PLD法制备BST铁电薄膜及其性质的研究
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摘要
钛酸锶钡(简称BST)是一种铁电／介电性能十分优越的材料,因其具有低漏导电流密度、低介质损耗、低介电常数温度、高介电常数、热释电系数及其居里点温度可依据Ba/Sr比值可调等特点,广泛应用于微波可调谐器件、动态随机存储器、热释电红外探测器等方面。随着科技的发展,及大规模集成电路应用技术的发展和需要,BST铁电薄膜材料因具备优异的电学和光学性能,本应在应用领域中大展身手,却因为薄膜制备技术上仍然无法达到领域内所期望的高性能指标。这一定程度上阻碍了相关的科技的发展,因此,BST铁电材料的性能改善的研究工作有很高的研究价值和需要。
为了解决上述问题,使得钛酸锶钡材料能够得到更加广泛的应用,本文主要研究了以下内容并将研究结果简单介绍如下：
1.采用脉冲激光沉积的方法,在不同的衬底上制备了BST薄膜样品,并通过改善工艺,如退火温度,衬底选择等,研究了这些工艺条件对薄膜的质量和性能包括物性和电学性质的影响。
2.XRD图谱表明衍射峰的相对强度随着沉积温度的增加而增加,随着沉积温度的上升,薄膜的生长质量和结晶状况得到改善。而在铂金衬底和镍酸镧衬底上制备的薄膜样品均良好结晶,呈现出不同的取向。在LNO衬底和Pt衬底上沉积的BST薄膜的晶粒尺寸分别为70nm和50nm。使用与钛酸锶钡材料晶格结构类似的镍酸镧薄膜作为衬底,薄膜的晶粒尺寸有明显的增加。结果也表明,衬底的选择的确对薄膜的取向和结晶性能有很显著的影响。
3.AFM的测试结果表明,在LNO和Pt衬底基片上生长的钛酸锶钡薄膜均结晶良好,表面光滑,无针洞裂纹。在LNO衬底上生长的薄膜样品,具备更平滑的表面形貌,更大的晶粒尺寸,以及更佳的结晶状况。在激光沉积法制备BST薄膜的过程中,BST的微晶成核和动态生长的过程明显对衬底的性质存在依赖。
4.XPS测试表明采用脉冲激光沉积工艺所制备的BST薄膜中Ba、Sr、Ti、O元素都以钙钛矿结构中各元素的相应的化学态而存在,而AES测试的结果表明样品出现了c元素的沾污,这是由于空气中的碳元素的污染造成的。
5.对所制备样品进行了电学特性分析,包括CF特性,介电损耗特性,CV特性,PE曲线特性等。分析表明,薄膜样品的介电特性受到频率的影响明显,尤其在高频区域。而不同衬底的选择,会对这一特性有明显的影响。这和薄膜的微结构,以及薄膜和电极间的过渡层的出现有关。
6.本文中着重研究了BST薄膜的介电特性的不对称性。结果表明,介电特性的不对称性是薄膜本身的性质造成,而非加诸的电压导致。而上下电极的不对称性,会加重其不对称性,为改善这一点,BST薄膜的上下电极均采用铂金电极后,显著改善了不对称性(薄膜的介电不对称率由50.38%减小至17.86%)。
7.BST薄膜材料在微波器件的研制领域有着广阔的前景。在本文的电学测试中,也着重研究了其介电可调谐性。结果表明,薄膜样品的可调谐度随着频率的上升而一定程度上出现下降趋势。且可调谐度受到底电极的影响较为明显。在LNO底电极上沉积的薄膜表现出更优良的可调谐度,在10kHz和100kHz下分别达到45.3％和39.5%,而沉积在Pt底电极上的薄膜的可调谐度均在30%左右,且随频率变化不大。薄膜样品的可调谐度随频率的变化与之前薄膜样品的C-F特性曲线相吻合。薄膜的微结构和介电性能,直接影响了其可调谐性能。要制备出可调谐性良好的薄膜材料,需要对薄膜的生长工艺和薄膜结构进行改进。
8.对于钛酸锶钡薄膜的铁电性也进行了测试和研究。在8V的测试电压下,电场强度约为400kv／cm,此时LNO上的样品的剩余极化强度Pr和矫顽电场Ec分别是0.409μC／cm2和22.04 kV／cm,而Pt上沉积的样品的对应的Pr和Ec分别为0.393μC／cm2和12.09 kV／cm。在Pt缓冲层上沉积的薄膜显示了更大和更明显的电滞回线,而在LNO缓冲层上沉积的薄膜的电滞回线沿着电场v=0轴线有一定程度的偏移,这一结果与CV测试结果相吻合这可能是由于薄膜的微畴结构上的差异造成的。结果表明,晶粒的大小和底电极的选择,对铁电性有明显的影响。
Barium strontium titanate (BST) have recently been widely investigated as promising candidates for the applications in tunable microwave devices, dynamic random access memories (DRAMs) and Uncooled Focal Plane Arrays (UFPA), due to their high dielectric constant, low dielectric loss, low leakage current, smooth film surface, and the highly nonlinear dependence of the dielectric constant with the applied electric field, namely, electric tunabilty, etc. With the development of technology and the requirement of VLSI industry, Barium strontium titanate Ferroelectric thin film should be widely used as for its high performance in both electric and optical properties. However, the film fabrication technology didn't reach the requirement of VLSI industry, which limits the application of BST film. For that reason, research in BST film preparation and the improvement of BST film properties is needed.
To solve these problems, this paper has concentrated on several aspects and the main results and conclusions can be summarized as following:
1. Using Pulsed Laser Deposition technology, BST film samples have been successfully prepared on different substrates. By changing the deposition condition and film preparation process, for example, the annealing temperature, choosing of substrates, etc, the influence in film properties including physicality and electric properties are studied.
2. XRD spectra shows that the intensity of BST peaks are increased as the depositon temperature changed from 700℃to 750℃, so are the film quality and the crystallization. Films deposited on LNO and Pt substrate are both well crystallized and show different orientation. The grain size of each film is 70nm and 50nm. Results show that with the using of LNO buffer layer, which is of almost the same crystal structure with BST, film's grain size is increased and the choosing of different substrate strongly influence the the orientation and crystallization of BST thin films.
3. The microscopic surface morphology for BST thin films deposited on Pt and LNO bottom electrodes was studied by atomic force microscopy (AFM). The results show that the two kinds of BST films are well-crystallized, pinhole-free, and crack-free. BST film deposited on LNO has larger and more uniform grain size (60-70nm) and larger than that grown on Pt, whose grain size is 40-50nm. Additionally, The surface roughness of BST/Pt/Ti/Si(100) (RMS=10.218nm) is apparently larger than that of BST/LNO/Si(100)(RMS=4.301nm). In whole, films growing on LNO electrode have smoother surface, larger grain size and better crystallization. Consequently,The nucleation and growth dynamics of the BST film show obvious bottom electrode dependence.
4. XPS spectra shows that the Ba, Sr, Ti,O element in BST film fabricated by PLD method represent in a typical chemical state of pervoskite structure. And the result given by AES spectra shows that C element pollution, which is caused by air contact.
5. We tested the electric properties of the BST samples, including the C-F curves, dielectric loss properties, C-V curves, hysteresis loops, etc. Results show that the dielectric properties depend on frequency, especially on high frequency area. Differences in buffer layer also show strongly influence.
6. In our study, the asymmetry of films' dielectric property is studied carefully. Results show that the asymmetry is the result of film property, not the testing method. And the difference in top electrode and bottom electrode will increase the asymmetry. In order to improve the asymmetry, using same electrode (Pt electrode), improved the asymmetry property remarkably, as the rate of asymmetry decreased from 50.38% to 17.86%.
7. BST thin films show great potential in microwave applications. In this paper, the dielectric tunability is also studied. Results show that LNO buffered sample show better tunability, at 10 kHz and 100 kHz, the tunability is 45.3% and 39.5%. However, tunability of sample deposited on Pt sub is about 30% during different testing frequency. The result of tunability is consistent with the result of C-V testing. Micro structure and dielectric property directly influence the performance of tunability. In order to fabricate high tunability BST films, film preparation process should be improved.
8. We also tested the ferroelectric properties of these film samples. Results show that with an applied voltage of 8 V, the remanent polarization (2Pr) and the coercive electric field (Ec) changed from 0.409μC/cm2 and 22.04kV/cm to 0.393μC/cm2 and 12.09kV/cm, as the bottom electrode changes from LNO to Pt, respectively. Film deposited on Pt electrode exhibits larger and clearer hysteresis loops, and the hysteresis loops of film deposited on LNO buffer layer apparently show a shift along the electric field axis, which is consistent with the C-V measurement. This suggests that the domain structure is different in these films, the grain size and using of buffer layer may cause the result.
为了解决上述问题,使得钛酸锶钡材料能够得到更加广泛的应用,本文主要研究了以下内容并将研究结果简单介绍如下：
1.采用脉冲激光沉积的方法,在不同的衬底上制备了BST薄膜样品,并通过改善工艺,如退火温度,衬底选择等,研究了这些工艺条件对薄膜的质量和性能包括物性和电学性质的影响。
2.XRD图谱表明衍射峰的相对强度随着沉积温度的增加而增加,随着沉积温度的上升,薄膜的生长质量和结晶状况得到改善。而在铂金衬底和镍酸镧衬底上制备的薄膜样品均良好结晶,呈现出不同的取向。在LNO衬底和Pt衬底上沉积的BST薄膜的晶粒尺寸分别为70nm和50nm。使用与钛酸锶钡材料晶格结构类似的镍酸镧薄膜作为衬底,薄膜的晶粒尺寸有明显的增加。结果也表明,衬底的选择的确对薄膜的取向和结晶性能有很显著的影响。
3.AFM的测试结果表明,在LNO和Pt衬底基片上生长的钛酸锶钡薄膜均结晶良好,表面光滑,无针洞裂纹。在LNO衬底上生长的薄膜样品,具备更平滑的表面形貌,更大的晶粒尺寸,以及更佳的结晶状况。在激光沉积法制备BST薄膜的过程中,BST的微晶成核和动态生长的过程明显对衬底的性质存在依赖。
4.XPS测试表明采用脉冲激光沉积工艺所制备的BST薄膜中Ba、Sr、Ti、O元素都以钙钛矿结构中各元素的相应的化学态而存在,而AES测试的结果表明样品出现了c元素的沾污,这是由于空气中的碳元素的污染造成的。
5.对所制备样品进行了电学特性分析,包括CF特性,介电损耗特性,CV特性,PE曲线特性等。分析表明,薄膜样品的介电特性受到频率的影响明显,尤其在高频区域。而不同衬底的选择,会对这一特性有明显的影响。这和薄膜的微结构,以及薄膜和电极间的过渡层的出现有关。
6.本文中着重研究了BST薄膜的介电特性的不对称性。结果表明,介电特性的不对称性是薄膜本身的性质造成,而非加诸的电压导致。而上下电极的不对称性,会加重其不对称性,为改善这一点,BST薄膜的上下电极均采用铂金电极后,显著改善了不对称性(薄膜的介电不对称率由50.38%减小至17.86%)。
7.BST薄膜材料在微波器件的研制领域有着广阔的前景。在本文的电学测试中,也着重研究了其介电可调谐性。结果表明,薄膜样品的可调谐度随着频率的上升而一定程度上出现下降趋势。且可调谐度受到底电极的影响较为明显。在LNO底电极上沉积的薄膜表现出更优良的可调谐度,在10kHz和100kHz下分别达到45.3％和39.5%,而沉积在Pt底电极上的薄膜的可调谐度均在30%左右,且随频率变化不大。薄膜样品的可调谐度随频率的变化与之前薄膜样品的C-F特性曲线相吻合。薄膜的微结构和介电性能,直接影响了其可调谐性能。要制备出可调谐性良好的薄膜材料,需要对薄膜的生长工艺和薄膜结构进行改进。
8.对于钛酸锶钡薄膜的铁电性也进行了测试和研究。在8V的测试电压下,电场强度约为400kv／cm,此时LNO上的样品的剩余极化强度Pr和矫顽电场Ec分别是0.409μC／cm2和22.04 kV／cm,而Pt上沉积的样品的对应的Pr和Ec分别为0.393μC／cm2和12.09 kV／cm。在Pt缓冲层上沉积的薄膜显示了更大和更明显的电滞回线,而在LNO缓冲层上沉积的薄膜的电滞回线沿着电场v=0轴线有一定程度的偏移,这一结果与CV测试结果相吻合这可能是由于薄膜的微畴结构上的差异造成的。结果表明,晶粒的大小和底电极的选择,对铁电性有明显的影响。
Barium strontium titanate (BST) have recently been widely investigated as promising candidates for the applications in tunable microwave devices, dynamic random access memories (DRAMs) and Uncooled Focal Plane Arrays (UFPA), due to their high dielectric constant, low dielectric loss, low leakage current, smooth film surface, and the highly nonlinear dependence of the dielectric constant with the applied electric field, namely, electric tunabilty, etc. With the development of technology and the requirement of VLSI industry, Barium strontium titanate Ferroelectric thin film should be widely used as for its high performance in both electric and optical properties. However, the film fabrication technology didn't reach the requirement of VLSI industry, which limits the application of BST film. For that reason, research in BST film preparation and the improvement of BST film properties is needed.
To solve these problems, this paper has concentrated on several aspects and the main results and conclusions can be summarized as following:
1. Using Pulsed Laser Deposition technology, BST film samples have been successfully prepared on different substrates. By changing the deposition condition and film preparation process, for example, the annealing temperature, choosing of substrates, etc, the influence in film properties including physicality and electric properties are studied.
2. XRD spectra shows that the intensity of BST peaks are increased as the depositon temperature changed from 700℃to 750℃, so are the film quality and the crystallization. Films deposited on LNO and Pt substrate are both well crystallized and show different orientation. The grain size of each film is 70nm and 50nm. Results show that with the using of LNO buffer layer, which is of almost the same crystal structure with BST, film's grain size is increased and the choosing of different substrate strongly influence the the orientation and crystallization of BST thin films.
3. The microscopic surface morphology for BST thin films deposited on Pt and LNO bottom electrodes was studied by atomic force microscopy (AFM). The results show that the two kinds of BST films are well-crystallized, pinhole-free, and crack-free. BST film deposited on LNO has larger and more uniform grain size (60-70nm) and larger than that grown on Pt, whose grain size is 40-50nm. Additionally, The surface roughness of BST/Pt/Ti/Si(100) (RMS=10.218nm) is apparently larger than that of BST/LNO/Si(100)(RMS=4.301nm). In whole, films growing on LNO electrode have smoother surface, larger grain size and better crystallization. Consequently,The nucleation and growth dynamics of the BST film show obvious bottom electrode dependence.
4. XPS spectra shows that the Ba, Sr, Ti,O element in BST film fabricated by PLD method represent in a typical chemical state of pervoskite structure. And the result given by AES spectra shows that C element pollution, which is caused by air contact.
5. We tested the electric properties of the BST samples, including the C-F curves, dielectric loss properties, C-V curves, hysteresis loops, etc. Results show that the dielectric properties depend on frequency, especially on high frequency area. Differences in buffer layer also show strongly influence.
6. In our study, the asymmetry of films' dielectric property is studied carefully. Results show that the asymmetry is the result of film property, not the testing method. And the difference in top electrode and bottom electrode will increase the asymmetry. In order to improve the asymmetry, using same electrode (Pt electrode), improved the asymmetry property remarkably, as the rate of asymmetry decreased from 50.38% to 17.86%.
7. BST thin films show great potential in microwave applications. In this paper, the dielectric tunability is also studied. Results show that LNO buffered sample show better tunability, at 10 kHz and 100 kHz, the tunability is 45.3% and 39.5%. However, tunability of sample deposited on Pt sub is about 30% during different testing frequency. The result of tunability is consistent with the result of C-V testing. Micro structure and dielectric property directly influence the performance of tunability. In order to fabricate high tunability BST films, film preparation process should be improved.
8. We also tested the ferroelectric properties of these film samples. Results show that with an applied voltage of 8 V, the remanent polarization (2Pr) and the coercive electric field (Ec) changed from 0.409μC/cm2 and 22.04kV/cm to 0.393μC/cm2 and 12.09kV/cm, as the bottom electrode changes from LNO to Pt, respectively. Film deposited on Pt electrode exhibits larger and clearer hysteresis loops, and the hysteresis loops of film deposited on LNO buffer layer apparently show a shift along the electric field axis, which is consistent with the C-V measurement. This suggests that the domain structure is different in these films, the grain size and using of buffer layer may cause the result.
引文
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[3]曾汉民等, 《高技术新材料要览》,中国科学技术出版社,1993。
[4]符春林,杨传仁等,电子元件与材料；Vol.22 No.5 May 2003
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