High loading of nanostructured ceramics in polymer composite thick films by aerosol deposition
© Kim and Nam; licensee Springer. 2012
Received: 26 July 2011
Accepted: 27 January 2012
Published: 27 January 2012
Low temperature fabrication of Al2O3-polyimide composite substrates was carried out by an aerosol deposition process using a mixture of Al2O3 and polyimide starting powders. The microstructures and dielectric properties of the composite thick films in relation to their Al2O3 contents were characterized by X-ray diffraction analysis. As a result, the crystallite size of α-Al2O3 calculated from Scherrer's formula was increased from 26 to 52 nm as the polyimide ratio in the starting powders increased from 4 to 12 vol.% due to the crushing of the Al2O3 powder being reduced by the shock-absorbing effect of the polyimide powder. The Al2O3-polyimide composite thick films showed a high loss tangent with a large frequency dependence when a mixed powder of 12 vol.% polyimide was used due to the nonuniform microstructure with a rough surface. The Al2O3-polyimide composite thick films showed uniform composite structures with a low loss tangent of less than 0.01 at 1 MHz and a high Al2O3 content of more than 75 vol.% when a mixed powder of 8 vol.% polyimide was used. Moreover, the Al2O3-polyimide composite thick films had extremely high Al2O3 contents of 95 vol.% and showed a dense microstructure close to that of the Al2O3 thick films when a mixed powder of 4 vol.% polyimide was used.
Keywordsaerosol deposition Al2O3 polyimide polymer composite integrated substrate high loading of ceramics system-on-package
Electronic devices have recently undergone rapid progress in terms of their multifunctionality, speed, and miniaturization. These desired properties have produced many studies into the technology and integration of components on substrates, such as printed circuit boards [PCB], multi-chip modules, and system-in-a-package methodologies [1–4]. As a next generation electronic packaging technology, system-on-package integrates both the active components (digital integrated circuits [ICs], analog ICs, memory modules, and MEMS) and the embedded passive components (capacitors, resistors, and inductors) into a multilayer-integrated substrate and provides an improved miniaturization through three dimensional [3-D] lamination [5–7]. Moreover, the high-frequency properties of the components have grown in importance due to rising demands on wireless communications. However, conventional polymer-based PCB substrates are not suitable for high-frequency applications, such as embedded RF, since these applications require high quality factors [Qs] . In comparison, ceramic substrates have high Qs, excellent thermal conductivity, and low coefficients of thermal expansion close to those of Si. However, the ceramics have some fundamentally weak characteristics, such as brittleness, poor plasticity, and a high processing temperature of over 1,000°C. The high processing temperature needed for ceramics is a critical problem that must be solved in order to achieve 3-D integration because the embedded metal transmission lines and polymer insulation films cannot tolerate high temperatures . For this reason, many studies have been carried out regarding low temperature processes for ceramic-based substrates. Polymer composites are a candidate for low temperature fabrication technology, but it is difficult to increase the ceramic content, which offers superior dielectric and thermal properties at levels above 60 vol.% [10–12].
In order to overcome this problem, our research group has studied the aerosol deposition method [AD]; based on its room-temperature process [13, 14], it can easily form composites in the submicron range using different kinds of materials, such as ceramics, polymers, or metals by simply mixing their starting powders [15–18]. In this study, we attempted to fabricate Al2O3-polyimide composite thick films with high Al2O3 contents of more than 60 vol.% and studied the characteristics of these composite thick films in relation to their contents of Al2O3.
The AD method is based on the principle of particle collision. A starting powder forms an aerosol in an aerosol chamber by mixing with the carrier gas controlled by a mass flow controller, and a vibration system under the aerosol chamber helps to generate the aerosol. The aerosol is transferred to a nozzle in the deposition chamber through a pipe line by a pressure difference generated by vacuum pumps. The aerosol is accelerated to a velocity of several hundred meters per second by the flow of the gas through a nozzle and then sprayed onto a substrate. In order to obtain uniform thick films, the substrate is continuously moved. Dense thick films are grown through the impact of the powder on the substrate in the deposition chamber at room temperature.
A commercial polyimide powder (BMI-5100, Daiwa Kasei IND, Wakayama, Japan) was milled to decrease the powder size by a planetary ball mill (Pulverisette 5, Fritsch, Idar-Oberstein, Germany) so that a polyimide starting powder with a 1-μm average diameter was obtained. We used α-Al2O3 powder with a 0.5-μm average diameter (99.4% purity, AL-160SG3, Showa-Denko K.K., Tokyo, Japan) as the ceramic starting powder. The Al2O3 powder was heated to 900°C for 2 h before deposition in order to improve its dielectric properties . The Al2O3 powder was mixed with the polyimide powder at volume ratios of 4%, 8%, and 12% using the ball mill.
The AD parameters for the Al2O3-polyimide composite thick films
Cu and glass
Size of nozzle orifice
10 × 0.4 mm2
Consumption of carrier gas
Distance between substrate and nozzle
10 × 10 mm2
Results and discussions
The Al2O3-polyimide composite thick films were deposited on Cu substrates by AD using mixed starting powders at room temperature. The crystallite size of α-Al2O3 in the composite thick films increased from 26 to 52 nm as the polyimide ratio in the mixed starting powders increased from 4 to 12 vol.%. The Al2O3 content was close to 95 vol.% when the mixed powder of 4 vol.% polyimide is used; however, the microstructure was close to that of the Al2O3 films. In the case of the mixed powder of 12 vol.% polyimide, the composite thick film showed a high loss tangent of close to 0.03 at 1 MHz and a large frequency dependence with a nonuniform microstructure. The Al2O3-polyimide composite thick films made using a mixed powder of 8 vol.% polyimide showed a uniform composite structure with a low loss tangent of less than 0.01 at 1 MHz and a high Al2O3 content of more than 75 vol.%.
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 using the research grant of Kwangwoon University in 2011.
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