铁电存储器用PZT薄膜的制备及性能研究
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摘要
近年来,将铁电材料与半导体器件相结合的新器件逐渐得到了广泛的应用。其中,新型非挥发铁电存储器(Ferroelectric Random Access Memories,FeRAM)与传统的EEPROM和FLASH非挥发存储器相比,具有操作电压低、功耗低、信息保持时间长、写操作速度快、抗辐射等优异的特性,因而非常适合嵌入式应用的要求。由于铁电薄膜材料是FeRAM器件的关键组成部分,所以本论文对FeRAM用铁电薄膜材料进行了一些实验研究。
考虑到材料的自身特性、工艺制备和商业化应用前途,确定了PZT(52/48)是较为理想的嵌入式FeRAM用材料。使用固相反应法制备了PbO过量20mol%的纯钙钛矿结构的PZT靶材。XRD测试表明陶瓷为纯钙钛矿相。用磁控溅射法制备出了一系列PZT铁电薄膜,讨论了溅射气压、基片温度、以及退火工艺对PZT薄膜性能的影响,并着重考查了基片温度对PZT铁电薄膜性能的影响。分析得出制备PZT薄膜的最优工艺条件是:工作气压3Pa,基片温度300oC,溅射气氛为纯Ar气,退火温度为650oC,退火时间为20分钟;由于单层的PZT薄膜很难满足FeRAM所提出的材料要求,我们在PZT铁电薄膜与电极之间加上了一层PLT晶种层来进一步提高薄膜的性能。经研究得出,PLT晶种层会增大薄膜的剩余极化,提高薄膜的疲劳特性,因而用PLT晶种层可以生长出符合要求的PZT铁电薄膜。Pt/PLT/PZT/PLT/Pt/TiO_2/SiO_2/Si是一种优化的,适用于FeRAM的铁电薄膜结构。由不同PLT晶种层厚度的PZT铁电薄膜的性能分析指出PLT厚度增加时,薄膜的铁电性能和疲劳特性变好。为了得出制备PLT晶种层的最佳工艺条件,我们考查了PLT制备条件对PLT/PZT/PLT薄膜结构铁电性能的影响,总结出了制备PLT薄膜的最佳工艺条件:工作气压3Pa,基片温度600oC,溅射气氛为:Ar/O_2=6:1。根据这个工艺条件,在Pt衬底上成功地制备了性能优异的PLT/PZT/PLT铁电薄膜。测试结果表明,薄膜具有良好的铁电性能;对于约500kV/cm的外加电场,薄膜具有较大的剩余极化值(2Pr=52.7μC/cm2),较小的矫顽场(2Ec=130kV/cm)。经1010开关极化后,剩余极化无明显的减少。
In recent years, the new devices combining ferroelectric materials with semiconductor devices have been widely used. Compared with the traditional EEPROM and FLASH memory, the new non-volatile Ferroelectric Random Access Memories (FeRAM) have many advantages, such as low operating voltage, low power consumption, long-time retention, quick writing-operation and outstanding anti-radiation characteristics, which make it very suitable for embedded applications. Therefore, this paper mainly focused on the fundamentally experimental research of the ferroelectric film materials, which are the key components of the embedded FeRAM.
Taking the intrinsic material properties and preparation conditions, as well as the commercial future into consideration, we chose PZT(52/48) to be the ideal ferroelectric film material for application to FeRAM. Ferroelectric ceramic PZT targets with 20mol% excess PbO were prepared by the conventional solid-state route. XRD patterns on the polished ceramic targets indicated pure perovskite phase. A series of ferroelectric thin films were prepared by rf magnetron sputtering. Fabrication parameters, such as work pressure, substrate temperature, and post annealing were examined in terms of their influences on the ferroelectric characteristics of the ferroelectric films. Experimental analysis showed that the optimal deposition conditions for PZT film are as follows, work pressure: 3Pa, substrate temperature: 300°C, sputtering atmosphere: pure argon, annealing temperature: 650°C, annealing time: 20 minutes. Considering it is hard for a single PZT film to satisfy the needs put forward by FeRAM, we inserted a thin PLT seed layer between PZT film and the electrode so as to improve the ferroelectric properties of PZT films. Experiment results indicated that PLT seed layer would increase the remnant polarization and enhance the fatigure resistance of the PZT films. Therefore, PZT films with satisfying electrical properties could be acquired through the prior deposition of a thin PLT seed layer. Pt/PLT/PZT/PLT/Pt/TiO_2/SiO_2/Si is an optimized structure, and suitable for FeRAM use. By analyzing the effects of various PLT seed layer thickness on the ferroelectric properties of PZT film, we found that when the thickness increased, the ferroelectric properties got better. In order to get the optimal fabrication conditions of the PLT seed layer, we examined the effects of PLT fabrication conditions on ferroelctric characteristics of PLT/PZT/PLT structure, and summarized the following optimal fabrication parameters of depositing PLT film, work pressure: 3Pa, substrate temperature: 600°C, sputtering atmosphere: Ar/O_2 = 6:1. In the end, optimized PLT/PZT/PLT ferroelectric structure with excellent characteristics was successfully prepared on Pt substrate. Measured results indicated that the films possessed excellent ferroelectric characteristics. When an electric field of approximate 500kV/cm was applied, the film had a larger remanant polarization (2Pr=52.7μC/cm2), lower coercive field (2Ec=130kV/cm). Moreover, PLT/PZT/PLT structure showed almost no polarization fatigue even after 1010 switching cycles.
考虑到材料的自身特性、工艺制备和商业化应用前途,确定了PZT(52/48)是较为理想的嵌入式FeRAM用材料。使用固相反应法制备了PbO过量20mol%的纯钙钛矿结构的PZT靶材。XRD测试表明陶瓷为纯钙钛矿相。用磁控溅射法制备出了一系列PZT铁电薄膜,讨论了溅射气压、基片温度、以及退火工艺对PZT薄膜性能的影响,并着重考查了基片温度对PZT铁电薄膜性能的影响。分析得出制备PZT薄膜的最优工艺条件是:工作气压3Pa,基片温度300oC,溅射气氛为纯Ar气,退火温度为650oC,退火时间为20分钟;由于单层的PZT薄膜很难满足FeRAM所提出的材料要求,我们在PZT铁电薄膜与电极之间加上了一层PLT晶种层来进一步提高薄膜的性能。经研究得出,PLT晶种层会增大薄膜的剩余极化,提高薄膜的疲劳特性,因而用PLT晶种层可以生长出符合要求的PZT铁电薄膜。Pt/PLT/PZT/PLT/Pt/TiO_2/SiO_2/Si是一种优化的,适用于FeRAM的铁电薄膜结构。由不同PLT晶种层厚度的PZT铁电薄膜的性能分析指出PLT厚度增加时,薄膜的铁电性能和疲劳特性变好。为了得出制备PLT晶种层的最佳工艺条件,我们考查了PLT制备条件对PLT/PZT/PLT薄膜结构铁电性能的影响,总结出了制备PLT薄膜的最佳工艺条件:工作气压3Pa,基片温度600oC,溅射气氛为:Ar/O_2=6:1。根据这个工艺条件,在Pt衬底上成功地制备了性能优异的PLT/PZT/PLT铁电薄膜。测试结果表明,薄膜具有良好的铁电性能;对于约500kV/cm的外加电场,薄膜具有较大的剩余极化值(2Pr=52.7μC/cm2),较小的矫顽场(2Ec=130kV/cm)。经1010开关极化后,剩余极化无明显的减少。
In recent years, the new devices combining ferroelectric materials with semiconductor devices have been widely used. Compared with the traditional EEPROM and FLASH memory, the new non-volatile Ferroelectric Random Access Memories (FeRAM) have many advantages, such as low operating voltage, low power consumption, long-time retention, quick writing-operation and outstanding anti-radiation characteristics, which make it very suitable for embedded applications. Therefore, this paper mainly focused on the fundamentally experimental research of the ferroelectric film materials, which are the key components of the embedded FeRAM.
Taking the intrinsic material properties and preparation conditions, as well as the commercial future into consideration, we chose PZT(52/48) to be the ideal ferroelectric film material for application to FeRAM. Ferroelectric ceramic PZT targets with 20mol% excess PbO were prepared by the conventional solid-state route. XRD patterns on the polished ceramic targets indicated pure perovskite phase. A series of ferroelectric thin films were prepared by rf magnetron sputtering. Fabrication parameters, such as work pressure, substrate temperature, and post annealing were examined in terms of their influences on the ferroelectric characteristics of the ferroelectric films. Experimental analysis showed that the optimal deposition conditions for PZT film are as follows, work pressure: 3Pa, substrate temperature: 300°C, sputtering atmosphere: pure argon, annealing temperature: 650°C, annealing time: 20 minutes. Considering it is hard for a single PZT film to satisfy the needs put forward by FeRAM, we inserted a thin PLT seed layer between PZT film and the electrode so as to improve the ferroelectric properties of PZT films. Experiment results indicated that PLT seed layer would increase the remnant polarization and enhance the fatigure resistance of the PZT films. Therefore, PZT films with satisfying electrical properties could be acquired through the prior deposition of a thin PLT seed layer. Pt/PLT/PZT/PLT/Pt/TiO_2/SiO_2/Si is an optimized structure, and suitable for FeRAM use. By analyzing the effects of various PLT seed layer thickness on the ferroelectric properties of PZT film, we found that when the thickness increased, the ferroelectric properties got better. In order to get the optimal fabrication conditions of the PLT seed layer, we examined the effects of PLT fabrication conditions on ferroelctric characteristics of PLT/PZT/PLT structure, and summarized the following optimal fabrication parameters of depositing PLT film, work pressure: 3Pa, substrate temperature: 600°C, sputtering atmosphere: Ar/O_2 = 6:1. In the end, optimized PLT/PZT/PLT ferroelectric structure with excellent characteristics was successfully prepared on Pt substrate. Measured results indicated that the films possessed excellent ferroelectric characteristics. When an electric field of approximate 500kV/cm was applied, the film had a larger remanant polarization (2Pr=52.7μC/cm2), lower coercive field (2Ec=130kV/cm). Moreover, PLT/PZT/PLT structure showed almost no polarization fatigue even after 1010 switching cycles.
引文
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