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Preparation of PZT thin films for low voltage application by Sol-Gel method

Preparation of PZT thin films for low voltage application by Sol-Gel method. N. Soyama, K. Maki, S. Mori, K. Ogi Mitsubishi Materials Corporation Applications of Ferroelectrics, 2000. ISAF 2000. Proceedings of the 2000 12th IEEE International Symposium on , Volume: 2 , 21 July-2 Aug. 2000.

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Preparation of PZT thin films for low voltage application by Sol-Gel method

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  1. Preparation of PZT thin films for low voltage application by Sol-Gel method N. Soyama, K. Maki, S. Mori, K. Ogi Mitsubishi Materials Corporation Applications of Ferroelectrics, 2000. ISAF 2000. Proceedings of the 2000 12th IEEE International Symposium on , Volume: 2 , 21 July-2 Aug. 2000 전기 재료 연구실 유 지 성

  2. Abstract • PZT ultra thin films with film thickness of 90nm were prepared by Sol-Gel method. • By using modified sol-gel solutions, 90nm-thick PZT films with good surface morphology were obtained even on Pt/SiO2/Si substrate. • Use of the modified solutions made it possible to make the film thinner without degradation of the properties. • PZT thin films from the modified solution exhibited potential for low voltage drive FeRAM. FeRAM (Ferroelectrics Random Access Memory)이란? 차세대 반도체로 주목 받고 있는 FeRAM은 전원을 끊더라도 데이터가 지워지지 않을 뿐만아니라 전력 소비량이 적고 고집적화가 가능하다는 장점을 지니고 있는 비휘발성 RAM으로 현재까지 개발된 반도체중 가장메모리 용량이 큰 반도체다.

  3. INTRODUCTION • PZT thin film의 advantage ♣ good surface morphology ♣ high remnant polarization (Pr) • PZT thin film의 disadvantage ♣ low-voltage operation에서 Bad saturation - 개선 방법 : decrease of film-thickness • 필름 두께 감소 시 문제점 ♣ 결정 grain이 커지며 secondary phase 발생 ♣ Ec (coercive field)증가  저 전압 구동이 어려워짐 ♧ 원인 –결정 구조가 불균일

  4. INTRODUCTION • 졸겔 용액의 제조에 있어서 가수분해 조건의 변화를 통해 PZT 박막 필름의 단점 개선  modified type A6 • 이 용액의 사용은 특성의 degradation 없이 두께를 줄 일 것으로 기대되며 두께를 120nm 이하로 만드는 것이 가능하다고 보고하였다. • 이 논문에서, 우리는 modified Sol-Gel solution으로 PZT 박막 필름(100nm 이하)을 제조하고 평가하였다. 그리고 필름의 두께와 특성의 의존성에 대해 논의 하였다.

  5. Sol-Gel method • 강유전성 박막 제조법 ♣ 기상 화학 증착법 (chemical vapor deposition) MOCVD method, PECVD method ♣ 용액 증착법 (solution deposition) Sol-Gel method, MOD method • Sol-Gel method ♣ 유동성을 갖는 액체가 여러 가지 과정을 거치면서 Gel로 전이되는 과정을 이용하여 원하는 재료의 양이온이 포함된 금속 알콕사이드로부터 가수분해, 축압, 성형, 건조 및 열처리에 의하여 원하는 형상을 구현하는 방법으로 코팅 방법에 따라 Dip 코팅과 Spin 코팅으로 나누어진다.

  6. Experimental (Preparation) • PZT films were deposited by conventional sol-gel method (spin coating). ♣ solution : modified type A6(Mitsubishi Materials Corporation) ♣ composition - Zr/Ti ratio : 20/80, 30/70, 40/60 - Pb/(Zr+Ti) : 1.15 ♣ substrate : Pt(200nm)/SiO2(500nm)/Si ♣ 열처리 : 200℃에서 5분 (repeat twice) -> 700℃에서 1분 (rapid thermal annealing)  PZT film with thickness of 90nm

  7. Experimental (Evaluation) • 미세구조 관찰 : FE-SEM(field-emission type scanning electron microscopy) • 결정구조 분석 : XRD(X-ray diffraction) • 전기적 특성 측정 ♣ Pt 전극(200nm) 증착 : 700℃에서 1분 ♣ P-V characteristic : Radiant Technology RT66A ♣ I-V characteristic : Keithley 236

  8. Result and Discussion (1) FE-SEM images showing microstructure of 90nm-thick PZT films with Zr-Ti ratio of (a)20/80 (b)30/70 (c)40/60 derived from PZT-A6 solution.

  9. Result and Discussion (2) • XRD는 X-ray 회절 분석으로 물체의 결정구조를 알아보기 위한 측정 방법으로 동일한 결정 구조 일수록 Intensity가 높게 나타난다. XRD pattern of 90nm-thick PZT films with Zr/Ti ratio of 20/80, 30/70 and 40/60

  10. Result and Discussion (3) P-V hysteresis loops for 90nm-thick PZT Films with Zr/Ti ratio 20/80, 30/70 and 40/60 Saturation properties of 90nm-thick PZT Films with Zr/Ti ratio 20/80, 30/70 and 40/60

  11. Result and Discussion (4) I-V characteristics of 90nm-thick PZT films with Zr/Ti ratio of 20/80, 30/70 and 40/60

  12. Result and Discussion (5) Film thickness dependence of 2Pr for PZT films with Zr/Ti ratio 20/80, 30/70 and 40/60. Film thickness dependence of 2Ec for PZT films with Zr/Ti ratio 20/80, 30/70 and 40/60.

  13. Conclusions • PZT modified sol-gel solution (A6) was effective to obtain a very thin film (below 90nm) with good ferroelectric performance. ♣ smooth surface and uniform grain ♣ P-V histeresis loop saturated at low voltage (Zr/Ti의 비율 30/70 일 때 거의 1V에서 saturated) PZT capacitor operate at low voltage. ♣ seeding layer가 없는 기판에도 박막 증착 가능 • It is expected that low voltage FeRAM can be realized by using A6 solution.

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