Tensile stress regulated microstructures and ferroelectric properties of Hf0.5Zr0.5O2 films

Siying Huo(霍思颖), Junfeng Zheng(郑俊锋), Yuanyang Liu(刘远洋), Yushan Li(李育姗),Ruiqiang Tao(陶瑞强), Xubing Lu(陆旭兵),†, and Junming Liu(刘俊明)

1Institute for Advanced Materials and Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials,South China Academy of Advanced Optoelectronics,South China Normal University,Guangzhou 510006,China

2Laboratory of Solid State Microstructures and Innovation Center of Advanced Microstructures,Nanjing University,Nanjing 210093,China

Keywords: HfO2,ferroelectric materials,tension stress,annealing

1.Introduction

In recent years, ferroelectric (FE) HfO2thin films have garnered increasing attention owing to their exceptional advantages.[1-4]They exhibit excellent compatibility with the standard CMOS process and possess the ability to maintain robust ferroelectric properties even at the nanoscale,distinguishing them from traditional perovskite ferroelectrics.[5-9]Prior theoretical and experimental research has conclusively demonstrated that the noncentrosymmetric orthorhombic phase (ophase,Pca21)is widely recognized as the fundamental source of ferroelectricity in HfO2thin films.[10-12]It has been widely acknowledged that rapid cooling during the annealing process plays a crucial role in the formation of the ferroelectric polar o-phase in FE-HfO2films.[13,14]During this annealing process, the doped HfO2film is initially heated to a high temperature (usually above 500◦C) to achieve crystallization.Subsequently,the crystalline HfO2undergoes a transition from tetragonal phase(t-phase)to o-phase via rapid thermal annealing.[15]However,if a prolonged annealing process is employed,the crystalline film usually experiences a transition from the t-phase to the m-phase,resulting in the absence of polar o-phase in doped HfO2film upon cooling down to room temperature.[16,17]The influence of cooling rate during the annealing process on the microstructures and ferroelectric properties of doped HfO2films still remains ambiguous.Gaining insight into this aspect would be valuable for a deeper understanding of the origin of the ferroelectricity in doped HfO2film.

In this study,we fabricated Zr-doped HfO2(Hf0.5Zr0.5O2:HZO) thin films using atomic layer deposition (ALD).The HZO thin films were subjected to annealing with different cooling rates through rapid thermal annealing.The microstructure and electric properties of these HZO films were systematically investigated.The results clearly demonstrated that the cooling rate during the annealing process significantly influences the phase ratio(m/t/o)and stress values in the HZO thin films.We discovered that the variations in phase ratio can be attributed to the intrinsic change in stress within the HZO film,ultimately resulting in distinct ferroelectric properties.

2.Experiments

Figure 1(a) depicts the schematic representation of a W/HZO/W structure fabricated through the following steps.Initially, a W bottom electrode was sputtered onto a Si substrate.Subsequently, HZO thin films were deposited by ALD at 280◦C on the W bottom electrode.Hf[N(CH3)2]4,Zr[N(CH3)2]4, and O3were employed as Hf-precursor, Zrprecursor, and oxygen source, respectively.The Hf:Zr ratio in the HZO film was estimated to be 1:1.Finally, a W top electrode with a diameter of 100 µm was deposited by sputtering through a shadow mask.The samples then underwent a rapid thermal annealing in a high-purity N2atmosphere.The rate of temperature increase during rapid thermal annealing was 30◦C/s.After reaching the desired temperature, the annealing process was maintained at 550◦C for 30 s, followed by cooling at various rates through program control.For the sake of simplicity, the cooling rate during the annealing procedure would be referred to as the annealing rate in the subsequent discussion.The annealing rate without program control (approximately 15◦C/s-18◦C/s), and the sample with programmed annealing rates of 5◦C/s, 3.3◦C/s, 1◦C/s, and 0.5◦C/s were nominated as natural rate, 5◦C/s, 3.3◦C/s,1◦C/s, and 0.5◦C/s, respectively.The thickness of the HZO thin films was determined by x-ray reflectivity (XRR) measurements.Atomic force microscopy (AFM) was employed to examine the surface morphology of the HZO thin films.The microstructures of HZO thin films were analyzed using grazing incidence x-ray diffraction(GIXRD)and transmission electron microscopy(TEM).Especially,the stress in the HZO thin films was quantitatively determined by the GIXRD sin2ψmethod.[18-20]The ferroelectric properties and leakage current of the HZO thin films were investigated using a ferroelectric test station(Radiant Precision)and a high-precision semiconductor analyzer(Agilent B1500A).

3.Results and discussion

Figure 1(b) presents the cross-sectional TEM image of the W/HZO/W capacitor structure, showcasing the excellent thickness uniformity of the HZO film deposited via ALD.Notably,the interface between the HZO film and the top/bottom W electrode is distinct, signifying the absence of noticeable interface diffusion.The W top and bottom electrodes exhibit a thickness of approximately 50 nm,while the HZO thin film measures around 15 nm,both closely aligned with the desired thickness control during film growth.XRR measurements are performed to ascertain the thicknesses of the HZO thin films at various annealing rates.Figure 1(c) illustrates a representative XRR curve, from which the thickness of the HZO thin film was determined to be approximately 15 nm,demonstrating an excellent consistency with the TEM result.Figures 1(d)and 1(e)depict the results of the AFM surface topography test for samples annealed at a natural rate and 0.5◦C/s.Upon careful observation, it is found that the sample annealed at the natural rate exhibits a smaller grain size compared to the sample annealed at 0.5◦C/s.Additionally, their RMS values are 732.1 pm and 912.8 pm, respectively.Figure 1(f) summarizes the RMS values of the HZO thin films annealed at various rates, clearly indicating the impact of the annealing rate on the surface morphology of the HZO thin films.As the annealing rate decreases, the surface roughness of the sample progressively increases, reaching its maximum value at 0.5◦C/s.A prolonged annealing process,particularly with an extended cooling time,should be favorable for the nucleation of the grains.Consequently, samples subjected to longer annealing time exhibit bigger grain sizes and, correspondingly,larger RMS values.

Fig.1.(a)The schematic diagram of a W/HZO/W capacitor.(b)TEM cross-sectional images of W/HZO/W capacitor.(c)A representative XRR curve of the HZO thin film.(d)AFM measurement result of natural rate HZO thin film.(e)AFM measurement result of 0.5 ◦C/s HZO thin films.(f)Root of mean square(RMS)values of sample surface morphology with different annealing rates.

Fig.2.(a)GIXRD patterns of natural rate,5 ◦C/s,3.3 ◦C/s,1 ◦C/s,and 0.5 ◦C/s samples.(b)A summary of relative o/t-phase fraction ratio in HZO thin films.(c)Deconvolution results of GIXRD spectra extracted from panel(a).

Figure 2(a)illustrates the GIXRD test results for the samples of natural rate, 5◦C/s, 3.3◦C/s, 1◦C/s, and 0.5◦C/s.The peaks observed at 28.5◦, 31.5◦, and 30.5◦correspond to the m(-111)phase, the m(111)phase, and the mixed peak of ferroelectric o(111)phase and t(011)phase,respectively.Figure 2(b)presents a summary of the phase ratios as a function of the annealing rate, obtained through the deconvolution of the GIXRD patterns illustrated in Fig.2(a).The detailed deconvolution results are shown in Fig.2(c).By calculating the area of each deconvoluted phase graph,the relative fractions of the o(111)/t(011)phase and m(-111)/(111)phase for samples with various annealing rates are determined.It is observed that the ratio of the o(111)/t(011) phase decreases as the annealing rate decreases.Specifically,the o(111)/t(011)phase ratios are found to be 78.1% (natural rate), 70.3% (5◦C/s), 68.6%(3.3◦C/s),55.5%(1◦C/s),and 45.9%(0.5◦C/s),respectively.Conversely, the ratio of m(-111)phase and m(111)phase increases with the decrease in annealing rate.These GIXRD results demonstrate that the polar o-phase and nonpolar m-phase of HZO films can be precisely modulated by adjusting the annealing rate.

Figures 3(a)-3(e) depict the polarization-voltage (P-V)curves of the HZO thin films with various annealing rates.The curves are obtained by sweeping voltages from 1 V to 6 V at a test frequency of 10 kHz.Upon reaching a scanning voltage of 3 V,ferroelectric hysteresis loops start to emerge,while theP-Vloop tends to become saturated at a scanning voltage of 6 V.Notably, the sample annealed at the natural rate exhibits exceptional ferroelectric properties, with a 2Prvalue as high as 62.8µC/cm2.Figure 3(f)presents a comparison of thePVcurves for samples annealed at different rates(natural rate,5◦C/s, 3.3◦C/s, 1◦C/s, and 0.5◦C/s) at a scanning voltage of 6 V(4 MV/cm).As the annealing rate increases,the polarization characteristic curve of the sample gradually contracts and becomes more saturated,exhibiting enhanced ferroelectric properties.

Figure 4(a) presents the 2Prvalues plotted against the sweeping voltages for all samples with various annealing rates,enabling a direct comparison of their ferroelectric properties.Notably,the 2Prvalue demonstrates a pronounced voltage dependence,exhibiting an increase with the rise in the scanning voltage.Additionally, at the same scanning voltages, the 2Prvalue of the HZO thin film exhibits a significant enhancement with increasing annealing rate.TheseP-Vtest results show a good consistency with the GIXRD findings.It is worth highlighting that the sample annealed at the natural rate,with the highest proportion of the o(111)/t(011) phase characterized by GIXRD,possesses the highest 2Prvalue.Conversely,the sample annealed at 0.5◦C/s, with the lowest ratio of the o(111)/t(011) phase, shows the smallest 2Prvalue.These results align well with the notion that the polar o-phase contributes to the ferroelectric behavior of the HfO2-based ferroelectric films.

Figure 4(b) presents theεr-Vcurves of the HZO thin films annealed at various rates, measured at a scanning voltage of 5 V and a frequency of 1.0 MHz.All of theεr-Vcurves exhibit the characteristic butterfly shape typical of ferroelectric materials,implying excellent ferroelectric properties of the prepared HZO thin films.Additionally,with a decrease in the annealing rate, there is a corresponding reduction in relative permittivity.According to literature reports,the dielectric constant of the t-phase falls within the range of 40 to 50,while that of the ferroelectric o-phase is between 30 and 40, and the mphase shows a range of 16 to 22.Consequently,we conclude that the slower annealing rate leads to a decrease in the o-phase and an increase in the m-phase in the thin film.This observation aligns with the findings from the GIXRD measurement,providing mutual confirmation of the results.

The fatigue properties of the HZO thin films were investigated using a bipolar triangular pulse wave with a 100 kHz frequency, a 2.5 V polarization reversal voltage, and a 3 V PUND reading voltage.As depicted in Fig.4(c),samples annealed at the natural rate, 5◦C/s, and 3.3◦C/s exhibited fatigue cycles exceeding 108cycles,whereas samples annealed at 1◦C/s and 0.5◦C/s showed fatigue cycles below 108cycles.The improved fatigue performance of the sample with a faster annealing rate can be attributed to the following mechanisms.Previous studies highlighted the strong correlation between the fatigue properties of HZO thin films and both leakage current and domain pinning effect caused by defects.[21,22]Samples subjected to a faster annealing rate show a smaller grain size and smoother surface roughness,as confirmed by AFM results demonstrated in Figs.1(d)-1(f).Consequently,the presence of smaller grains and fewer grain boundaries in the film favored lower leakage current.Furthermore,the sample with reduced surface roughness should exhibit better electrode/HZO interface quality, resulting in fewer interface defects and reduced interface domain pinning effects.

Figure 4(d)displays the polarization retention characteristics with time decay,as measured using write/read pulses of 4 V in magnitude and 1 ms in pulse width.The schematic representation of the measurement cycle details can be observed in the inset of Fig.4(d).Even after a retention time as long as 2×104s,the polarization values of the natural rate,5◦C/s,3.3◦C/s,1◦C/s,and 0.5◦C/s samples remain remarkably high at 91.6%,96.4%,97.3%,84.7%,and 93.7%,respectively.Extrapolating the polarization retention of samples with various annealing rates over a period of 10 years demonstrates the outstanding reliability and retention performance of this HZO thin film.Fig.4.Comparison of electric properties of HZO thin film samples with different annealing rates.(a)2Prvalues under scanning voltage from 1 V to 6 V.(b)Dielectric constant variation with sweeping voltage.(c)Endurance measurement results.(d)Retention properties results.The inset shows pulse details from a single cycle measurement taken during the retention analysis,wherePTandPBdenote the recorded top and bottom polarizations obtained from the read pulses,respectively.

Fig.3.The P-V hysteresis loops of samples with various annealing rates: (a)natural rate,(b)5 ◦C/s,(c)3.3 ◦C/s,(d)1 ◦C/s,(e)0.5 ◦C/s samples.(f)A comparison of P-V hysteresis loops of HZO samples with various annealing rates at a sweeping voltage of±6 V.

Figures 5(a)-5(e) present the results of GIXRD sin2ψmethod stress tests conducted on HZO at various annealing rates.In this research, CuKαrays were used as the ray source,with a 2θscanning range of 25◦-35◦,an incident angle of 2◦, and a rotation setting range of 0◦-50◦.As the rotation angle increased from 0◦to 50◦, a significant reduction in the diffraction peaks of the samples was observed.In addition, the diffraction peak positions displayed a distinct tendency to shift to the left, indicating the presence of tensile stress within the film.Based on the GIXRD sin2ψresults,the residual stress values of HZO thin films annealed at different rates were calculated using the method proposed by Zhuet al.,[23]as depicted in Fig.5(f).The residual stresses of the samples annealed at natural rate, 5◦C/s, 3.3◦C/s, 1◦C/s,and 0.5◦C/s were determined to be 4.63 GPa, 4.236 GPa,3.6715 GPa, 3.4205 GPa, and 2.524 GPa, respectively.All residual stress values fall within the range of 2.5 GPa to 5 GPa,consistent with reported values for HZO thin films.[24]It is evident that the film’s residual stress decreases with a lower annealing, demonstrating the significant influence of annealing rate on stress.A faster annealing rate induces larger tension stress in the samples.Notably,stress in the HfO2-based ferroelectric materials has been reported to critically affect phase stability.[25,26]In this study, a high stress level in the HZO film is assumed to be favorable for the o-phase stability,while low stress favors the stability of the m-phase.Consequently,samples subjected to a faster annealing rate and higher stress have a high ratio of o-phase.Conversely,decreasing stress in the film leads to a reduced o-phase ratio and an increased mphase ratio.These stress measurement findings provide a reasonable explanation for the distinct microstructure and ferroelectric properties observed in samples with various annealing rates.

4.Conclusion

We explored the influence of tension stress,regulated by the annealing rate during the rapid thermal annealing process,on the microstructure and ferroelectric properties of the HZO thin films.AFM measurements revealed that samples with a faster annealing rate exhibited smaller grain sizes and improved surface roughness.GIXRD measurements indicated precise modulation of the polar o-phase and nonpolar m-phase of HZO films by adjusting the annealing rate,with faster rates leading to a higher o-phase ratio within the film.Electrical measurements indicated that samples with faster annealing rate showed enhanced ferroelectric properties, including higher polarization values and better fatigue cycles.GIXRD stress measurements showed that a faster annealing rate induced larger tension stress in the samples.These findings highlighted the critical role of stress within the HZO films in impacting their ferroelectric properties of HZO thin films.

Acknowledgments

Project supported by the National Natural Science Foundation of China (Grant Nos.62174059 and 52250281), the Science and Technology Projects of Guangzhou Province of China (Grant No.202201000008), the Guangdong Science and Technology Project-International Cooperation (Grant No.2021A0505030064), and the Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials(Grant No.2020B1212060066).