Summary

According to above research, after the best process time was decided from the C-V and J-V characteristics, the improvement effect of the ICP plasma process to the reliability of pure HfO2 thin films was verified from the hysteresis, SILC and CVS characteristics. For different ICP plasma nitridation process, the influence would be diverse. In conclusion, the ICP N2

plasma nitridation process could achieve the best performance to the reliability of HfO2 thin films. The EOT of HfO2 changes from 3.6 nm to 2.3 nm and the gate leakage current density changes from 3.74 × 10^-4 A/cm² to 2.22 × 10^-5 A/cm² after N2 plasma nitridation.

Furthermore, the plasma nitridation could be also used to improve the thermal stability of the HfO2 thin films to bear the high temperature process at 850 ℃ for at least 30 sec.

Figure 2.1 The C-V characteristics of the HfO2 thin films treated in N2 plasma for different process times.

Figure 2.2 The J-V characteristics of the HfO2 thin films treated in N2 plasma for different process times.

Figure 2.3 The C-V characteristics of the HfO2 thin films treated in NH3 plasma for different process times.

Figure 2.4 The J-V characteristics of the HfO2 thin films treated in NH3 plasma for different process times.

Figure 2.5 The C-V characteristics of the HfO2 thin films treated in N2O plasma for different process times.

Figure 2.6 The J-V characteristics of the HfO2 thin films treated in N2O plasma for different process times.

Figure 2.7 The hysteresis characteristics of the HfO2 thin films nitrided by different ICP plasma process.

Figure 2.8 The SILC characteristics of the HfO2 thin films nitrided by ICP N2 plasma.

Figure 2.9 The SILC characteristics of the HfO2 thin films nitrided by ICP NH3 plasma.

Figure 2.10 The SILC characteristics of the HfO2 thin films nitrided by ICP N2O plasma.

Figure 2.11 The leakage current shift curves of the HfO2 thin films nitrided by ICP N2 plasma.

Figure 2.12 The leakage current shift curves of the HfO2 thin films nitrided by different plasma.

-2.0 -1.5 -1.0 -0.5 0.0 0.0

2.0x10^-7 4.0x10^-7 6.0x10^-7 8.0x10^-7 1.0x10^-6 1.2x10^-6

Voltage (V)

Ca pac itance (F/cm

)

Origin after 850⁰C N₂ 30 s after 850⁰C N₂ 90 s

Original sample N₂ 90 s after 850⁰C

Figure 2.13 The C-V characteristics of the HfO2 gate dielectrics treated by different plasma nitridation process and PDA.

-2.0 -1.5 -1.0 -0.5 0.0 10^-10

10^-8 10^-6 10^-4 10^-2 10⁰ 10² 10⁴ 10⁶

Origin after 850⁰C Original sample N2 30 s after 850⁰C N

2 90 s after 850⁰C N₂ 90 s

Gate L e akag e (A/ c m

)

Voltage (V)

Figure 2.14 The J-V characteristics of the HfO2 gate dielectrics treated by different plasma nitridation process and PDA.

26 25 24 23 22 21 20 19 18 17 16

Figure 2.15 The XPS analysis of the Hf 4f electronic spectra of the samples treated in ICP N2

plasma for 60 sec.

Chapter 3

The Improvement Effect of the Plasma Nitridation Process to the Electrical Properties, the Reliability and the

Thermal Stability of HfAlO

thin films

3.1 Introduction

Gate dielectric scaling of complementary metal-oxide-semiconductor (CMOS) structure accomplishes increase in speed and pack density for modern integrated circuits. However, dielectric layer scaling companies the excessive off-state leakage current that could cause intolerable power consumption and overheat of the devices. To solve the issue about leakage current, a major resolution is to replace the silicon dioxide with a thicker dielectric layer that has a higher dielectric constant and maintains relatively low equivalent oxide thickness (EOT) [1]. There are many kinds of high-k dielectrics have been considered to substitute as the gate dielectrics for the advanced CMOS technologies [2-4]. Among these dielectrics, HfO2 is considered as a suitable gate dielectric material because of the acceptable band gap (~ 5.7 eV), the large dielectric constant (~ 25) and relatively high free energy of reaction with Si (47.6 kcal/mole at 727 ^oC) [2-4, 6]. However, there are still some issues which have to be solved in order to integrate these Hf-based dielectrics into an industrial CMOS process flow like the thermal instability and the reliability of these dielectrics [7, 30]. The crystallization temperature of pure HfO2 is quite low, so the thermal process, such as Source/Drain activation, would cause the high leakage current after the deposition of HfO2 thin films. In order to increase the crystallization temperature, Al could be added to HfO2 to form Hf aluminates [8].

Recently, several studies have used nitrogen incorporation in high-k gate stacks to improve thermal stability and increase dielectric constant [16, 38, 46]. However, thermal nitridation is usually performed at high temperature and hydrogen-containing species that act as electron traps could be added into the thin film. On the other hand, nitrogen could be incorporated into the dielectric layer by plasma nitridation process at lower temperature than by thermal nitridation process [22]. The objective of this study is to discuss the effect of different ICP plasma nitridation processes to the electrical properties, the reliability and the thermal stability of HfAlOx thin films.

3.2 Experimental

After a standard initial RCA cleaning, a 6-nm HfAlOx layer was deposited on the p-type wafers by the MOCVD system. The samples were then annealed at 800 ℃ for 60 sec in pure N2 gas by rapid temperature annealing (RTA) process and nitrided by an ICP process at the substrate temperature of 300 ℃. The process pressure of the plasma nitridation process was set as 1.33 × 10^-4 bar. Ar was added into the chamber to activating the plasma containing nitrogen while the nitridation process was carried out. The flow rate of Ar was set as 10 sccm and the flow rate of the gas containing nitrogen, which is N2, NH3 or N2O, was set as 100 sccm. After the plasma nitridation, there was an annealing process whose condition was at 600 ℃ for 60 sec in pure N2 to eliminate the plasma damage caused by nitridation process [35]. A 40-nm Ti film was deposited by dual e-gun evaporation and an Al film of 400 nm was thermally evaporated. The top electrodes were defined by a lithography process. Finally, the backside native oxide was stripped with diluted HF solution and the backside aluminum electrodes were evaporated by a thermal evaporation. The gate area of the Al/Ti/HfAlOx/Si MOS capacitors is 5000 μm². The capacitance-voltage (C-V) and the current density-voltage (J-V) characteristics of the MOS structures were measured by using a C-V measurement (HP

4284) and an Agilent 4156C semiconductor parameter analyzer. The experimental condition of the SILC measurement that was carried out in this study was set as constant voltage of -3 V for 180 sec. The stress condition of CVS measurement implemented in this study was set as -3 V.

3.3 Results and Discussion

In the beginning of this study, different process times were tested to determinate the most suitable process condition to the C-V and the J-V characteristics of the samples for various plasma nitridation processes. After the process time had been decided, we would examine the effect of the plasma nitridation process to the reliability of HfAlOx thin films.

3.3.1 The Most Suitable Process Time

Figure 3.1 presents the C-V characteristics of the HfAlOx gate dielectrics nitrided in ICP N2 plasma for different process times. The frequency used in the high frequency C-V measurement was set as 50 kHz. The capacitors treated for 10 and 30 sec perform the larger capacitance density among these samples. The factor of improvement might be from that the nitrogen incorporation in the HfAlOx dielectrics. First, the incorporated nitrogen could enhance the electronic polarization as well as the ionic polarization, so the dielectric constant of the HfAlOx thin films increases just as Hf-silicate thin films and SiO2 thin films [40-41].

Secondly, the growth of the interfacial layer between HfAlOx thin film and silicon could be suppressed by incorporated nitrogen [47]. A properly nitrided Hf-based dielectric could act as an oxygen diffusion barrier, perhaps by filling grain boundaries or vacancies [20]. Besides, the capacitance density of the samples treated for 60 sec and 90 sec is degraded because of the damage caused by the N2 plasma.

The J-V characteristics of the HfAlOx capacitors nitrided by ICP N2 plasma with different process times from 0 V to -2 V are shown in Fig. 3.2. The gate leakage current density is restrained apparently while the treatment condition was 30 sec. The reduction of the leakage current could be attributed to that the defects in Hf-based dielectric could be passivated in nitridation process and the post deposition annealing (PDA) [37-38]. The gate leakage current density of the samples not treated in ICP N2 plasma at Vg of -1 V is about 2.5 × 10^-4 A/cm² and the gate leakage current density of the capacitors treated in ICP N2 plasma for 30 sec at Vg of -1 V is about 1.36 × 10^-5 A/cm². Moreover, the leakage current densities of the samples treated in N2 plasma for shorter or longer time are larger than the one treated for 30 sec. The improvement effect of the nitridation process with shorter process time might be not enough.

On the other hand, while the nitridation process time is longer than 30 sec, the plasma damage from the plasma nitridation could be obvious. In summary, the best process time of the plasma nitridation for ICP N2 process is set as 30 sec. The samples treated in N2 plasma for 30 sec display the best value (the EOT of the samples is about 2.6 nm).

For ICP NH3 and ICP N2O nitridation process, the best process time of the plasma nitridation could be set as 30 sec from similar analysis process.

In Fig. 3.3 and Fig. 3.4, the C-V and the J-V characteristics of the HfAlOx gate dielectrics treated by three kinds of ICP plasma are demonstrated. The EOT of HfAlOx thin films changes from 3.7 nm to 2.9 nm after NH3 plasma nitridation and changes from 3.7 nm to 2.6 nm after N2O plasma nitridation. For the N2O plasma nitridation process, a high concentration of oxygen vacancies would cause electrons to be generated and a large leakage current to flow, so treatment with plasma that contains oxygen could reduce oxygen vacancies to improve the quality of dielectric films [34]. As mentioned above, the gate leakage current density of the

samples treated in ICP N2O plasma for 30 sec would be the smallest among all samples. It is about 4.40 × 10^-6 A/cm² at Vg of -1 V. Besides, the gate leakage current density of HfAlOx

thin films changes from 2.50 × 10^-4 A/cm² to 7.98 × 10^-6 A/cm² at -1 V after NH3 plasma nitridation. In summary, the C-V and the J-V characteristics of the samples with various ICP nitridation processes are better than the one with simply PDA.

3.3.2 Reliability

Figure 3.5 displays the hysteresis characteristics of the HfAlOx gate dielectrics treated in different kinds of plasma containing nitrogen. Hysteresis measurement was started from positive to negative bias, and then swept back from negative to positive bias at a frequency of 50 kHz. The hysteresis phenomenon of the C-V curves is caused by the existence of negative charges trapped in the dielectric defect states when the samples are stressed. These defect states are called slow trapping sites [45]. The reason for the lower hysteresis is the thin film has lower number of defects, and consequently there is less charge trapped. The voltage shift in the C-V curve of the samples nitrided by N2 or N2O plasma is about 6 mV and the voltage shift in the C-V curve of the sample nitrided by NH3 plasma is about 7 mV. That is, the hysteresis phenomenon of pure HfAlOx dielectrics could be restrained to be less than 10 mV by various nitridation processes.

The SILC curves of p-type HfAlOx gate dielectrics treated with N2 plasma process are shown in Fig. 3.6. After the sample was stressed by constant voltage of -3 V for 180 sec, the degradation of the leakage current could reflect the reliability of the MOS structures. Figure 3.6 performs that the degradation could be suppressed effectively by N2 plasma nitridation process with suitable process time. The increase of gate leakage current at -1 V of HfAlOx

thim film changes from 94.00 % to 60.29 % after N2 plasma nitridation. On the other hand,

when the nitridation process time is too long, the leakage current would increase and the variation of leakage current caused by the stress voltage would be apparent again due to the plasma damage.

The SILC curves of p-type HfAlOx gate dielectrics nitrided with NH3 plasma process and N2O plasma process are displayed in Fig. 3.7 and Fig. 3.8. As shown in Fig. 3.7, the SILC characteristics of p-type HfAlOx gate dielectrics nitrided by NH3 plasma process present that the shift of leakage current because of the CVS could also be restrained by NH3 plasma nitridation. But the improvement effect of the NH3 plasma nitridation process to the SILC phenomenon of the HfAlOx thin films seems unapparent in comparison with other plasma nitridation process. Figure 3.8 describes the SILC curves of p-type HfAlOx gate dielectrics treated with N2O plasma process. As Fig. 3.8 demonstrates, the shift of leakage current due to the CVS could also be restrained obviously by N2O plasma nitridation.

The CVS characteristics of samples nitrided by N2 plasma are described in Fig. 3.9. The stress voltage was set as -3 V. All the gate current shifts of the samples with nitridation could be decreased. Among several process times, the N2 plasma nitridation process for 30 sec performs the best result. After N2 plasma nitridation 30 sec, the gate leakage shift shrinks as 7.71 %. This result could correspond to the electrical characteristics that have been discussed above.

Figure 3.10 demonstrates the gate current shift of p-type HfAlOx capacitors treated with NH3 plasma for different process time. Comparing to other samples, the gate current shift of the sample treated for 30 sec is the smallest. After NH3 plasma nitridation 30 sec, the gate leakage shift shrinks as 8.86 %. This result is consistent with the above discussion about the C-V and the J-V characteristics. Figure 3.11 presents the gate current shift of p-type HfAlOx

thin films treated with N2O plasma for different process time. The current shift of the sample nitrided for 30 sec is the slightest and shrinks as 7.29 %. This result is also consistent with the C-V and the J-V characteristics.

In summary, the improvement effect of the various ICP nitridation processes to the reliability of HfAlOx thin films has been verified. According the above discussions, although there is not enormous difference, the improvement effect of the ICP N2O nitridation process would be the most obvious to both the electrical characteristics and the reliability of HfAlOx

thin films.

3.3.3 Thermal stability

The nitrogen was incorporated into the dielectric could maintain an amorphous homogeneous film without phase separation at high temperature [21]. In Figure 3.12 and Figure 3.13, the C-V and the J-V characteristics of the HfAlOx gate dielectrics treated by different plasma nitridation processes and thermal treatments are shown. As demonstrated in Figure 3.12, for the samples which were just deposited and not nitrided, the C-V characteristic of the samples with the high temperature process (in N2 gas at 900 ℃ for 30 sec) degenerated because of the recrystallization of the HfAlOx thin films. So from the electrical characteristic, the original samples could not sustain the high temperature annealing. In the meantime, for the samples which went through the PDA, N2 plasma nitridation and the post-nitridation annealing (PNA), the C-V characteristic of the samples without the high temperature process was very similar to the ones with the process. So it seemed to prove that the nitridation process could improve the thermal stability of the HfAlOx thin films. In Figure 3.13, we observed that the J-V curve of the sample with nitridation which suffered high temperature annealing could maintain a lower value than the one without nitridation. Besides,

while the capacitance density of the samples nitrided by ICP plasma was close to the one of the samples just went through the PDA process, the gate leakage density of the nitrided samples was smaller than the one of the samples without nitridation. The above electrical characteristics could also confirm the improved effect of the plasma nitridation to the thermally stability of the HfAlOx thin films.

3.4 Summary

Based on above results, the nitridation effect and the plasma damage might need to be traded off to achieve the optimum result. According to our study, the whole plasma nitridation process, which includes the post-deposition annealing and the post-nitridation annealing, could be used to strengthen the HfAlOx thin films in order to enhance the C-V characteristic and suppress the gate leakage from the as-deposited samples. After the process time was decided from the C-V and the J-V characteristics, the improvement effect of the ICP nitridation process to the reliability of HfAlOx thin films was verified from the hysteresis, the SILC and the CVS characteristics. For different kinds of ICP plasma nitridation processes, the influence would be diverse. In conclusion, for the electrical characteristics and the reliability of HfAlOx thin films, the ICP N2O process could be a suitable nitridation process. The EOT of HfAlOx changes from 3.7 nm to 2.6 nm and the gate leakage current density changes from 2.50 × 10^-4 A/cm² to 4.40 × 10^-6 A/cm² after N2 plasma nitridation. Moreover, the plasma nitridation could be used to improve the thermal stability of the HfAlOx thin films to bear the high temperature process at 900 ℃ for at least 30 sec.

Figure 3.1 The C-V characteristics of the HfAlOx thin films treated in N2 plasma for different process times.

Figure 3.2 The J-V characteristics of the HfAlOx thin films treated in N2 plasma for different process times.

Figure 3.3 The C-V characteristics of the HfAlOx thin films treated by different kinds of plasma.

Figure 3.4 The J-V characteristics of the HfAlOx thin films treated by different kinds of plasma.

Figure 3.5 The hysteresis characteristics of the samples nitrided by different kinds of plasma containing nitrogen.

Figure 3.6 The SILC characteristics of the HfAlOx thin films nitrided by N2 plasma.

Figure 3.7 The SILC characteristics of the HfAlOx thin films nitrided by NH3 plasma.

Figure 3.8 The SILC characteristics of the HfAlOx thin films nitrided by N2O plasma.

Figure 3.9 The leakage current shift curves of the HfAlOx thin films nitride by N2 plasma.

Figure 3.10 The leakage current shift curves of the HfAlOx thin films nitride by NH3 plasma.

Figure 3.11 The leakage current shift curves of the HfAlOx thin films nitride by N2O plasma.

1.0 0.5 0.0 -0.5 -1.0 0.0

2.0x10^-7 4.0x10^-7 6.0x10^-7 8.0x10^-7 1.0x10^-6 1.2x10^-6

Capacitance (F/cm

)

Voltage(V)

HfAlO_x

original + 600⁰C 30 s original + 800⁰C 30 s N₂

N₂ + 900⁰C 30 s original + 900⁰C 30 s

Figure 3.12 The C-V characteristics of the HfAlOx gate dielectrics treated by different plasma nitridation process, PDA and high temperature process.

2.0 1.5 1.0 0.5 0.0 10^-14

10^-11 10^-8 10^-5 10^-2 10¹

Gate Leakage (A/cm

)

HfAlO_x

original + 600⁰C 30 s original + 800⁰C 30 s N2

N₂ + 900⁰C 30 s original + 900⁰C 30 s

Voltage(V)

Figure 3.13 The J-V characteristics of the HfAlOx gate dielectrics treated by different plasma nitridation process, PDA and high temperature process.

Chapter 4

The Improvement Effect of the Plasma Nitridation Process and the Plasma fluorination Process to HfO

thin films

4.1 Introduction

To solve the problem of the excessive leakage, Hafnium-based dielectrics have emerged as the promising high-κ candidates to replace the SiON dielectrics for the advanced CMOS technologies [1, 3-4]. The pure HfO2 is considered as a suitable gate dielectric material because of the acceptable band gap (6 eV) and the large dielectric constant (about 25). The band gap of Hafnium-based dielectrics is not too small to cause the large gate leakage which forms the large power consumption. In the meantime, the dielectric constant of hafnium-based dielectrics is large enough to increase the physical thickness of the gate dielectric and maintain the relatively low effective oxide thickness (EOT). However, there are several challenges which have to be considered in order to integrate these high-κ dielectrics into a conventional CMOS process flow such as the interface SiO2 regrowth and the thermal stability of these dielectrics [7]. The nitridation process has been shown to improve the thermal stability of hafnium-silicate thin films [21]. On the other hand, there are some recent studies show that fluorine passivation could be use to improve the reliability of high-k dielectric MOS field effect transistors [25, 27, 48]. Plasma fluorination could be an effective method to make incorporation of fluorine into the hafnium-based high-k dielectrics [49-50].

The plasma nitridation effect to pure HfO2 thin films has already been examined in the chapter 2 of this dissertation. In this chapter, we would try to apply plasma fluorination process to improve the plasma nitridation effect of pure HfO2 thin films.

4.2 Experimental

After initial standard RCA cleaning, the wafers were placed into the chamber and the HfO2 layers were deposited on the wafers by the metal organic chemical vapor deposition

After initial standard RCA cleaning, the wafers were placed into the chamber and the HfO2 layers were deposited on the wafers by the metal organic chemical vapor deposition