Study on Characteristics of Various RF Transmission Line Structures on PES Substrate for Application to Flexible MMIC

Abstract

In this work, the coplanar waveguide is fabricated on a PES (poly[ether sulfone]) substrate for application to a flexible monolithic microwave integrated circuit, and its RF characteristics were thoroughly investigated. The quality factor of the coplanar waveguide on PES is 40.3 at a resonance frequency of 46.7 GHz. A fishbone-type transmission line (FTTL) structure is also fabricated on the PES substrate, and its RF characteristics are investigated. The wavelength of the FTTL on PES is 5.11 mm at 20 GHz, which is 55% of the conventional coplanar waveguide on PES. Using the FTTL, an impedance transformer is fabricated on PES. The size of the impedance transformer is 0.318 mm × 0.318 mm, which is 69.2% of the size of the transformer fabricated by the conventional coplanar waveguide on PES. The impedance transformer showed return loss values better than -12.9 dB from 5 GHz to 50 GHz and an insertion loss better than -1.13 dB in the same frequency range.

Basic RF Characteristics of Fishbone-Type Transmission Line Employing Comb-Type Ground Plane (FTLCGP) on PES Substrate for Use in Flexible Passive Circuits

“…The electrical properties of PES were also compared with the silicon substrate. According to our previous results, [4]- [5], it was found that the insertion loss of PES was much lower than that of silicon substrate. To reduce the size of the RF device on semiconducting substrate, we should use a transmission line with a short wavelength.…”

“…To reduce the size of the RF device on semiconducting substrate, we should use a transmission line with a short wavelength. However, the wavelength of the transmission line on PES was much longer than that on conventional semiconducting substrate, such as silicon, due to its low effective permittivity [4]. Usually, a transmission line with low impedance is required for the impedance matching of a transistor in high frequency due to the low input and output impedance of transistors.…”

“…For this reason, the passive components on a PES substrate have to be a little larger in size than those on a conventional semiconducting substrate. To solve the above problem, the fishbone-type transmission line (FTTL) was proposed, and its RF characteristics were evaluated [4]. According to the results in [4], the FTTL on the PES showed an effective permittivity ( eff ) that was much higher than that shown by a conventional coplanar waveguide on PES, and this resulted in a reduction in wavelength.…”

“…Therefore, a transmission line with a very wide line width is required to perform the impedance matching of transistors with low input and output impedance [4]. For this reason, the passive components on a PES substrate have to be a little larger in size than those on a conventional semiconducting substrate.…”

“…In addition, PES shows good waterresistant qualities [2]. For these reasons, PES has been employed in flexible Monolithic Microwave Integrated Circuits (MMICs) [4]- [5]. The coefficient of thermal expansion and glass transition temperature of PES is 49.1 ppm/K and 228°C, respectively, and the dielectric loss tangent and relative permittivity of PES is 0.001 and 3.5, respectively.…”