Relation between electrical properties of aerosol-deposited BaTiO3 thin films and their mechanical hardness measured by nano-indentation
© Kim et al.; licensee Springer. 2012
Received: 6 February 2012
Accepted: 22 May 2012
Published: 22 May 2012
To achieve a high capacitance density for embedded decoupling capacitor applications, the aerosol deposition (AD) process was applied as a thin film deposition process. BaTiO3 films were fabricated on Cu substrates by the AD process at room temperature, and the film thickness was reduced to confirm the limit of the critical minimum thickness for dielectric properties. As a result, the BaTiO3 thin films that were less than 1-μm thick showed unstable electric properties owing to their high leakage currents. Therefore, to overcome this problem, the causes of the high leakage currents were investigated. In this study, it was confirmed that by comparing BaTiO3 thin films on Cu substrates with those on stainless steels (SUS) substrates, macroscopic defects and rough interfaces between films and substrates influence the leakage currents. Moreover, based on the deposition mechanism of the AD process, it was considered that the BaTiO3 thin films on Cu substrates with thicknesses of less than 1 μm are formed with chinks and weak particle-to-particle bonding, giving rise to leakage currents. In order to confirm the relation between the above-mentioned surface morphologies and the dielectric behavior, the hardness of BaTiO3 films on Cu and SUS substrates was investigated by nano-indentation. Consequently, we proposed that the chinks and weak particle-to-particle bonding in the BaTiO3 thin films with thicknesses of less than 0.5 μm on Cu substrates could be the main cause of the high leakage currents.
KeywordsBaTiO3 thin film aerosol deposition nano-indentation leakage current
The ever increasing functionality, portability, and switching frequencies of consumer electronics, which cause parasitic inductance effects such as electromagnetic interference (EMI) and simultaneous switching noise (SSN) [1, 2], are putting tremendous pressure on designers and manufacturers to pack more circuitry into a smaller space at the same time as suppressing SSN and EMI. Because of these demands, the three-dimensional integration of passive components should be realized instead of relying on existing surface-mounted passive components, i.e., two-dimensional integration, owing to the occupation of space by the passives over the entire printed circuit board area . Among the passive components requiring three-dimensional integration, the development of an embedded decoupling capacitor is particularly important for miniaturization and the suppression of EMI and SSN [4, 5].
As candidates for an embedded decoupling capacitor, there are low-temperature co-fired ceramics (LTCCs), polymer composites, films deposited by sputtering, and so on . However, LTCCs still require high-temperature processes around 850°C, which pose a critical problem for the embedding of passive components , and polymer composites have poor dielectric properties . Also, ferroelectric films deposited by sputtering must be annealed at a high temperature in order to achieve the desired crystalline phase and orientation . Therefore, to overcome the processing problems, a new technological approach has been attempted using an aerosol deposition (AD) process that enables us to fabricate dense ceramic thick films at room temperature. Although the AD process was invented for the growth of ceramic thick films , we applied the AD process as a thin film fabrication process to achieve a high capacitance density of over 1,000 nF/cm2.
In our previous research, to produce a high capacitance density, BaTiO3 was deposited on Cu substrates by the AD process at room temperature, and the film thickness was reduced to confirm the critical minimum thickness for dielectric properties. Consequently, all BaTiO3 thin films that were less than 0.5-μm thick showed short-circuit characteristics owing to high leakage currents, while the BaTiO3 thin films that were 0.5- to 1-μm thick had only partial dielectric properties, which was an obstacle in achieving a satisfactory thin film process using the AD process [11–13]. However, the BaTiO3 films that were more than 1-μm thick exhibited a dense morphology from which defects were markedly absent, which was considered to be due to the rigidity of the surfaces of the hard ceramic BaTiO3 films. From the above consideration, in order to clarify the role of substrate hardness in growing the deposited films, BaTiO3 films were fabricated on stainless steel (SUS) substrates which were harder than the Cu substrates. As a result, the BaTiO3 films on SUS substrates had a lower critical minimum thickness of 0.2 μm, a fewer macroscopic defects, and a lower interface roughness than the BaTiO3 films on Cu substrates. Thus, we discussed that the high leakage currents were related to macroscopic defects and a rough interface .
In this study, in order to reveal the influence of the macroscopic defects and the rough interface on the leakage currents, a current image of the macroscopic defects was obtained by conductive atomic force microscopy (C-AFM) analysis, and the roughness of the interface between the films and the substrates was also observed by the focused ion beam (FIB) technique. Moreover, based on the deposition mechanism of the AD process, it was considered that BaTiO3 thin films of less than 1 μm in thickness are formed on Cu substrates with chinks and weak particle-to-particle bonding, and these surface morphologies are related to the presence of leakage currents. Therefore, the hardness, which depended on the surface morphologies of the BaTiO3 films on Cu and SUS substrates, was measured by nano-indentation, and the relation between the hardness and the electrical properties of the BaTiO3 films on Cu and SUS substrates was investigated to identify another cause of the high leakage currents.
The AD process is based on shock-loading solidification due to the impact of ceramic particles. During the AD process, fine ceramic particles are accelerated by gas flow in the nozzle up to a velocity of several hundred meters per second and are sprayed onto the substrates. The details of the AD process apparatus can be referred to elsewhere . BaTiO3 films were deposited by the AD process using a commercial BaTiO3 powder (BT-045 J, Samsung Fine Chemicals Co., Ltd., Ulsan, South Korea) with a particle size of 0.45 μm as a starting powder. The particles were aerosolized in an aerosol chamber and transported into a deposition chamber using He gas at a flow rate of 5 L/min. The transported BaTiO3 powder was continuously ejected through the nozzle and deposited onto the Cu and SUS substrates, respectively. The orifice size of the nozzle, the deposition area, the distance between the nozzle and the substrate, the working pressure, and the deposition time were 10 × 0.4 mm2, 10 × 10 mm2, 10 mm, 3.4 Torr, and 1 to 10 min, respectively.
The dielectric properties of the BaTiO3 films with thicknesses of 0.1 to 2.2 μm on Cu and SUS substrates were measured using an impedance analyzer (HP 4194A, Agilent Technologies, Inc., Santa Clara, CA, USA) and their dielectric behaviors were confirmed. The crystallinity and crystallite size of the BaTiO3 films on the Cu and SUS substrates were analyzed using an X-ray diffractometer (XRD; X’Pert PRO, PANalytical, Almelo, The Netherlands). Moreover, in order to confirm the causes of high leakage currents, the roughness values of the interfaces between deposited films and substrates were observed through a FIB system (Helios 600i, FEI, Hillsboro, OR, USA), and the surface morphology and current images were observed by C-AFM (SPM 9600, Shimadzu Corporation, Kyoto, Japan). During C-AFM analysis, a voltage of 5 V was applied. In addition, a field-emission scanning electron microscopy (S-4700, Hitachi, Ltd., Chiyoda-ku, Japan) analysis was conducted, and the hardness of the BaTiO3 films was investigated by nano-indentation (TriboIndenter, Hysitron, Inc., Eden Prairie, MN, USA) which has been established as a powerful method to characterize the near-surface mechanical properties of materials. For the measurement of the hardness of the BaTiO3 films, a Berkovich tip was used, the diameter of which was about 100 nm, which can avoid the influence of the surface roughness of the BaTiO3 films on the measured data. The details of the nano-indentation can be referred to elsewhere [16–20].
Results and discussions
Influence of rough interface and macroscopic defects on leakage currents
In our previous research, BaTiO3 films with thicknesses of 0.1 to 3.0 μm were fabricated on Cu substrates by an AD process at room temperature and compared with BaTiO3 films on SUS substrates. As a result, all BaTiO3 thin films with a thickness of less than 0.5 μm showed short-circuit characteristics owing to high leakage currents, and BaTiO3 thin films of 0.5 to 1 μm in thickness had partial dielectric properties [11–13]. In addition, the BaTiO3 thin films on Cu substrates exhibited rough interfaces between the films and the substrates as well as macroscopic defects such as pores and not-fully-crushed particles. On the other hand, the BaTiO3 films on SUS substrates had a lower critical minimum thickness of 0.2 μm, a lower interface roughness, and a fewer macroscopic defects than the BaTiO3 films on Cu substrates. From these results, we proposed that the rough interfaces and the macroscopic defects in the BaTiO3 thin films with thicknesses of less than 0.5 μm on Cu substrates lead to high leakage currents .
Relationship of dielectric properties with substrate type and thickness of BaTiO 3 films (at 1 MHz)
Film thickness (μm)
tan δ (%)
Capacitance density (nF/cm2)
Dielectric behavior (%)
50 to 80
50 to 80
90 to 100
tan δ (%)
Capacitance density (nF/cm2)
Dielectric behavior (%)
90 to 100
90 to 100
90 to 100
90 to 100
90 to 100
Surface morphology of BaTiO 3 films without macroscopic defect areas
From the above results, it was confirmed that the roughness of the interface and the macroscopic defects have a direct influence on the leakage currents. However, we consider that the causes of the leakage currents are not only the rough interface and the macroscopic defects, but also the chinks and weak particle-to-particle bonding that arise due to the deposition mechanism of the AD process. The bonding of ceramic particles in the AD process is based on shock-loading solidification, resulting from the impact of ceramic particles. The densification of the ceramic films is formed by the continuous impaction of ceramic particles onto the pre-impacted particles or substrate. Therefore, it is considered that the impaction of ceramic particles is not sufficient for the densification of the BaTiO3 thin films on Cu substrates, and therefore, these BaTiO3 thin films have chinks and weak particle-to-particle bonding. On the other hand, in the case of the BaTiO3 thin films on SUS substrates, it is considered that even if the impaction of ceramic particles is not high enough, the ceramic particles are sufficiently fractured to form dense BaTiO3 thin films due to the use of harder substrates than Cu substrates. From the calculated crystallite sizes based on the XRD data, the smaller crystallite sizes of the 0.2-μm-thick BaTiO3 thin films on SUS substrates compared to those of the 0.2-μm-thick BaTiO3 thin films on Cu substrates support the above consideration.
Relation between electrical properties and film hardness
From the surface morphologies of the BaTiO3 films on Cu and SUS substrates, it was considered that the chinks and weak particle-to-particle bonding of the BaTiO3 thin films bring about an increase in the leakage currents. However, it was difficult to reveal the relation between the surface morphologies and the leakage currents by using only the AFM and SEM analyses. Therefore, in order to exhibit the above-mentioned relation, the hardness of the BaTiO3 films with thicknesses of 0.2 to 2.2 μm on Cu and SUS substrates was measured by nano-indentation, and then the tendencies of the hardness and the dielectric behavior of the BaTiO3 films were compared.
In order to clarify the causes of high leakage currents, BaTiO3 films with thicknesses of 0.1 to 2.2 μm were fabricated on Cu and SUS substrates by an AD process at room temperature. The critical minimum thicknesses of the BaTiO3 films on Cu and SUS substrates were 0.5- and 0.2-μm thick. From the FIB technique, it was exhibited that the 0.2-μm-thick BaTiO3 thin films on Cu substrates had higher interface roughness than the 0.2-μm-thick BaTiO3 thin films on SUS substrates, which can cause high field concentrations and therefore give rise to high leakage currents. In addition, it was confirmed from the C-AFM analysis that the leakage currents largely flow through the macroscopic defects. Chinks and incompact surface morphologies of the 0.2-μm-thick BaTiO3 thin films on Cu substrates were observed, and the hardness of BaTiO3 thin films with thicknesses of less than 0.5 μm on Cu substrates was below 3 GPa. On the other hand, the BaTiO3 films with thicknesses over 0.5 μm on Cu substrates and the BaTiO3 films of 0.1 to 2.2 μm in thickness exhibited dense surface morphologies and hardness values of more than 3 GPa. It was confirmed that the tendency of the hardness of the BaTiO3 films on Cu and SUS substrates matched that of their electrical properties. We explained the above results by concluding that the chinks and weak particle-to-particle bonding bring about leakage currents based on the relation between hardness and the electrical properties.
This research was supported by a grant from the Consumer-connected Components and Materials Technology Development program funded by the Ministry of Knowledge Economy, Republic of Korea. The present research has been conducted by the research grant of Kwangwoon University in 2012.
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