Electric Properties

7-1 Introduction

The capacitance and leakage current of a-C:F films can be measured by HP 4280 and HP 4156. We can calculate the dielectric constant and estimate the conductivity of the films. The higher flow ratio a-C:F films have a lower dielectric constant and a low leakage current. The electric properties a-C:F films will change when being annealed at 300℃. The dielectric constant increases at the rise of temperature, while the leakage current declines. The dielectric constant decreases when fluorinated concentration goes up. A low dielectric constant of around 1.5 and high dielectric strength beyond 35 MV/cm were obtained from R = 0.98 a-C:F films in the as-deposited and annealed conditions.

7-2 Experiment

The metal-insulator-metal (MIM) structure (Pt/a-C:F/Pt) was used for electric property measurements (Fig. 7-1). The bottom Pt electrodes were deposited by sputter tool, and the top electrodes were deposited by electron gun (e-gun) tool. Current-voltage (I-V) measurements were performed with the HP4156, and the capacitance (C-V) characteristic was measured with the HP4280 at a frequency of 1MHz. The flowchart of measurement method is shown in Fig. 7-2.

Figure 7-1 shows the schematic structure of MIM (Pt/a-C:F/Pt).

Figure 7-2 shows the measuring methods of electronic properties.

7-2-1 Deposition of Insulator

We used SiO₂ which was grown on silicon wafer as an insulator layer in this study. The 1500Å thermal oxide was grown at 950℃ in a steam atmosphere (ASM furnace). The metal-insulator-metal structure was used to measure the electronic properties of a-C:F films.

7-2-2 Deposition of Metal Thin Film

The MIM (metal/insulator/metal) structure was used for electronic properties measurement. We sputtered 1000 Å Pt as the bottom electrode, and used E-gun to deposit 600 Å Pt as a top electrode.

7-3 Results and Discussion

The measured dielectric constant (ε) of the as-deposited and the annealed films is displayed in Fig. 7-3, where ε declines monotonically with increasing CF₄ flow ratio until it reaches as low as ~1.5 at R = 0.98.

Such a low ε value may be partly due to, in addition to the high F content of the films, the low-density film structure containing nano-voids as evidenced in Fig. 6-10. Increase of the dielectric constant is observed in all of the samples after heat treatment. Detailed analysis has been carried out on the dielectric properties of a-C:F films annealed in a N₂ atmosphere¹. The increase of the ε value was due to the enlarged orientational polarization arising from the thermally generated trapped radicals, and the minor increase of electronic polarization was caused by the reduction of the F content. In our case, one additional factor contributing to the

increase of the dielectric constant is probably the shrinkage of voids which enhanced the film density.

Figure 7-3. Dielectric constant of the films as function of the CF₄ flow ratio R.

Among the measured I-V curves of the as-deposited films, many of them showed either a huge transient current spike or a breakdown at field intensity depending on the samples (Fig. 7-4). The current surges are normally attributed to a low-resistance path formed in the films during discharge^2,3, where the low-resistance path, in the case of the transient spike, would form a open-circuit, then, a self-healing process. In general, microstructure defects presumably work to trigger the discharge and initiate the formation of the low-resistance path. In the current study, an inhomogeneous, high local field intensity, which is capable of triggering

0.9 0 0 .92 0.9 4 0 .96 0.9 8

the discharge, may be induced near the irregular morphology of the nano-voids when an external field is applied across the films. The shrinkage of voids after annealing might be responsible for the deletion of breakdowns in the I-V characteristics of the annealed a-C:F films, which demonstrated superior dielectric strength compared with that of SiO₂ films.

The annealed sample of R = 0.98 with a thickness of around 400 Å showed a high dielectric strength beyond 35 MV/cm, while for SiO₂ film with the same thickness, the highest field strength it could sustain is around 10 MV/cm⁴.

Fig. 7-4. Leakage current of the a-C:F films of (a) R = 0.98, (b) R = 0.97,

The magnitude of the current surges decreases when the flow ratio goes down, and eventually diminishes in the sample with R = 0.90. The above variation is consistent with the fact that fewer nano-voids are found in the as-deposited samples at a lower flow ratio. In contrast, the increase of normal leakage current at a decreasing flow ratio was observed in the annealed films (Fig. 7-4), and the variation was expressed in terms of averaged electrical conductivity versus flow ratio as shown in Fig. 7-5.

Similar results have been reported in previous studies^5,6, where the higher conductivity of a-C:F films was attributed to their higher sp²/sp³ bonding fraction (sbf),considering that the graphitic, sp² bonded carbon, is more conductive than the diamond-like, sp³ bonded carbon. Consistent results for the measured sbf of the annealed films are displayed in Fig. 7-6. It is noted that the conductivity increased slowly with an increasing sbf in the CF₄ flow ratio from R = 0.98 to 0.95. The conductivity then increases rapidly at around sbf = 40%, which indicates a threshold fraction of sp² bonded carbons for charge conduction. Further studies are needed to delineate the dependence of charge carrier concentration and mobility on the sp²/sp³ bonding fraction.

Fig. 7-5. Electrical conductivity of the annealed films as a function of the CF₄ flow ratio R.

Fig. 7-6. sp²/sp³ bonding fraction of the annealed films as a function of the CF₄ flow ratio R.

0.90 0.92 0.94 0.96 0.98

0.0 1.0x10^-15 2.0x10^-15 3.0x10^-15 4.0x10^-15

Conductivity (1/Ω−cm)

R

0.90 0.92 0.94 0.96 0.98

20 30 40 50

sp2 bonding (%)

R

7-4 Summary

By heat treatment the fluorine concentration will be decreased and the orientational polarization will be enlarged. The dielectric constant of a-C:F films rise at a negative rate in CF₄ flow ratio. The shrinkage of voids after annealing is responsible for the deletion of breakdowns in the I-V characteristics of the annealed a-C:F films. The higher flow ratio in a-C:F films generates more sp³ structures which lowers the electric conductivity.

The dielectric constant of the as-deposited films can be as low as 1.5 for those prepared at a high flow ratio, and the dielectric strength of the annealed films is beyond 30 MV/cm, far superior to that of SiO₂ films with the same thickness. The occurrence of a huge transient current spike or breakdown during the I-V test was attributed to the nano-voids found in the as-deposited films, which worked to trigger the discharge. The averaged conductivity of the films was related to the measured sp²/sp³ bonding fraction which indicated a threshold fraction of around 40%.

Reference

1 K. Endo and T. Tatsumi, J. Appl. Phys. 86, (1999) 2739.

2 N. Klein, Thin Solid Films 7, (1971) 149.

3 J. C. Jackson, T. Robinson, O. Oralkan, D. J. Dumin and G. A. Brown, J.

Electrochem. Soc. 145, (1998) 1033.

4 P. Solomon, J. Vac. Technol. 14, (1997) 1122.

5 H. Yang, D. J. Tweet, Y. Ma and T. Nguyen: Appl. Phys. Lett. 73 (1998) 1514.

6 J. P. Chang, H. W. Krautter, W. Zhu, R. L. Opila and C. S. Pai: J. Vac.

Sci. & Technol. A. 17 (1999) 2969.

Chapter 8