Figure 2-5 shows the room-temperature leakage current density for the as-fabricated as well as 400oC-annealed Al-gated and TaN/Cu-gated MIS capacitors of various α-SiCN and α-SiN dielectric films. The measurements were carried out with the MIS capacitors biased in the accumulation region. For each dielectric film, very little difference in leakage current was observed between the Al-gated and TaN/Cu-gated MIS capacitors, whether as-fabricated or 400oC-annealed. This implies that all the dielectrics were capable of preventing Cu permeation at temperatures up to 400oC. The BTS test was used to further explore the barrier property of the TaN/Cu-gated MIS capacitors. Prior to the BTS test, all MIS capacitors were thermally annealed at 400oC for 30 min in N2 ambient to repair the plasma-induced damage that occurred during the metal electrode sputtering and also to provide the driving force for Cu diffusion. The BTS test was performed in N2 ambient to
prevent the Cu electrode from oxidizing at elevated temperatures as well as moisture uptake into the dielectric films. Figure 2-6 shows the leakage current density versus stress time for various TaN/Cu-gated and Al-gated MIS capacitors under BTS at 200oC with an applied electric field of 1 MV/cm. It can be seen that the TaN/Cu-gated SC3 sample failed after being subjected to the BTS for 30 min, whereas all other samples remained stable under the BTS up to at least 15 h. The breakdown of the TaN/Cu-gated SC3 sample is presumably due to the penetration of Cu into the SC3 dielectric rather than due to wear out of the SC3 film from the BTS. Figure 2-7 illustrates the leakage current density versus electric field for various MIS capacitors measured at 200oC before and immediately after the BTS test.
There is a significant increase in leakage current for the TaN/Cu-gated SC3 sample after application of the BTS for 1 h, while all other samples show no obvious change in leakage current after application of the BTS. Figure 2-8 shows the SIMS depth profiles of Cu in the TaN/Cu-gated SC3, SC2, and SC1 samples after removal of the TaN/Cu electrode.
The depth profile of Cu clearly indicates the permeation of Cu into only the SC3 dielectric for the TaN/Cu-gated SC3 sample after one hour of BTS. Thus, we may conclude that the spiking in the leakage current of the TaN/Cu-gated SC3 sample during the BTS test (Fig.
2-6a) and the significant increase of leakage current in the TaN/Cu-gated SC3 sample after application of BTS for one hour (Fig. 2-7a), resulted from the penetration of Cu into the SC3 dielectric.
The poor barrier property of the SC3 film against Cu penetration might be attributed to
21
enhance the porosity of α-SiCN films [7,9], resulting in easier penetration of Cu into the film. Figure 2-10 illustrates a schematic microstructure of a porous α-SiCN film. The open pore is a cavity or a channel that can communicate with the surface of the dielectric film, while the closed pore is a cavity not communicating with the surface [14]. A highly porous film will lead to more adsorption of moisture into the film. After the α-SiCN films were immersed in boiling water for 1 h, the signal of H2O was observed in the TDS spectrum for the SC3 sample, as shown in Fig. 2-11. It is obvious that the SC3 film stored the most amount of moisture (H2O) with a major mass peak at 18. Moisture in the dielectric will enhance the drift of Cu ions in the dielectric film during the BTS test [15].
Thus, the penetration of Cu into the SC3 film in the TaN/Cu/SC3/Si MIS capacitor during the BTS test is presumably due to the porosity enrichment caused by an abundant amount of carbon atoms in the SC3 dielectric.
2-5 Summary
Three α-SiCN films with different carbon and nitrogen concentrations and dielectric constants of less than 5.5 were investigated with respect to the thermal stability and physical and barrier properties. For a comparison, an α-SiN film with a k-value of 7.2 was also studied. It is found that the dielectric constant of α-SiCN films decreases with increasing content of carbon and decreasing content of nitrogen in the films. All of the dielectrics, including the three α-SiCN and the one α-SiN films, are thermally stable up to 500oC (for 30 min in N2 ambient). However, degraded barrier capability and moisture resistance were observed for the α-SiCN film with a k-value of 3.5, which has a C/Si atomic ratio of 0.875. This is presumably due to the poorly crosslinked molecular structure and the porosity enrichment caused by the abundant amount of carbon atoms in the α-SiCN film.
References
[1] L. Peters, Semicond. Int. (June 2000) p. 122.
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Loboda, IEEE IITC Technol. Dig. (1999) p. 109.
[3] M. J. Loboda, Microelectronic Engineering, 50 (2000) p. 15.
[4] M. J. Loboda, J. A. Seifferly, and F. C. Dall, J. Vac. Sci. Technol. A, 12 (1994) p. 90.
[5] M. J. Loboda, J. A. Seifferly, C. M. Grove, and R. F. Schneider, Mat. Res. Soc. Symp.
Proc., 447 (1997) p. 145.
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[15] J. D. McBrayer, R. M. Swanson, and T. W. Sigmon, J. Electrochem. Soc., 133 (1986) p. 1242.
Table 2-1 Compositions, dielectric constants, refractive indices, and deposition temperatures of α-SiCN and α-SiN films.
α-SiCN α-SiN
Sample identification
SC3 SC2 SC1 SN
C/Si ratio N/Si ratio
0.875 0.111
0.718 0.333
0.513 0.467
0.040 1.111 Dielectric constant
@ 1 MHz 3.5 4.5 5.4 7.2
Refractive index @
633 nm 1.65 1.93 1.83 1.95 Deposition
temperature (oC) 350 350 400 400