with Dual Plasma Treatment
2.4 Electrical characteristics of dual plasma treatment .1 C-V and J-V characteristics
Fig. 2-4 shows the C-V characteristics of the HfO2 thin films treated in CF4
plasma for different process durations and N2 plasma for 120 sec. The frequency used in the high frequency C-V measurement was set at 50 kHz. The sample was treated only in N2 plasma for 120 sec (RF power = 50 W) shows much higher capacitance
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density than the sample with no treatment. The higher capacitance could be attributed to the PDA process [21-23] and the nitrogen incorporation in the HfO2 thin film. The nitrogen incorporation could enhance the electronic polarization as well as the ionic polarization, which result in the increase of dielectric constant [12, 24]. On the other hand, the capacitance density and interface characteristics show further improvement with the combination of CF4 pre-treatment for 10 sec and N2 post-treatment for 120 sec. With CF4 pre-treatment, fluorine atoms would pile up at the HfO2/Si interface, improve the quality of interface [19], and suppress the IL formation [20]. Besides, for CF4 pre-treatment times longer than 10 sec, the plasma damage caused the degradation of HfO2/Si interface and the degradation of capacitance density.
Fig. 2-5 shows the J-V characteristics of the HfO2 thin films treated in CF4
plasma for different process durations and N2 plasma for 120 sec. Compared with the sample with no treatment, the gate leakage current decreased by about 4 orders of magnitude for the sample with CF4 pre-treatment for 10 sec and N2 post-treatment for 120 sec. The reduction of the gate leakage could be attributed to defect passivation.
Oxygen vacancy related states and interface states could be passivated by nitrogen and fluorine atoms [13, 16]. On the other hand, for CF4 pre-treatment times longer than 10 sec, the plasma damage caused the degradation of interface and the increase of gate leakage current. The best condition of dual plasma treatment is as follows: CF4
pre-treatment (time=10s, RF Power=20W) and N2 post-treatment (time=120s, RF Power=50W). The gate leakage of the sample with best condition is 1.05 × 10-5 A/cm2 at Vg ＝ -1.5 V. Table 2-1 illustrates the basic electrical characteristics.
In Fig. 2-6 and Fig. 2-7, the C-V and the J-V characteristics of the HfO2 gate dielectrics, treated in CF4 plasma for different process durations and NH3 plasma for 120 sec, are presented. The RF power of NH3 post-treatment was set at 40 W. As
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mentioned before, the reason of the improvement in the NH3 plasma nitridation process could be the same as the one in the N2 plasma nitridation process. From the similar analysis, the best condition of dual plasma treatment is as follows: CF4 pre-treatment (time=10s, RF Power=20W) and NH3 post-treatment (time=120s, RF Power=40W). The gate leakage of the sample with best condition is 1.62 × 10-5 A/cm2 at Vg ＝ -1.5 V. In summary, C-V and I-V Characteristics could be further improved by dual plasma treatment. Table 2-2 illustrates the basic electrical characteristics.
2.4.2 Hysteresis
Fig. 2-8 demonstrates the hysteresis characteristics of the HfO2 thin films treated in CF4 plasma for different process durations and N2 plasma for 120 sec. Hysteresis measurement was measured from accumulation to inversion (-2 V to 1 V) and backward form inversion to accumulation (1V to -2 V) by sweeping the voltage.
Because positive and negative carrier might be trapped at the inner interface during voltage sweeping [19], the hysteresis phenomenon could be observed for all the samples. Although single N2 plasma treatment could improve the hysteresis characteristic, the hysteresis characteristic could be further improved by dual plasma treatment (CF4 pre-treatment for 10 sec and N2 post-treatment for 120 sec). The C-V curve shift of the sample treated by the combination of CF4 pre-treatment for 10 sec and N2 post-treatment for 120 sec is about 14 mV, which is smaller than the fresh sample (27 mV) and the sample with N2 plasma treatment (17 mV). Moreover, the fresh sample shows a hump in the depletion region of the C-V characteristics, due to higher Dit at the interface [38]. After dual plasma treatment (CF4 pre-treatment for 10 sec and N2 post-treatment for 120 sec), the hump could be effectively eliminated. It can be speculated that interface quality could be enhanced by dual plasma treatment.
On the other hand, the hump and hysteresis became worst while CF4 pre-treatment
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time is longer than 10 sec due to plasma damage at the HfO2/Si interface. Similarly, the sample treated by the combination of CF4 pre-treatment for 10 sec and NH3
post-treatment for 120 sec shows the best hysteresis and interface characteristics than other samples, as shown in Fig. 2-9.
2.4.3 Frequency dispersion characteristics
The C-V characteristics of the HfO2 thin films, treated with CF4 plasma for different process durations and N2 plasma for 120 sec, have been measured as a function of frequency as shown in Fig. 2-10 to Fig. 2-15. The measurements were made in the frequency range of 1-100 kHz (1, 10, and 100 kHz). Frequency dispersion could be observed because of the response of trap charges to signal frequency. At low frequencies, interface traps generated the additional capacitance because some of traps could follow the change of gate voltage [25]. The frequency dispersion in the accumulation region and the hump in the depletion region are significant for the sample with no treatment. The sample treated by N2 plasma showed relatively smaller frequency dispersion and smaller hump than the sample with no treatment. On the other hand, it was obvious that the sample treated by CF4 pre-treatment for 10 sec and N2 post-treatment for 120 sec exhibited nearly no dispersion in the accumulation region and nearly no hump in the depletion because interface states could be improved effectively [25-28] by dual plasma treatment. However, the frequency dispersion and the hump became severe again when the CF4 pre-treatment time is longer than 10 sec owing to plasma damage at interface.
Fig. 2-16 to Fig. 2-20 display the C-V frequency dependence of the HfO2 thin films treated in CF4 plasma for different process durations and NH3 plasma for 120 sec. The sample treated by CF4 pre-treatment for 10 sec and NH3 post-treatment for
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120 sec exhibited nearly no dispersion and nearly no hump during C-V measurement.
This indicated that dual plasma treatment could effectively eliminate interface states and greatly enhance IL quality than single plasma treatment.
2.4.4 Constant voltage stress characteristics
In order to study the trapping properties of the Al/Ti/HfO2/Si MIS capacitors, the constant voltage stress (CVS) was applied on the gate electrode, leading to the stress induced flat-band voltage shift. The stress voltage was set at -3 V. The stress times were made in a range from 0 to 500 sec. As shown in Fig. 2-21 to Fig. 2-27, all the C-V curves shift to left as stress time increase indicated that there were positive charges trapped in the high-κ dielectric layer. Trapping of positive charges could be explained by Anode hole injection model [29]. At higher electrical field, electrons might tunnel through a triangular potential barrier of the gate dielectric film by the mechanism of Fowler-Nordheim (F-N) tunneling and arrive at the anode terminal, resulting in the generation of electron-hole pairs, as shown in Fig. 2-28 [18].
During constant negative bias stress at a fixed gate voltage, the injected electrons traveled through dielectric and arrived at the interface. These electrons gained energy to liberate the hydrogen at the interface, leading to the generation of Si dangling bond and released hydrogen atom. These dangling bonds were interface defects, which usually called the pb centers. The liberated hydrogen diffused into the dielectric through the oxide field, trapped in the dielectric, leading to the creation of positively charged centers [30, 31].
SiH e Si Si H e
Si3 3 . (1) Compared to Fig. 2-22 and Fig. 2-23, the C-V curves had smaller Vfb shift and less distortion for samples with dual plasma treatment, indicating that samples with
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dual plasma treatment had less interface trap charges generated at the dielectric/Si interface and had better reliability properties than samples with single plasma treatment.