Effects of mechanical properties of polymer on ceramic-polymer composite thick films fabricated by aerosol deposition
© Kwon et al.; licensee Springer. 2012
Received: 29 March 2012
Accepted: 22 May 2012
Published: 22 May 2012
Two types of ceramic-polymer composite thick films were deposited on Cu substrates by an aerosol deposition process, and their properties were investigated to fabricate optimized ceramic-based polymer composite thick films for application onto integrated substrates with the advantage of plasticity. When polymers with different mechanical properties, such as polyimide (PI) and poly(methyl methacrylate) (PMMA), are used as starting powders together with α-Al2O3 powder, two types of composite films are formed with different characteristics - surface morphologies, deposition rates, and crystallite size of α-Al2O3. Through the results of micro-Vickers hardness testing, it was confirmed that the mechanical properties of the polymer itself are associated with the performances of the ceramic-polymer composite films. To support and explain these results, the microstructures of the two types of polymer powders were observed after planetary milling and an additional modeling test was carried out. As a result, we could conclude that the PMMA powder is distorted by the impact of the Al2O3 powder, so that the resulting Al2O3-PMMA composite film had a very small amount of PMMA and a low deposition rate. In contrast, when using PI powder, the Al2O3-PI composite film had a high deposition rate due to the cracking of PI particles. Consequently, it was revealed that the mechanical properties of polymers have a considerable effect on the properties of the resulting ceramic-polymer composite thick films.
KeywordsAerosol deposition Al2O3 PMMA PI Integrated substrates Composite films
Due to the continuous demand for the miniaturization and integration of electronic devices for ubiquitous and digital convergence, system-on-package (SOP) technology is emerging as an alternative concept to overcome the technological limits of conventional technologies, such as system-on-chip and system-in-package. The SOP is a new concept for the three-dimensional integration of active and passive devices onto a single system to provide multifunctionality . In order to integrate active and passive devices onto a single package for SOP, a fabrication technology for integrated substrates must be established. Ceramics are widely used for the integrated substrates since they have favorable characteristics, such as high reliability and good dielectric properties. Nevertheless, ceramics have a fundamental weakness in that they are easily fractured and require high-temperature processes for the fabrication of integrated substrates.
In the present technologies, two methods which overcome these fundamental weaknesses of ceramics for the fabrication of integrated substrates have been attempted. One is the development of a ceramic-polymer composite, which provides plasticity to the ceramics. The other is the use of low-temperature processes. However, during the formation of the ceramic-polymer composite, it was hard to increase the ceramic content above 60 vol.%. Even when the ceramic content in the composite was over 60 vol.%, the dielectric properties of the composites were worsened compared to those of pure ceramics. Furthermore, although low-temperature co-fired ceramics are well known in the low-temperature fabrication of ceramics, they still require high-temperature processes at around 850°C [2, 3]. Taking these factors into consideration, the aerosol deposition (AD) process could be a good candidate method due to its various merits, such as room-temperature processing and the ability to form heterogeneous junctions consisting of different kinds of materials [4, 5]. As reported in earlier studies, the AD process is based on shock-loading solidification via the impact of ultrafine particles with a surface. The consolidation and densification phenomena are affected by the mechanical properties of the starting powder [6, 7]. To fabricate optimized ceramic-based polymer composite thick films by the AD process, it is necessary to understand how the mechanical properties of the starting powder affect the characteristics of the composite films. Thus, our focus was concentrated on the mechanical properties of the polymer powder. Poly(methyl methacrylate) (PMMA) and polyimide (PI) were chosen as the polymer powders for two reasons. First of all, PMMA and PI are widely used as matrix components in composite films due to their good electrical, physical, and mechanical properties [8–12]. However, these two polymers have different mechanical properties. It is reported that the tensile strength, fracture toughness, and elongation of PMMA are 48 to 76 MPa, 1.21 to 1.76 MPa·m1/2, and 2% to 10%, respectively. In comparison, the tensile strength, fracture toughness, and elongation of PI are 75 to 90 MPa, 1.65 to 5.4 MPa·m1/2, and 4% to 8%, respectively .
In this study, ceramic-polymer composite thick films were fabricated by the AD process and the effects of the mechanical properties of the polymer powder were examined for the two types of polymer. To confirm the effect of the mechanical properties of the polymer, Vickers hardness testing was carried out. Moreover, to understand how the mechanical properties affect the deposition characteristics, a high mechanical energy was applied to the two types of polymer powder using a planetary mill, and additional modeling was also carried out.
Deposition conditions of composite thick films fabricated by the AD process
Consumption of carrier gas
1 to 10 L/min
Scanning speed of the nozzle motion
5 to 20 Torr
Size of nozzle orifice
10 mm × 0.4 mm
Distance between substrate and nozzle
5 to 15 mm
10 to 30 min
10 mm × 20 mm
Results and discussions
Fabrication of Al 2 O 3 -PMMA composite thick films
Fabrication of Al 2 O 3 -PI composite thick films
However, it has not been sufficiently clarified which properties of the polymer powder affect the characteristics of the deposited films. Therefore, as mentioned above, we focused on the mechanical properties of the polymer powder based on the mechanism of the AD process.
Comparison of Al 2 O 3 -PMMA and Al 2 O 3 -PMMA composite thick films
Al2O3-PMMA and Al2O3-PI composite thick films were successfully fabricated on Cu substrates by the AD process; however, they had considerably different properties depending on the mechanical properties of their component polymers. The deposition rate and the crystallite size of α-Al2O3 in the Al2O3-PMMA composite film were decreased, and its surface morphology is similar to that of the Al2O3 film. In contrast, the Al2O3-PI composite film had a high deposition rate, its crystallite size was increased, and its surface morphology tended to be similar to that of the PI film. Through observations of the microstructures after planetary milling and additional modeling tests, it was revealed that the mechanical properties of polymers have a considerable effect on the properties of the ceramic-polymer composite thick films. In addition, we explained that the two types of polymer-ceramic composite films had different growth mechanisms: PMMA particles were distorted by the impact of Al2O3 particles during deposition, whereas PI particles were fragmented by the impact of Al2O3.
This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials 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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