is consistent with the theoretical predictions [28].
Y films can wet well to CNTs and make Ohmic contact [32].
However, it can also be easily oxidized, and indeed this nature has been applied to obtain high-performance gate dielectrics for graphene-based devices [33].