диафрагмированные волноводные фильтры / d057d6a5-84c3-4952-ac34-e06fa0f713c4

Abstract - A metallic waveguide is one of effective media for millimeterand submillimeter-waves because of the advantage of low-loss. This paper describes trial fabrication of a PTFE-filled waveguide bandpass filter (BPF) with the use of the SR (synchrotron radiation) direct etching process of PTFE, sputter deposition of metal, and electroplating. PTFE is known as a difficult material to process at high precision. However, it has been reported that PTFE microstructures can be fabricated by the direct exposure to SR. In this paper, an iris-coupled waveguide BPF with 5-stage Chebyshev response is designed at Q-band, and the fabrication procedure for the PTFE-fiIled waveguide BPF is described. The measurement results of the fabricated BPF are shown.

Index Terms - X-ray lithography, waveguide filters, waveguide components dielectric waveguides, bandpass filters.

I. INTRODUCTION

Microparts such as mechatronics, optics, and fluidics have been developed by various micromachining technologies such as laser cutting, lithography, etc. In the field of microwave and millimeter-wave engineering, trial fabrications of hollow waveguide hom antennas at W-band based on the fabrication process with SU-8 photoregist [1], a coplanar transmission line and its filter at microwave band based on the LIGA process [2], and a hollow waveguide of laser-curing resin with copper-electroplating at W-band [3] have been reported.

PTFE (Polytetrafluoroethylene) is widely used as a microwave and millimeter-wave material. It is also known that PTFE is a difficult material to process at high precision by conventional machining, or even by laser cutting or electrical discharge machining. However, it has been reported that PTFE microstructures can be fabricated by direct exposure to synchrotron radiation (SR) [4][5]. Recently, a trial fabrication of a PTFE-filled waveguide was attempted using the SR direct etching process [6][7]. In the trial fabrication, a straight section and a mitered-bend section of simple structure were obtained at Q-band. It has been shown by measurement results that they can work as millimeter-wave waveguides.

The metal-covered waveguide, especially H- and E-plane waveguide components suit with the applications, because the structures are very simple and various desired performances are realized by optimization of the contour of component structure [8][9]. These properties are favorable to the sub-

Imillimeter-wave applications requiring refined configurations.

In this paper, an iris-coupled PTFE-filled waveguide bandpass filter (BPF) with 5-stage Chebyshev response that contains narrow slit patterns is designed at Q-band. The purpose is to examine the validity of the fabrication process for the practical components. The BPF is selected as an example. First, the structure and the frequency characteristics of the S-parameters are shown. The PTFE-fiIled waveguide BPF is designed using formulas in [10]. Next, the outline of the fabrication procedure for the PTFE-filled waveguide BPF is described. The fabrication process consists of the SR etching of the PTFE pattern and coating Au film on it by sputter deposition and electroplating. Finally, the measurement results of the S-parameters of the fabricated PTFE-fiIled waveguide BPF are shown. For the measurement, PTFE-filled waveguide to standard waveguide transformers are necessary to connect the PTFE waveguide to a network analyzer. The frequency characteristics of the S-parameters inclusive of the transformers are simulated using Ansoft's HFSS. The validity of the fabricated PTFE-filled waveguide BPF is confirmed by comparing with the results obtained using HFSS.

Fig. 1. Iris-coupled PTFE-filled waveguide BPF.

(b)

Fig. 4. Sputter deposition. (a) Coating Au all over the surface of the PTFE pattern. (b) Photograph after sputtering. Adhesive tape of the right side is removed, and the original PTFE surface is seen.

automatically carried out. Consequently, the waveguide pattern can be obtained.

B. Sputter deposition and electroplating

Next procedure is sputter deposition of metal and electroplating. A waveguide structure covered with thin metal film is realized by sputtering Au all over the surface of the PTFE pattern etched in the previous section as shown in FigA(a). First, the etched PTFE pattern is cleaned for preparation. If some PTFE fragments are remained after the

etching process, they should be removed. Due to the hydrophobic property of PTFE, it is difficult to form a film of

metal unless surface modification of PTFE is introduced. Therefore, the surface of the PTFE pattern is exposed to Ar plasma for several minutes. It is expected that anchor effect increase the adhesive strength. Subsequently, the Au film is deposited on all the surface of it by sputtering. In this work, the Au film of 500nm thick was formed. FigA (b) shows a photograph of the PTFE pattern after sputtering. Adhesive tape of the right side is removed, and the original PTFE surface is seen. Both sides of the PTFE sheet are sputtered. The deposition thickness must be determined with consideration of the skin depth for millimeter-wave in order not to leak the electromagnetic wave. The sufficient thickness than the skin depth is required to work as a waveguide.

For that purpose, electroplating is used to increase the thickness of the Au film up to about lOllm. Because the skin depth of Au at Q-band is OAllm, it is considered that the sufficient thickness is accomplished. Finally, removing the unnecessary frame of the pattern, the fabrication of the PTFE­ filled waveguide BPF is completed.

IV. FABRICATION AND MEASUREMENT

Based on the above process, the iris-coupled PTFE-filled waveguide BPF with 5-stage Chebyshev response shown in

Fig. 5. PTFE pattern of the filter obtained through the SR direct etching process.

(b)

Fig. 6. PTFE pattern of the filter covered with Au film by sputter deposition and electroplating. (a) Over view. The frame is masked to avoid electroplating. (b) Enlarged photograph of the irises.

Fig. 2 was fabricated. Figure 5 displays the PTFE pattern of the filter obtained through the SR direct etching process. In the exposure process, the substrate temperature was kept at about 220°C in the evacuated chamber. The amount of X-rays exposed is 3600 A*sec. Because the NewSUBARU runs at a storage current of 220 rnA on average, it takes about 4.5 hours to obtain the PTFE pattern in Fig. 5. It is found that the filter pattern can be etched directly.

Figure 6 shows photographs of the PTFE pattern covered with Au film resulting from the sputter deposition and electroplating process. The Au film maintains adhesive force enough to prevent exfoliation for general handling. After forming the Au film, removing the unnecessary frame, the fabrication of the PTFE-filledwaveguide BPF is completed.

Because the cross section of the PTFE-filled waveguide is smaller than that of the standard Q-band waveguide, the PTFE-filled waveguide to standard waveguide transformer is utilized for measurement. Figure 7 shows the configuration of the fabricated filter connecting the transformers at both ends.

Fig. 7. Circuit configuration of PTFE-filled waveguide BPF and transformers to be measured. (a) Model. (b) Photograph. Top covers of the transformers are removed.

The transformer consists of a widening section (the PTFE waveguide of 4mm width to the Q-band standard waveguide of 5.70mm), a matching section using finite-length window, and a 1.14 transformer in the waveguide E-plane to raise the height 1mm to 2.85mm.

The frequency characteristics of the S-parameters of the PTFE-filled waveguide BPF were measured by the network analyzer (Agilent E8361A). Figure 8 shows the measured results of the PTFE-filled waveguide BPF, which contain the characteristics of the two transformers. The frequency characteristics of the filter including the transformers simulated using HFSS are also shown in Fig. 8. Although the center frequency is shifted toward the higher frequency by about 180 MHz (0.4 % deviation against 42 GHz) and the insertion loss of 3dB is observed in S2h the measured results agree with the design results. The frequency deviation is equivalent to shortening the resonator lengths by 0.5 - 1 %. It can be explained by the accuracy of the mask, or by the thermal expansion of PTFE (about 2 % at 200°C) due to irradiation process. S21 simulated with the conductor and the dielectric losses becomes 2.04 dB (QIl == 346) without the transformers. The measurement results of the through connection of the two transformers (back-to-back connection) show the insertion loss of 1.2 - 2 dB [7]. Judging from these facts, the insertion loss of the filter itself is about 2 dB and it is comparable to the theoretical value. Figure 9 displays the measured group delay of the PTFE-filled waveguide BPF. It is found that a linear phase response in the passband is realized.

v. CONCLUSION

The fabrication of the PTFE-filled waveguide BPF with 5- stage Chebyshev response was demonstrated by applying the SR direct etching process, sputter deposition process, and electroplating. The trial circuit was fabricated at the frequency range of Q-band. It was confirmed that functional circuits

41.5 42.0 42.5 43.0 Frequency[GHz]

Fig. 8. Measured frequency characteristics of S-parameters of iriscoupled PTFE-filled waveguide BPF designed at 42GHz.

Frequency[GHz]

Fig. 9. Measured group delay.

making use of the PTFE-filled waveguide could be realized. The present fabrication process is useful to construct the waveguide components for millimeter-wave frequencies and also for submillimeter-wave frequencies.

ACKNOWLEDGEMENT

Part of this work was supported by KAKENHI (21760263).

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