摘要
铁电随机存储器(FRAM)的工作原理是利用铁电薄膜材料剩余极化双稳态的特点,它具有非易失性、高速度、高密度、抗辐射等优点,被认为半导体存储器的终结者。ABO_3型Pb(Zr_(1-x)Ti_x)O_3(0＜x＜1,PZT)和铋层状SrBi_2Ta_2O_9(SBT)薄膜是铁电存储器应用最主要的两种材料。本论文以PZT铁电存储器的制备和特性为研究目的,研究了2T2C结构FRAM和新型FFET结构FRAM,在国内外首先验证了MFPIS结构的FFET存储器件并在国内外学术刊物进行了研究报道。
主要研究内容如下:
1、采用磁控溅射工艺,分别在Pt/Ti/SiO_2/Si及poly-Si/SiO_2/Si两种衬底上制备PZT铁电薄膜。研究了传统缓慢退火方式和快速退火方式及退火温度对PZT铁电薄膜结构的影响,并根据这些实验来确定合理的退火方式和温度。研究表明,在650℃热处理时,传统退火方式的PZT薄膜为(100)晶向择优,而快速退火的PZT薄膜为(111)晶向择优。Pt/Ti/SiO_2/Si衬底上的PZT薄膜的衍射峰强度明显强于poly-Si/SiO_2/Si衬底上PZT薄膜的衍射峰强度。
2、对不同衬底上PZT薄膜的铁电性能进行了测试和分析。结果表明:Pt/Ti/SiO_2/Si衬底上PZT薄膜的剩余极化大于在poly-Si/SiO_2/Si上PZT薄膜的剩余极化值。研究了不同退火温度下的PZT薄膜的铁电特性,650℃为PZT铁电薄膜退火的最佳温度。
3、研究了铁电存储器集成工艺,针对铁电存储器集成工艺存在的PZT薄膜起泡问题,开展了一系列研究工作。提出了采用先刻蚀出Pt电极图形,后制备PZT薄膜的工艺技术。并且对PZT薄膜采用先500℃预处理,再650℃快速退火的方式,以解决PZT薄膜起泡问题。提出了优化的集成铁电存储器工艺流程。研究了PZT样品中铅在不同温度下的挥发性。
4、研究了集成铁电电容面积和HF湿法腐蚀对铁电电容性能的影响。研究了2T2C单元结构的铁电存储器的存储性能。设计了1Kb铁电存储器电路和版图,并最终研制出了集成铁电存储器的测试芯片。
5、研究了Pt/PZT/Pt/Ti/SiO_2/Si(MFMIS)结构的FET器件。器件的顺时针C-V滞回曲线和逆时针Ⅰ-Ⅴ滞回曲线表明MFMIS结构n沟道PZT铁电场效应晶体管可以实现极化存储。栅电压V_g在-5V和+5V之间获得了2.1V的存储窗口。经过10~(11)次读写循环后,FFET的存储窗口从2.1V变为1.6V,变化幅度较小,具有实际应用的能力。MFMIS结构的n沟道PZT铁电场效应晶体管适合在于未来大规模、高密度、高速度铁电存储器上使用。
6、在上述研究的基础上,提出了Pt/PZT/poly-Si/SiO_2/Si(MFPIS)结构的FET器件,并在国际上首先进行了报道。研究了MFPIS结构的C-V特性和Ⅰ-Ⅴ特性。制备的MFPIS-FET的顺时针C-V特性曲线和逆时针的I_d-V_g特性曲线表明由于PZT铁电薄膜的极化作用,FFET能够实现存储的功能。研究表明存储窗口随着工作电压的增加而增加。I_d-V_g曲线表明当V_g为5V时,FFET的存储窗口为2.6V。通过测试在1MHz、±5V的脉冲方波下器件的阈值电压的变化值,来检测器件的疲劳特性。实验表明器件具有较好的抗疲劳性。与MFMIS-FET相比,这种结构和工艺更为简单,Poly-Si作为下电极可以代替金属Pt,Poly-Si还可以阻挡PZT成分向衬底扩散,更有利于铁电工艺和半导体工艺的集成。本论文研究表明,本文所提出的MFPIS-FET器件有希望成为下一代高密度存储器侯选结构。
The ferroelectric random access memory(FRAM) is a type of the ferroelectric random access memory using a ferroelectric thin film as the memory material.The ferroelectric thin film is polarized by the electric field applied from an external source and remains polarized even as the external electric field is removed.FRAM is considered a candidate for next generation memory applications for the characteristics of non-volatile,high speed,high density and radiation resistant.Attributed to the spontaneous polarization of the ferroelectric thin films,such as ABO_3 structured Pb(Zr_(1-x)Ti_x)O_3(0＜x＜1,PZT) and bismuth layered SrBi_2Ta_2O_9(SBT) are now known as ferroelectric films used for the ferroelectric memory.This dissertation is aimed to deal with the fabrication and the characteristics of the PZT ferroelectric thin films and their applications in the ferroelectric memory.The 2T2C cell and the novel FFET cell structure FRAMs were studied.The Pt/PZT/poly-Si/SiO_2/Si(MFPIS) structure FFET was proposed and demonstrated.The research results were reported in the international journals.
The main contents are as following:
1.The PZT thin films were deposited on the Pt/Ti/SiO_2/Si substrates and the poly-Si/SiO_2/Si substrates by the radio frequency magnetron sputtering.The effects of the annealing method and the annealing temperature on the crystal phase were studied. The PZT thin films annealed in the conventional method exhibited the preferential orientation of(100) while those of rapid thermal annealing exhibited the(111) preferential orientation at 650℃.The XRD peak intensity of the PZT thin films deposited on Pt/Ti/SiO_2/n-Si was distinctly higher than that of on the poly-Si/SiO_2/Si.
2.The ferroelectric characteristics of the PZT thin films on the different substrates were measured and analyzed.The remnant polarizations on Pt/Ti/SiO_2/Si were greater than those of on poly-Si/SiO_2/Si.The ferroelectric characteristics of the PZT thin films annealed at different temperatures were investigated.The best temperature in terms of the ferroelectric characteristics was 650℃.
3.The hillock formation of the PZT films on the Pt bottom electrode was the problem during the integrated process.A series of experiments were done to solve the hillock problem.The new technology was provided in which the Pt bottom electrode was patterned and then the PZT films was formed on the bottom electrode.The PZT thin films were pre-annealed at 500℃and then rapidly annealed at 650℃.The optimized integrated ferroelectric memory technology was contrived.
4.The integrated ferroelectric capacitors were fabricated and studied.The effects of the area of the PZT capacitors and the HF wet etching on the ferroelectric characteristics of the PZT films were investigated.The ferroelectric memory of the 2T2C cell structure was studied and the test chip of the integrated ferroelectric memory was successfully fabricated.
5.The n-channel field-effect-transistor(FET) with the Pt/PZT/Pt/SiO_2/Si substrates (MFMIS) structure was fabricated by using the PZT thin film on Si substrates.The clockwise C-V and the counterclockwise I_d-V_g hysteresis loops of the n channel FFET demonstrated that the FFET could realize a memory effect due to the ferroelectric polarization of the PZT thin film.The memory widow of the FFET was 2.1V observed from the I_d-V_g hysteresis curves with V_g swing between -5V and +5V.After 10~(11) cycles, the memory windows decreased from 2.1V to 1.6V.The change was little and satisfied the practical application.The MFMIS-FET could be used for the very large scale,high density and high speed ferroelectric memory in the future.
6.The Pt/PZT/poly-Si/SiO_2/Si(MFPIS) structure FET was proposed under the base of the experiments mentioned above and reported in the international journals.The C-V and the I-V characteristics of the device were studied.The clockwise C-V hysteresis loops and the counterclockwise hysteresis loops in the I_d-V_g characteristics demonstrated that FFET could realize a memory function due to the ferroelectric polarization of the PZT thin film.The memory window of the FFET was about 2.6 V as observed from the I_d-V_g characteristics,with the V_g swinging between -5V and 5V.The endurance characteristics of the threshold voltage changes of the MFPIS-FET after 1 MHz±5 V writing with different switching cycles were given out.The structure of the MFPIS FET was compact compared with the MFMIS FET,and the poly-Si substituted Pt as the bottom electrode.And the poly-Si could avoid the component of PZT diffusing to the substrate in favor of the integration of the ferroelectric technology and semiconductor process.The experiments and results showed that the MFPIS-FET proposed in this paper was the promising candidate structure for the application of the next generation high-density memory.
引文
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