The properties of barium titanate (BaTiO3, BT), such as high dielectric constant and resistivity, allow it to find numerous applications in the field of microelectronics. In this work silicon metal-insulator-semiconductor field effect transistor (MISFET) structures with BaTiO3 thin films (containing La2O3 admixture) acting as gate insulator were investigated. The films were produced by means of radio frequency plasma sputtering (RF PS) of sintered BaTiO3 + La2O3 (2% wt.) target. In the paper transfer and output I−V, transconductance and output conductance characteristics of the obtained transistors are presented and discussed. Basic parameters of these devices, such as threshold voltage (VTH) are determined and discussed

Firek, Piotr

Szmidt, Jan

Biblioteka Cyfrowa Instytutu Łączności / National Institute of Telecomunications: Digital Library

Paper Technology of MISFETwith SiO2/BaTiO3 System as a Gate InsulatorPiotr Firek and Jan SzmidtAbstract— The properties of barium titanate (BaTiO3 , BT),such as high dielectric constant and resistivity, allow it to findnumerous applications in the field of microelectronics. In thiswork silicon metal-insulator-semiconductor field effect transis-tor (MISFET) structures with BaTiO3 thin films (containingLa2O3 admixture) acting as gate insulator were investigated.The films were produced by means of radio frequency plasmasputtering (RF PS) of sintered BaTiO3 + La2O3 (2% wt.) tar-get. In the paper transfer and output I−V , transconductanceand output conductance characteristics of the obtained tran-sistors are presented and discussed. Basic parameters of thesedevices, such as threshold voltage (VT H ) are determined anddiscussed.Keywords— barium titanate, I–V characteristics, MISFETstructures, radio frequency plasma sputtering.1. IntroductionBarium titanate (BaTiO3, BT) ceramics have been exten-sively used in the field of electronic applications. Mul-tilayer ceramic capacitors (MLCCs) [1], [2], embeddedcapacitances in printed circuit boards [2], optical waveg-uides [3], electrooptic modulators [4], micromechanical [5]and humidity sensor [6] devices, positive temperature co-efficient of resistivity thermistors [7], gas sensors [8] werefabricated using BT. In all those applications BaTiO3 wasused in the form of either bulk material or thick layer.BT shows ferroelectric and piezoelectric properties as wellas a high dielectric constant that make it a promisingmaterial for potential applications in dynamic access ran-dom memories (DRAM) [9], [10] or non-volatile memories(NVM) [9], [11].Thin barium titanate films for microelectronic applicationsare usually either amorphous or polycrystalline and havesignificantly worse electrical properties than bulk or thick-film material. It is difficult, for example, to obtain uni-form composition, the piezoelectric effect is weaker, andthe values of the dielectric constant are lower (typicallyless then 50) [12]. On the other hand, its dielectric con-stant is usually still much higher than that of silicon dioxidealthough thin BT layers are typically plagued with higherleakage current and lower dielectric strength.2. Experimental DetailsThe fabrication process of metal-insulator-semiconductorfield effect transistor (MISFET) structures is presentedFig. 1. MISFET fabrication process with cross-sectional view ofthe structures.61Piotr Firek and Jan Szmidtin Fig. 1. Its first step is thermal oxidation in order to obtainfield oxide of about 440 nm. The p-type silicon < 100 >oriented substrate with the resistivity of 6− 8 Ωcm wasused. After cleaning processes 40 nm thick SiO2 film wasgrown thermally and then a thin (approximately 80 nm) bar-ium titanite film was deposited by means of radio frequencyplasma sputtering (RF PS) of sintered BaTiO3 + La2O3(2% wt.) target.A schematic diagram of the RF PS setup is shown in Fig. 2.The BaTiO3 layer was deposited as a result of 30 minlong process (280 V self-bias voltage, argon flow rate ofFig. 2. Schematic diagram of the setup for radio frequencyplasma sputtering deposition processes.Fig. 3. Silicon wafer and MISFET topography.10 ml/min and 15 mm distance between the Si substrateand the sputtered target). Next, a photoresist mask foretching in a buffer solution of hydrofluoric acid was pre-pared by means of photolithography. As a last step contactsfor metallization were opened and aluminum was evapo-rated. The described fabrication process is presented inFig. 2. Silicon wafer and transistor topography are shownin Fig. 3.3. Results and DiscusionsThe dielectric constant (k) of about 20 was extractedfrom capacitance-voltage measurements of a MIS structureFig. 4. Transfer current-voltage characteristics of the fabricatedstructures.Fig. 5. Output current-voltage characteristics of the fabricatedstructures.containing BaTiO3 dielectric. The current-voltage (I−V )characteristics of MISFETs were measured with KeithleySMU 236/237/238. The obtained transfer and output char-acteristics are presented in Figs. 4 and 5.62Technology of MISFET with SiO2/BaTiO3 System as a Gate InsulatorThreshold voltage (UT ) is one of the most important pa-rameters of a transistor since it represents the gate voltageat which the MISFET channel is turned on. The thresholdvoltage ranged from −6 V to −8 V.The transconductance gm and output conductance of thestructures are presented in Figs. 6 and 7, respectively.Fig. 6. Transconductance characteristics of the fabricated struc-tures.Fig. 7. Output conductance characteristics of the fabricated struc-tures.It can be seen that the values of transconductance are rel-atively low when compared to a typical silicon MOSFET.Taking into consideration that these structures contain ma-terial (BaTiO3) that is relatively novel for this type of ap-plications and demanding from the technological point ofview, the obtained results in our opinion are very satisfy-ing. Postprocessing (e.g., annealing) or better purity of thefilms should improve the results significantly.The dispersion of the obtained parameters (e.g., thresh-old voltage, trans- and output conductance) may be causedmainly by the variations of BT thickness and its composi-tion in the area under the gate. The structure of the layer,i.e., the grain size or the presence of amorphous phase,the quality of the interface between the BT layer and SiO2and the influence of plasma during fabrication may leadto different levels of the effective charge in the dielectricand at the aforementioned interface. As a consequence, thetransistors show different values of flat-band voltage andthreshold voltage.4. ConclusionsThe obtained BT films show good adhesion to SiO2 layerson silicon substrate. Their relatively low dielectric con-stant (k) (for BT) is due to their amorphous nature. Highvalues of the threshold voltage are a consequence of chargepresence at the SiO2/BaTiO3 interface. A better controlof the deposition process (e.g., purity) may significantlyimprove the film properties. Our investigations confirmthat the RF PS method is suitable for obtaining BT layersthat may exhibit several very interesting electronic prop-erties, especially for future IS (ion sensitive) FET struc-tures.References[1] J. Zhen, Z. Yue, G. Yousong, W. Sen, L. Lingfeng, X. Zhigang,and W. Yanbin, “Non-reducible BaTiO3-based dielectric ceramicsfor Ni-MLCC synthesized by soft chemical method”, Ceram. Int.,vol. 32, no. 4, pp. 447–450, 2006.[2] A. Rae, M. Chu, and V. Ganine, “Barium titanate-past, present andfuture”, Ceram. Trans., vol. 100, pp. 1–12, 1999.[3] D.-G. Sun, Z. Liu, Y. Huang, S.-T. Ho, D. J. Towner, andB. W. Wessels, “Performance simulation for ferroelectric thin-filmbased waveguide electro-optic modulators”, Opt. Commun., vol. 255,no. 4–6, pp. 319–330, 2005.[4] P. Tang, D. J. Towner, T. Hamano, A. L. Meier, and B. W. Wessels,“Electrooptic modulation up to 40 GHz in a barium titanate thin filmwaveguide modulator”, Opt. Expr., vol. 12, no. 24, pp. 5962–5967,2004.[5] D. L. Polla and L. F. Francis, “Ferroelectric thin films in micro-electromechanical systems applications”, MRS Bull., vol. 21, no. 7,pp. 59–65, 1996.[6] V. M. Fuenzalida, M. E. Pilleux, and I. Eisele, “Adsorbed water onhydrothermal BaTiO3 films: work function measurements”, Vacuum,vol. 55, no. 1, pp. 81–83, 1999.[7] L. Aﬄeck and C. Leach, “Microstructures of BaTiO3 based PTCthermistors with Ca, Sr and Pb additions”, J. Eur. Ceram. Soc.,vol. 25, no. 12, pp. 3017–3020, 2005.[8] M. Kumar, S. Rani, M. C. Bhatnagar, and S. C. Roy, “Struc-ture, ferroelectric and gas sensing properties of sol-gel derived(Ba, Sr)(Ti, Zr)O3 thin films”, Mater. Chem. Phys., vol. 107, no. 2–3,pp. 399–403, 2008.[9] J. F. Scott, “Device physics of ferroelectric thin-film memories”,Jap. J. Appl. Phys., vol. 38, no. 4B, pp. 2272–2274, 1999.[10] R. Ramesh, S. Aggarwal, and O. Auciello, “Science and technol-ogy of ferroelectric films and heterostructures for non-volatile fer-roelectric memories”, Mater. Sci. Eng., vol. 32, no. 6, pp. 191–236,2001.[11] T. Kuroiwa et al., “Dielectric properties of (BaxSr1−x)TiO3 thin filmsprepared by RF sputtering for dynamic random access memory ap-plication”, Jap. J. Appl. Phys., vol. 33, no. 9B, pp. 5187–5191,1994.63Piotr Firek and Jan Szmidt[12] R. Thomas, D. C. Dube, M. N. Kamalasanan, and N. Deepak Kumar,“Electrical properties of sol-gel processed amorphous BaTiO3 thinfilms”, J. Sol-Gel Sci. Technol., vol. 16, no. 1–2, pp. 101–107, 1999.Piotr Firek was born in RawaMazowiecka, Poland, in 1977.He received the M.Sc. degree inmicroelectronics from the Fac-ulty of Electronics and Informa-tion Technology, Warsaw Uni-versity of Technology (WUT),Poland, in 2004, where he iscurrently finishing a Ph.D. the-sis. His research concentrateson fabrication, characterization,processing and application of thin and thick film materials(e.g., barium titanate, boron nitride, DLC, diamond) in mi-croelectronic devices.e-mail: pfirek@elka.pw.edu.plInstitute of Microelectronics and OptoelectronicsWarsaw University of TechnologyKoszykowa st 7500-662 Warsaw, PolandJan Szmidt received the M.Sc.degree in electronics from theFaculty of Electronics, War-saw University of Technology(WUT), Poland, in 1976. Fromthe same university, he receivedthe Ph.D. and D.Sc. degrees in1984 and 1995, respectively. In1999, he became an AssociateProfessor. In 2002 he becamean Associate Dean for Develop-ment of the Faculty of Electronics and Information Tech-nology, Warsaw University of Technology. Since 2006 hehas been the Head of Electronic Materials and Microsys-tem Technology Division, Institute of Microelectronics andOptoelectronics, WUT and since 2008 – the Dean of theFaculty of Electronics and Information Technology WUT.His research interests concentrate on technology and char-acterization of thin films for electronics, especially for mi-croelectronics and nanoelectronics, as well as on their ap-plication in microelectronic and nanoelectronic structures.e-mail: j.szmidt@elka.pw.edu.plInstitute of Microelectronics and OptoelectronicsWarsaw University of TechnologyKoszykowa st 7500-662 Warsaw, Poland64

Technology of MISFET with SiO2/BaTiO3 System as a Gate Insulator, Journal of Telecommunications and Information Technology, 2009, nr 4

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Technology of MISFET with SiO2/BaTiO3 System as a Gate Insulator, Journal of Telecommunications and Information Technology, 2009, nr 4

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