Growth of High-Dielectric-Constant TiO 2 Films in Capacitors with RuO 2 Electrodes K. Fröhlich, a, * ,z M. Ťapajna, a A. Rosová, a E. Dobročka, a K. Hušeková, a J. Aarik, b and A. Aidla b a Institute of Electrical Engineering, Centre of Excellence CENG, Slovak Academy of Sciences, Dúbravská cesta 9, 841 04 Bratislava, Slovakia b Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia Titanium dioxide thin films were grown on RuO 2 layers by atomic layer deposition. The stabilizing effect of the bottom rutile-type RuO 2 layer resulted in growth of the TiO 2 rutile films at temperatures above 275°C. Stabilization of the TiO 2 rutile phase occurred due to local epitaxial growth of the polycrystalline RuO 2 /TiO 2 /RuO 2 structure, as revealed by transmission electron microscopy. A dielectric constant as high as 155 and equivalent oxide thickness EOT as low as 0.5 nm were determined from the capacitance–voltage measurements for the TiO 2 films grown above 275°C. A leakage current density of 10 -3 A/cm 2 at 1 V bias voltage was obtained for the films with EOT equal to 0.5 nm. © 2008 The Electrochemical Society. DOI: 10.1149/1.2898184 All rights reserved. Manuscript submitted January 31, 2008; revised manuscript received February 25, 2008. Available electronically March 21, 2008. Titanium dioxide is an attractive dielectric material for micro- electronic applications. Depending on its growth conditions, TiO 2 can be most easily prepared in amorphous, anatase, and/or rutile phases. The rutile phase exhibits very high dielectric constant, rang- ing from 90 to 170, depending on the lattice orientation. 1 Due to the high dielectric constant, the TiO 2 rutile phase is considered a prom- ising material for capacitors in future generations of dynamic ran- dom access memories DRAMs. 2 Unfortunately, rutile often coexists in thin films with lower di- electric constant TiO 2 phase, i.e., anatase, thereby resulting in a reduction of effective dielectric constant. Postdeposition annealing at temperatures above 800°C has to be performed to obtain pure rutile phase films. 3-5 However, it was shown that the growth of phase-pure rutile films at low temperatures can be stabilized by choice of an appropriate substrate. For instance, by using atomic layer deposition ALD and TiCl 4 and H 2 O as precursors, pure rutile films have been grown on 1–102-oriented sapphire substrates at 425°C. 6 Recently, Kim, et al. demonstrated ALD growth of TiO 2 rutile film at even lower temperatures on a Ru substrate pretreated by O 3 . 7 This pretreatment of the Ru electrode resulted in a thin surface film of RuO 2 with the structure compatible to that of the TiO 2 rutile phase. Using this approach, TiO 2 films with a dielectric constant of 100 were prepared at temperature as low as 250°C. 8 In a DRAM capacitor dielectric film should be combined with conductive electrodes. As the affinity of TiO 2 is about 4 eV, 9 metals with high work function should be used as electrodes to prevent excessive leakage currents due to Schottky emission. We have re- cently demonstrated that the work function of ruthenium oxide is more than 5 eV. 10 RuO 2 has high conductivity and crystallizes in the rutile structure with the lattice parameters a RuO 2 = 0.4499 nm, c RuO 2 = 0.3107 nm close to that of the TiO 2 rutile phase a TiO 2 = 0.4593 nm, c TiO 2 = 0.2959 nm. Therefore, RuO 2 is a promising material for electrodes in capacitors with TiO 2 dielectric. In our work we have extended the approach of Kim et al. 7 to use RuO 2 as a seed layer for the TiO 2 rutile phase growth. Instead of oxidizing the Ru surface to obtain RuO 2 , we have used bottom polycrystalline RuO 2 electrode grown by metallorganic chemical vapor deposition MOCVD. Consequently, TiO 2 rutile films with very high permit- tivity were obtained on top of the MOCVD-grown RuO 2 electrode at temperatures above 275°C. Thin RuO 2 films were deposited by MOCVD in a low-pressure, hotwall quartz reactor operated at a pressure of 2 Torr. Precursor bis2,2,6,6-tetramethyl-3,5-heptanedionato1,5-cyclooctadiene ru- thenium was dissolved in iso-octane concentration 0.035 M and injected into the evaporation chamber using electromagnet micro- valve. The injector was opened for 3 ms with a frequency of 0.33 Hz. Oxygen was used as a reactant gas with flow rate of 170 sccm, while argon was used as a carrier gas with a flow rate of 21 sccm. The films were grown at a deposition temperature 290°C on Si100 substrates covered by a 100 nm SiO 2 layer or on a top of RuO 2 /TiO 2 bilayer. The TiO 2 films were grown by ALD in a flow-type reactor 11 at temperatures from 150 up to 600°C. In order to synthesize the films, the substrates were exposed to the TiCl 4 vapor for 2 s, purged in the flow of pure nitrogen for 2 s, exposed to the H 2 O vapor for 2 s, and again purged in the flow of pure nitrogen for 5 s. The ALD cycle was repeated 200–1500 times to obtain 10–80 nm thick TiO 2 films. TiO 2 films were deposited in the same run on 10  10 mm 2 pieces of bare Si100 wafers, as well as on the Si substrates covered by polycrystalline RuO 2 films. Crystallographic phases were identified by X-ray diffraction XRD on Bruker AXS-D8 Discover equipment in grazing incidence  1.5° mode using Cu K radiation. The microstructure was studied by transmission electron microscopy TEM on a JEOL JEM 1200 EX microscope with 120 kV accelerating voltage after specimen preparation using mechanical grinding, polishing, and ion milling. The thickness of the TiO 2 films was determined by X-ray reflectivity on samples grown on Si substrate. Capacitance–voltage and current– voltage characteristics were measured using an Agilent 4284A LCR meter and Keithley 2400 Source Meter, respectively, on samples with a top RuO 2 electrode defined by optical lithography and pat- terned by ion milling. After patterning the structures received short annealing 20 min at 300°C in flowing oxygen. Figure 1 shows XRD patterns of TiO 2 films grown on bare Si bottom and on the Si with a RuO 2 layer top at temperatures from 150 to 600°C. TiO 2 films grown at 150°C were amorphous with traces of anatase phase. Increase of TiO 2 deposition temperatures to 275°C resulted in growth of anatase phase on Si substrate, while the films grown on the RuO 2 layer contained only TiO 2 rutile phase. After further increase of the deposition temperature to 425°C traces of the rutile phase emerged also in the TiO 2 films deposited on the Si substrate. The amount of the rutile phase became more important in the TiO 2 films grown at 600°C on the bare silicon. The TiO 2 films grown at 425°C on RuO 2 contained pure rutile phase. Although the seed RuO 2 layer was transformed to Ru during the growth of TiO 2 at 600°C, the top TiO 2 films contained only the rutile phase. Com- parison of the TiO 2 phase composition of the films grown on silicon and RuO 2 clearly indicates the stabilization of the rutile phase growth by the bottom RuO 2 seed layer. Cross-sectional TEM imagery reveals columnar growth of the whole RuO 2 /TiO 2 /RuO 2 structure, left upper part of Fig. 2. Grain * Electrochemical Society Active Member. z E-mail: karol.frohlich@savba.sk Electrochemical and Solid-State Letters, 11 6 G19-G21 2008 1099-0062/2008/116/G19/3/$23.00 © The Electrochemical Society G19