Chapter 3 Electrical Characteristics of Al/HfO 2 /Si MIS Capacitors
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：
The optimum condition will be used to the structures of our MOS device in the future.
Table
Table 1-1 The time of intel corporation found a solution for high-k and metal gate to keep continuation of Moor’s Law
Table 1-2: Material requirements of high-k dielectrics
Table 1-3 Comparison of relevant properties for various high- k candidates [32].
aCalculated by Robertson.
bMono.=monoclinic.
cTetrag.=tetragonal.
Table 2-1 Comparison of deposition techniques: Sputter, ALCVD, and MOCVD [53].
Figure-chapter 1
Figure 1-1 Illustration of Moore’s law: number of transistors integrated in the different generations of Intel’s microprocessors vs. the production year of these circuits.
Figure 1-2 Trend of device scaling: Transistor physical gate length will reach
~ 15nm before end of this decade and ~ 10nm early next decade.
Fig. 1-3 With the marching of technology nodes, gate dielectric has to be shrunk and five silicon atoms thick of gate dielectric is predicted for 2012.[2]
Fig. 1-4 Measured and simulated Ig-Vgcharacteristics under inversion condition for nMOSFETs. The dotted line indicates the 1A/cm2 limit for the leakage current. [3]
Fig. 1-5 Conduction mechanism in oxide for the MOS structure.
Figure 1-6 (a) Energy band chart of NMOS device (b) The influence of poly-Si depletion for capacitance density.
Fig. 1-7 High-k+ metal gate transistors provide significant performance increase and leakage current reduction , ensuring continuation of moor’s law.
Figure 1-8 Power consumption and gate leakage current density comparing to the potential reduction in leakage current by an alternative dielectric exhibiting the same equivalent oxide thickness [5].
(a) Schottky Emission (SE)
(b) Frenkel-Poole Emission (FP)
(c) Fowler-Nordheim Tunneling (F-N)
Figure 1-9 (a) Schottky Emission (SE) (b) Frenkel-Poole Emission (FP) (c)Fowler-Nordheim Tunneling (F-N) current transport mechanism.
Figure 1-10 schemes of important regions in gate stack of a field effect transistor
Figure-chapter 2
Fig. 2-1 Schematic diagram of MOCVD system structure.
Fig. 2-2 The ICP plasma system that was used in this experiment.
Fig.2-3 (1)Si substrate RCA clean (2)6 nm HfAlO was deposited on the sub-Si by MOCVD.
Fig.2-4 (1) PDA by RTA (2) Plasma treatment (3) PNA by RTA
Fig.2-5 40 nm Ti was deposited on the HfAlO layer by dual e-gun evaporation system .
Fig.2-6 400 nm Al was deposited on the Ti layer as top electrode by thermal evaporation coater.
Fig.2-7 Undefined Al was removed by wet etching .
Fig.2-8 Undefined Ti was removed by wet etching (1%HF).
Fig.2-9 Al was deposited on the back side of sub-Si as bottom electrode by thermal evaporation coater.
Fig. 2-10 MOS diode capacitance structure
Fig. 2-11 The energy band plot and electric charges distribution of MOS diode capacitance under bias voltage.
Fig. 2-12 The capacitance-voltage curve of three different conditions
Figure-chapter 3
Fig.3-1 The capacitance-voltage (C-V) characteristics of HfAlO gate dielectrics treated with N2 plasma treatment for different process time.
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
Fig.3-2 The capacitance-voltage (C-V) characteristics of HfAlO gate dielectrics treated with NH3 plasma treatment for different process time.
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
Fig.3-3 The capacitance-voltage (C-V) characteristics of HfAlO gate dielectrics treated with N2Oplasma treatment for different process time.
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
Fig.3-4 The capacitance-voltage (C-V) characteristics of HfAlO gate dielectrics treated with N2 plasma treatment, NH3 plasma treatment and N2O plasma treatment all for 30 sec.
-2.0 -1.5 -1.0 -0.5 0.0
Fig. 3-5 The J-V characteristics of p-type HfAlOcapacitors treated by N2 plasma with different process time from 0 V to -2 V.
-2.0 -1.5 -1.0 -0.5 0.0
Fig. 3-6 The J-V characteristics of p-type HfAlOcapacitors treated by NH3 plasma with different process time from 0 V to -2 V.
-2.0 -1.5 -1.0 -0.5 0.0
Fig. 3-7 The J-V characteristics of p-type HfAlOcapacitors treated by N2Oplasma with different process time from 0 V to -2 V.
-2.0 -1.5 -1.0 -0.5 0.0
Fig. 3-8 The J-V characteristics of HfAlOgate dielectrics treated with N2 plasma treatment, NH3 plasma treatment and N2Oplasma treatment all for 30 sec.
Figure-chapter 4
Fig. 4-1 The hysteresis of p-type HfAlOgate dielectrics treated without PDA, plasma treatment, PNA.
Fig. 4-2 The hysteresis of p-type HfAlOgate dielectrics treated with PDA, N2 plasma treatment, PNA.
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
Fig. 4-3 The hysteresis of p-type HfAlO gate dielectrics treated with PDA, NH3 plasma treatment, PNA.
Fig. 4-4 The hysteresis of p-type HfAlO gate dielectrics treated with PDA, N2O plasma treatment, PNA.
-2.0 -1.5 -1.0 -0.5 0.0
SILC after CVS -3V for 180sec
Voltage(V) 8000C-60sec+N2-10sec+6000C-60sec+SILC 8000 8000C-60sec+N2-60sec+6000C-60sec+SILC 8000
Fig. 4-5 The SILC curve of p-type HfAlOgate dielectrics treated with N2 plasma treatment for different process time.
-2.0 -1.5 -1.0 -0.5 0.0
SILC after CVS -3V for 180sec
Voltage(V) Gate Leakage ( A/cm2 )
Fig. 4-6 The SILC curve of p-type HfAlOgate dielectrics treated with NH3 plasma treatment for different process time.
-2.0 -1.5 -1.0 -0.5 0.0
SILC after CVS -3V for 180sec
Voltage(V) Gate Leakage ( A/cm2 )
Fig. 4-7 The SILC curve of p-type HfAlOgate dielectrics treated with N2Oplasma treatment for different process time.
0 20 40 60 80 100 120 140 160 180
Gate Leakag e shift ( A/cm
2)
Fig. 4-8 The gate current shift of p-type HfAlOgate dielectrics treated with N2 plasma treatment for different process time as a function of stress time during Vg = -3 V CVS stress.
0 20 40 60 80 100 120 140 160 180
Fig. 4-9 The 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.
Fig. 4-10 The gate current shift of p-type HfAlOgate dielectrics treated with N2O plasma treatment for different process time as a function of stress time during Vg = -3 V CVS stress.
0 20 40 60 80 100 120 140 160 180
Gate L eakag e shi ft ( A/ cm
2)
Fig. 4-11 The CVS compare of HfAlO gate dielectrics treated with N2 plasma treatment , NH3 plasma treatment and N2Oplasma treatment all for 30 sec.
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
Fig. 4-12 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.
-2.0 -1.5 -1.0 -0.5 0.0
Fig. 4-13 The J-V characteristics of HfAlOgate dielectrics treated with PDA, N2, NH3, N2O plasma treatment all for 30 sec, PNA plus 950℃ 30 sec.
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℃.
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