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 HfO2 gate dielectrics treated without plasma treatment. Fig. 4-2 shows the hysteresis of p-type HfO2 gate dielectrics treated with N2 plasma treatment for different process time. Hysteresis of p-type HfO2 capacitors are changed with the increase of plasma treatment time. First, about the case of 10 sec and 30 sec we can see the hysteresis happened. When the treatment time has achieved 1min, the hysteresis is suppressed by means of the fixing ability at the interface. Until the time continues for 3 min the hysteresis becomes worse again and it attributes to plasma damage.
Fig. 4-3 shows the hysteresis of p-type HfO2 gate dielectrics treated with N2O plasma treatment for different process time. The tendency of hysteresis is similar with the case of N2 plasma treatment. Fig. 4-4 shows the hysteresis of p-type HfO2 gate dielectrics treated with O2 plasma treatment for different process time. It also shows a likely tendency. As a consequence, the plasma treatment can improve the reliability of hysteresis for the shorter process time for fear of the plasma damage brought by long process duration. Among these samples, we can find that the hysteresis of N2 plasma treatment for 1 min is the smallest. Therefore, we can speculate that the quality of interfacial oxide is not very well 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 HfO2 gate dielectrics treated with N2
plasma treatment for different process time. After the stress of constant voltage ( 1V ) for 30 second, the gate leakage current become larger then 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 no-treated capacitor.
Second, it is considered that the SILC of 1 min-treated sample which has the largest C value and the lowest leakage shows a very small degradation. On the other hand, it is also can be noticed that the SILC of 3 min-treated sample become worse due to the plasma damage.
Fig. 4-6 and Fig. 4-7 display the SILC curve of p-type HfO2 gate dielectrics treated with N2O plasma treatment and O2 plasma treatment respectively. They all show the distinct improvement as long as they are treated with plasma treatment. So the plasma treatment including of N2, N2O, and O2 as source gas can intensify the reliability of devices to suppress SLIC. Fig. 4-8 shows the SILC compare 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. The sample treated by N2 plasma treatment for 1 min shows the smallest SILC degradation because of it’s best improvement of interface quality.
4.3 Constant Voltage Stress (CVS)
To study the reliability of HfO2 film, 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 HfO2 film. Fig. 4-9 shows gate current shift of p-type HfO2
gate dielectrics treated with N2 plasma treatment for different process time as a function of stress time during Vg = 1 V CVS stress. From the condition of 10 sec to 1min, 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 3 min. Fig. 4-10 shows gate current shift of p-type HfO2 gate dielectrics treated with N2O plasma treatment for different process time as a function of stress time during Vg = 1 V CVS stress. It has similar behavior about the trend. Fig. 4-11 shows gate current shift of p-type HfO2
gate dielectrics treated with O plasma treatment for different process time as a
function of stress time during Vg = 1 V CVS stress. While the 30-sec treated sample presents the lowest current shift, the 1-min treated sample become to be destroyed by the plasma damage. Fig. 4-12 shows the CVS compare 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 verified that the sample using by N2 plasma treatment for 1 min has the best quality of thin film.
4.4 Charge to Breakdown ( Q
BD)
Another important issue of reliability is to investigate the breakdown behavior of the gate dielectrics. As long as we inject large number of charge by the stress at constant voltage or constant current for a long period, we can find the breakdown profile and calculate the count of QBD.
Fig. 4-13 shows the charge-to-breakdown characteristics ( QBD ) of p-type HfO2
gate dielectrics treated with N2 plasma treatment for different process time. The charge to breakdown characteristics was measured at a constant current of -1 A/cm2. As we respect, the capacitor treated for 1 min shows the larger QBD and it means that the capacitor more hardly begins to breakdown. Fig. 4-14 shows the charge-to breakdown-characteristics ( QBD ) of p-type HfO2 gate dielectrics treated with N2O plasma treatment for different process time. Although the capacitor treated for 10 second shows the largest QBD, the capacitor treated for 1 min has become degradation.
Fig. 4-15 shows the charge-to-breakdown characteristics ( QBD ) of p-type HfO2 gate dielectrics treated with O2 plasma treatment for different process time. Like CVS curve, the capacitor treated for 30 sec shows the best characteristics compared to other process time. And then, we try to compare the three plasma treatment process of
different source gas. Fig. 4-16 shows the charge to breakdown characteristics ( QBD ) of p-type HfO2 gate dielectrics treated with N2 plasma treatment for 1min, N2O for 30 sec, and O2 for 30 sec. It is indicated that the sample of N2 plasma has larger QBD then that of other plasma treatment. We thought that it is because of well structure of interface fixed by N2 plasma for a long time.