Low-temperature Fabrication Of HfO₂-based Ferroelectric Thin Films And Performance Optimization Of Their FeFETs

Ferroelectric memory,as a new type of memory devices,has the advantages of nanosecond operation speed,low power consumption,high endurance,non-volatility,high radiation tolerance,etc.From the concept of ferroelectric memory to the realization of industrialization,the biggest bottlenecks encountered by researchers are the incompatibility between traditional ferroelectric materials and the current CMOS process,and the scaling limit beyond 130 nm technology node.In 2011,B?scke et al.observed ferroelectric properties in Si doped hafnium oxide(HfO₂)thin films.Compared with traditional perovskite ferroelectric materials,doped HfO₂ ferroelectric thin films have many significant advantages:(1)Atomic layer deposition(ALD)process could be adopted,which is the mainstream deposition technology of the gate insulating layer on the CMOS process line;(2)Ferroelectric properties are still excellent when the thickness is below 10 nm;(3)With a large coercive electric field E_c(1?2 MV/cm)and a low dielectric constant(?30),which is conducive to the realization of non-volatile memory devices with excellent performance;(4)It is a simple and pollution-free material.Recently,Global Foundries made the 64 kbit and 32 Mbit HfO₂ based ferroelectric field-effect transistor(Fe FET)test chips based on 28 nm CMOS process and 22 nm fully depleted silicon on insulator(FDSOI)process,respectively,and the retention characteristics could reach to 10 years.However,the process temperature of the gate stacks for HfO₂ based Fe FETs is higher than 1000 ^oC.Such a high process temperature is not conducive to the high density integrated ferroelectric memory.Therefore,it is necessary to reduce the process temperature of the gate stacks for HfO₂ based Fe FETs.With the wide application of ferroelectric memory,the demand for ferroelectric memory performance is increasing,especially in harsh conditions,and the development of flexible electronics has significantly enhanced the application of ferroelectric memory in the emerging Internet of Things(Io T)field.Besides,in order to improve the competitive advantage of HfO₂-based ferroelectric memory in the field of non-volatile memory,in addition to high integration density and fast writing speed,HfO₂-based Fe FET is required to have a large memory window(MW),good retention and fatigue performance.Presently,as the related study is in early stage,there are still a lot of problems to be studied on the low temperature deposition and reliability of HfO₂based ferroelectric thin films and fatigue performance optimization of their Fe FETs.In this work,the low temperature deposition of Hf_0.5Zr_0.5O₂(HZO)thin film and its ferroelectricity were studied in detail.Then,the ferroelectric properties,temperature stability,bending resistance and anti-radiation of HZO thin film on the flexible substrate were systematically studied.Next,based on the low temperature deposition and gate-last process,HZO-based Fe FET was prepared and its electrical characteristics were studied in detail.On the basis of the preparation technology and the electrical properties of HZO-based Fe FET,a method of introducing a Zr O₂ seed layer at the interface of Si O₂ and HZO was proposed to realize the MW and endurance improvement of HZO-based Fe FET.This dissertation includes four parts:1.Study of low temperature deposition of HZO thin film and its ferroelectricityHZO thin films with different thicknesses(6 nm,12 nm and 20 nm)were successfully deposited by ALD at a temperature lower than 550?.As the thickness of HZO thin film increases,remenent polarization(P_r)of the HZO thin film first increases and then decreases.The P_r of the HZO thin film is 20?C/cm²when the thickness is 12 nm.The polarization switching characteristics of HZO thin film were studied by changing the parameters of write pulse(t _Write and V_Write).The results show that the P_SW depends on t _Write and V_Write.In addition,the HZO thin film shows the nanosecond-scale polarization switching.The HZO thin film exhibits the typical ferroelectric characteristic of the butterfly?_r-V curve at different frequencies.With the increase of the frequency,the?_r peak decreases.And the HZO thin film shows better fatigue properties at higher frequencies.2.Study of ferroelectric characteristics of HZO thin film on the flexible substrateWe integrate highly scalable ferroelectric HZO thin films on potential flexible mica substrates using ALD to form flexible MFM capacitors and investigate the ferroelectric properties,the retention behaviors,and endurance characteristics of the Ta N/HZO/Ta N/mica flexible MFM capacitors under various tensile and compressive bending radii.The 2P_r and2E_c values show no significant change after 1000 bending cycles,and the 2P_r values are all higher than those of the HfO₂ based ferroelectric films on polyimide.In addition,the thermal stability of the Ta N/HZO/Ta N/mica flexible MFM capacitor was also examined in a wide temperature range from 25 up to 125°C.Furthermore,the flexible devices display robust ferroelectric performance against radiation of ⁶⁰Co?-rays with a total dose of 1 Mrad(Si).This stable performance is substantially better than traditional ferroelectric materials such as PZT and SBT reported so far and is consistent with the reported results of Y doped HfO₂based ferroelectric thin films.3.Preparation and performance investigation of HZO-based Fe FETBased on the gate-last process,HZO-based Fe FET was prepared and its DC and pulse operating characteristics were studied.Owing to the ferroelectric polarization switching,typical counter-clockwise hysteresis loops were observed in the I_DS–V_GScurves of HZO-based Fe FET.As the maximum sweeping voltage increases,the hysteresis loops become wider.The HZO-based Fe FET shows a MW of approximately 1.6 V at the maximum sweeping voltage of 3 V.By changing the amplitude and width of the gate voltage pulses,the programming/erasing characteristics of the HZO-based Fe FET were studied.The HZO-based Fe FET exhibits a initial MW of 0.8 V and a 10-year extrapolated MW of 0.6 V after applying a program pulse of 7 V/100 ns and an erase pulse of–7 V/100 ns.Moreover,the reliability with respect to the endurance of the HZO-based Fe FET was estimated,and it shows a MW of 0.35 V up to 1×10⁴ program/erase(P/E)cycles.4.Investigation of HZO-based Fe FET with Zr O₂ seed layerWe present the fabrication and the characterization of HZO-based Fe FET devices with Zr O₂seed layers and investigate the effects of Zr O₂ seed layers on the ferroelectric properties of HZO thin films and their Fe FETs.The results showed that the crystalline Zr O₂ seed layer could indeed improve the crystalline quality and suppress the formation of monoclinic phase in HZO thin films.Due to the improved ferroelectric properties,the Fe FET with the Zr O₂seed layer shows a MW of approximately 2.8 V at the maximum sweeping voltage of 3 V characterized by DC voltage sweep method,which is approximately 0.8 times larger than that(1.6 V)for the Fe FET without the Zr O₂ seed layer.Meanwhile,the HZO-based Fe FET with the Zr O₂ seed layer exhibits a large initial MW of 1.4 V and a 10-year extrapolated MW of 0.9 V characterized by fast voltage pulse measurements,larger than the initial MW(0.8V)of the HZO-based Fe FET without the Zr O₂ seed layer.Moreover,the MW of the HZO-based Fe FET with the Zr O₂ seed layer is still larger than 0.9 V up to 1×10⁴(P/E)cycles,which is approximately 1.6 times larger than that(0.35 V)of the HZO-based Fe FET without the Zr O₂ seed layer.