High loading of nanostructured ceramics in polymer composite thick films by aerosol deposition

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.


Introduction
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-apackage methodologies [1][2][3][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][6][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 highfrequency applications, such as embedded RF, since these applications require high quality factors [Qs] [8]. 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 [9]. 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][11][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][16][17][18]. In this study, we attempted to fabricate Al 2 O 3polyimide composite thick films with high Al 2 O 3 contents of more than 60 vol.% and studied the characteristics of these composite thick films in relation to their contents of Al 2 O 3 .

The experiment
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-Al 2 O 3 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 Al 2 O 3 powder was heated to 900°C for 2 h before deposition in order to improve its dielectric properties [15]. The Al 2 O 3 powder was mixed with the polyimide powder at volume ratios of 4%, 8%, and 12% using the ball mill.
The Al 2 O 3 -polyimide composite thick films were deposited on Cu and glass substrates by AD at room temperature. Table 1 shows the deposition conditions. The microstructures of the composite thick films were examined by scanning electron microscopy [SEM] and transmission electron microscopy [TEM]. An X-ray diffraction [XRD] analysis was performed to confirm the existence of α-Al 2 O 3 in the composite thick films and to examine the variations in crystallinity according to the changes in the mixing ratio. The crystallite size of the α-Al 2 O 3 in the films was calculated using Scherrer's formula. The dielectric properties were measured from 1 kHz to 10 MHz using an impedance analyzer. In order to measure the dielectric properties of the deposited thick films, Au electrodes of 1.5 mm in diameter were sputtered onto the surface of the composite thick films. Finally, the Al 2 O 3 content in the fabricated composite thick films was calculated from the relative permittivity using the Hashin-Shtrikman theory [19] and electrostatic simulations. Previous research showed that Al 2 O 3 -based polymer composite thick films fabricated by AD were well matched at the bottom limits of the Hashin-Shtrikman bounds with errors of less than 5% [16].

Results and discussions
The Al 2 O 3 -polyimide composite thick films were deposited on Cu substrates using the mixed starting powders by AD at room temperature. The Al 2 O 3 thick films and polyimide thick films were also fabricated to compare the crystallinity and dielectric properties of the films. Figure 1 shows the XRD patterns of deposited films with different mixing ratios of the Al 2 O 3 starting powder. The α-Al 2 O 3 phase of the Al 2 O 3 starting powder could be confirmed in the deposited Al 2 O 3 thick films as well as in all of the composite thick films. The diffraction pattern of the Al 2 O 3 thick film showed peak broadening and decreased intensity in comparison with that of the Al 2 O 3 starting powder as shown in Figure  1a. This result can be explained by the presence of nanocrystallites in the films, which were generated by particle crushing during the deposition [13]. In comparison, the peak patterns of the Al 2 O 3 -polyimide composite thick films became sharp and strong as the polyimide ratio in the starting powders increased.
Also, the crystallite size of α-Al 2 O 3 calculated from Scherrer's formula was increased from 26 to 52 nm as the polyimide ratio in the starting powders increased  Figure 2. This result can be attributed to the decrease of the crystallite size after deposition due to the crushing of the starting powder being reduced by the shock-absorbing effect of the polyimide. Figure 3 shows the dielectric properties of the films fabricated by AD. The relative permittivity of the Al 2 O 3polyimide composite thick films decreased as the polyimide ratio in the starting powders increased. For the loss tangent, all composite thick films showed a low loss tangent of less than 1%, except for the composite thick film that was made using the starting powder of 12 vol. % polyimide. The Al 2 O 3 -polyimide composite thick film made using the starting powder of 12 vol.% polyimide showed a high loss tangent of close to 3% and a large frequency dependence. In order to confirm the cause of the increased loss tangent in this film, the microstructures of the films were analyzed through SEM observations. Figure 4 shows the microstructures of the Al 2 O 3 -polyimide composite thick films fabricated by AD. The surface roughness increased as the polyimide ratio increased in the starting powder as shown in Figure 4a, c,e. The cross-sectional SEM observations showed more clearly the structural changes in the Al 2 O 3 -polyimide composite thick films caused by the increase of the polyimide content. The Al 2 O 3 -polyimide composite thick film made using the starting powder of 4 vol.% polyimide showed a dense microstructure close to that of the Al 2 O 3 thick films. In the case of the composite film made by using the starting powder of 8 vol.% polyimide, there were submicron Al 2 O 3 particles with dense microstructure in the composite film. In the case of the composite film made using the starting powder of 12 vol.% polyimide, however, the film density was deteriorated and the porosity was increased due to the excessive amount of polyimide. It was estimated that the increased loss tangent in the composite thick films made using the starting powder of 12 vol.% polyimide was caused by the rough surface and increased porosity of these films. The increased surface area and open pores could have caused the increase in the loss tangent by facilitating the absorption of moisture [20].
The TEM images of the Al 2 O 3 -polyimide composite thick films showed the differences between these films and the Al 2 O 3 thick films more clearly. Figure 5 shows the TEM images of the Al 2 O 3 thick film and the Al 2 O 3polyimide composite thick film made using the starting powder of 8 vol.% polyimide. As shown in Figure 5a, the   microstructure of the Al 2 O 3 thick film showed a polycrystalline structure consisting of nanocrystallites with sizes between 5 and 20 nm. It has been suggested that the nanocrystallites are formed by the fracturing of the Al 2 O 3 starting powder during the film growth. In comparison, the Al 2 O 3 -polyimide composite thick films included large Al 2 O 3 crystallites that are greater in size than 100 nm as shown in Figure 5b. It is thought that the relatively soft polyimide powders prevent the crushing of the Al 2 O 3 particles when the Al 2 O 3 particles collide with the substrate.
Finally, the Al 2 O 3 content in the composite thick films was calculated from the relative permittivity using the Hashin-Shtrikman bounds and the electrostatic simulation. As a result, the possible range of the Al 2 O 3 volume fraction in the Al 2 O 3 -polyimide composite thick films can be calculated as shown in Figure 6a. In our previous research, the dielectric properties of the Al 2 O 3 -based composite thick films were close to the bottom limits of the Hashin-Shtrikman bounds [16]. As a result, the relationship between the Al 2 O 3 volume fractions in the Al 2 O 3 -polyimide composite thick films and the Al 2 O 3 volume fractions in the starting powders was obtained from the bottom limits of the Hashin-Shtrikman bounds as shown in Figure 6b. The starting powder of 4 vol.% polyimide could achieve the highest Al 2 O 3 content in the composites of close to 95 vol.%. However, we did not expect any relief of brittleness due to the dense microstructure of almost the Al 2 O 3 thick films. Except for the above result, the Al 2 O 3 -polyimide composite thick films showed a high Al 2 O 3 content of close to 75 vol.% with a uniform composite structure when the starting powder of 8 vol.% polyimide was used. As the result of this study, we could confirm the structural variations of the composite films according to the polyimide ratio and the possibility of AD as a solution for the high loading of ceramics in polymer composites.

Conclusion
The Al 2 O 3 -polyimide composite thick films were deposited on Cu substrates by AD using mixed starting powders at room temperature. The crystallite size of α-Al 2 O 3 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 Al 2 O 3 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 Al 2 O 3 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 Al 2 O 3 -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 Al 2 O 3 content of more than 75 vol.%.