3.1 Capacitance-Voltage Characteristics
In order to measure the C-V characteristics of our MIS capacitors we used HP2484A LCR meter in our experiments. We swept the gate bias from inversion region to accumulation region to obtain the curve at the frequency of 10 kHz. There are three kinds of plasma treatment with different source gas ( i.e. N2, N2O, and O2 ) and they were treated for different process time ( i.e. 10 sec, 30sec, 1min, 3min, and 5 min). Firstly, the relationship of difference process time in one kinds of plasma treatment will be discussed. Then compare with the effect of different source gas.
Fig. 3-1 reveals the capacitance-voltage (C-V) characteristics of HfO2 gate dielectrics treated with N2 plasma treatment for different process time. The capacitor treated for 1 minute shows the maximum capacitance among these conditions of process time. In addition, the capacitor treated for 10 second and 30 second both show the good C values which are larger then the capacitor with the condition of no treatment. This phenomenon indicates that the N2 plasma treatment was workable to improve the capacitance. Maybe it is caused of the intensifying of the interface structure or high-k bulk itself. The growing of interfacial oxide has also been restrained. On the other hand, the capacitance treated for 5 minute is very low and it is
even lower than the no-treated sample. It is seems that the plasma damage occur and then destroy the structure of high-k capacitance when the duration of plasma treatment is too long. The degradation of capacitance also can be found at the case of 3 min-treatment time, although the C value is still larger then the case without plasma treatment.
Fig. 3-2 shows the capacitance-voltage (C-V) characteristics of HfO2 gate dielectrics treated with N2O plasma treatment for different process time. Just like the group of N2 plasma treatment. The improvement of capacitance and the damage cause by excessive plasma treatment both can be seen. At this condition, the capacitance treated with N2O plasma treatment for 10 second shows the largest value. Then, the capacitance becomes worse and worse with the increase of the treatment time. By the way, the samples besides 3 min and 5 min all perform well about larger capacitance then the sample without treatment. It is indicated that N2O plasma treatment is also a practicable method to improve the capacitance-voltage characteristics of HfO2 gate dielectrics.
Fig. 3-3 shows the capacitance-voltage (C-V) characteristics of HfO2 gate dielectrics treated with O2 plasma treatment for different process time. The experiment of plasma treatment only with oxygen radical is wanted to see if it is still existed the improvement of capacitance, even if without nitridation. Consequently, it is shown that the capacitors treated for 10 sec and 30 ses have larger capacitances than the no-treated sample, especially for 30ses provided the maximum capacitance.
Take the view of 1min condition, its capacitance has begun lower than the sample which is no treated. As the same time of 1min conditions, the other kinds of treatments still remain good capacitances then no-treated sample. So it can be know
that there are some reasons besides plasma damage occurred at the condition of 1min treatment. It is suggested that plasma treatment with oxygen radical may cause additional oxidation followed by repairing of the interface structure. Because the interfacial oxide provides lower k value, the total capacitance was be effected and become lower.
Fig. 3-4 shows the capacitance-voltage (C-V) characteristics of HfO2 gate dielectrics treated with N2 plasma treatment for 1 min, N2O plasma treatment for 10 sec, and O2 plasma treatment for 30 sec. It is indicated that the capacitance treated with N2 plasma treatment for 1 min shows the most excellent value ( i.e. 50%
increasing about capacitance). Among these samples, the capacitance treated with O2
plasma treatment is the worst because the growing of interfacial oxide is unavoidable while the oxygen atoms become radical and enter the interface. Besides this, the reason why the sample treated with N2O plasma has lower capacitance then N2
plasma treatment is complex.
It is may be the growing of interfacial oxide made the capacitance degradation. Thus, the capacitance improvement by interface repair was easily eliminated by the interfacial oxide which has lower k value. So, the capacitance becomes degradation when the process time only exceeds 10 second.
3.2 Current-Voltage Characteristic
Fig. 3-5 shows the J-V characteristics of p-type HfO2 capacitors treated by N2
plasma with different process time from 0 V to -1 V. We observed that the gate leakage current density is suppressed while treatment conditions are 10sec, 30sec,
1min, and 3 min. It is indicated that N2 plasma treatment supply an effective barrier against the leakage current. The lower leakage shows that the weak structure of interface must be fixed by the plasma nitridation, especially for 1 min capacitor which both has the lowest leakage and largest capacitance value from Fig. 3.1. Gate leakage current density of no treatment insulator at VG = 1 V is about 3.25×10-6 A/cm2. From fig.3-5, however, gate leakage current density of the capacitor treated for 1 min N2 plasma at VG = -1 V is only about 1.35×10-8 A/cm2. It has less gate leakage than no treatment insulator about 2 orders. Furthermore, we notices that the 3 min capacitor although has little leakage, its capacitance has become degradation. This is an interesting phenomenon. Even though the plasma damage has begun to reduce C value, the amount of leakage current is still kept very well. It means that the capacitance value is more easily affected by plasma damage than leakage current.
Fig. 3-6 shows the J-V characteristics of p-type HfO2 capacitors treated by N2O plasma with different process time from 0 V to -1 V. After N2O plasma treatment, we can see the reduction of leakage current in contrast of no treatment samples. It is worthy to be noticed that the capacitors treated by 10 sec N2O plasma which has the best C value also performs a low leakage current about 5.97×10-8 A/cm2. In addition, we find that the leakage currents of 3 min and 5 min treatment are larger then 1min.
But for counterpart, they are not larger then no treatment sample. Relative to the case of N2 plasma, we can see that the level of leakage current increasing obviously mitigate. It is possibly due to the additional oxidation layer formed by oxygen radical.
The interfacial oxidation layer will let the dielectric thicker to prevent from gate leakage.
Fig. 3-7 shows the J-V characteristics of p-type HfO2 capacitors treated by O2
plasma with different process time from 0 V to -1 V. All of the samples depict the presence of the reduction in leakage current. It is indicated that there are not only the effect of improving interface quality but also another effect to suppress the leakage current in the case. According to the discussion about Fig. 3-3, we know that the growth of interfacial oxide layer will decrease the C value. Now the interfacial layer introduces a hard barrier to suppress leakage current. Consequently, the leakage current all displays a lower value including the capacitor treated by O2 plasma for 5 min even if it is must be damaged by plasma.
Fig. 3-8 shows the J-V characteristics of HfO2 gate dielectrics treated with N2
plasma treatment for 1 min, N2O plasma treatment for 10 sec, and O2 plasma treatment for 30 sec. 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 lowest leakage current. It is showed that the N2 plasma treatment for 1 min is the best one. It is proved that without thick oxidation layer, it can also reach the smallest leakage current when there is enough long-time treatment.
3.3 Summary and SIMS profile
In order to deeply realize the effect of the plasma treatment, we take SIMS ( Secondary Ion Mass Spectrometer ) analysis to verify the phenomenon observed from CV and JV curve. Fig. 3-9 and Fig. 3-10 show the SIMS profiles of different plasma treatment conditions. Fig. 3-9 shows the counts of Hf-N bonds with different conditions which are N2 for 1 min, N2O for 10 sec, and O2 for 30 sec. And Fig. 3-10 shows the counts of oxygen secondary ion. These three samples which show good electrical characteristic are the best process time in their own gas conditions
respectively. So we take these three samples to do SIMS analysis. First, from the Fig.
3-9 and Fig. 3-10 the sample of N2 for 1 min shows the most Hf-N bonds and the less oxygen concentration. This explains why the sample of N2 for 1 min has the best C value. Moreover, it is the evidence of the oxidation suppression by N2 plasma treatment compared with no treatment sample. With the proof we can understand why the sample of N2 for 1 min has the best C value and less leakage: the suppression of oxidation and the Hf-N bonds which appear at the interface fix the interface structure and strengthen it.
On the other hand, although the sample of N2O for 10 sec shows a mount of Hf-N bonds at interface, there is still oxidation at interface so that the sample shows the less C value compared with N2 plasma treatment. However, if we take a look at the sample of O2 for 30 sec, we can find that the oxidation phenomenon is more serious so that its C value is the less.
As a consequence, the N2 , N2O, and O2 plasma treatment all shows better electrical properties than no treatment sample. Furthermore, the N element and O element all can fix the interface and promote the electrical properties include of CV curve and JV curve. But for the reason of oxidation caused by oxygen radical, the N2O and O2 plasma treatment samples shows the lower C value. Just because the oxidation phenomenon, the films will become thicker so that the plasma damage will not easily effect the leakage current profile.