A transmission line employing a periodic structure was fabricated on a polyether sulfone (PES) substrate and its RF characteristics were investigated. A fishbone-type transmission line, employing a comb-type ground plane (FTLCGP) structure, was designed to reduce the wavelength. The FTLCGP structure on PES substrate showed a wavelength much shorter than for a conventional coplanar waveguide on PES. The FTLCGP structure on the PES substrate showed an attenuation constant alower than 0.2 Np/mm, up to 40 GHz, which was much lower than for a transmission line on a commercial silicon substrate. Using the FTLCGP structure, a miniaturized impedance transformer was fabricated on a PES substrate for flexible RFIC applications. The size of the impedance transformer was 0.45 mm2, which was 57.5 percent of the size of the transformer fabricated by conventional coplanar waveguide on PES substrate. The return and insertion losses were 43 and 0.74 dB, respectively, at a center frequency of 22 GHz and the return loss values were better than 10 dB and the insertion loss was better than 1.1 dB up to 46.4 GHz

Figure 1

Figure 1 The FTLCGP structure (a) and a typical coplanar waveguide (b).

Flexible electronics have drawn significant attention, owing to their variety of applications, such as flexible displays, smart tags and wearable products.1 In the development of a transparent flexible display for mobile communications, RF devices should be integrated into a transparent flexible substrate. Recently, polyether sulfone (PES) has drawn attention for flexible displays, due to its good heat-resisting property, high transparency and good flexibility.2,3 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.2,3 For a short time, 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), soldering and bonding processes of electronic devices on PES can be easily performed, which facilitates the packaging process. In addition, a very thin PES substrate, with a thickness of less than 100 µm, can be used for fabrication of electronic devices due to its stability, 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 for a microelectromechanical system (MEMS). Besides the above-mentioned properties, the PES shows good water resisting qualities. For this reason, PES is well suited for transparent flexible displays used in mobile communications and several groups have employed the PES substrate to evaluate the electrical properties of oxide films.4

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In mobile communication flexible display applications, RF passive components5,6 as well as active devices should be integrated in PES substrates. An impedance transformer7 is a key device for impedance matching between RF devices. In this work, a miniaturized impedance transformer, employing a periodic structure, was fabricated on a PES substrate for impedance matching on a flexible RFIC and its RF characteristics were investigated. This is the first known report of an impedance transformer fabricated on a flexible PES substrate.

RF Characteristics of Transmission Line Employing Periodic Structure on PES

According to previous results,8 a coplanar waveguide on a PES substrate showed a wavelength much longer than the one on commercial silicon substrate, due to the low permittivity of PES, which is unfavorable to integrate  RF passive components on a PES substrate because of its large circuit size. The wavelength at 20 GHz, for a coplanar waveguide on silicon and PES substrate, was 5.71 and 9.29 mm, respectively. Here, a transmission line employing a periodic structurewas fabricated on PES to reduce the wavelength. A fishbone-type transmission line employing a comb-type ground plane (FTLCGP) was designed. Until now, various types of periodic structures have been studied for application to RF circuits. The FTLCGP structure was also fabricated on PCB for low impedance transmission lines in S-Band.6 The FTLCGP structure fabricated on the conventional PCB successfully operated as a transmission line up to S-Band. However, according to measured results, the FTLCGP structure on a conventional PCB such as Teflon showed a very narrowband characteristic at millimeter wave frequencies and it could not be used as a transmission line. In this work, the FTLCGP structure was fabricated on a PES substrate, and it successfully operated as a transmission line on a PES substrate up to 50 GHz.

Table 1

Figure 1 shows the structures of a FTLCGP and a typical coplanar waveguide without periodic structure on the PES substrate. As shown, the FTLCGP consists of a fishbone-type signal line and comb-type ground planes. The fishbone-type signal line consists of a central line with periodic metal strips (PMS) and the comb-type ground plane consists of a ground plane and periodic ground strips (PGS). The PMSs are placed alternately with the PGSs. The conventional coplanar waveguide has only a periodical capacitance (Ca) between the line and ground plane, while the FTLCGP has additional capacitances, Cb as well as Ca, due to the electromagnetic coupling between PMS and PGS. In addition, the FTLCGP has a periodical shunt capacitance Cc because PMS operates as an open stub at the operating frequency. Therefore, the FTLCGP exhibits a wavelength (λg) shorter than the conventional coplanar waveguide, because λg is inversely proportional to the periodical capacitance between signal line and ground. In other words, λg =1/[f(LC)0.5].

Table 1 shows the wavelength of transmission lines on PES. For a fabrication of transmission lines on PES, Au/Ti was deposited on PES substrate 200 µm thick and the thickness of the Au/Ti was 2 µm. For the FTLCGP, the length and width of PMS/PGS are 160 and 30 µm, respectively, and the distance between PMS and PGS is 30 µm. The signal line width is 70 µm. As shown in the table, the FTLCGP structure shows wavelengths much shorter than for the conventional coplanar waveguide. The wavelength at 20 GHz of the FTLCGP structure on PES is 4.81 mm, which is shorter than for a coplanar waveguide on silicon substrate. For example, the wavelength at 20 GHz of a conventional coplanar waveguide on silicon substrate with a thickness of 600 µm is 5.71 mm.

Figure 2

Figure 2 Measured attenuation constant α.

In order to investigate the suitability of the FTLCGP structure for RF applications, the loss of the transmission line was measured at 20 GHz. Figure 2 shows the measured attenuation constant aof the FTLCGP structure on PES. The insertion loss was measured for a 50 Ω impedance and it was normalized to the characteristic impedance of the transmission line. For comparison, the data for a coplanar waveguide on commercial silicon substrate was also plotted, because a silicon substrate is the most popular semiconducting substrate for commercial RFIC applications. As shown, the FTLCGP structure on PES shows a comparatively low loss, compared with the silicon substrate. It shows an attenuation constant alower than 0.2 Np/mm up to 40 GHz. This low loss of the transmission line on PES originates from its excellent electrical insulating properties. In the case of the coplanar waveguide on silicon substrate, there is a current flowing from line to ground plane through the silicon substrate due to a relatively high conductivity of the silicon substrate, which causes a relatively high loss of electromagnetic energy.5 In the case of the transmission line on PES, however, there does not exist a current flowing from line to ground plane through the PES substrate due to its good electrical insulating characteristic.

RF Characteristics of Impedance Transformer Employing a Periodic Structure on PES

Using the FTLCGP structure on PES, a miniaturized impedance transformer was developed for flexible RFIC applications. Figure 3 shows a photograph of the single section λ/4 impedance transformer on a PES substrate. Au/Ti was deposited on a PES substrate 200 µm thick and the thickness of the Au/Ti was 2 µm. The characteristic impedance Z0 of the transformer is given by Z0 = (Zc1Zc2) 0.5 where Zc1 and Zc2 are the source and load impedance, respectively.7 In this work, the impedance transformer was designed to transform an impedance of 70 Ω into a standard impedance of 50 Ω. Therefore, Z0 is 59 Ω. For a Z0 of 59 Ω, the length of PMS/PGS is 160 µm and signal line width is 0.45 mm. At a center frequency of 22 GHz, the length of the λ/4 transformer is 1 mm. Therefore, the size of the impedance transformer is 0.45 mm2, which is 57.5 percent of the size of a transformer fabricated with a conventional coplanar waveguide on PES. If a λ/4 transformer with a Z0 of 59 Ω is fabricated with a conventional coplanar waveguide on a PES substrate, the signal line width and length are 0.34 and 2.3 mm, respectively, and its size is 0.782 mm2. The sizes of impedance transformers on a PES substrate are summarized in Table 2.

Figure 3

Figure 3 Photograph of the single section ?/4 impedance transformer using the FTLCGP structure on a PES substrate.

Figure 4

Figure 4 Measured RF performance of the impedance transformer on a PES substrate, return loss (a) and insertion loss (b).

Figure 4 shows the measured return loss and insertion loss of the transformer. The insertion and return loss were measured at a port impedance of 50 Ω, and it was normalized by source and load impedance, Zc1 and Zc2. As shown, an excellent RF performance can be observed from the transformer. The return and insertion losses are 43 and 0.74 dB, respectively, at a center frequency of 22 GHz, return loss values better than 10 dB up to 46.4 GHz and insertion loss better than 1.1 dB in the same frequency range.

Table 2

Conclusion

A FTLCGP structure was fabricated on a PES substrate and its RF characteristics were investigated. The FTLCGP structure on PES showed a wavelength much shorter than conventional coplanar waveguide on PES. The wavelength of the conventional coplanar waveguide on PES is 9.29 mm at 20 GHz, while the wavelength of the FTLCGP structure on PES is 4.81 mm at the same frequency. The FTLCGP structure on PES showed an attenuation constant a lower than 0.2 Np/mm up to 40 GHz, which was much lower than for a transmission line on commercial silicon substrate. Using the FTLCGP structure on a PES, a miniaturized impedance transformer was fabricated on a PES substrate for flexible RFIC applications. The impedance transformer was designed to transform an impedance of 70 Ω into a standard impedance of 50 Ω. The size of the impedance transformer was 0.45 mm2, which was 57.5 percent of the size of the transformer fabricated with a conventional coplanar waveguide on PES. Excellent RF performance could be observed from the impedance transformer. The return and insertion losses were 43 and 0.74 dB, respectively, at a center frequency of 22 GHz. The return loss values were better than 10 dB and the insertion loss were better than 1.1 dB up to 46.4 GHz.

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 work was financially supported by the Ministry of Knowledge Economy (MKE) and the Korea Institute for Advancement of Technology (KIAT) through the Workforce Development Program in Strategic Technology.

References

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Young Yun received his bachelor’s degree in electronic engineering from Yonsei University, Seoul, Korea in 1993, his master’s degree in electrical and electronic engineering from Pohang University of Science and Technology, Pohang, Korea in 1995 and his Ph.D. in electrical engineering from Osaka University, Osaka, Japan in 1999. From 1999 to 2003, he worked as an engineer with Matsushita Electric Industrial Company Ltd. (Panasonic), Osaka, Japan, where he was engaged in the research and development of monolithic microwave ICs (MMIC) for wireless communications. In 2003, he joined the Dept. of Radio Sciences and Engineering, Korea Maritime University, Busan, Korea. He is currently a professor and his research interests include design and measurement for RF/microwave and millimeter-wave IC and design and fabrication for HEMT and HBT.

Hong-Seung Kim received his bachelor’s, master’s and Ph.D. degrees in materials science and engineering from Korea Advanced Institute of Science and Technology in 1990, 1993 and 1999, respectively. He joined the Electronics and Telecommunications Research Institute, Korea in 1999 and has worked on the fabrication and development of SiGe heterojunction bipolar transistor (HBT) and InP/InGaAs HBT for OEIC. From 2001 to 2002, he was a post doctoral research associate in electrical engineering at Cornell University, Ithaca, NY, where he worked on three-dimensional integration. In 2003, he joined the Dept. of Nano Semiconductor Engineering, Korea Maritime University, Busan, Korea. He is currently a professor and his research interests include optoelectronic properties of ZnO based devices such as UV LED and transparent transistors.

Nak-Won Jang received his bachelor’s, master’s and Ph.D. degrees in electrical engineering from the Yonsei University, Korea, in 1990, 1992 and 1999, respectively. From 1992 to 1995, he was with Samsung Electronics, Korea, where he was involved in the design of video signal driver circuits for p-Si TFT LCD. After his Ph.D., he worked as a senior engineer in the semiconductor R&D division of Samsung Electronics,where he was engaged in the research and development of 4-Mb and 32-Mb FRAM. He joined the Korea Maritime University as a professor in the Dept. of Electrical and Electronics Engineering on September 2003. He is currently a professor and his research interests are the design and fabrication for LED and ZnO TFT.