Ferroelectric memory devices using hafnium aluminum oxides and remote plasma-treated electrodes for sustainable energy-efficient electronics

In this work, we adopt a low-temperature sustainable plasma treatment approach for the fabrication of ferroelectric memory devices. From our experimental results, we found that the ferroelectric polarization characteristics of HfAlOx ferroelectric device could be further improved by using low-temperature nitrogen plasma treatment on bottom TiN electrode for surface modification. The low-temperature nitrogen plasma treatment on TiN bottom electrode not only prevent electrode oxidation, but also lowers the generation of defect traps at the interface between ferroelectric HfAlOx and TiN bottom electrode during high-temperature ferroelectric annealing process. Besides, the nitrogen-treated bottom electrode also can improve bias-stress induced instability and cycling endurance of HfAlOx ferroelectric devices due to the effective suppression of randomly distributed defect traps or oxygen vacancies near the surface of bottom electrode.

Compared to traditional ferroelectric materials [14][15][16], HfO 2 -based ferroelectric materials exhibit significant advantages in terms of material thickness reduction, fast speed and long cycling endurance, making them highly competitive in the development and applications of 3D high-density memory devices and sustainable neuromorphic computing [17,18] in the future.Although there is potential for the application of HfO 2 -based ferroelectric materials in the fields of logic and memory device technologies, there are still some issues in device manufacturing that need to be addressed.One of the most prominent concerns is the interface domain pinning problem caused by interface defects between ferroelectric materials and electrodes [19,20].Interface domain pinning possibly leads to various issues affecting ferroelectric switching behavior and polarization strength, such as wake-up effect, switching uniformity, fatigue effect, and more.Remote plasma technology for sustainable energy production in atomic layer deposition (ALD) system can provide an effective surface modification during nanodevice fabrication.
The previous literature indicated that TiN/HfZrO/TiN capacitors treated with nitrogen plasma can maintain a certain amount of polarization without wake-up process, and there is no apparent degradation in leakage current after cycling operation due to interface defect reduction [21].The ammonia (NH 3 ) plasma treatment on HfZrO capacitor to improve the voltage stress immunity also has been demonstrated [22].However, the study of remote nitrogen plasma treatment for ferroelectric HfAlO x capacitor is less mentioned.To address this issue, this paper proposes a remote nitrogen plasma treatment on TaN/HfAlO x /TiN MFM capacitor to investigate the thermal stability of bottom electrode surface.The nitrogen remote plasma treatment aims to alleviate the oxidation of bottom elecctrode and the formation of excessive interface defects during hightemperature ferroelectric phase transitions.In addition to investigating the ferroelectric polarization properties under different plasma treatment conditions, the electrical reliabilities of the ferroelectric devices using the constant voltage stress (CVS) method and endurance cycling test were simultaneously evaluated.

Experiments
The ferroelectric metal-ferroelectric-metal (MFM) device process was described as the following.First, a 20-nmthick TiN metal was deposited as a bottom electrode.In order to study the plasma treatment effect on the TiN electrode, a remote nitrogen plasma treatment with the powers of 150 ∼ 200 W were performed on the bottom TiN electrode.Then, a 9.8-nm-thick HfAlO x films with an Al doping ratio of 6.5% was deposited by ALD system.After HfAlO x ferroelectric film deposition, the rapid thermal annealing temperatures of 500 °C ∼ 700 °C were carried out for achieving ferroelectric orthorhombic phase transformation.Finally, a 200-nm-thick TaN metal was deposited by sputtering as a top electrode.Figure 1 shows the TEM image, energy band diagram and device fabrication process flow of TaN/HfAlO x /TiN MFM capacitor.To further investigate the impact of plasma treatment on ferroelectric polarization and leakage current characteristics, the electrical stress reliability and cycling endurance test were also performed.

Results and discussion
In figure 2(a), different ferroelectric hysteresis curves under various annealing temperatures are depicted.From the ferroelectric hysteresis curves, it can be observed that the ferroelectric polarization strength increases with increasing the annealing temperature from 500 °C to 700 °C under an operating voltage of ± 4.5 V. Additionally, as the temperature increases, the ferroelectric domains involved in polarization reversal increase, leading to an increase in instantaneous current, as shown in figure 2(b).Figure 2(c) illustrates the leakage current characteristics of the HfAlO x ferroelectric capacitor.In the positive bias voltage, it can be observed that the ferroelectric domains undergo gradual polarization reversal, resulting in a larger current peak near the coercive electric field while the applied voltage gradually increases to 1.5 V.Moreover, the leakage current increases with the annealing temperature.The increased leakage current in HfAlO x ferroelectric device can be attributed to not only the improved crystallinity at elevated annealing temperatures, but also the increased interface defects between the ferroelectric layer and the bottom electrode.The high PDA temperature would lead to the increase of interface defects near the electrode, which may result in the larger leakage current, especially under high electrical field.
Therefore, to improve the high-temperature interface stability between the HfAlO x ferroelectric layer and TiN bottom electrode, we employed a remote nitrogen plasma treatment method to modify the surface of TiN bottom electrode.The figure 3 shows the ferroelectric hysteresis curve comparison of the HfAlO x ferroelectric capacitor with and without 150 W nitrogen plasma treatment.From the measured results, it is evident that the HfAlO x ferroelectric capacitor treated with nitrogen plasma for 15 s exhibits an increased remanent polarization compared to the condition with 5 s at the annealing temperature of 500 °C.Moreover, compared to the untreated device, there is a significant increase in ferroelectric remanent polarization for plasma-treated samples.The nitrogen plasma treatment was used to enhance the high-temperature oxidation resistance of TiN surface.Enhancing the surface oxidation resistance of TiN bottom electrode not only prevents electrode oxidation, but also lowers the generation of interface defects during the high-temperature ferroelectric crystalline phase transition.However, the difference in ferroelectric remanent polarization between the HfAlO x MFM devices without and with 150 W nitrogen plasma treatment becomes insignificant, while the PDA temperature increases to 700 °C.This indicates that the optimized process temperature for the 150 W nitrogen plasma treatment is 600 °C.The excessively high annealing temperatures may lead to the degradation of interface thermal stability and accelerate the oxidation of bottom electrode.Thus, an appropriate annealing temperature and remote plasma treatment condition are both important for stabilizing ferroelectric phase transformation of HfAlO x film and interface quality near TiN bottom electrode.
Figure 4 shows the ferroelectric hysteresis curves of the HfAlO x ferroelectric capacitor with 200 W nitrogen plasma treatment.From the measurement data, it is evident that the ferroelectric remanent polarization increases with increasing the PDA temperatures from 500 °C to 700 °C.When the annealing temperature reaches 700 °C, a significant increase in ferroelectric remanent polarization accompanied with the contribution of non-remnant polarization.Therefore, the optimal annealing temperature for HfAlO x ferroelectric thin films is determined to be 600 °C under the consideration of cycling endurance and electrical reliability.This PDA temperature strikes a balance between achieving a desirable ferroelectric remanent polarization, and minimizing the unwanted non-remnant polarization to ensure the best ferroelectric performance of the HfAlO x thin film.
Figure 5(a) presents a comparative chart of ferroelectric remanent polarization under different nitrogen plasma processing conditions.It is evident that the improvement in interface quality and ferroelectric remanent polarization are both achieved by using nitrogen plasma treatments of 150 W and 200 W for 15 s under 500 °C and 600 °C PDA temperatures.Even at the higher PDA temperature of 700 °C, the ferroelectric remanent polarization still can be further enhanced by using nitrogen plasma treatment condition of 200 W for 15 sec.Furthermore, there is an improvement in leakage current of HfAlO x ferroelectric capacitor after using nitrogen plasma processing at 150 W and 200 W, as shown in figures 5(b) and (c).This substantiates that nitrogen plasma treatment on the bottom electrode not only stabilizes the formation of the ferroelectric phase during hightemperature annealing but also enhances the oxidation resistance of TiN electrode to reduce the generation of interface defects, thereby improving ferroelectric polarization strength and leakage current.
In order to evaluate the ferroelectric switching behavior and electrical reliability of HfAlO x ferroelectric capacitors using nitrogen-plasma-treated bottom electrode, the constant voltage stress (CVS) tests and endurance tests were carried out.Figure 6 presents the CVS results under different stress voltages from 2.6 V to   3 V for HfAlO x ferroelectric capacitor devices using 150 W and 200 W plasma treatment conditions.From the experimental results, we can clearly observe that the leakage current at the smaller voltage condition of 2.6 V significantly decreases with increasing stress time.This is attributed to electron trapping behavior within the film stack, leading to a gradual reduction in leakage current.However, as the CVS voltage is increased to 3 V, the leakage current gradually rises, which is associated with the hole generation under high electric field.
Additionally, within the CVS voltage range of 2.6 V to 3 V, the HfAlO x ferroelectric capacitor devices treated with 200 W nitrogen plasma condition exhibits the lowest and most stable output current without apparent hole generation.This indicates that hole generation are suppressed in the nitrogen-processed TiN.The reduction in hole generation at high CVS voltage and the detrapping of generated holes at low CVS voltage can be ascribed to the improved thermal stability and enhanced oxidation resistance of bottom electrode after nitrogen plasma treatment.In contrast, for MFM devices without nitrogen plasma treatment or subjected to inappropriate nitrogen plasma treatment conditions, the significant fluctuations in leakage current occur under these stress voltage conditions.The bias-stress-induced instability is associated with the randomly distributed defects generated near the bulk of the ferroelectric layer or the TiN interface, leading to charge trapping/detrapping behavior.Figure 7 presents the cycling endurance results for HfAlO x ferroelectric capacitor devices under cyclic operation voltages of 3.5 V and 4 V.As shown in figures 7(a) and (b), the HfAlO x ferroelectric capacitor devices with and without nitrogen plasma treatment exhibit significant remanent polarization decay after 10 5 cycles under the cyclic operation voltage of 3.5 V.Even so, the endurance characteristics of ferroelectric devices still can reach up to 10 8 cycles.When the cyclic operation voltage is increased to 4 V, there is no significant fast decay in remanent polarization during cycling endurance.The cycling endurance of HfAlO x ferroelectric capacitor using nitrogen-plasma-treated bottom electrode can reach 6.3 × 10 7 cycles, while the cycling endurance of HfAlO x ferroelectric devices only reach 3.8 × 10 7 cycles.The mechanism responsible for the improved cycling endurance can be ascribed to remote nitrogen plasma treatment on bottom electrode, alleviating the influence of electrode oxidation and interface defects and extending the operating cycle.

Conclusion
Compared to control sample without nitrogen plasma treatment, the TaN/HfAlO x /TiN MFM device using bottom-electrode plasma treatment significantly presents the better ferroelectric polarization strength, stronger stress resistance and longer endurance cycle, which can be ascribed to the reduction of randomly distributed defects or oxygen vacancies at the HfAlO x /TiN interface.Thus, the low-temperature remote plasma treatment is beneficial for the sustainable nanodevice manufacturing, especially for the surface modification of nano-scale thin film.

Figure 1 .
Figure 1.(a) TEM image, (b) energy band diagram and (c) device fabrication process flow of HfAlO x ferroelectric MFM capacitors.

Figure 4 .Figure 5 .
Figure 4. Polarization-voltage characteristics of HfAlO x ferroelectric MFM capacitors without and with nitrogen plasma treatment at a power of 200 W under the PDA temperatures of (a) 500 °C, (b) 600 °C and (c) 700 °C.

Figure 7 .
Figure 7. Endurance characteristics measured at the voltages of (a) 3.5 V and (b) 4 V in HfAlO x ferroelectric MFM capacitors without and with plasma power of 200 W.