Effects of starting powder and thermal treatment on the aerosol deposited BaTiO3 thin films toward less leakage currents
© Yao et al.; licensee Springer. 2014
Received: 17 July 2014
Accepted: 20 August 2014
Published: 27 August 2014
To prepare high-density integrated capacitors with low leakage currents, 0.2-μm-thick BaTiO3 thin films were successfully deposited on integrated semiconductor substrates at room temperature by the aerosol deposition (AD) method. In this study, the effects of starting powder size were considered in an effort to remove macroscopic defects. A surface morphology of 25.3 nm and an interface roughness of less than 50 nm were obtained using BT-03B starting powder. The nano-crystalline thin films achieved after deposition were annealed at various temperatures to promote crystallization and densification. Moreover, the influence of rapid thermal annealing process on the surface morphology and crystal growth was evaluated. As the annealing temperature increased from room temperature to 650°C, the root mean square (RMS) roughness decreased from 25.3 to 14.3 nm. However, the surface was transformed into rough performance at 750°C, which agreed well with the surface microstructure trend. Moreover, the crystal growth also reveals the changes in surface morphology via surface energy analysis.
KeywordsSulfur hexafluoride Post-annealing treatment Inductively coupled plasma etching Barium titanate
Recently, to meet the modern communication system demands of miniaturization and high frequency, high-density integrated capacitors have attracted increasing industry interest, which has been driven by thin-film integrated passive devices (IPDs) [1–3], electromagnetic interference (EMI) protection , high-electron-mobility transistor (HEMT) input-/output-matching circuit blocks , and digital and mixed signal applications . Several semiconductor technologies, such as low-temperature co-firing ceramics (LTCC)  and sputtering , can be used to fabricate materials with high relative permittivity. However, both LTCC and sputtering need sintering at approximately 850°C to form the desired crystallite structure, which is a critical problem for embedding passive devices. Consequently, a new, green, and environmentally friendly approach called aerosol deposition (AD), which can deposit ceramic films at room temperature, has attracted great interest. The pioneering work was published in 2001 , and various ceramic films fabricated by AD have been studied quite intensively in recent years.
In previous research, ferroelectric BaTiO3 was employed in high-density embedded decoupling capacitors using the AD method. BaTiO3 films with thicknesses of 0.1 to 2.2 μm were deposited on Cu and stainless steel (SUS) substrates [10–13]. The BaTiO3 films with a thickness of less than 0.5 μm on Cu substrates and 0.2 μm on SUS substrates exhibited conductor properties because of their high leakage currents. The leakage current mechanisms for aerosol-deposited BaTiO3 thin films and the causes of the high leakage currents were determined in previous research [10, 12]. However, the densification mechanism of BaTiO3 films deposited by AD has yet to be identified.
In this study, we applied 0.2-μm-thick BaTiO3 thin films deposited by AD onto an integrated substrate suitable for thin-film IPDs. To overcome the macroscopic defects and rough interface between the BaTiO3 films and substrates, the influence of starting powders with difference particle sizes was investigated by scanning electron microscopy (SEM) and focused ion beam (FIB). In addition, the densification of AD-deposited BaTiO3 thin films and stronger particle-to-particle bonding could be obtained using rapid thermal annealing treatment. The surface morphology of post-annealed BaTiO3 thin films was examined using atom force microscopy (AFM) to reveal the effect of rapid thermal annealing (RTA) treatment on leakage currents.
The AD method is a very attractive deposition process for integrating ceramic thin films. During the deposition process, the raw particles are mixed with a N2 carrier gas to form an aerosol flow and then ejected through a nozzle and coated onto the substrate in the deposition chamber at room temperature. The detailed fabrication apparatus has been described in elsewhere . The BaTiO3 thin films were successfully deposited on Pt/Ti/SiO2/Si integrated substrates with a thickness of 200 nm and a deposition area of 10 × 10 mm2 using a similar AD apparatus in this paper. The thickness of the Pt/Ti layer is 150/10 nm. During the deposition process, to clarify the influence of the starting powder on the morphology of the bottom Pt interface, different BaTiO3 powders BT-045J and BT-03B (Samsung Fine Chemicals Co., Ltd., Ulsan, South Korea) with particle sizes of 0.45 and 0.30 μm, respectively, were used as starting powders. The surfaces of the as-deposited thin films were evaluated using SEM (S-4300SE; Hitachi Ltd, Tokyo, Japan), and the cross-section of the interface between the BaTiO3 thin films and Pt substrate deposited using different starting powders was observed using a FIB system (Nova 600 Nanolab, FEI, Hillsboro, OR, USA).
Results and discussion
In our previous work, BaTiO3 films of 0.1 to 2.2 μm in thickness were deposited on Cu and SUS substrate by the AD method. All of the samples with thicknesses of less than 0.5 μm on Cu substrates and 0.2 μm on SUS substrates were electrically shorted, which can be a result of high leakage currents caused by macroscopic defects and rough interfaces between films and substrates .
Interface between BaTiO3 thin films and substrates
Effect of rapid thermal annealing on surface morphology and crystal growth
In this study, BaTiO3 thin films with thickness of 0.2 μm were deposited on platinum-coated silicon substrates at room temperature by AD. Different thin films deposited using starting powders of various sizes were investigated, and the results confirmed that the macroscopic defects such as pores and incompletely crushed particles could be reduced by employing BT-03B starting powder. An interface roughness of less than 50 nm and a minimum surface roughness of 14.3 nm were obtained after RTA treatment at 650°C. As the annealing temperature increased from room temperature to 650°C, the calculated crystalline size increased from 11.3 to 16.3 nm. Thus, the surface morphology and the densification of AD-deposited BaTiO3 thin films can be controlled by appropriate choice of RTA temperature to achieve a low leakage current.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) No. 2013-067321 and a grant supported from the Korean government (MEST) No. 2012R1A1A2004366 and (MSIP) No.2014R1A1A1005901. This work was also supported by a Research Grant of Kwangwoon University in 2014. Also, we would like to thank Mr. Ho-Kun Sung from Korea Advanced Nano Fab Center (KANC) for his technical support with the materials and circuit fabrications during this work.
- Wang C, Lee WS, Kim NY: Practical integrated passive device technology on GaAs. Microwave J 2012, 55: 94–106.
- Wang C, Zhang F, Kim NY: Development and characterization of metal-insulator-metal capacitors with SiNx thin films by plasma-enhanced chemical vapor deposition. Chinese Phys Lett 2010, 27: 078101. 10.1088/0256-307X/27/7/078101View Article
- Wang C, Lee WS, Zhang F, Kim NY: A novel method for the fabrication of integrated passive devices on SI-GaAs substrate. Int J Aav Manuf Tech 2011, 52: 1011–1018. 10.1007/s00170-010-2807-zView Article
- Robutel R, Martin C, Buttay C, Morel H, Mattavelli P, Boroyevich D, Meuret R: Design and implementation of integrated common mode capacitors for SiC JFET inverters. IEEE T Power Electr 2013, 29: 3625–3636.View Article
- Wang C, Kim NY: Electrical characterization and nanoscale surface morphology of optimized Ti/Al/Ta/Au ohmic contact for AlGaN/GaN HEMT. Nanoscale Res Lett 2012, 7: 1–8. 10.1186/1556-276X-7-1View Article
- Ramadass YK, Fayed AA, Chandrakasan AP: A fully-integrated switched-capacitor step-down DC-DC converter with digital capacitance modulation in 45 nm CMOS. IEEE J Solid-ST Circ 2010, 45: 2557–2565.View Article
- Naghib-Zadeh H, Glitzky C, Oesterle W, Rabe T: Low temperature sintering of barium titanate based ceramics with high dielectric constant for LTCC applications. J Eur Ceram Soc 2011, 31: 589–596. 10.1016/j.jeurceramsoc.2010.10.003View Article
- Saravanan KV, Raju KCJ: Quasi-rapid thermal annealing studies on barium strontium titanate thin films deposited on fused silica substrates. J Alloy Compd 2013, 571: 43–49.View Article
- Akedo J, Lebedev M: Aerosol deposition method (ADM): a novel method of PZT thick films producing for microactuators. Recent Res Dev Mater Res 2001, 2: 51–77.
- Kim HK, Oh JM, Kim SI, Kim HJ, Lee CW, Nam SM: Relation between electrical properties of aerosol-deposited BaTiO3 thin films and their mechanical hardness measured by nano-indentation. Nanoscale Res Lett 2012, 7: 1–8. 10.1186/1556-276X-7-1View Article
- Oh JM, Nam SM: Thickness limit of BaTiO3 thin film capacitors grown on SUS substrates using aerosol deposition method. Thin Solid Films 2010, 518: 6531–6536. 10.1016/j.tsf.2010.03.159View Article
- Oh JM, Nam SM: Role of surface hardness of substrates in growing BaTiO3 thin films by aerosol deposition method. Jpn J Appl Phys 2009, 48: 09KA07.
- Oh JM, Kim NH, Choi SC, Nam SM: Thickness dependence of dielectric properties in BaTiO3 films fabricated by aerosol deposition method. Mat Sci Eng: B 2009, 161: 80–84. 10.1016/j.mseb.2009.01.028View Article
- Akedo J: Room temperature impact consolidation (RTIC) of fine ceramic powder by aerosol deposition method and applications to microdevices. J Therm Spray Techn 2008, 17: 181–198. 10.1007/s11666-008-9163-7View Article
- Lee DW, Kim HJ, Nam SM: Effects of starting powder on the growth of Al2O3 films on Cu substrates using the aerosol deposition method. J Korean Phys Soc 2010, 57: 1115–1121. 10.3938/jkps.57.1115View Article
- Hatono H, Ito T, Matsumura A: Application of BaTiO3 film deposited by aerosol deposition to decoupling capacitor. Jpn J Appl Phys 2007, 46: 6915–6919. 10.1143/JJAP.46.6915View Article
- Kim HK, Lee SH, Kim SI, Lee CW, Yoon JR, Lee SG, Lee YH: Dielectric strength of voidless BaTiO3 films with nano-scale grains fabricated by aerosol deposition. J Appl Phys 2014, 11: 1–6.
- Cao GZ: Nanostructures and Nanomaterials: Synthesis, Properties and Applications. London: Imperial College Press; 2004.View Article
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