Chapter 1 Introduction
2.7 The theorem of MIS Capacitors measurement
As shown in Fig.2-10, the theorem of we use on capacitance measurement is:
we add a constant DC bias on the metal gate and then add a small AC signal of different frequency. Due to AC signal is changing constantly so the electric charges of being attracted are changing constantly, too. As the result of the above-mentioned, we will get the capacitance using the changing amount of the electric charges divided by the changing amount of the voltage (2.1).
We will illustrate the three different conditions of ideal MOS diode capacitance under three different bias voltage. As shown in Fig. 2-11, the first condition, we add a minus bias voltage on the metal gate and then the energy band of the interface between oxide layer and semiconductor bend upward. The interface will attract some holes to accumulate around and we name this condition: accumulation. This is real oxide capacitance.
The second condition, we add a small positive bias voltage and then the energy band bend downward. The holes of around interface will be repel and form a depletion layer. We name this condition: depletion. Due to the measurement capacitance is the oxide capacitance series connect depletion capacitance so the capacitance is smaller. The third condition, we add a large positive bias voltage on the metal gate and then the energy band bend downward drastically. The interface around will not only form depletion layer but also attract some electrons. When the numbers of minority electrons are more than majority holes and the surface will form inversion phenomenon. We name this condition is: inversion. If the measurement frequency is high frequency, the capacitance is constant due to the width of depletion layer up to maximum. If the measurement frequency is low frequency, the recombination and generation rate of the minority carrier will catch up with the changing of low
frequency. The carrier changing will happen at interface around totally and the capacitance is larger and larger up to oxide capacitance [55, 56]. The capacitance-voltage curves of three conditions are shown in Fig. 2-12.
Chapter 3
Electrical Characteristics of Al/Ti/HfAlO/Si MIS Capacitors
3.1 Capacitance-Voltage Characteristics
In order to measure the C-V characteristics of our MIS capacitors we used HP2484C LCR meter in our experiments. We swept the gate bias from inversion region to accumulation region to obtain the curve at the frequency of 50 kHz. There are three kinds of plasma treatment with different source gas ( i.e. N2, N2O, and NH3 ) and they were treated for different process time ( i.e. 10 sec, 30sec, 60 sec,90 sec).
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 HfAlOgate dielectrics treated with N2 plasma treatment for different process time. The capacitor treated for 10 sec and 30 sec shows almost the same maximum capacitance among these conditions of process time. In addition, the capacitor treated for 60 second show the good C values which are larger then the capacitor with the condition of origin. 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 90 sec 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 60 sec 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 HfAlO gate dielectrics treated with NH3 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 NH3 plasma treatment for 30 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 60 sec and 90 sec all have larger capacitance than the original sample. It is indicated that NH3 plasma treatment is also a practicable method to improve the capacitance-voltage characteristics of HfAlOgate dielectrics.
Fig. 3-3 shows the capacitance-voltage (C-V) characteristics of HfAlO gate dielectrics treated with N2O 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. Consequently, it is shown that the capacitors treated for 10 sec, 30 sec, 60 sec and 90 sec have larger capacitances than the origin sample, especially for 30ses provided the maximum capacitance. Take the view of 90 sec condition, its capacitance curve shift larger than the other samples. Besides 90 sec conditions, the other samples still remain good capacitances than origin sample. So it can be know that the long plasma time will damage the sample. 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 affected and become lower.
Fig. 3-4 shows the capacitance-voltage (C-V) characteristics of HfAlO gate dielectrics treated with N2, NH3 andN2Oplasma treatment all for 30 sec. It is indicated that the capacitance treated with N2 plasma treatment for 30 sec shows the most excellent value (i.e. 31.1% increasing about capacitance). Among these samples, the capacitance treated with NH3 plasma treatment is the worst because NH3 has the least N atoms to repair defects and dangling bonds. NH3 has hydrogen atoms to bond with dangling bonds but the bond is weak. The capacitance treated with N2O plasma treatment is the second because N2O has oxygen atoms and the growing of interfacial oxide is unavoidable while the oxygen atoms become radical and enter the interface.
This is the reason why the sample treated with N2O plasma has lower capacitance than N2 plasma treatment. 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.
3.2 Current-Voltage Characteristic
Fig. 3-5 shows the J-V characteristics of p-type HfAlOcapacitors treated by N2
plasma with different process time from 0 V to -2 V. We observed that the gate leakage current density is suppressed while treatment conditions are 10sec, 30sec, 60 sec. 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 30 sec capacitor which both has the lowest leakage and largest capacitance value from Fig. 3.1. Gate leakage current density of origin insulator at VG = -1 V is about 2.5×10-4 A/cm2. From fig.3-5, however, gate leakage current density of the capacitor treated for 30 sec N2 plasma at VG = -1 V is only about 1.36×10-5A/cm2. It has less gate leakage than origin insulator
about 2 orders. Furthermore, we notices that the 60 sec 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. By the way, the 90 sec sample has the larger leakage current value than original that because the too long plasma time will damage the sample.
Fig. 3-6 shows the J-V characteristics of p-type HfAlO capacitors treated by NH3 plasma with different process time from 0 V to -2 V. After NH3 plasma treatment, we can see the reduction of leakage current in contrast of original sample. It is worthy to be noticed that the capacitors treated by 30 sec NH3 plasma which has the best C value also performs a low leakage current about 7.98×10-6A/cm2. In addition, we find that the leakage current of 10 sec treatment are larger than the other sample, but they are all not larger than original sample. Relative to the case of N2 plasma, we can see that the level of leakage current increasing obviously mitigate.
Fig. 3-7 shows the J-V characteristics of p-type HfAlOcapacitors treated by N2Oplasma with different process time from 0 V to -2 V. Besides 60 sec sample, the other 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 N2O plasma for 90 sec even if it is must be damaged by plasma.
Fig. 3-8 shows the J-V characteristics of HfAlOgate dielectrics treated with treated with N2, NH3 andN2Oplasma treatment all for 30 sec. We find that N2Oplasma treatment all for 30 sec has the lowest leakage current compared with N2, NH3 both for 30 sec. It is possibly due to the additional oxidation layer formed by oxygen atom.
The interfacial oxidation layer will let the dielectric thicker to prevent from gate leakage. It is proved that without thick oxidation layer, it can also reach the smallest leakage current when there is suitable time treatment.
3.3 Summary
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. But from Fig. 3-4 and Fig. 3-8 show that no any plasma gas has the both advantages, we must find a relative optimum condition. It is showed that the N2 plasma treatment for 30 sec has the maximum C value and the third leakage current compared with N2 and NH3 both for 30 sec. On the other hand, it is showed that the NH3 plasma treatment for 30 sec has the second C value and the second leakage current compared with N2 and N2O both for 30 sec. Finally, N2O plasma treatment for 30 sec has the second C value and the lowest leakage current compared with N2 and NH3 both for 30 sec. We can determine the N2O sample to be the relative optimum condition among our all samples.
If we take a look at all the samples, we find that the N2, N2O, and O2 plasma treatment all shows better electrical properties than original sample. Furthermore, the N element 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 plasma treatment samples shows the lower C value than N2. Just because the oxidation phenomenon, the films will become thicker so that the plasma damage will not easily affect the leakage current profile.
Chapter 4
Reliability of Al/HfAlO/Si MIS Capacitors
4.1 Hysteresis
The name of Hysteresis was borrowed from electromagnetics. It is means that when a ferromagnetic material is magnetized in one direction, it will not relax back to zero magnetization when the applied magnetizing field is removed. It must be driven back to zero by the additional opposite direction magnetic field. If an alternating magnetic field is applied to the material, its magnetization will trace out a loop called a hysteresis loop [34].
The hysteresis phenomenon is similar in the C-V curve of the MIS capacitor device. When we apply a voltage in opposite direction, it will not fit the original C-V curve measured previously. It is due to the traps of interface which can trap charges to influence the flat band voltage and C-V curve. [23] Fig. 4-1 shows the hysteresis of p-type HfAlOgate dielectrics treated without PDA, plasma treatment and PNA and the hysteresis is 83 mV. Fig. 4-2 shows the hysteresis of p-type HfAlOgate dielectrics treated with N2 plasma treatment for 30 sec process time. Hysteresis of p-type HfAlO capacitors is changed with the plasma treatment and it’s value is 6 mV. The hysteresis is suppressed by means of the fixing ability at the interface .
Fig. 4-3 shows the hysteresis of p-type HfAlOgate dielectrics treated with NH3
plasma treatment for 30 sec process time. The tendency of hysteresis is similar with
the case of N2 plasma treatment and it’s value is 7 mV. Fig. 4-4 shows the hysteresis of p-type HfAlOgate dielectrics treated with N2Oplasma treatment for 30 sec process time and it’s value is 8 mV. It also shows a likely tendency. As a consequence, the plasma treatment can improve the reliability of hysteresis all for the different plasma gas treatment. Among these samples, we can find that the hysteresis of N2 plasma treatment for 30 sec is the smallest but three hysteresis values are almost the same.
Therefore, we can speculate that the sample without plasma treatment which is not very good at quality of interface oxide layer so that the charge was be trapped at the interface and introduce hysteresis.
4.2 Stress Induced Leakage Current (SILC)
In order to investigate the reliability of MIS capacitor device, the stress induced leakage current is a common experiment. The machine about SILC is the stress induced trap density in the bulk in thin film. The trap density introduce new leakage path. Fig. 4-5 shows the SILC curve of p-type HfAlOgate dielectrics treated with N2 plasma treatment for 30 sec process time. After the stress of -3 constant voltage for 180 second, the gate leakage current become larger than before. The degree of leakage current degradation can be judged for the reliability of MIS capacitor. From Fig. 4-5, it displays the improvement of SILC compared with the capacitor of the original sample. Almost all the samples are considered that have smaller increasing of leakage current after SILC than original sample. On the other hand, it is also can be noticed that the SILC of 90 sec treated sample become worse due to the plasma damage.
Fig. 4-6 and Fig. 4-7 display the SILC curve of p-type HfAlOgate dielectrics treated with NH3 plasma treatment and N2Oplasma treatment respectively. They all
show the distinct improvement as long as they are treated with plasma treatment. So the plasma treatment including of N2, NH3, and N2Oas source gas can have the reliability of devices to suppress SLIC.
4.3 Constant Voltage Stress (CVS)
To study the reliability of HfAlOfilm, stressing the film with a constant voltage or a constant current are two common methods. The machine about CVS is the charge trapping by the interfacial trap density which is caused by stress for long time.
Furthermore, the mount of charges cause more interface trap density and from new leakage path to add leakage. In our experiments, we use constant voltage stress (CVS) to test the reliability of HfAlOfilm. Fig. 4-8 shows gate current shift of p-type HfAlO gate dielectrics treated with N2 plasma treatment for different process time as a function of stress time during Vg = -3 V CVS stress. From the condition of 10 sec to 30 sec, the current shift is smaller and smaller. Then the current shift begins to become great by the damage of plasma at the process time of 60 sec and 90 sec . Fig.
4-9 shows gate current shift of p-type HfAlOgate dielectrics treated with NH3 plasma treatment for different process time as a function of stress time during Vg = -3 V CVS stress. It has similar behavior about the trend compared with N2. Fig. 4-10 shows gate current shift of p-type HfAlOgate dielectrics treated with N2Oplasma treatment for different process time as a function of stress time during Vg = -3 V CVS stress. While the 30-sec treated sample presents the lowest current shift, the 90 sec treated sample become to be destroyed by the plasma damage. Fig. 4-11 shows the CVS compare of HfAlOgate dielectrics treated with N2 plasma treatment , NH3 plasma treatment and N2Oplasma treatment all for 30 sec. We find the leakage current shift of three plasma treatment almost the same.
4.4 Thermal reliability
Fig. 4-12 shows the capacitance-voltage (C-V) characteristics of HfAlOgate dielectrics treated with PDA, N2, NH3, N2O plasma treatment all for 30 sec, PNA plus 950℃ 30 sec. We find that after 950℃ 30 sec the capacitance of all the samples are decreasing smaller than 10%. Fig. 4-13 shows the J-V characteristics of HfAlOgate dielectrics treated with PDA, N2, NH3, N2O plasma treatment all for 30 sec, PNA plus 950℃ 30 sec. We find that after 950℃ 30 sec the capacitance of all the samples are increasing about one order. Fig. 4-14 The J-V characteristics of HfAlOgate dielectrics treated with N2 plasma treatment , NH3 plasma treatment and N2Oplasma treatment all for 30 sec and then measured at 25℃ and 125℃. We find that the leakage current is increasing slightly.
Chapter 5
Conclusions and Future work
5.1 Conclusions
In this thesis, we used the post-deposition annealing, plasma treatment and post-nitridation to enrich the HfAlOfilm quality. The plasma treatment conditions are N2, NH3, and N2Oplasma for 10 sec, 30 sec, 60 sec, 90 sec individually. Several important phenomena were observed and summarized as follows. First of all, improvement in the electrical characteristics of Al/Ti/HfAlO/Si MIS capacitors using plasma treatment has been demonstrated in this work. All of the plasma treatment can promote the electrical characteristics and reliability until the plasma damage happened.
Among these treatments, the sample using N2, NH3 and N2Oplasma all for 30 sec represent fairly great improvement, such as good capacitance (31.1 %, 19.0 % and 25.1 % increasing respectively at -2 V ), reduced leakage current (about 2 order reduction ). It is showed that the formation of interfacial layer has been suppressed and the weak structure of interface has been repaired by N2, NH3 and N2Oplasma respectively. Besides, the sample treated by N2, NH3, and N2Oplasma all for 30 sec also show excellence promotion about reliability issue, such as smaller hysteresis ( 6 mV, 7 mV, 8 mV respectively), less SILC and better CVS curve. These advancements were ascribed to the good interface quality. On the other hand, the N2O plasma treatment has the lowest leakage current. The reason is that the samples using N2O plasma treatment will introduce oxygen bonding to form additional interfacial layer so
that the capacitance will be lower than N2. But for another hand, the thicker oxidation layer becomes a good resistance against leakage current. Finally, in this thesis, the point we focus on both the improvement of capacitance and leakage current. The treatment of N2Oplasma for 30 sec is the relative optimum condition because it has the second capacitance improvement (25.1 % increasing) and the first leakage current improvement (about 2 orders reduction). Simultaneously, its reliability also represents a excellent progress.
5.2 Future work
1. The goal of low leakage current：
We must try to research the other new process and the other gas plasma treatment to reduce the defects and suppress the leakage current in HfAlO thin film further.
2. More potential interfacial layer investigation：
The quality of the interfacial layer still must be improved. Moreover, in order to improve the quality at high-k/Si-substrate interafce, other more potential interfacial layers maybe can be investigated in the future. For example：HfSiON.
3. Devices fabrication with the above results：
3. Devices fabrication with the above results：