基于BST的多层薄膜及其可调器件的研究
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摘要
当代无线电技术与微电子技术朝着小型化、集成化方向发展。铁电材料因其具有介电性、压电性、热释电、铁电性等重要特性,越来越受到研究者的重视。Ba_xSr_(1-x)TiO_3具有高介电常数、较低的损耗等良好的性能,成为研究的热点之一。顺电相下BST材料具有介电常数随外加偏压而改变的特性,且在微波范围仍具有很大的介电常数,这使得BaxSr1-xTiO3材料成为移相器、滤波器、变容器等微波射频器件潜在的功能材料,具有很好的应用前景。但是在实际应用和研究中还存在着一些问题:如与硅基半导体工艺的集成,介电可调性能与损耗性能存在着矛盾。
MgO具有和BST相近的晶格参数,并且具有很好的微波特性和稳定性,已经被广泛应用于微波领域。本文通过在硅衬底上射频磁控溅射MgO,并对其工艺进行优化,以此作为缓冲层生长Ba_(0.5)Sr_(0.5)TiO_3。本文加工了插指式电容,并利用HP4192测量其介电性能,同时对比了LNO/MgO/BST复合缓冲层结构。实验研究表明:MgO缓冲层不仅改善了Ba_(0.5)Sr_(0.5)TiO_3的结晶性能,而且缓冲层的插入提高了Ba_(0.5)Sr_(0.5)TiO_3的介电可调性,可调率从原来的14.7%(1MHz)增加到61.5%,而且介电损耗从原来的0.15减小到0.05左右。同时在前期研究基础上,制备了LNO/MgO/BST多层复合薄膜结构,复合缓冲层结构具有更高的可调性,但同时损耗也较大,这可能与介面层的低性能有关。在此基础上,利用微细加工技术制备了共面波导结构,借助网络分析仪Agilent 8722ES S-parameter Network Analyzer测试了共面波导的传输性能。
With the trend of wireless communication and microelectronics technology becoming smaller and integrated, the ferroelectric material has received much more attention for its dielectric piezoelectricity, pyro-electricity and Ferro-electricity. Ba_xSr_(1-x)TiO_3 has aroused great interest with its high dielectrics and low dielectric loss under high frequency. The BaxSr1-xTiO3 at paraelectric phase has dielectric tunability under biased electrical field and comparatively high permittivity under microwave band, which make BaxSr1-xTiO3 a promising functional material for tunable microwave devices, such as phase shifter, voltage tunable filter etc. However, there still exist some problems in utilization and research: the integration of BST with silicon semiconductor fabrication and the conflict between high tunability and low dielectric loss.
MgO has excellent microwave properties and stability, which has been widely utilized in RF and microwave area. Also, the parameter of crystallization of MgO is close to that of BST. In this study, the Ba_(0.5)Sr_(0.5)TiO_3 was deposited on Si with MgO and LNO/MgO as the buffer layer which were deposited by rf. Magnetron sputtering. Then the interdigital capacitors have been produced by photolithographic techniques.
The structural and electric properties of the multi-layer films were investigated. It has been shown that the MgO buffer layer not only enhanced the crystallization of Ba_(0.5)Sr_(0.5)TiO_3, but also improved the tunability from 14.7% (1MHz) to 61.5%, and the dielectric loss was decreased from 0.15 to around 0.05. The LNO/MgO/BST composite films had higher tunability with comparatively higher dielectric loss, which depended on the better crystallization of Ba_(0.5)Sr_(0.5)TiO_3 and the bad interface of composite films. The CPW structure has been produced with micro-fabrication technology, and the property of CPW was tested by network analyzer Agilent 8722ES S-parameter Network Analyzer.
MgO具有和BST相近的晶格参数,并且具有很好的微波特性和稳定性,已经被广泛应用于微波领域。本文通过在硅衬底上射频磁控溅射MgO,并对其工艺进行优化,以此作为缓冲层生长Ba_(0.5)Sr_(0.5)TiO_3。本文加工了插指式电容,并利用HP4192测量其介电性能,同时对比了LNO/MgO/BST复合缓冲层结构。实验研究表明:MgO缓冲层不仅改善了Ba_(0.5)Sr_(0.5)TiO_3的结晶性能,而且缓冲层的插入提高了Ba_(0.5)Sr_(0.5)TiO_3的介电可调性,可调率从原来的14.7%(1MHz)增加到61.5%,而且介电损耗从原来的0.15减小到0.05左右。同时在前期研究基础上,制备了LNO/MgO/BST多层复合薄膜结构,复合缓冲层结构具有更高的可调性,但同时损耗也较大,这可能与介面层的低性能有关。在此基础上,利用微细加工技术制备了共面波导结构,借助网络分析仪Agilent 8722ES S-parameter Network Analyzer测试了共面波导的传输性能。
With the trend of wireless communication and microelectronics technology becoming smaller and integrated, the ferroelectric material has received much more attention for its dielectric piezoelectricity, pyro-electricity and Ferro-electricity. Ba_xSr_(1-x)TiO_3 has aroused great interest with its high dielectrics and low dielectric loss under high frequency. The BaxSr1-xTiO3 at paraelectric phase has dielectric tunability under biased electrical field and comparatively high permittivity under microwave band, which make BaxSr1-xTiO3 a promising functional material for tunable microwave devices, such as phase shifter, voltage tunable filter etc. However, there still exist some problems in utilization and research: the integration of BST with silicon semiconductor fabrication and the conflict between high tunability and low dielectric loss.
MgO has excellent microwave properties and stability, which has been widely utilized in RF and microwave area. Also, the parameter of crystallization of MgO is close to that of BST. In this study, the Ba_(0.5)Sr_(0.5)TiO_3 was deposited on Si with MgO and LNO/MgO as the buffer layer which were deposited by rf. Magnetron sputtering. Then the interdigital capacitors have been produced by photolithographic techniques.
The structural and electric properties of the multi-layer films were investigated. It has been shown that the MgO buffer layer not only enhanced the crystallization of Ba_(0.5)Sr_(0.5)TiO_3, but also improved the tunability from 14.7% (1MHz) to 61.5%, and the dielectric loss was decreased from 0.15 to around 0.05. The LNO/MgO/BST composite films had higher tunability with comparatively higher dielectric loss, which depended on the better crystallization of Ba_(0.5)Sr_(0.5)TiO_3 and the bad interface of composite films. The CPW structure has been produced with micro-fabrication technology, and the property of CPW was tested by network analyzer Agilent 8722ES S-parameter Network Analyzer.
引文
[1]符春林.铁电薄膜材料及其应用.北京:科学出版社, 2009
[2]殷之文.电介质物理学(第二版).北京:科学出版社,2003
[3]张福学,王丽坤.现代压电学(中册).北京:科学出版社,2002
[4]符春林,杨传仁,陈宏伟等.钛酸锶钡(BST)薄膜的组成、结构与性能研究进展.真空科学与技术,2003,23(4):255~263
[5] R.Waser, J.Korean. Phys.Soc.32(1998)Spp,1340
[6]王永龄.功能陶瓷性能与应用.北京:科学出版社,2003
[7]蔡毅,胡旭.红外成像寻的红外探测器现状与发展趋势.红外与激光工程,2006,35(1):7~11
[8]陈实,张海波,姜胜林等.热释电非制冷红外焦平面现状及发展趋势.红外与激光工程,2006,35(4):419~423
[9] Cho H J, Kang C S, Hwang C S et al. Structure and electrical properties of Ba_(0.5)Sr_(0.5)TiO_3 thin films on Ir and IrO2 electrodes. Jpn. J. Appl. Phys. 1997,36(7A):874~876
[10] Kawakubo T, Komatsu S, Abe K et al. Ferroelectric properties of SrRuO3/BST/ SrRuO3 epotaxial capacitor. Jpn.J. Appl. Phys.1998,37(9B):5108~5111
[11]王东,杨传仁,陈宏伟等.钛酸锶钡微波移相器研究现状.电讯技术,2003,43(3): 68~76
[12]余慧春,徐爱兰,惠春. BST薄膜铁电移相器研究进展.电子元件与材料,2005,24(3):59~62
[13] Fu C, Pan F, Chen H et al. Coplanar phase shifters based on ferroelectric thin films. Int J. Infrared Mill. 2007, 28(3): 229~235
[14] O. Auciello, S. Saha, D.Y. Kaufman, Science and Technology of High Dielectric Constant Thin Films and Materials Integration for Application to High Frequency Devices, Journal of Electroceramics, 2004,12, 126~127
[15] K.H. Yoon, J.C. Lee, J. Park, D.H. Kang, C.M. Song, Y.G. Seo, Japan Journal Applied Physics, 2001, 40, 5497.
[16] S.Y. Chen, H.W. Wang, L.C. Huang, Japan Journal Applied Physics, 2001, 40, 4974.
[17] Chun-Sheng Liang, Electrical properties of W-doped (Ba_(0.5)Sr_(0.5))TiO_3 thin films, Journal of Crystal Growth, 2005, 274, 173~177
[18] Kyoung-Tae Kim, The effect of Cr doping on the microstructural and dielectric properties of (Ba_(0.6)Sr_(0.4))TiO_3 thin films, Thin Solid Films,2005, 472,26~30
[19] J.K. Kim, S.S. Kim, W.J. Kim et al. Improved ferroelectric properties of Cr-doped Ba_(0.7)Sr_(0.3)TiO_3 thin films prepared by wet chemical deposition, Materials Letters, 2006, 60, 2324
[20] Tonglai Chen, Xiaomin Li, W.B.Wu et al. Electrical conduction transition and largely reduced leakage current in aluminum-doped barium strontium titanate thin films heteroepitaixally grown on Ir/MgO/Si(100), Appl. Phys. Lett. 86 (2005) 132902.
[21] Improving the dielectric losses of (Ba,Sr)TiO_3 thin films using a SiO_2 buffer layer. Ceram. Int. 2004,30:1085~1087
[22] Kim W J, Chang W, Qadri S B et al. Microwave properties of tetragonally distorted (Ba_(0.5)Sr_(0.5))TiO_3 thin films. Appl. Phys. Lett. 2000, 76(9): 1185~1187
[23] Joo J H, Jeon Y C, Seon J M et al. Effects of post-annealing on the conduction properties of Pt/(Ba,Sr)TiO_3/Pt capacitors for dynamic random access memory applications. Jpn. J. Appl. Phys. 1997, 36(7A):4382~4385
[24] Cao L.Z., Meng Q.D., Fu W.Y. et al.Effect of annealing on the crystal structure and dielectric properties of Ba_(0.6)Sr_(0.4)TiO_3 thick films. Physica B 393 (2007) 175~178
[25] Liu Y.R., Lai P.T. , Li G.Q. et al. Effects of post-deposition oxygen annealing on tuning properties of Ba_(0.8)Sr_(0.2)TiO_3 thin-film capacitors for microwave integrated circuits. Materials Chemistry and Physics 94 (2005) 114~118
[1] Young-Hoon Chun, Jia-Sheng Hong, et al. BST Varactor Tuned Bandstop Filter with Slotted Ground Structure. Microwave Symposium Digest, 2008 IEEE MTT-S International. 2005: 1115~1118
[2] Hein M.A., Hirst P.J. et al. Nonlinear Microwave Properties of Superconducting YBaCuO and Dielectric MgO. 33rd European Microwave Conference - Munich 2003:1303~1306
[3] S. Y. Lee and T. Y. Tseng. Electrical and dielectric behavior of MgO doped Ba_(0.7)Sr_(0.3)TiO_3 thin films on Al_2O_3 substrate. Appl. Phys. Lett. 2002, 80: 1797
[4] Tae-Soon Yun, Hyun-Suk Kim et al. New tunable microwave Ba_(0.6)Sr_(0.4)TiO_3/MgO phase shifters using interdigital capacitors and slit-inductors. Jpn. J. Appl. Phys. 2007,46:5245~5248
[5] R.Pinto, J.I.Poothra et al. J.Vac.Sci.Technol.A. 1991,9: 2670~2674
[6] Prasama, Srinivasan et al. Jounal of Microelectromechanical Systems, 2008, 17:653~667
[1]胡耀志,黄光周,于继荣.机电产品微细加工技术与工艺.广州,广东科技出版社,1993.
[2]詹娟,唐国洪,张佐兰.大规模集成电路工艺原理.南京:东南大学出版社,1989.
[3]史西蒙著,史常忻等译.超大规模集成电路工艺学.上海:上海交通大学出版社,1987.
[4] Jack W Judy, Microelectromechanical systems(MEMS): fabricaition, design and applications, smart materials and structures, 10(2001) 1115~1134
[5] Orphee Cugat, Jerome Delamare, and Gilbert Reyne, Mannetic Micro-Actuators and systems(MAGMAS), IEEE Transactions on magnetics, 2003, vol.39, No.5
[6] D.Niarchos, Magnetic MEMS: key issues and some applications, Sensors and Actuators, 2003, 106:255~262
[7]殷之文.电介质物理学(第二版).北京:科学出版社,2003
[8] Tagantsev A.K., Sherman V.O. el al. Ferroelectric materials for microwave tunable applications. Journal of Electroceramics. 2003, 11: 39~41
[9]符春林.铁电薄膜材料及其应用.北京:科学出版社, 2009
[10] Vorobiev A., Rundqvist P. et al. Silicion integreated high Q-factor parallel-plate ferroelectric varactors for microwave/millimeterwave qpplications. 2003, 83:3144~3146
[11] Zhang Tianjin, Wang Jinzhao et al. Bottom electrodes dependence of microstructures and dielectric properties of compositionally graded (Ba1-xSrx)TiO3 thin films. 2008, 43: 700~706.
[12] Gevorgian, S.S., Martinsson, T. et al. CAD models for multilayered substrate interdigital capacitors. IEEE Transactions on microwave theory and techniques. 1996, 44:896~904
[13] Sun X.F., Ding G.F. et al. Design and fabrication of a new micromachines interdigital coplanar waveguide. Microelectronics Journal. 2006, 37: 1379~1383
[14]孟庆瑞,孙立功等.宽带双周期性BST电容加载共面波导移相器. 2007,46:38~40
[15] Chun Young-Hoon, Hong J.S. et al. An electronically tuned bandstop filter using BST varactors. Proceedings of the 38th Eruropean microwave conference. 2008, 1699~1702.
[16] Stengel M. and Spaldin N.A. Origin of the dielectric dead layer in nanoscale capacitors. Nature Letters. 2006, 443:679~682
[17] G. Catalan, D.O’Neill. et al. Relaxor features in ferroelectric superlattices: a Maxwell-Wagner approach. Applied physics Letters. 2000, 77:3078~3080
[18] O’Neill D., Bowman R.M. et al. Dielectric enhancement and Maxwell-Wagner effects in ferroelectric superlattice structures. Applied physics Letters. 2000, 77:1520~1522
[19] Shen M.R., Ge S.B. et al. Dielectric enhancement and Maxwell-Wagner effects in polycrystalline ferroelectric multilayered thin films. Journal of Physics D: Applied Phys. 2001, 34:2935~2938
[20] Zhang X.Y.,Song Q. et al. Dielectric dispersion of Ba_xSr_(1-x)TiO_3 thin film with parallel-plate and coplanar interdigital electrodes. Journal of Physics D: Applied Phys. 2009, 42: 065411
[21] Lee S. J. Kang K.Y et al. Low-frequency dielectric relaxation of BaTiO_3 thin-film capacitors. Applied Physics Letters. 1999, 75:1784~1786
[1] C.P. Wen,“Coplanar Waveguide: A surface Strip Transmission Line Suitable for Nonreciprocal Gyromagnetic Device Applications,”IEEE Trans. Microwave Theory Tech., Vol.17, No.12, pp.1087-1090,Dec.1969
[2] Spartak Gevorgian, L. J. Peter Linner CAD Models for Shielded Multilayered CPW, IEEE Transactions on Microwave Theory and Techniques, VOL. 43, No. 4, 77
[2]殷之文.电介质物理学(第二版).北京:科学出版社,2003
[3]张福学,王丽坤.现代压电学(中册).北京:科学出版社,2002
[4]符春林,杨传仁,陈宏伟等.钛酸锶钡(BST)薄膜的组成、结构与性能研究进展.真空科学与技术,2003,23(4):255~263
[5] R.Waser, J.Korean. Phys.Soc.32(1998)Spp,1340
[6]王永龄.功能陶瓷性能与应用.北京:科学出版社,2003
[7]蔡毅,胡旭.红外成像寻的红外探测器现状与发展趋势.红外与激光工程,2006,35(1):7~11
[8]陈实,张海波,姜胜林等.热释电非制冷红外焦平面现状及发展趋势.红外与激光工程,2006,35(4):419~423
[9] Cho H J, Kang C S, Hwang C S et al. Structure and electrical properties of Ba_(0.5)Sr_(0.5)TiO_3 thin films on Ir and IrO2 electrodes. Jpn. J. Appl. Phys. 1997,36(7A):874~876
[10] Kawakubo T, Komatsu S, Abe K et al. Ferroelectric properties of SrRuO3/BST/ SrRuO3 epotaxial capacitor. Jpn.J. Appl. Phys.1998,37(9B):5108~5111
[11]王东,杨传仁,陈宏伟等.钛酸锶钡微波移相器研究现状.电讯技术,2003,43(3): 68~76
[12]余慧春,徐爱兰,惠春. BST薄膜铁电移相器研究进展.电子元件与材料,2005,24(3):59~62
[13] Fu C, Pan F, Chen H et al. Coplanar phase shifters based on ferroelectric thin films. Int J. Infrared Mill. 2007, 28(3): 229~235
[14] O. Auciello, S. Saha, D.Y. Kaufman, Science and Technology of High Dielectric Constant Thin Films and Materials Integration for Application to High Frequency Devices, Journal of Electroceramics, 2004,12, 126~127
[15] K.H. Yoon, J.C. Lee, J. Park, D.H. Kang, C.M. Song, Y.G. Seo, Japan Journal Applied Physics, 2001, 40, 5497.
[16] S.Y. Chen, H.W. Wang, L.C. Huang, Japan Journal Applied Physics, 2001, 40, 4974.
[17] Chun-Sheng Liang, Electrical properties of W-doped (Ba_(0.5)Sr_(0.5))TiO_3 thin films, Journal of Crystal Growth, 2005, 274, 173~177
[18] Kyoung-Tae Kim, The effect of Cr doping on the microstructural and dielectric properties of (Ba_(0.6)Sr_(0.4))TiO_3 thin films, Thin Solid Films,2005, 472,26~30
[19] J.K. Kim, S.S. Kim, W.J. Kim et al. Improved ferroelectric properties of Cr-doped Ba_(0.7)Sr_(0.3)TiO_3 thin films prepared by wet chemical deposition, Materials Letters, 2006, 60, 2324
[20] Tonglai Chen, Xiaomin Li, W.B.Wu et al. Electrical conduction transition and largely reduced leakage current in aluminum-doped barium strontium titanate thin films heteroepitaixally grown on Ir/MgO/Si(100), Appl. Phys. Lett. 86 (2005) 132902.
[21] Improving the dielectric losses of (Ba,Sr)TiO_3 thin films using a SiO_2 buffer layer. Ceram. Int. 2004,30:1085~1087
[22] Kim W J, Chang W, Qadri S B et al. Microwave properties of tetragonally distorted (Ba_(0.5)Sr_(0.5))TiO_3 thin films. Appl. Phys. Lett. 2000, 76(9): 1185~1187
[23] Joo J H, Jeon Y C, Seon J M et al. Effects of post-annealing on the conduction properties of Pt/(Ba,Sr)TiO_3/Pt capacitors for dynamic random access memory applications. Jpn. J. Appl. Phys. 1997, 36(7A):4382~4385
[24] Cao L.Z., Meng Q.D., Fu W.Y. et al.Effect of annealing on the crystal structure and dielectric properties of Ba_(0.6)Sr_(0.4)TiO_3 thick films. Physica B 393 (2007) 175~178
[25] Liu Y.R., Lai P.T. , Li G.Q. et al. Effects of post-deposition oxygen annealing on tuning properties of Ba_(0.8)Sr_(0.2)TiO_3 thin-film capacitors for microwave integrated circuits. Materials Chemistry and Physics 94 (2005) 114~118
[1] Young-Hoon Chun, Jia-Sheng Hong, et al. BST Varactor Tuned Bandstop Filter with Slotted Ground Structure. Microwave Symposium Digest, 2008 IEEE MTT-S International. 2005: 1115~1118
[2] Hein M.A., Hirst P.J. et al. Nonlinear Microwave Properties of Superconducting YBaCuO and Dielectric MgO. 33rd European Microwave Conference - Munich 2003:1303~1306
[3] S. Y. Lee and T. Y. Tseng. Electrical and dielectric behavior of MgO doped Ba_(0.7)Sr_(0.3)TiO_3 thin films on Al_2O_3 substrate. Appl. Phys. Lett. 2002, 80: 1797
[4] Tae-Soon Yun, Hyun-Suk Kim et al. New tunable microwave Ba_(0.6)Sr_(0.4)TiO_3/MgO phase shifters using interdigital capacitors and slit-inductors. Jpn. J. Appl. Phys. 2007,46:5245~5248
[5] R.Pinto, J.I.Poothra et al. J.Vac.Sci.Technol.A. 1991,9: 2670~2674
[6] Prasama, Srinivasan et al. Jounal of Microelectromechanical Systems, 2008, 17:653~667
[1]胡耀志,黄光周,于继荣.机电产品微细加工技术与工艺.广州,广东科技出版社,1993.
[2]詹娟,唐国洪,张佐兰.大规模集成电路工艺原理.南京:东南大学出版社,1989.
[3]史西蒙著,史常忻等译.超大规模集成电路工艺学.上海:上海交通大学出版社,1987.
[4] Jack W Judy, Microelectromechanical systems(MEMS): fabricaition, design and applications, smart materials and structures, 10(2001) 1115~1134
[5] Orphee Cugat, Jerome Delamare, and Gilbert Reyne, Mannetic Micro-Actuators and systems(MAGMAS), IEEE Transactions on magnetics, 2003, vol.39, No.5
[6] D.Niarchos, Magnetic MEMS: key issues and some applications, Sensors and Actuators, 2003, 106:255~262
[7]殷之文.电介质物理学(第二版).北京:科学出版社,2003
[8] Tagantsev A.K., Sherman V.O. el al. Ferroelectric materials for microwave tunable applications. Journal of Electroceramics. 2003, 11: 39~41
[9]符春林.铁电薄膜材料及其应用.北京:科学出版社, 2009
[10] Vorobiev A., Rundqvist P. et al. Silicion integreated high Q-factor parallel-plate ferroelectric varactors for microwave/millimeterwave qpplications. 2003, 83:3144~3146
[11] Zhang Tianjin, Wang Jinzhao et al. Bottom electrodes dependence of microstructures and dielectric properties of compositionally graded (Ba1-xSrx)TiO3 thin films. 2008, 43: 700~706.
[12] Gevorgian, S.S., Martinsson, T. et al. CAD models for multilayered substrate interdigital capacitors. IEEE Transactions on microwave theory and techniques. 1996, 44:896~904
[13] Sun X.F., Ding G.F. et al. Design and fabrication of a new micromachines interdigital coplanar waveguide. Microelectronics Journal. 2006, 37: 1379~1383
[14]孟庆瑞,孙立功等.宽带双周期性BST电容加载共面波导移相器. 2007,46:38~40
[15] Chun Young-Hoon, Hong J.S. et al. An electronically tuned bandstop filter using BST varactors. Proceedings of the 38th Eruropean microwave conference. 2008, 1699~1702.
[16] Stengel M. and Spaldin N.A. Origin of the dielectric dead layer in nanoscale capacitors. Nature Letters. 2006, 443:679~682
[17] G. Catalan, D.O’Neill. et al. Relaxor features in ferroelectric superlattices: a Maxwell-Wagner approach. Applied physics Letters. 2000, 77:3078~3080
[18] O’Neill D., Bowman R.M. et al. Dielectric enhancement and Maxwell-Wagner effects in ferroelectric superlattice structures. Applied physics Letters. 2000, 77:1520~1522
[19] Shen M.R., Ge S.B. et al. Dielectric enhancement and Maxwell-Wagner effects in polycrystalline ferroelectric multilayered thin films. Journal of Physics D: Applied Phys. 2001, 34:2935~2938
[20] Zhang X.Y.,Song Q. et al. Dielectric dispersion of Ba_xSr_(1-x)TiO_3 thin film with parallel-plate and coplanar interdigital electrodes. Journal of Physics D: Applied Phys. 2009, 42: 065411
[21] Lee S. J. Kang K.Y et al. Low-frequency dielectric relaxation of BaTiO_3 thin-film capacitors. Applied Physics Letters. 1999, 75:1784~1786
[1] C.P. Wen,“Coplanar Waveguide: A surface Strip Transmission Line Suitable for Nonreciprocal Gyromagnetic Device Applications,”IEEE Trans. Microwave Theory Tech., Vol.17, No.12, pp.1087-1090,Dec.1969
[2] Spartak Gevorgian, L. J. Peter Linner CAD Models for Shielded Multilayered CPW, IEEE Transactions on Microwave Theory and Techniques, VOL. 43, No. 4, 77