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.
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.
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