Investigations On Memory Of PZT Ferroelectric Films

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 ABO₃ structured Pb(Zr_1-xTi_x)O₃(0＜x＜1,PZT) and bismuth layered SrBi₂Ta₂O₉(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/SiO₂/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/SiO₂/Si substrates and the poly-Si/SiO₂/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/SiO₂/n-Si was distinctly higher than that of on the poly-Si/SiO₂/Si.2.The ferroelectric characteristics of the PZT thin films on the different substrates were measured and analyzed.The remnant polarizations on Pt/Ti/SiO₂/Si were greater than those of on poly-Si/SiO₂/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/SiO₂/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 10¹¹ 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/SiO₂/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.