Electrical characteristics for HfO 2 with nitridation plasma treatment 28

There are three kinds of plasma treatment with different source gas (i.e. N2, N2O, NH3) and they were treated for different process time (i.e.

30 sec, 60sec, 90sec, 120sec, 150sec and 180sec).

4.1.1 Electrical characteristics for HfO2 with N2 plasma treatment for different process time

Fig. 4-1 reveals the capacitance-voltage (C-V) characteristics of HfO2 gate dielectrics treated in N2 plasma and DC bias 50W for different process time. The capacitor treated in N2 for 120 sec shows the maximum capacitance density among these samples with different process time. In addition, other samples which treated in N₂ plasma all present the larger values than the capacitors without whole plasma nitridation process. This phenomenon indicates that the N₂ plasma treatment was workable to improve the capacitance. The factor of improvement might be from that the PDA process and the nitrogen incorporation in the HfO₂ dielectrics,

29

which could enhance the electronic polarization as well as the ionic polarization, so the dielectric constant of the HfO2 thin films increases just as Hf-silicate thin film and SiO2 thin film. Besides, the capacitance density of the samples treated in N2 plasma for 150 sec and 180 sec are degraded. The reason could be the damage caused by the N2 plasma.

The J-V characteristics of the HfO2 capacitors treated by N2

plasma and DC bias 50W with different process time from 0V to -2V are described in Fig. 4-2. It can be observed that the samples which treated in N2 plasma all present the smaller leakage current density than the leakage current density without whole plasma nitridation process. The gate leakage current density treated in N2 for 120 sec shows the minimum current density among these conditions. The lower leakage shows that the weak structure of interface must be fixed by the plasma nitridaiton. In Fig.

4-1 and Fig. 4-2, it appears that the samples treated in N2 plasma for 120 sec display the most excellent value. While the nitridation process time is longer than 120 sec, the plasma damage from the plasma nitridation could cause the increase of the gate leakage density.

The hysteresis of C-V characteristics are shown in Figs. 4-3, 4-4, 4-5, 4-6, 4-7, 4-8, and 4-9 for the samples without treatment, and with 30, 60, 90, 120, 150, and 180 sec N2 plasma treatment , respectively. The hysteresis phenomenon of the C-V curves can be observed for all samples, which is caused by the existence of negative charges trapped in the dielectric defect states when the capacitors are stressed. The hysteresis characteristic could be improved by various plasma nitridation process. In Figs.4-3, 4-4, and 4-5, the hysteresis was a slightly reduced after N₂

30

plasma treatment because the nitrogen incorporation in the HfO2

dielectrics and reduced the interface trap state, thus improving hysteresis.

In Fig.4-6, 4-7, 4-8, and 4-9, the hysteresis was a slightly enhanced more than other samples with N2 plasma treatment due to plasma damage.

However, these hysteresis voltage are very close to each other and can be acceptable.

4.1.2 Electrical characteristics for HfO₂ with N₂O plasma treatment for different process time

Fig4-10 reveals the capacitance-voltage (C-V) characteristics of HfO2

gate dielectrics treats in N2O plasma and DC bias 40W for different process time. The capacitor treated in N2O 10 sec shows the maximum capacitance among these samples with different process time, just like the group of N2 plasma treatment. In addition, other samples which treated in N2O plasma all present the larger values than the capacitors without whole plasma nitridation process. This phenomenon indicates that the N2O plasma treatment was workable to improve the capacitance. The factor of improvement might be from that the PDA process and the nitrogen incorporation in the HfO2 dielectrics, which could enhance the electronic polarization as well as the ionic polarization, so the dielectric constant of the HfO₂ thin films increases just as Hf-silicate thin film and SiO2 thin film. Besides, the capacitance density of the samples treated in N₂O plasma for 150 sec and 180 sec are degraded. The reason could be the damage caused by the N2 plasma.

31

The J-V characteristics of the HfO2 capacitors treated by N2O plasma and DC bias 40W with different process time from 0V to -2V are described in Fig. 4-11. It can be observed that the samples which treated in N2 plasma all present the smaller leakage current density than the leakage current density without whole plasma nitridation process. The gate leakage current density treated in N2O for 120 sec shows the minimum current density among these conditions. The lower leakage shows that the weak structure of interface must be fixed by the plasma nitridaiton. In Fig. 4-10 and Fig. 4-11, it appears that the samples treated in N2O plasma for 120 sec display the most excellent value. While the nitridation process time is longer than 120 sec, the plasma damage from the plasma nitridation could cause the increase of the gate leakage density.

The hysteresis of C-V characteristics are shown in Figs. 4-12, 4-13, 4-14, 4-15, 4-16, and 4-17 for the samples without treatment, and with 60, 90, 120, 150, and 180 sec N2 plasma treatment respectively. The hysteresis phenomenon of the C-V curves can be observed for all samples, which is caused by the existence of negative charges trapped in the dielectric defect states when the capacitors are stressed. The hysteresis characteristic could be improved by various plasma nitridation process.

It can be observed that the samples which treated in N₂O plasma all present the smaller values than the leakage current density without whole plasma nitridation process. The hysteresis was a slightly reduced after N₂ plasma treatment because the nitrogen incorporation in the HfO2

dielectrics and reduced the interface trap state, thus improving hysteresis.

32

However, these hysteresis voltage are very close to each other and can be acceptable.

4.1.3 Electrical characteristics for HfO2 with NH3 plasma treatment for different process time

Fig. 4-18 shows the capacitance-voltage (C-V) characteristics of HfO2 gate dielectrics treated in NH3 plasma and DC bias 40W for different process time. The capacitor treated in NH3 for 120sec shows the maximum capacitance density among these samples with different process times. In addition, other samples which treated in NH3 plasma all present the larger values than the capacitors without whole plasma nitridation process. This phenomenon indicates that the NH3 plasma treatment was workable to improve the capacitance. The factor of improvement might be from that the PDA process and the nitrogen incorporation in the HfO2 dielectrics, which could enhance the electronic polarization as well as the ionic polarization, so the dielectric constant of the HfO2 thin films increases just as Hf-silicate thin film and SiO2 thin film. Besides, the capacitance density of the samples treated in NH3

plasma for 150 sec and 180 sec are degraded. The reason could be the damage caused by the NH3 plasma.

The J-V characteristics of the HfO₂ capacitors treated by NH₃ plasma and DC bias 40W with different process time from 0V to -2V are described in Fig. 4-19. It can be observed that the samples which treated in N2 plasma all present the smaller leakage current density than the leakage current density without whole plasma nitridation process. The

33

gate leakage current density treated in NH3 for 120 sec shows the minimum current density among these conditions. The lower leakage shows that the weak structure of interface must be fixed by the plasma nitridaiton. In Fig. 4-18 and Fig. 4-19, it appears that the samples treated in NH3 plasma for 120 sec display the most excellent value. While the nitridation process time is longer than 120 sec, the plasma damage from the plasma nitridation could cause the increase of the gate leakage density.

The hysteresis of C-V characteristics are shown in Figs. 4-20, 4-21, 4-22, 4-23, 4-24, and 4-25 for the samples without treatment, and with 60, 90, 120, 150, and 180 sec NH3 plasma treatment respectively. The hysteresis phenomenon of the C-V curves can be observed for all samples, which is caused by the existence of negative charges trapped in the dielectric defect states when the capacitors are stressed. The hysteresis characteristic could be improved by various plasma nitridation process.

In Figs.4-20, 4-21, and 4-22, the hysteresis of NH3 plasma treatment for 60 sec and 90 sec were both larger than the sample of no plasma treatment. The reason possibly was the plasma process times not enough.

In Figs.4-23, 4-24, and 4-25, the hysteresis was a slightly reduced after NH3 plasma treatment because the nitrogen incorporation in the HfO2

dielectrics and reduced the interface trap state, thus improving hysteresis.

However, these hysteresis voltage are very close to each other and can be acceptable.

34

4.1.4 Short summary

Fig. 4-26 shows the capacitance-voltage (C-V) characteristics of HfO2 gate dielectrics treated in N2, N2O, and NH3 plasma at optimal condition. It is indicated that the capacitance treated in N2 plasma for 120 sec shows the most excellent value.

The J-V characteristics of the HfO2 capacitors treated in N2, N2O, and NH3 plasma at optimal condition in Fig. 4-27. It is indicated that the capacitance treated in N2O plasma for 120 sec shows the most excellent value.

By the compare of the samples which has the best capacitance in their own gas, we can realize the most suitable treatment condition which both has the best capacitance and lower leakage current. Hence, we significantly find a relative optimal condition among above discussion. It is proved that without thick oxidation layer, it can also reach the smallest leakage current when there is suitable time treatment.

4.2 Electrical characteristics for HfO2 with fluorination