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37, NO. 2, FEBRUARY 2016. Room Temperature Fabrication of MIMCAPs via Aerosol Deposition. C. Wang, Member, IEEE, H. J. Kim, F. Y. Meng, Member, IEEE, ...
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IEEE ELECTRON DEVICE LETTERS, VOL. 37, NO. 2, FEBRUARY 2016

Room Temperature Fabrication of MIMCAPs via Aerosol Deposition C. Wang, Member, IEEE, H. J. Kim, F. Y. Meng, Member, IEEE, H. K. Kim, Y. Li, Student Member, IEEE, Z. Yao, Student Member, IEEE, and N. Y. Kim, Member, IEEE

Abstract— We report the microwave dielectric properties of BaTiO3 thin films fabricated via aerosol deposition, which is capable of applying functional ceramic films to Si-based semiconductor fabrication at room temperature. As the starting powder, BaTiO3 powder with an average particle size of 300 nm afforded a dense thin film with a thickness of 500 nm as well as a smooth interface with the Pt/Ti/SiO2 /Si substrate. In comparison, the interface roughness of BaTiO3 thin film was degraded (>100 nm) when BaTiO3 powder with an average particle size of 450 nm was used as the starting powder, owing to the excessive impact energy during film growth. Metal–insulator–metal capacitors were realized on a Si wafer via electron beam evaporation and inductively coupled plasma etching in order to determine the relative permittivity, loss tangent, and current–voltage characteristics. The average values of the relative dielectric permittivity and loss tangent of the BaTiO3 thin film were 78 and 0.03, respectively, in the frequency range of 1–6 GHz. Index Terms— Aerosol deposition, BaTiO3 , microwave dielectric properties, inductively coupled plasma etching.

I. I NTRODUCTION ITH the rapid development of portable electronic devices such as smartphones, which offer multifunction and miniaturization, integration technologies for components have emerged as a key issue in the electronics industry. In particular, high-frequency applications have witnessed significant growth owing to the increasing demand for wireless communications. These factors have led to numerous studies on integration technologies such as multi-chip modules [1], system-in-package [2], and system-on-package [3]. Realizing integration technologies essentially requires embedded thin-film technology [4] for passive components such as R, L, and C, which occupy more than 60% of the circuit board area. As representative materials for passive components, ceramics have superior dielectric and thermal properties.

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Manuscript received November 24, 2015; accepted December 3, 2015. Date of publication December 7, 2015; date of current version January 25, 2016. This work was supported in part by the Research Grant in 2016 from Kwangwoon University and in part by the National Research Foundation of Korea within the Ministry of Science, ICT and Future Planning through the Korean Government under Grant 2011-0030079, Grant 2014R1A1A1005901, and Grant 2015R1D1A1A09057081. The review of this letter was arranged by Editor C. P. Yue. C. Wang, Y. Li, Z. Yao, and N. Y. Kim are with the Department of Electronic Engineering, Kwangwoon University, Seoul 139-701, Korea (e-mail: [email protected]). H. J. Kim and H. K. Kim are with the Department of Electronic Materials Engineering, Kwangwoon University, Seoul 139-701, Korea. F. Y. Meng is with the Department of Microwave Engineering, Harbin Institute of Technology, Harbin 150-001, China. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2015.2506406

However, their application to embedded thin-film components is limited owing to their high sintering temperature (>1000 °C). Therefore, most embedded ceramic films have been investigated as ceramic-polymer composites [5], [6]. Aerosol deposition (AD) [7]–[9], which is capable of fabricating dense ceramic films at room temperature, has been widely investigated as a promising embedded ceramic thin-film technology for microelectromechanical systems that employ PZT-based piezoelectric materials [10], Al2 O3 -based integrated substrates [11], BaTiO3 -based embedded decoupling capacitors [12], and so on. In particular, the application of BaTiO3 thin films to embedded decoupling capacitors could effectively suppress electromagnetic interference and simultaneous switching noise in high-frequency applications. Furthermore, BaTiO3 thin films can be applied to high-κ dielectric layers in complementary metal-oxide semiconductors. Several deposition methods have been adopted for fabricating BaTiO3 thin films, such as the sol-gel method [13], molecular beam epitaxy [14], and RF sputtering [15]. However, these methods entail high processing costs because they require high-vacuum systems. In addition, they have limited potential for mass production owing to their low deposition rate. In comparison, AD entails relatively low costs because it involves low-vacuum systems. Moreover, it allows for the fabrication of BaTiO3 films with high deposition rates (>100 nm/min). BaTiO3 films grown via AD show superior dielectric properties despite room-temperature deposition [8]. Furthermore, there have been reports on the enhancement of their dielectric properties with heat treatment [16]. However, most existing studies have investigated dielectric properties in the sub-MHz region; microwave dielectric properties have not been reported thus far. In this study, BaTiO3 thin films are grown via AD and their microwave dielectric properties are investigated. In addition, micropatterning of BaTiO3 thin films is carried out to fabricate metal-insulator-metal capacitors (MIMCAPs) via inductively coupled plasma (ICP) etching. II. E XPERIMENT A. Room-Temperature Growth of BaTiO3 Thin Films via AD BaTiO3 thin films were grown on Pt/Ti/SiO2 /Si substrates via AD at room temperature. Crystalline BaTiO3 powders (average particle size: 300 nm and 450 nm) were used as starting powders to compare the deposition efficiency and uniformity of film growth. Fig. 1(a) shows the general apparatus

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WANG et al.: ROOM TEMPERATURE FABRICATION OF MIMCAPs VIA AEROSOL DEPOSITION

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Fig. 1. Fabrication of MIM microcapacitors via AD and ICP etching. (a) AD apparatus for room-temperature growth of BaTiO3 thin films. (b) TEM analysis of BaTiO3 thin film grown at room temperature with selected area diffraction patterns (inset). (c) Using Joint Committee on Powder Diffraction Standards card 00-031-0174, the BaTiO3 structure can be identified by XRD patterns. (d) Schematic of micropatterning procedure for MIMCAPs.

for AD, which can be referred to elsewhere [7]. For detailed analysis, the BaTiO3 thin films were subjected to transmission electron microscopy (TEM) and X-ray diffraction (XRD). As can be seen in Fig. 1(b), TEM showed highly crystalline BaTiO3 nanostructures synthesized during the AD process, and the fabricated 0.5-μm-thick BaTiO3 thin film subsequently acquired a crystalline structure. As shown in Fig. 1(c), XRD confirmed that the BaTiO3 film has a cubic crystal system with a single perovskite phase due to its small crystallite size and non-uniform distortion during impaction in the AD process, which decreases the permittivity of BaTiO3 [7]. The average synthesized BaTiO3 nanostructure, calculated as 11.3 nm based on the Scherrer equation, was obtained using the full-width at half-maximum intensity of the (110) diffraction peak [8]. B. Analysis of BaTiO3 Thin Films In order to confirm the dielectric properties of the BaTiO3 films, MIMCAPs were fabricated by conventional integrated passive device process, as shown in Fig. 1(d) [17]. The crosssectional microstructures of the BaTiO3 films and powders were observed via scanning electron microscope (SEM). The dielectric properties of the BaTiO3 thin film were measured from 1–6 GHz with a vector network analyzer (VNA, 85225GE09, Agilent) using the fabricated MIMCAP. In addition, the current-voltage characteristics were analyzed using a current-voltage meter (4200-SCS/F, Keithley). III. R ESULTS AND D ISCUSSION A. Effect of BaTiO3 Particle Size on Deposition Rate and Interface Roughness In order to obtain optimum AD conditions for growing the BaTiO3 thin film, the deposition rate under different flow rates of N2 carrier gas was investigated using BaTiO3 starting powder with average particle sizes of 300 nm and 450 nm, as shown in Fig. 2(a) and (b), respectively. Fig. 2(c)-(e) and (f)-(h) show cross-sectional SEM images of BaTiO3 films deposited on Pt/Ti/SiO2 /Si substrate under

Fig. 2. Cross-sectional SEM images of BaTiO3 thin films deposited on the Pt/Ti/SiO2 /Si substrate under various flow rates of carrier gas. BaTiO3 starting powders with mean a size of (a) 300 nm and (b) 450 nm for the AD process. BaTiO3 thin films using 300-nm starting powder deposited at a flow rate of (c) 6 slm, (d) 8 slm, and (e) 10 slm. BaTiO3 thin films using 450-nm starting powder deposited at a flow rate of (f) 6 slm, (g) 8 slm, and (h) 10 slm. (i) Deposition rate of BaTiO3 thin films and etching rate of Pt layer for 300-nm and 450-nm starting powders under various flow rates carrier gas.

different flow rates of N2 carrier gas. The initial thickness of the Pt/Ti layer of the substrate was 130 nm, which confirmed that the Pt layer was damaged and etched during the deposition of BaTiO3 thin films owing to collisions with the BaTiO3 particles, as shown in Fig. 2(c) and (f). In the case of BaTiO3 powder with an average particle size of 450 nm, BaTiO3 thin film with a thickness of ∼100 nm was grown at a flow rate of 6 slm, but the interface roughness was relatively high compared to the thickness of film (see Fig. 2(f)). Furthermore, the deposition rate decreased as the flow rate of N2 carrier gas increased beyond 8 slm because the deposited BaTiO3 layer was etched owing to the excessive impact energy of the BaTiO3 particles (see Fig. 2(i)). Interface roughness is caused by the impact of particles during deposition, which forms a mechanically strong bonding layer (“anchoring layer”). The typical range of interface roughness is 100-200 nm for ceramic thick films deposited via AD [7], which is too high for thin-film applications. Moreover, a rough interface can also increase the leakage current of thin films [18]. On the other hand, in the case of BaTiO3 powder with an average particle size of 300 nm, the BaTiO3 thin film exhibited a comparably smooth interface with the substrate, as shown in Fig. 2(c)-(e). However, it was confirmed that the BaTiO3 particles were not fully crushed and remained in the BaTiO3 film deposited at a flow rate of 6 slm (see Fig. 2(c)). This implies that a flow rate of 6 slm was insufficient for forming a dense BaTiO3 film with the 300-nm BaTiO3 . In comparison, the BaTiO3 film deposited at a flow rate of 8 slm exhibited a dense microstructure with a higher deposition rate (see Fig. 2(d)).

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IEEE ELECTRON DEVICE LETTERS, VOL. 37, NO. 2, FEBRUARY 2016

Fig. 3. (a) SEM images and cross section (inset) of BaTiO3 MIMCAP. (b) The fitting result of contact resistance of BaTiO3 MIMCAP. (c) Microwave dielectric properties of BaTiO3 thin film with correction of parasitic resistance from contact resistance. The frequency dependence of the dissipation factor (tanδ) was remarkably reduced after correction of the contact resistance. (d) Current-voltage (I-V) characteristics of BaTiO3 films of various thicknesses.

Fig. 2(i) shows the deposition rate of the BaTiO3 films and the etched thickness of the Pt layer as a function of the flow rate of N2 carrier gas. The etched thickness of the Pt layer showed a tendency to increase with the flow rate of N2 carrier gas, while the particle size of the BaTiO3 powder increased owing to the increase in the impact energy. The BaTiO3 thin film exhibited the highest deposition rate at a flow late of 8 slm with a smooth interface when 300-nm BaTiO3 powder was used. Fig. 3(a) shows the MIMCAP fabricated on an Si wafer; the thickness of the BaTiO3 dielectric layer was ∼500 nm. Both the bottom metal and top electrodes are 0.5-μm thick with Ti of 50 nm and Au of 450 nm deposited by e-beam evaporation. B. Microwave Dielectric Properties of BaTiO3 Thin Film The S-parameters of the MIMCAP were obtained via two-port VNA measurement at room temperature. They were subsequently converted into impedances as follows: 1 + S11 = R − jX (1) 1 − S11 where Z0 is the characteristic impedance (50 ), S11 is the reflective scattering parameter measured by the VNA, and R and X are the real and imaginary parts of the impedance, respectively. In general, the equivalent circuit of the capacitor is considered as the parallel resistance of the capacitance and the intrinsic resistance of the dielectric film. In addition, the contact resistance (Rc ) between the electrodes and the microprobe should be considered as a series resistance with the equivalent circuit of the capacitor. Therefore, the dissipation factor (tanδ) of the dielectric film can be expressed as follows: Z = Z0

tanδ =

R − Rc X

(2)

1 × tan δ + Rc (3) ωC where C is the capacitance of the dielectric film. Further, Rc is the same as R when ω is infinite, as shown in Eq. (3). R = X × tan δ + Rc = −

Fig. 4. AFM top images of the surface morphology of AD deposited BaTiO3 thin film with different thickness, (a) 300 nm, (b) 400 nm, and (c) 500 nm. (a-i) to (c-i) are 3D view of the fabricated surface roughness. (a-ii) to (c-ii) show the cross-section line profiles, acquired from (a) to (c) indicated by the red lines.

R at ω = ∞ was obtained via extrapolation by employing the linear fitting of the measured S11 data, as shown in Fig. 3(b). The fitting frequency range was selected over 1–6 GHz owing to the large noise and frequency dependence of the dielectric losses caused by instrument sensitivity and other parasitic effects. Therefore, tanδ can be calculated from Eq. (2), and the relative permittivity, εr , can be calculated from the capacitance of the MIMCAP. Fig. 3(c) shows the dielectric properties of the BaTiO3 thin film with a thickness of 500 nm obtained from the dielectric measurements of the MIMCAP in the frequency range of 1–6 GHz. The contact resistance was calculated by the extrapolation of the total resistance (Fig. 3(b)) and then was subtracted from the raw loss tangent of BaTiO3 film by Eq. (1)-(3). The relative permittivity and dissipation factor were constant at the measured frequency, with average values of 78 and 0.03, respectively. The dielectric properties of BaTiO3 in the microwave range investigated in this study were similar to those of BaTiO3 thick films in MHz regions as reported in our previous studies [3], [8]. This implies that BaTiO3 films grown via AD have constant εr and tanδ in MHz to GHz regions without frequency dependence. Fig. 3(d) shows the I-V characteristics of the BaTiO3 films with various thicknesses. The leakage current shows a tendency to increase as the thickness of the films decreases owing to the rough interface, which can lead to concentration of the electric field [19], [20]. In the case of the BaTiO3 film with a thickness of 500 nm, the leakage current density was 3.3×10−5A/cm2 at 100 kV/cm. Figure 4 shows atomic focus microscopy (AFM) images of 2D views, 3D views and surface line profiles of deposited BaTiO3 thin films by AD method. At the very beginning of the AD process, an anchoring layer should be deposited by high deposition speed to increase the adhesion between substrate and BaTiO3 thin films [21]. In the anchoring layer, the starting powders cannot fully crash, therefore, the roughness of this anchoring layer is not very smooth. The root mean square (RMS) of the BaTiO3 thin film with a thickness of 300 nm is 65.5 nm, as shown in Fig. 4(a). As the films thickness increased to 400 nm and 500 nm, the RMS reduced

WANG et al.: ROOM TEMPERATURE FABRICATION OF MIMCAPs VIA AEROSOL DEPOSITION

to 52.0 nm (Fig. 4(b)) and 33.5 nm (Fig. 4(c)), respectively. The selected line profile indicated that the starting powders crushed into a smaller size and led to a smoother surface compared with the anchoring layer, which corresponds to the I-V characteristics, as shown in Fig 3. (d). In our previous research, the crystallite sizes decreased as the BaTiO3 films grew thicker resulted in the dense BaTiO3 films with higher hardness, which also leading to a decrease of leakage current [22]. We believe that aerosol-deposited high-κ dielectric thin films could be applied in various fields of microelectronics. Further investigation is necessary to decrease interface roughness and enhance electrical properties such as the insulating property. IV. C ONCLUSION In this study, the microwave dielectric properties of BaTiO3 thin films fabricated via AD were investigated in the frequency range of 1–6 GHz. In order to fabricate the thin BaTiO3 films, crystalline BaTiO3 powders with average particle sizes of 300 nm and 450 nm were used; the former afforded smoother interfaces substrate as well as higher deposition rates. The average relative permittivity and loss tangent of the BaTiO3 thin film with a thickness of 500 nm were 78 and 0.03, respectively. Furthermore, the leakage current density of the BaTiO3 film with a thickness of 500 nm was 3.3×10−5A/cm2 at 100 kV/cm. R EFERENCES [1] R. Laroussi and G. I. Costache, “Finite-element method applied to EMC problems (PCB environment),” IEEE Trans. Elect. Comp., vol. 35, no. 2, pp. 178–184, May 1993. DOI: 10.1109/15.229423 [2] F. Roozeboom, A. L. A. M. Kemmeren, J. F. C. Verhoeven, F. C. van den Heuvel, J. Klootwijk, H. Kretschman, T. Friˇc, E. C. E. van Grunsven, S. Bardy, C. Bunel, D. Chevrie, F. LeCornec, S. Ledain, F. Murray, and P. Philippe, “Passive and heterogeneous integration towards a Si-based system-in-package concept,” Thin Solid Films, vol. 504, nos. 1–2, pp. 391–396, May 2006. DOI: 10.1016/j.tsf.2005.09.103 [3] R. R. Tummala, P. M. Raj, S. Atmur, S. Bansal, S. Banerji, F. Liu, S. Bhattacharya, V. Sundaram, K.-I. Shinotani, and G. White, “Fundamental limits of organic packages and boards and the need for novel ceramic boards for next generation electronic packaging,” J. Electroceram., vol. 13, no. 1, pp. 417–422, Jul. 2006. DOI: 10.1007/s10832-004-5135-6 [4] W. Jillek and W. K. C. Yung, “Embedded components in printed circuit boards: A processing technology review,” Int. J. Adv. Manuf. Technol. Rev., vol. 25, no. 3, pp. 350–360, Feb. 2005. DOI: 10.1007/s00170-003-1872-y [5] S. K. Bhattacharya and R. R. Tummala, “Integral passives for next generation of electronic packaging: Application of epoxy/ceramic nanocomposites as integral capacitors,” J. Microelectron., vol. 32, no. 1, pp. 11–19, Jan. 2001. DOI: 10.1016/S0026-2692(00)00104-X [6] J. Xu, S. Bhattacharya, P. Pramanik, and C. P. Wong, “High dielectric constant polymer-ceramic (epoxy varnish-barium titanate) nanocomposites at moderate filler loadings for embedded capacitors,” J. Electron. Mater., vol. 35, no. 11, pp. 2009–2015, Nov. 2006. DOI: 10.1007/s11664-006-0307-6

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