Evaluating the Robustness of Complementary Channel Ferroelectric FETs Against Total Ionizing Dose Toward Radiation-Tolerant Embedded Nonvolatile Memory

In this work, a thorough assessment of the robustness of complementary channel HfO2 ferroelectric FET (FeFET) against total ionizing dose (TID) radiation is conducted, with the goal of determining its suitability for use as high-performance and energy-efficient embedded nonvolatile memory (eNVM) for space applications. We demonstrate that: i) ferroelectric HfO2 thin film is robust against X-ray and proton irradiation; ii) FeFET exhibits a polarization state dependent radiation sensitivity where the high-<inline-formula> <tex-math notation="LaTeX">${V} {_{\text {TH}}}$ </tex-math></inline-formula> (HVT) state sees noticeable negative <inline-formula> <tex-math notation="LaTeX">${V} {_{\text {TH}}}$ </tex-math></inline-formula> shift and low-<inline-formula> <tex-math notation="LaTeX">${V} {_{\text {TH}}}$ </tex-math></inline-formula> (LVT) is immune to irradiation, irrespective of the channel type; iii) the state dependence is ascribed to the depolarization field in the HVT, which points toward the channel and facilitates the transport and trapping of radiation-generated holes close to the channel. In the future, radiation hardening techniques need to be considered.


I. INTRODUCTION
T HE recent surge in the space industry requires robust electronics to ensure the safety and effectiveness of space exploration and utilization as a major challenge of electronics posed by harsh space environment is the degradation caused by various radiation, e.g., electrons, protons, heavy ions, and electromagnetic radiation, e.g., X-ray and γ -ray [2], [3].Total ionizing dose (TID) irradiation causes accumulated degradation in the device [4], [5], [6].Therefore, understanding the degradation mechanisms is critical for any type of electronic devices to be deployed in space.
In this work, the TID effects in the FeFETs, as embedded nonvolatile memory (eNVM), are studied.To date, various types of NVM have been evaluated for their tolerance to TID.For example, Flash memory saves data by maintaining an electric charge on a floating gate or within a charge trapping layer.It's particularly vulnerable to TID effect, as highenergy radiation can disrupt this stored charge [7] but still can be mitigated through defect engineering as depicted in [8].Phase change memory, by storing information as the phases of the medium, demonstrates a high robustness against TID irradiation even at 30 Mrad(SiO 2 ) [9], [10].Resistive random access memory (ReRAM) also shows its robustness up to 10 Mrad(SiO 2 ) as radiation generated charges are incapable of forming/dissolving conducting filaments [11], [12], [13].Magnetic memory, such as spin transfer torque magnetic random access memory (STT-MRAM), also shows strong TID robustness up to 1 Mrad(SiO 2 ) by leveraging the spin degree of freedom, rather than charge [14], [15], [16].The aforementioned resistive memories, are all promising NVM candidates for space applications.However, they are typically power-hungry with their current-driven inefficient write process, which could limit their wide adoption in space.In contrast, ferroelectric memory based on HfO 2 , has excellent energy efficiency due to its electric field driven write 0741-3106 © 2023 IEEE.Personal use is permitted, but republication/redistribution requires IEEE permission.
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mechanisms [17], [18], [19].Evaluating the TID sensitivity of ferroelectric memory for space applications is critical and the focus of this work.Ferroelectric capacitor based memory (FeRAM), has long been touted as robust NVM against radiation [20], [21].But its destructive read operation places stringent requirements on the device endurance.FeFET, on the other hand, is particularly attractive due to its nondestructive read and transistor structure.There have been several studies on the TID effects in HfO 2 nFeFETs [22], [23], [24], [25].It has been discovered that the high-V TH (HVT) is more susceptible than low-V TH (LVT) state because radiation generated holes in the ferroelectric tend to move towards the channel under the depolarization field (E DEP ) at the HVT state [23].In addition, remnant polarization degradation is observed under γ -ray irradiation, which is ascribed to the radiation induced lattice distortion and oxygen vacancies [24].To date, however, few works have compared the TID response between nFeFET and pFeFET, though it is known that pFeFET has better read-after-write performance related with less electron trapping [26], [27], [28].If the state dependence of the degradation indeed depends on the depolarization field, similar degradation should be observed in pFeFET as well, which has not been validated before.To bridge the gap, this work studies the TID effect in both nFeFET and pFeFET by investigating memory state dependent degradation.Additionally, the study will explore the radiation tolerance of metal-ferroelectric-metal (MFM) capacitors to X-ray and proton irradiation to investigate the intrinsic ferroelectric film robustness.

II. EXPERIMENTAL DETAILS
nFeFETs/pFeFETs with W/L= 500nm/500nm integrated on 28 nm high-κ metal gate (HKMG) technology are used, as shown in Fig. 1(a) [1].The I D -V G curves of nFeFET (Fig. 1(b)) and pFeFET (Fig. 1(c)) for LVT and HVT states show a memory window more than 1V.Note that in this work, HVT refers to the memory state programmed with a -4V write pulse, regardless of nFeFET or pFeFET.For radiation experiments on MFM capacitors with 10nm Hf 0.5 Zr 0.5 O 2 thin film using X-ray and proton, the capacitor is grounded during irradiation and the radiation is interrupted at certain times for Q FE -V FE and C FE -V FE measurements.For the TID experiments of FeFETs, the dose rate is set to 29krad(SiO 2 )/min.All FeFETs are initially programmed to a clearly defined initial state and are grounded during the X-ray exposure.The exposure is temporarily halted for device electrical measurements to be conducted.The devices are left in dark and recovery after irradiation is measured.To differentiate the intrinsic retention degradation from TID induced degradation, a control experiment is conducted where the X-ray exposure is replaced with a simple grounded retention without radiation, while keeping all other parameters constant.The corresponding retention times for total doses of 100krad, 500krad, 1Mrad, and 3Mrad are 3.45 minutes, 17.2 minutes, 34.38 minutes, and 103.4 minutes, respectively.

III. RESULTS AND DISCUSSIONS A. Radiation Response of MFM Capacitors
Fig. 2(a)-(c) show the Q FE -V FE , I FE -V FE , and C FE -V FE characteristics with different X-ray doses up to 2 Mrad(SiO 2 ).No noticeable change is observed, suggesting that the ferroelectric thin film is instrincally robust against radiation.Additionally, the Q FE -V FE loops remained unchanged even after  proton irradiation up to 10 14 cm -2 fluence as demonstrated in Fig. 2(d), which can cause potential displacement damage and creates significant amount of oxygen vacancies.These results confirm that the ferroelectric HfO 2 thin film or capacitor is robust against radiation.Hence, the overall FeRAM robustness will then be determined by the peripheral circuitry [29].

B. TID Response of FeFET
Fig. 3(a) and (c) show the HVT state degradation for nFeFET and pFeFET, respectively.Conventional MOSFETs' TID response typically displays a negative V TH shift due to hole trapping in oxides, as the highly mobile electrons in the insulator are quickly released if trapped [6], [30].Both types of FeFET show a significant negative V TH shift, indicating similar behavior [30].In contrast, the LVT state in both FeFETs experience a negligible degradation, as shown in Fig. 3(b) and Fig. 3(d), respectively.This suggests that regardless of the channel type, the radiation response depends only on the polarization state.To rule out the intrinsic retention degradation, Fig. 4(a) and (b) show the I D -V G characteristics for nFeFET and pFeFET, respectively, after the same duration as the X-ray exposure.The retention induced V TH shift is minor, compared with that in Fig. 3.The corresponding V TH shift is summarized in Fig. 5(a)-(d) for both X-ray irradiation and control.The results suggest that the LVT state is very resistant to radiation, while the HVT state experiences noticeable degradation that is not recovered even after a 30-minute recovery following irradiation.From these results, it shows that the memory window reduces by 0.6V after 3Mrad(SiO 2 ) irradiation, leaving the final memory window around 0.6V.Note that the pFeFET in the LVT state shows a positive V TH shift during controlled retention, which could be related to Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.the depolarization field induced retention loss.Interestingly, radiation seems to improve the LVT state stability of the pFe-FET, likely because the radiation-induced holes compensate the depolarization field induced V TH shift.It is interesting to explore the hardening techniques in the future, such as depolarization field engineering and scaling of ferroelectric thickness [30].It has been discovered that the endurance of HZO-based FeFETs degrades with the irradiation [24], [31], which is probably related with radiation induced charge trapping and pinning of the domains.
Band diagrams extracted from TCAD simulations for nFe-FET at HVT (Fig. 6(a)) /LVT (Fig. 6(b)) and pFeFET at HVT(Fig.6(c))/LVT(Fig.6(d)) are presented.For the grounded HVT state, the depolarization field in the ferroelectric points towards the channel which can separate the electron-hole pairs generated by radiation and drive holes towards the channel.In contrast, at the LVT state, the depolarization field is pointing towards the gate, which hinders hole transport towards the channel.This leads to less degradation at the LVT state.Besides the hole trapping in the bulk, interface defect (D it ) generation [22] and subsequent charge trapping is possible, as observed by small but observable subthreshold swing (SS) increase (∼30mV/dec).One possible approach is to decrease the depolarization field at the HVT state by adjusting the operational bias or modifying the threshold voltage (V TH ).In addition, an alternative approach is to reduce the thickness of the ferroelectric layer while ensuring sufficient sensing margin.With the deduction of ferroelectric thickness, bulk trapping induced V TH shift will decrease, thus improving the robustness [30].
The distinct radiation responses of MFM capacitors and Fe-FETs may stem from their unique sensing mechanisms: MFM capacitors sense current through their stack, while FeFETs measure channel current orthogonal to the gate stack, affecting their sensitivity to trapped charges that can alter electric field distribution and domain behavior in MFM capacitors.However, the impact of the trapped charge on MFM Q FE -V FE hysteresis loops remains negligible as long as the trapped charge density remains much smaller than the polarization.There is almost negligible horizontal shift of the Q FE -V FE hysteresis loops.Conversely, in FeFETs, the trapped charge directly modulates the channel carrier density.During sensing, the polarization and trapped charge density are expected to remain undisturbed, and thus their influence on the channel carrier density is detected.Consequently, even a small amount of trapped charge can cause a substantial V TH shift in FeFETs.

IV. CONCLUSION
In conclusion, thorough research has revealed that while ferroelectric material and LVT states are highly resistant to radiation, the HVT states of FeFETs degrade under radiation due to depolarization fields.Future radiation hardening strategies may include modifying the depolarization field in HVT states or reducing the thickness of the ferroelectric layer.

Fig. 2 .
Fig. 2. Radiation tolerance of MFM capacitors to X-ray and proton irradiation.(a)/(b)/(c) Q FE -V FE loops/I FE -V FE /C FE -V FE show immunity to X-ray irradiation.(d) It is also immune to proton irradiation up to 10 14 cm -2 fluence.

Fig. 3 .
Fig. 3.The I D -V G curves of nFeFET and pFeFET under X-ray irradiation are shown in (a)/(b) and (c)/(d) for the HVT/LVT states, respectively.The HVT state experiences greater degradation compared to the LVT state.

Fig. 4 .
Fig.4.The I D -V G curves of (a) nFeFET and (b) pFeFET during a control experiment, where the devices were initialized to LVT/HVT state and grounded without any radiation exposure for the same duration as the X-ray experiment.

Fig. 5 .
Fig.5.V TH shift for HVT/LVT state in nFeFET, respectively.(c)/(d) V TH shift for HVT/LVT state in pFeFET, respectively.After irradiation or grounded retention, no recovery was found for both nFeFET and pFeFET in either LVT or HVT state.

Fig. 6 .
Fig. 6.Band diagrams explaining TID degradation.(a)/(c) nFe-FET/pFeFET at HVT state.Radiation-generated holes are driven to FE/IL interface following the depolarization field, causing more degradation.(b)/(d) show nFeFET/pFeFET at LVT state.Holes are less prone to be trapped close to the channel, thus negligible degradation.