Effect of thickness on electrical properties of bismuth ... - koasas

JOURNAL OF APPLIED PHYSICS 101, 084114 共2007兲

Effect of thickness on electrical properties of bismuth-magnesium niobate pyrochlore thin films deposited at low temperature Cheng-Ji Xian, Jong-Hyun Park, and Soon-Gil Yoona兲 School of Nano Science and Technology, Chungnam National University, Daeduk Science Town, Daejeon 305-764, Korea

Jin Seok Moon, Sung Taek Lim, Seung Hyun Sohn, Hyung Mi Jung, Yee-na Shin, and Woon Chun Kim Samsung Electro-Mechanics Co. Ltd., 314 Maetan3-Dong, Yeongtong-Gu, Suwon, Gyunggi-Do 442-743, Korea

Min-Ku Jeon and Seong-Ihl Woo Center for Ultramicrochemical Process Systems, Korea Advanced Institute of Science and Technology, 373-1 Kusong-dong, Yusong-gu, Daejeon 305-701, Korea

共Received 4 December 2006; accepted 26 January 2007; published online 30 April 2007兲 Bi2Mg2/3Nb4/3O7 共BMN兲 pyrochlore thin films were deposited at 25 and 100 ° C on Cu/ Ti/ Si substrates by pulsed laser deposition. Dielectric and leakage current properties of BMN films are investigated as a function of film thickness. The critical thicknesses showing the thickness dependence of dielectric constant are approximately 50 and 70 nm in BMN films deposited at 25 and 100 ° C, respectively. The capacitances of interfacial layers in the films deposited at 25 and 100 ° C are approximately 5.5 and 3.9 pF, respectively. The thickness dependence of leakage current characteristics was attributed to the copper diffusion into the BMN films. An intrinsic conduction of BMN films was controlled by Schottky emission and the barrier height was estimated as 0.9– 1.2 eV in the temperature range from 25 to 100 ° C. Film thickness in terms of leakage current characteristics is limited above 100 nm for embedded capacitor applications. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2715546兴 I. INTRODUCTION

As microsystems move towards higher speed and miniaturization, the requirement of electronic components and devices grows consistently. They should be fabricated in smaller size with maintaining and/or even improving the overall performance. The miniaturization, especially, becomes even more important today since more and more devices are required to be made portable. Embedding of capacitors into polymer-based printed circuit boards 共PCB兲 is becoming an important strategy for electronic system design and fabrication. Embedded components have advantages over surface mounted technology, including improved reliability due to the reduced number of solder contacts,1,2 reduced direct materials, and reduced device thickness and mass.3 The bismuth-based pyrochlores were investigated by Safronov et al.4 and Cann et al.5 Studies on bismuth-based dielectric thin films for its preparation, characterization, and properties were also performed by many research groups.6,7 In our recent work,8,9 dielectric properties and leakage current densities as well as structural properties in bismuthbased pyrochlore thin films were systematically investigated for embedded capacitor applications. In this study, Bi2Mg2/3Nb4/3O7 共BMN兲 thin films were deposited on Cu/ Ti/ Si substrates at 25 and 100 ° C by pulsed laser deposition. The structural and electrical propera兲

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ties of BMN films were investigated for various film thicknesses. Especially, effect of interfacial layer on dielectric properties and leakage current behaviors was systematically investigated. An intrinsic conduction mechanism was investigated for BMN films deposited on Pt/ TiO2 / Si substrates. II. EXPERIMENT

BMN thin films with various thicknesses were prepared by pulsed laser deposition 共PLD兲 using a pulsed KrF excimer laser 共248 nm, Lambda Physik COMPexPro 201兲. A ceramic of 1 in. diameter was used as a target. The base pressure of the chamber used was approximately 5.3⫻ 10−4 Pa. The laser density and repetition rate were 1.5 J / cm2 and 4 Hz, respectively. Oxygen was added into the chamber during deposition and the oxygen pressure was maintained at 30 mTorr. Thin films were deposited on Cu/ Ti/ SiO2 / Si substrates at temperatures of 25 and 100 ° C. 20-nm-thick Ti layers in Cu/ Ti/ SiO2 / Si substrates were used in order to improve the adhesion between Cu and SiO2. Film thicknesses were varied from 20 to 300 nm at deposition temperatures of 25 and 100 ° C. Surface morphologies and compositions of the films were characterized by atomic force microscopy 共AFM兲 and Rutherford backscattering spectroscopy 共RBS兲, respectively. The elemental distributions at interface between BMN and Cu bottom electrodes were characterized for various film thicknesses by secondary ion mass spectroscopy 共SIMS兲. For electrical measurement, Pt top electrodes with dimensions of 100⫻ 100 ␮m2 were exactly patterned by lift-off lithography and sputtered by dc magnetron sputtering. The dielectric

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FIG. 1. 共a兲 Variations in dielectric constant and dissipation factor as a function of film thickness and 共b兲 the relationship between ␧0S / C and film thickness in films deposited at 25 and 100 ° C.

properties of Pt/pyrochlore films/Cu metal-insulator-metal 共MIM兲 capacitors were evaluated as a function of film thickness by impedance analyzer 共HP 4194A兲. The leakage current characteristics of MIM capacitors were investigated by HP4145B semiconductor parameter analysis. III. RESULTS AND DISCUSSION

Figure 1共a兲 shows variations in dielectric constant and dissipation factor as a function of film thickness in films deposited at 25 and 100 ° C. The elemental compositions of the films deposited at 100 ° C using Bi6Mg2Nb4O21 ceramic target are confirmed as Bi, 6.2; Mg, 2.2; Nb, 4.0; and O, 15 by RBS, resulting in stoichiometric composition similar to target materials. As shown in Fig. 1共a兲, dielectric constant in the films deposited at 100 ° C maintains constant values of approximately 45 with decreasing film thickness up to 70 nm. On the other hand, below 70 nm thickness, dielectric constant was slightly reduced with decreasing film thickness and exhibits approximately 30 at thickness of 20 nm. Dielectric constant with film thickness in BMN films deposited at 25 ° C was varied with a similar trend to the films deposited at 100 ° C, as shown in Fig. 1共a兲. A slight decrease of dielectric constant below a critical thickness was attributed to the formation of a low dielectric constant layer at interface in crystallized films. However, BMN films deposited at low temperature showed partially crystallized morphologies with nanosized grains, as shown in previous works.10 It is important to ascertain the existence of a low dielectric constant layer even though BMN films deposited at low temperature do not show fully crystallized phases. Figure 1共b兲 shows the relationship between ␧0S / C and film thickness d in BMN films deposited at 25 and 100 ° C, where ␧0 was the permittivity of the free space, S was the dielectric area, and C was the measured capacitance. When a film has a uniform dielectric constant, the ␧0S / C vs d plot becomes linear and crosses through the origin. However, when a low dielectric constant layer exists in series with a high dielectric constant layer, the linear relationship is observed between the film thickness and ␧0S / C, and the normal extrapolation 共fitted line兲 of ␧0S / C from d = 300 nm to d = 0 nm gives a positive value of ␧0S / C at d = 0.11,12 As shown in Fig. 1共b兲, fitted lines of films deposited at 25 and 100 ° C exhibit the positive values of ␧0S / C at d = 0, resulting in the formation of a low dielectric constant layer at interface. The interfacial capacitances in films deposited at 25 and 100 ° C are approximately 5.5 and

J. Appl. Phys. 101, 084114 共2007兲

FIG. 2. Variations in leakage current density vs applied voltage as a function of thickness in the films deposited on the Cu electrode at 共a兲 25 ° C and 共b兲 100 ° C.

3.9 pF, respectively. Figures 2共a兲 and 2共b兲 show variations in leakage current density versus applied voltage as a function of thickness in films deposited at 25 and 100 ° C, respectively. As shown in Fig. 2, films below 100 nm thickness show an abrupt increase of leakage current densities with increasing applied voltage irrespective of films deposited at 25 and 100 ° C. Films above 100 nm thickness exhibit typical leakage current characteristics having stable breakdown strength and leakage current densities of approximately 10−8 A / cm2. Especially, as shown in Fig. 2共b兲, films above 200 nm thickness deposited at 100 ° C exhibit stable leakage current characteristics of 1 ⫻ 10−8 A / cm2 up to 10 V. Root mean square 共rms兲 roughness of the films deposited at 100 ° C in the range from 20 to 200 nm thickness was varied from 0.5 to 0.8 nm. To verify the origin for thickness dependence on leakage current density in bismuth-based pyrochlore films deposited at low temperature is important in order to improve the leakage properties in embedded capacitor applications using bismuth-based pyrochlore films. Figure 3 shows SIMS depth profiles of BMN films deposited at 100 ° C for various thicknesses. Depth profiles of BMN films with 30 nm 关Fig. 3共a兲兴 and 60 nm 关Fig. 3共b兲兴 thicknesses show a strong pileup of Mg and Nb elementals at interface between BMN and Cu bottom electrode. From SIMS profiles, Mg and Nb elementals piled up at interface

FIG. 3. SIMS depth profiles of BMN films with 共a兲 30 nm, 共b兲 60 nm, 共c兲 100 nm, and 共d兲 200 nm thicknesses deposited at 100 ° C.


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FIG. 4. 共a兲 log J vs E1/2 and 共b兲 log J / E vs E1/2 plots of 200-nm-thick BMN films deposited at 25 ° C.

are considered to exist as metal state rather than as oxide state. On the other hand, as shown in Figs. 3共c兲 and 3共d兲, magnesium and niobium elementals show a homogeneous distribution within the films with 100 and 200 nm thicknesses, respectively. The pileup of Mg and Nb elementals in an interface of ultrathin BMN films was found irrespective of the bottom electrodes such as Cu, Pt, and Ni. This result was considered to be related to the peculiarity of an analysis by SIMS in BMN ultrathin films and should be studied in detail. As shown in Fig. 3, diffusion of copper into the BMN films was found in all the films with different thicknesses. The portion of the diffusion length by copper diffused into the BMN films increases with decreasing film thickness and as a result, leakage current density of the films increases with decreasing film thickness. Films above 100 nm thickness showed typical leakage characteristics having lower leakage current density because the BMN is thicker than the diffusion length of copper. This result suggests that films deposited on the copper bottom electrode are limited above 100 nm in terms of leakage current characteristics in embedded capacitor applications. To identify the conduction mechanism of BMN films, log 共leakage current density兲 versus 共electric field兲1/2 of 200-nm-thick BMN films deposited on Pt/ TiO2 / SiO2 / Si substrates at room temperature was plotted, as shown in Fig. 4. When the leakage current density of BMN films was plotted as a function of electric field, it is observed that they follow either Poole-Frenkel or Schottky emission. The difference between two mechanisms is that the former is a film bulk-controlled process but the latter is interface controlled.13 If the conduction in BMN films is governed by Schottky emission at a constant temperature, log J 共leakage current density兲 versus E1/2 共electric field兲 shows a linear relationship. On the other hand, if the conduction is controlled by Poole-Frenkel emission, log J / E 共leakage current density/electric field兲 versus E1/2 共electric field兲 has a linear relationship. Figures 4共a兲 and 4共b兲 show relationships between log J vs E1/2 and log J / E vs E1/2, respectively. As shown in Fig. 4共a兲, leakage current density of the films linearly increases with increasing electric field. However, as shown in Fig. 4共b兲, log J / E does not show a linear relationship with the electric field. As a result, conduction of BMN films was governed by Schottky emission. Because Schottky emission was controlled by interface between the electrode and BMN films, Schottky barrier height can be controlled by

J. Appl. Phys. 101, 084114 共2007兲

FIG. 5. 共a兲 The leakage current density 共J兲 vs applied voltage as a function of temperature and 共b兲 log 共J / T2兲 vs 1 / T at the various applied voltages.

the interface states of the films. The Schottky barrier height at a reverse biased junction can be estimated using the following equation:14 ln关⌬共J/T2兲兴 = q⌽B/K关⌬共1/T兲兴,

共1兲

where ⌽B is Schottky barrier height. Figure 5共b兲 shows the leakage current density 共J / T2兲 vs 1 / T as a function of applied voltage. Figure 5共b兲 was derived from Fig. 5共a兲 for applied voltages between 1.0 and 4.0 V. From the slope of J / T2 vs 1 / T plot, ⌽B was estimated as 0.9– 1.2 eV in the temperature region from 25 to 100 ° C. From the conduction mechanism of the BMN films, leakage properties of the films are mainly controlled by an interface between BMN and electrodes. In order to ascertain the origin that 50-nm-thick BMN films deposited on the Cu electrode exhibit a continuous increase of leakage current density with increasing applied voltage 关see Figs. 2共a兲 and 2共b兲兴, the platinum bottom electrode was used instead of the Cu electrode and the films deposited on the Pt bottom electrode were characterized for leakage current behavior. The relationship between leakage current density and an applied field in Pt/ BMN共50 nm兲 / Pt/ TiO2 / SiO2 / Si and Pt/ BMN共50 nm兲 / Cu/ Ti/ SiO2 / Si capacitors was shown in Fig. 6. Compared with a continuous increase of leakage current density with increasing applied field in the films of 50 nm thickness deposited on the Cu electrodes, films deposited on the Pt bottom electrode exhibited the leakage characteristics observed in typical dielectrics and showed leakage current density of three orders of mag-

FIG. 6. Leakage current density as a function of applied field in Pt/ BMN共50 nm兲 / Pt and Pt/ BMN共50 nm兲 / Cu capacitors.


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characteristics was related to the diffusion length of copper diffused into the BMN films. Film thickness in terms of leakage current characteristics is limited above 100 nm for embedded capacitor applications. The electrical conduction of BMN films was controlled by Schottky emission and the Schottky barrier heights were approximately 0.9– 1.2 eV. ACKNOWLEDGMENTS FIG. 7. SIMS depth profiles of 50-nm-thick BMN films deposited on 共a兲 Pt and 共b兲 Cu bottom electrodes.

nitude lower than films on the Cu the electrode. The improved leakage current characteristics in 50-nm-thick films deposited on Pt were clearly identified by an elemental distribution shown in Fig. 7. As shown in Fig. 7共a兲, films deposited on the Pt bottom electrode show stable interface characteristics in BMN/Pt structure. On the other hand, in BMN/Cu structure shown in Fig. 7共b兲, copper was diffused into the BMN films and increased the diffusion length of copper compared to bulk films, resulting in higher leakage current characteristics, as shown in Fig. 6. However, 200-nm-thick BMN films deposited on the Cu bottom electrode exhibited improved leakage current characteristics compared with films deposited on the Pt bottom electrode.15 IV. CONCLUSIONS

Dielectric and leakage current properties of BMN films deposited at 25 and 100 ° C are investigated as a function of film thickness. The critical thicknesses showing the thickness dependence of dielectric constant are approximately 50 and 70 nm in BMN films deposited at 25 and 100 ° C, respectively. The capacitances of interfacial layers in the films deposited at 25 and 100 ° C are approximately 5.5 and 3.9 pF, respectively. The thickness dependence of leakage current

This work was financially supported by Research Foundation of the Samsung-Electro Mechanics Co. Ltd., by the Center for Ultramicrochemical Process Systems sponsored by KOSEF, by the Brain Korea 21 Project in 2006, and by the Korea Science and Engineering Foundation through the Research Center for Advanced Magnetic Materials at Chungnam National University. 1

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