Porous methylsiloxane gel thick film for millimeter-wave antenna substrate prepared by gap filling method
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Porous methylsiloxane gel thick film for millimeter-wave antenna substrate prepared by gap filling method Takeharu Tanaka1, Nobuyuki Kawakami2, Takayuki Hirano2, Yoshito Fukumoto3, Tetsuo Suzuki1, Kazuyoshi Kanamori4, and Kazuki Nakanishi4 1 Mechanical Engineering Research Laboratory, KOBE STEEL, LTD., Kobe Japan. 2 Electronics Research Laboratory, KOBE STEEL, LTD., Kobe Japan. 3 Production Systems Research Laboratory, KOBE STEEL, LTD., Kobe Japan. 4 Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto, Japan. ABSTRACT A novel process to fabricate a thick silica gel film on an aluminum substrate, named the "gap filling method", has been developed. The process is based on a sol-gel method accompanied by phase separation in a submillimeter thick space defined by two facing substrates. Dielectric thick film integrated on a metal ground plate is successfully formed by using a SiO2 coated aluminum substrate and a cap plate covered with an amorphous carbon thin film, which control the hydrophilicity and the hydrophobicity of the plates, respectively. A thin continuous layer of less than 0.5 µm thick is formed at the interface between the porous film and the cap plate, which gives the film a smooth surface preferable for patterning metal circuits of an antenna. The observed dielectric constants (εr) at 60 GHz measured by the Fabry-Perot resonator method are in the range between 1.4 and 1.6. Methyl groups homogeneously distributed in the siloxane network give a hydrophobic siloxane gel film, thus the obtained films have enough environmental stability. These results show that the gap filling method is applicable to the preparation of the porous thick film that is needed for antenna applications. INTRODUCTION The prosperity of ubiquitous computing in recent years has produced a demand for high performance devices in millimeter wave application. To realize high-speed communications for mobile use, one of the key devices is an antenna. Small and lightweight antennas that operate in millimeter-waves are in demand to realize the next generation high-speed wireless communications. In such a high frequency range, a patch array antenna formed on the dielectric substrate is suitable in terms of portability. The antenna gain is restricted to be low due to the dielectric loss. The dielectric loss is determined by the dielectric constant and the loss tangent of the substrate material. It is necessary to develop a process to fabricate a thick film with low dielectric constant on a large metal substrate. Porous methylsiloxane gels are one of the promising candidates for low dielectric conatant films, because of their low dielectric constant and pores filled with air [1-2]. A bicontinious structure which is preferable for thick film formation can be formed via the sol-gel technique accompanied by phase separation, i.e., spinodal decomposition. This kind of siloxane gel processes well-defined continuous pores with micrometer range, which results in high porosity over 70 % [3-4]. In the present study, p
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