In order to elucidate the leakage mechanism, a plot of the leakage current density versus the square root of the applied electric field in the high electric field regime was sketched, as shown in Figure 6.11. To make the plots concise and clear, only high field region is shown.
Figure 6.11(a), (b), and (c) show ln (J/E) and E1/2 characteristics of the Ta/HfO2/Ta, Al/HfO2/Ta, and Cu/HfO2/Ta MIM capacitors, respectively. We found that all leakage current densities of Ta, Al, and Cu top electrode capacitors are increased with temperature, revealing a temperature dependence on the leakage behavior. In addition, all leakage current densities are linearly related to square root of the applied electric field. These linear variations of current densities, after careful calculation, correspond to Frenkel-Poole (FP) type conduction
mechanism [142], linear relationship between ln J and 1/T, capable of calculating the height of trapping potential well φ FP Cu top electrode, respectively. The calculated conduction mechanism of Frenkel-Poole type for all samples indicates that the current conduction is essentially via the trap state. However, some variation still exists in the extracted height of trap potential with respect to different top electrodes. This implies that the traps at and around the interface rather than the traps at deep level play the major role to the conduction mechanism.
6.5 Conclusions
The IPD and MIM capacitors have been successfully fabricated with HfO2 as the dielectric layer. The measurement results show high capacitance density compared to the data shown in previous literatures. The leakage currents of the IPD and MIM capacitors are very small. Although the resistivity of the electrodes are larger because of the nitrogen
incorporated. The frequency dispersion effect is still not very serious for the MIM capacitors.
For the IPD, the thermal stability is very good and the temperature coefficient is very small.
These show that the HfO2 dielectric is very suitable for IPD and MIM applications.
HfO2 MIM capacitors with different metal top electrodes have been investigated in the second part. The MIM capacitor with Al top electrode exhibits the lowest capacitance density, while that with Cu top electrode exhibits the highest capacitance value of 3.4 fF/µm2. Due to the Al2O3 layer formed between Al and HfO2, the capacitance density and the leakage current density were reduced to 2.9 fF/µm2 and 5.0×10 –9 A/cm2 (at 5 V), respectively. On the other hand, although the MIM capacitor with Cu top electrode shows larger leakage current density at higher electric field, the successful fabrication of the Cu top electrode capacitor implies the possibility of integrating Cu with HfO2 dielectrics. Thus indicates that it is very suitable for HfO2 dielectric to use in silicon IC applications.
Table 6.1 Voltage linearity coefficients α (ppm/V2) and β (ppm/V) as a function of frequency for the HfO2 MIM capacitors with Ta, Al, and Cu top electrode.
Tantalum (Ta) Aluminum (Al) Copper (Cu) Frequency α
(ppm/V2)
β (ppm/V)
α (ppm/V2)
β (ppm/V)
α (ppm/V2)
β (ppm/V)
1 MHz 65.1 150.3 35.5 83.3 55.4 47.7
100 kHz 70.2 181.0 36.0 105.2 53.6 79.1
10 kHz 74.4 125.8 41.2 93.7 230.1 84.5
1 kHz 87.0 130.1 161.5 200.6 153.4 −33.7
Voltage (V)
0 1 2 3 4 5
Leakage Current (A/cm
2)
10-11 10-10 10-9 10-8 10-7 10-6
IPD MIM
Fig. 6.1 Current-voltage (J-V) characteristics of the IPD and MIM capacitors.
Temperature (
oC)
0 25 50 75 100 125 150 175 Capacitance Density (fF/ µm
2)
4 6 8 10 12
1MHz 100kHz 10kHz 1kHz 100Hz
Fig. 6.2 Capacitance density of the MIM capacitor as a function of temperature at frequencies varied from 100Hz to 1MHz.
Temperature (
oC)
Fig. 6.3 Capacitance density of the IPD as a function of temperature at frequencies varied from 100Hz to 1MHz.
Fig. 6.4 Capacitance density of the IPD as a function of frequency at temperatures varied from 25°C to 200°C.
Voltage (V)
-5 -4 -3 -2 -1 0 1 2 3 4 5
Capacitance Density (fF/ µm
2)
2 3 4 5 6
1MHz 100kHz 10kHz 1kHz
Fig. 6.5 Capacitance-voltage (C-V) characteristics of HfO2 MIM capacitors with Ta electrodes at the frequencies from 1 kHz to 1 MHz.
Voltage (V)
-5 -4 -3 -2 -1 0 1 2 3 4 5
Capacitance Density (fF/ µm
2)
2 3 4 5 6
Al / HfO
2/ Ta Ta / HfO
2/ Ta Cu / HfO
2/ Ta
Fig. 6.6 Capacitance-voltage (C-V) characteristics of HfO2 MIM capacitors with Al, Ta, and Cu top electrodes at the frequency of 100 kHz.
Voltage (V)
0 1 2 3 4 5
Leak age Current Densi ty (A /c m
2)
10
-11Fig. 6.7 Current density-voltage (J-V) characteristics of the HfO2 MIM capacitors with the top electrodes of Al, Ta, and Cu.
Fig. 6.8 Loss tangent as a function of frequency for the MIM capacitors with Al, Ta, and Cu top electrodes.
Frequency (Hz)
Fig. 6.9 Capacitance density of the MIM capacitor with Ta top electrode as a function of frequency after thermal stress from 25°C to 125°C.
Frequency (Hz)
Fig. 6.10 Capacitance density of the MIM capacitor with Al and Cu top electrode as a function of frequency after thermal stress from 25°C to 125°C.
E 1/2 (MV/cm) 1/2
0.88 0.89 0.90 0.91 0.92 0.93 0.94
J / E (A/MVcm)
10
-910
-810
-725
oC 50
oC 75
oC 100
oC 125
oC Ta / HfO
2/ Ta
(a)
E 1/2 (MV/cm) 1/2
0.82 0.84 0.86 0.88 0.90 0.92 0.94
J / E (A/MVcm)
10
-810
-725
oC 50
oC 75
oC 100
oC 125
oC
Al / HfO
2/ Ta
(b)
E 1/2 (MV/cm) 1/2
0.87 0.88 0.89 0.90 0.91 0.92 0.93 0.94
J / E (A/MVcm)
10
-910
-810
-725
oC 50
oC 75
oC 100
oC 125
oC Cu / HfO
2/ Ta
(c)
Fig. 6.11 Poole-Frenkel plot showing the current density versus electric field characteristics at five measurement temperatures from 25°C to 125°C for HfO2
MIM capacitor with (a) Ta, (b) Al, and (c) Cu top electrode.
Chapter 7
Conclusions and Suggestions for Future Work
7.1 Conclusions of This Study
The hot carrier injection reliability of nMOSFET’s with ultrathin plasma nitrided gate oxide is investigated in this thesis. The devices with plasma nitrided oxide suffer more transconductance reduction and threshold voltage shift. The degradation is direct proportional to the plasma nitridation time. For channel hot-carrier stressing, the most efficient stressing condition is located at Vg = Vd, rather than maximum substrate current or Vg = Vd / 2 which is often utilized for traditional nMOSFETs. For substrate hot-carrier stressing, the raising of substrate voltage can enhance the device degradation considerably. We report, for the first time, an enhanced degradation under negative substrate bias in nMOSFETs with ultrathin plasma nitrided gate dielectric. The enhanced degradation is attributed to the introduction of paramagnetic electron trap precursors during plasma nitridation. Similar to NBTI in pMOSFETs, our findings are important for nMOSFETs from the reliability point of view. For channel hot-carrier stressing of pMOSFET’s, the most efficient stressing condition is located at Vg = Vd, which is corresponding to the region of maximum gate current. The negative threshold voltage shift indicates a positive charge build-up in the gate dielectric. The positive charge can result from either hole trapping in the dielectric or the creation of positively charged interface states at the dielectric interface. For negative bias temperature stressing, appreciable enhancement of the threshold voltage shift can be observed through the raising of temperature. The enhanced device degradation is attributed to the H-related species and the
interface trap generated during NBT stressing. Even though the incorporation of nitrogen into thermal oxide is advantageous in many respects, our findings suggest that careful attentions need to be paid to ensure that plasma-nitrided gate dielectric meets the reliability requirements for the sub-100nm device technology node.
The second part in this thesis is high-k gate dielectrics related to hafnium oxide. Hafnium oxide film was evaluated as possible candidate material to replace SiO2 for gate dielectric in complementary metal-oxide-semiconductor technology. Both sputtering and MOCVD methods were used as the tool of thin-film HfO2 deposition. In view of the film uniformity and interfacial layer growth, however, sputtering maybe not sufficient to be a production tool in the future. AVD-deposited HfO2 capacitors using Cu and Al as the gate electrode have been fabricated and investigated for the first time. The counterparts with thermally grown SiO2
dielectric were also constructed for comparison. Our results clearly show that HfO2 dielectric depicts superior resistance against Cu diffusion after BTS test, compared to SiO2. Moreover, the presence of Cu metal in direct contact with HfO2 has negligible impact on the reliability of the HfO2 capacitor. The fact that HfO2 can behave as a good barrier against Cu diffusion is attributed to its considerably high density. This finding is important as it suggests the feasibility of a Cu integration process from the gate electrode to BEOL interconnect, which will allow the use of the gate electrode as the first-level metal simultaneously, resulting in a simplified process.
HfO2 MIM capacitors with different metal top electrodes have also been investigated.
The MIM capacitor with Al top electrode exhibits the lowest capacitance density, while that with Cu top electrode exhibits the highest capacitance value of 3.4 fF/µm2. Due to the Al2O3
layer formed between Al and HfO2, the capacitance density and the leakage current density were reduced to 2.9 fF/µm2 and 5.0×10 –9 A/cm2 (at 5 V), respectively. On the other hand,
although the MIM capacitor with Cu top electrode shows larger leakage current density at higher electric field, the successful fabrication of the Cu top electrode capacitor implies the possibility of integrating Cu with HfO2 dielectrics. Thus indicates that it is very suitable for HfO2 dielectric to use in silicon IC applications.