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Characterization of Sputtered Nano-Crystalline Zirconium Carbide as a Diffusion Barrier for Cu Metallization

September 17, 2007 – 7:10 am

Zirconium carbide (ZrC) films were grown on Si (100) substrates using magnetron sputtering where the growth temperature (T^sub s^) was varied from 25°C to 290°C. The microstructure and resistivity of the as-deposited ZrC films were examined. The results reveal that nano-crystalline ZrC films with grain size less than 5 nm were fabricated only at 29°C, which can be explained by a repeated nucleation mechanism. For thermal stability characterization, the stacked structure of Cu/ZrC/Si was subsequently subject to thermal treatments at temperatures from 300°C to 900°C for 30 min in a vacuum tube. The stacked samples were shown to be thermally stable up to about 800°C from Auger electron spectroscopy (AES) and x-ray diffraction (XRD). The diffusion coefficient and activation energy of Cu and Si in the ZrC barrier were also derived. It indicated that Si has a lower activation energy than Cu resulting in faster diffusion. The device completely fails at 900°C, and the mechanism is discussed in this paper.

Copper is known to diffuse quickly through dielectric and Si layers. Thus, it is essential to introduce a diffusion barrier between Cu and dielectric layers. Although many studies have been reported in the literature on the barrier properties of transition metal nitrides such as TaN,1 TiN,2 WN,3 and ZrN,4 transition metal carbides have also been promising materials in preventing Cu diffusion. Thus far, the carbides being investigated for the diffusion barrier application include TaC,5 WC,6 and TiC.7 However, most of these layers can only prevent Cu diffusion at temperatures below 750°C. On the other hand, ZrC is an advanced ceramic due to both superior covalent properties such as high melting point (Tn, = 3445°C) compared to TiN (T^sub m^ = 2950°C), TaN (T^sub m^ = 2980°C), ZrN (T^sub m^ = 2982°C), TaC (T^sub m^ = 2785°C), TiC (T^sub m^ = 3076°C), and WC (T^sub m^ = 2785°C), great hardness, excellent mechanical stability, lower work function,8 and a metallic behavior in electrical and optical properties.9 Therefore, ZrC films have been largely applied as refractory materials for cutting tools,10 crucibles in mechanical and nuclear industry,11,12 and in electronic devices.13 ZrC thin films have been synthesized by e-beam bombardment,8 chemical vapor deposition,14 laser cladding,15 plasma-assisted metal organic chemical vapor deposition,16 and pulsed laser ablation deposition.17 However, despite its technology interests, ZrC has not yet been fabricated by simple magnetron sputter deposition. Therefore, detailed studies on the microstructure and related properties of ZrC films are worth systematic examination with growth parameters. Here, we report on nanocrystalline ZrC film synthesis by sputtering and thermal stability of the ZrC thin films under various annealing temperatures as a diffusion barrier.

EXPERIMENTAL PROCEDURE

The ZrC films were deposited on Si (100) substrates using a stoichiometric ZrC target (99.5% in purity) by d.c. magnetron sputtering. The distance between the target and the substrate holder was fixed at 60 mm. A d.c. power of 150 W was employed, while the base pressure of the deposition chamber was 3 × 10^sup -6^ Torr. During deposition, the working pressure was fixed at 8 × 10^sup -3^ Torr with a carrier gas of Ar for a total flux of 50 seem. The sizes of the target and the substrate were 5 cm and 4 cm in diameter, respectively, and the deposition time was about 6 min to maintain the ZrC film thickness in the range of 150-200 nm. The growth temperature, T8, was varied from 25°C to 290°C in order to study the effect of the substrate temperature. A 200-nm Cu overlayer was then sputter deposited onto ZrC at room temperature to form the Cu/ZrC/Si stacks in the same chamber, where the applied power and the substrate bias were 50 W and -90V, respectively.

The specimens were then ex-situ annealed at various temperatures up to 900°C for 30 min in a vacuum tube of 3 × 10^sup -5^ Torr, where the heating and cooling rates were set to be 10°C/min and -26°C/min, respectively. We employed four-point probe, x-ray diffraction (XRD), and scanning electron microscopy (SEM) to characterize resistivity, microstructure, and surface morphology of the films, respectively. Auger electron spectroscopy (AES) was used to characterize the distribution of each constituent atom of the stacks in depth.

RESULTS AND DISCUSSION

Microstructure Characteristics of the ZrC Thin Films

Diffusion Barrier Property of the ZrC in Cu/ZrC/Si System

In this study, we chose the ZrC sample synthesized at T^sub s^ = 180°C, which exhibits the lowest resistivity in the sample series. Upon annealing the Cu/ZrC/Si stack at various temperatures, XRD patterns in Fig. 4 show that ZrC (111) and (200) peaks coexist, but the relative intensity of (111) monotonically increases from room temperature to annealing temperature at 700°C (not shown). The intensity of ZrC (111) and (200) peaks dramatically decreased at 800°C, possibly because the crystallinity becomes worse, signifying the onset of element interdiffusion; however, no other new phases are present. Basically, the ZrC films can still remain stable upon annealing up to 800°C, and the entire structure fails at 900°C, where many new phases are produced. This includes Cu^sub 3^Si and Zr-Si compounds at the expanse of Cu at 900°C annealing as shown in Fig. 4. Figure 5 shows the variation of the sheet resistance of the Cu/ZrC/Si structures with annealing temperature. It is evident that the sheet resistance slightly decreased up to 700°C due to grain growth. A slight increase in sheet resistance was observed after 800°C annealing resulting from the broken Cu film (Fig. 6), consistent with the XRD results, where the intensity of the Cu peak decreases. However, the sheet resistance increases drastically when the annealing temperature is 900°C. As is also supported by the XRD analysis, strong reactions between Cu and Si, which result in Cu^sub 3^Si phase, is responsible for the sharp increase in the sheet resistance. Figure 6 shows the plan-view SEM images of the as-deposited and annealed Cu/ZrC/Si samples. Apparently, from Fig. 6, Cu grain size becomes larger with temperature and a few voids form at 800°C, without any obvious precipitates, which conforms to the previous resistivity results.