A coplanar waveguide was fabricated on a polyether sulfone (PES) substrate for application to a flexible Radio Frequency Integrated Circuit (RFIC), and its RF characteristics were thoroughly investigated. According to the measured results, the coplanar waveguide on PES showed a very low loss, compared with a conventional silicon substrate. It showed an insertion loss lower than 0.6 dB/mm and an attenuation constant α lower than 0.065 Np/mm up to 50 GHz. The Q-factor of a coplanar waveguide on PES was 40.3 at a resonance frequency of 46.7 GHz. It showed a wavelength of 3.9 to 6.2 mm from 30 to 50 GHz, and an effective dielectric constant εeff of 2.35 to 2.9 from 10 to 50 GHz. The coplanar waveguide on PES showed a very weak frequency dispersion characteristic compared with silicon substrate. This work is the first known report of the investigation of RF signal transmission characteristics on PES substrate.

Flexible electronics have drawn significant attention, owing to their variety of applications such as flexible displays, smart tags and wearable products.1-3 Especially for the development of a transparent flexible display for a mobile communication function, RF devices should be integrated into a transparent flexible substrate. Recently, polyether sulfone (PES) has drawn attention for application to transparent flexible display, due to its good heat-resisting property, high transparency and good flexibility.4-7 The glass transition temperature (Tg) of PES is 230°C and it shows stable electrical and mechanical properties at high temperature, which enables the fabrication of electron devices at a relatively high temperature.4-7 Because of a heat-resisting property, the electrical and mechanical properties of the PES do not change at even 300°C. Therefore, unlike other flexible substrates such as polycarbonate (PC) and polyethylene terephthalate (PET), the soldering and bonding processes for the electron devices on PES can be easily performed, which facilitates the packaging process of electronic devices. In addition, a very thin PES substrate, with a thickness less than 100 µm, can be used to fabricate electronic devices due to its tenacity, which is very effective for the miniaturization of RF components. Furthermore, the PES shows a contraction ratio less than 0.2 percent, even if it is exposed to a high temperature environment for a long time, which enables precise processes, such as a microelectromechanical system (MEMS) process. Besides the mentioned properties, the PES shows good water resistance.

For these reasons, PES is suitable for applications such as a transparent flexible display for mobile communications. Several groups have employed a PES substrate to evaluate the electrical properties of oxide films.8 For applications to mobile communication flexible displays, RF passive components as well as active devices could be integrated onto PES substrates. For integration of RF passive components on a PES substrate, the RF signal transmission characteristic on a PES substrate should be thoroughly investigated. To date, no known studies of the RF characteristics of PES substrates has been performed. In this work, a coplanar waveguide was fabricated on PES substrate and its RF signal transmission characteristics were thoroughly investigated.

Figure 1

Figure 1 Photograph of a coplanar waveguide fabricated on PES.

Figure 2

Figure 2 Loss characteristics of coplanar waveguide on PES and silicon substrates.

Loss Characteristics of a Coplanar Waveguide Fabricated on PES

Figure 3

Figure 3 Measured insertion loss of a quarter wavelength coplanar waveguide fabricated on PES.

Figure 1 shows a photograph of the coplanar waveguide fabricated on PES. Pt/Au was deposited on a PES substrate 200 µm thick, and the thickness of the Au/Ti was 2 µm. The line width W is 20 µm, and the distance between line and ground plane is 80 µm. Figure 2 shows the measured insertion loss per 1 mm and the attenuation constant α of the coplanar waveguide on PES. The insertion loss was measured with a 50 Ω impedance system and was normalized to the characteristic impedance of the coplanar waveguide. For comparison, data for the same size coplanar waveguide on silicon substrate is also plotted, because silicon substrate is the most popular semiconducting substrate for RFIC applications. As shown, the coplanar waveguide on PES shows very low loss, compared to the silicon substrate. It shows an insertion loss lower than 0.6 dB/mm, and an attenuation constant alower than 0.065 Np/mm up to 50 GHz. The low loss of the coplanar waveguide on PES originates from its excellent electrical insulating properties. In the case of coplanar waveguide on silicon substrate, there is a current flowing from the line to the ground plane, through the silicon substrate, due to the relatively high conductivity of silicon substrate, which causes a relatively high loss of electromagnetic energy. In the case of coplanar waveguide on PES, there is no current flowing from line to ground plane through the PES substrate, due to its excellent electrical insulating characteristic.

The quality (Q) factor of the coplanar waveguide on PES was also investigated. There are several methods to calculate the Q-factor.10 Here, the transmission line Q-factor was derived from a resonator using a quarter wavelength long line at the resonance frequency f0. The Q-factor was extracted by the ratio of –3 dB bandwidth to f0, in other words, Q = f0/Δf3dB. Figure 3 shows the insertion loss S21 of a quarter wavelength coplanar waveguide fabricated on PES. The Q-factor was 40.3 at a resonance frequency of 46.7 GHz. Table 1 shows the Q-factors of transmission lines on PES and silicon substrate. As shown in this table, the transmission line on PES shows a Q-factor much higher than silicon substrate. The results indicate that the RF passive components on PES substrate can be used in applications at millimeter wave as well as microwave, due to its very low loss.

Table 1

Basic RF Characteristics of Coplanar Waveguide Fabricated on PES

Figure 4 shows the wavelength of the coplanar waveguide on PES to be 3.9 to 6.2 mm from 30 to 50 GHz. In a highly integrated RF circuit, bulky passive components as well as transistors could be integrated onto the PES substrate. One of the largest RF components is a branch-line coupler, which is usually fabricated on PCB, not on semiconducting substrate due to its large size. To evaluate the integration capacity of RF passive components on PES substrates, the size of branch-line couplers consisting of λ/8 transmission lines12 was calculated from the wavelength and is listed in Table 2. As shown, the size of branch-line couplers is much less than 1 mm2 over a frequency range of 30 to 50 GHz, which indicates that bulky passive components can be integrated onto a PES substrate with a small chip size in the millimeter wave frequency range.

Figure 4

Figure 4 Measured wavelength of a coplanar waveguide on PES.

Table 2

Figure 5 shows the characteristic impedance Z0 of coplanar waveguides on PES as a function of line width. A characteristic impedance Z0 of 50 to 160 Ω is obtained for a line width range of 10 to 110 µm. From this result, it can be seen that coplanar waveguides of various characteristic impedances can be realized on a PES substrate by only changing the line width W. They can be used for matching between RF components of various impedances.

Figure 5

Figure 5 Measured wavelength of a coplanar waveguide on PES.

Figure 6

Figure 6 Measured propagation constant ß of the coplanar waveguide on PES.

Figure 6 shows the propagation constant β of the coplanar waveguide on PES. The coplanar waveguide on PES shows a β of 0.35 to 1.61 rad/mm from 10 to 50 GHz. Figure 7 shows the high frequency effective permittivity εeff of the coplanar waveguide on PES. For a comparison, the εeff of the coplanar waveguide on silicon substrate is also plotted. As shown, the εeff of the coplanar waveguide on PES is lower than that of the silicon substrate. The coplanar waveguide on PES shows an εeff of 2.35 to 2.9 from 10 to 50 GHz, while the coplanar waveguide on silicon substrate shows an εeff of 5.9 to 13.6 from 10 to 50 GHz. Due to the lower εeff, the physical length of coplanar waveguide on PES is longer than on a silicon substrate for the same electrical length. Table 3 shows the length of a λ/8 coplanar waveguide on PES and silicon substrate. The coplanar waveguide on a silicon substrate shows strong frequency dispersion characteristics. This tendency originates from the existence of various propagation modes on the oxide/silicon substrate. The skin-effect and slow-wave mode of propagation exists, in addition to the quasi-transverse electromagnetic (quasi-TEM) mode on oxide/silicon substrates, which causes a large change of εeff between 1 and 20 GHz.9 On the other hand, the coplanar waveguide on PES shows a very weak frequency dispersion characteristic, compared to a silicon substrate, because the quasi-TEM mode propagates dominantly on the metal/high-insulating substrate structure.13 The above results indicate that the coplanar waveguide on PES is suitable for broadband applications, due to its very weak frequency dispersion characteristic.

Figure 7

Figure 7 Effective permittivity εeff of the coplanar waveguide on PES and silicon substrates.

Table 3

Conclusion

In this work, a coplanar waveguide was fabricated on a PES substrate and its RF characteristics were thoroughly investigated for the first time. The coplanar waveguide on PES showed a very low loss compared with a conventional silicon substrate. It showed an insertion loss lower than 0.6 dB/mm and an attenuation constant alower than 0.065 Np/mm up to 50 GHz. The transmission line Q-factor was derived from a resonant tank built using a one-quarter wavelength long line at the resonance frequency f0. According to a measured result, it was found that the Q-factor of a coplanar waveguide on PES was 40.3 at a resonance frequency of 46.7 GHz. The coplanar waveguide on PES showed a wavelength of 3.9 to 6.2 mm from 30 to 50 GHz, which means that bulky passive RF components can be integrated onto a PES substrate with a small area in the high frequency range. The sizes of conventional branch-line couplers consisting of λ/8 transmission lines on PES are smaller than 0.619 mm2 from 30 to 50 GHz. Compared with a silicon substrate, the coplanar waveguide on PES showed lower εeff. The coplanar waveguide on PES shows an εeff of 2.35 to 2.9 from 10 to 50 GHz, which caused a physical length of transmission line longer than for a silicon substrate. The coplanar waveguide on PES showed a very weak frequency dispersion characteristic compared with a silicon substrate, which means that the coplanar waveguide on PES is suitable for broadband applications. The results indicate that the PES is a promising candidate for application to flexible RFIC at high frequencies. This work is the first known report of an investigation of RF signal transmission characteristics on a PES substrate.

Acknowledgment

This research was financially supported by the Ministry of Education, Science Technology (MEST) and National Research Foundation of Korea (NRF), through the Human Resource Training Project for Regional Innovation. This research was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0007452).

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