диафрагмированные волноводные фильтры / 2e5158a5-5cdd-4c32-9bde-0b6f5988ba74

Design of a dual-band waveguide filter based on micromachining fabrication process

ISSN 1751-8725

Received on 30th July 2015 Revised on 14th November 2015 Accepted on 15th December 2015 doi: 10.1049/iet-map.2015.0489 www.ietdl.org

calculated as

convenience of simulation process, the width of the irises w0 is fixed at 50 μm.

For n = 4, the element values for Chebyshev low-pass filter

(9)prototypes (0.01 dB ripple) are g1 = 0.7128, g2 = 1.2003, g3 =

1.3212, g4 = 0.6476, g5 = 1.1007. Based on the K-inverter transformation, it can be calculated that

The first step is to simulate the unloaded quality factor with different metal thickness and roughness. The metal type is set as Au with its corresponding conductivity of 4.1 × 107 S/m. The EM simulation software used is HFSS and the solution type is set to be eigenmode. The quality factor is calculated based on the TE101 resonant mode of the rectangular metallic cavity. As shown in Fig. 3, with the increase of the metal thickness, the skin effect becomes less dominant. Above 1.4 μm, the quality factor is almost not affected by the thickness and the target Au thickness in the fabrication process is about 3 μm. As shown in Fig. 4, the quality factor drops dramatically after 1.6 μm, this is because surface roughness can increase the resistance of the metal layer, and high resistance causes more conductor loss, which eventually lowers the quality factor. Based on the fabrication process data provided by the manufacturer, the roughness can be controlled below 0.8 μm. Hence, from the analysis above, the fabricated Au layer can satisfy the quality factor requirements based on simulation.

The next step is to determine the coupling coefficients and the phase shift of the irises. In our design, four resonators are employed and coupled together for each one passband, as shown in Fig. 5. The standard waveguide WR-3 with the width of 0.8636 mm and height 0.4318 mm is chosen, and for the

K01 = K45 = 0.439, K12 = K34 = 0.149, K23 = 0.109 (10)

Let m be the gap width between two irises, and the coupling coefficient and the phase shift are simulated from m = 200–550 μm at 240 GHz. The simulated results are shown in Fig. 6. To satisfy (10), three points m1 = 535 μm, m2 = 390 μm and m3 = 360 μm can be found, then their corresponding phase shifts are arg(S21)1 = −148.6°, arg(S21)2 = −115.0° and arg(S21)3 = −148.6°. Based on (9), at 240 GHz, lg = 1.810 mm, and then from (8), the two amended

lengths can be calculated as len1 = 0.571 mm and len2 = 0.665 mm. Then, the simulated S-parameters of the first passband are shown in

Fig. 7. In this figure, it is shown that S11 is below 15 dB, therefore the results need further optimisation.

The design of the second passband is quite similar to the design of the first one. The K-inverters can be calculated as

K01′ = K45′ = 0.333, K12′ = K34′ = 0.085, K23′ = 0.063 (11)

The coupling coefficient and the phase shift are simulated from m = 200–450 μm at 300 GHz. The simulated results are shown in Fig. 8. Also in order to satisfy (11), three points m1′ = 430 μm, m2′ = 290 μm and m3′ = 360 μm can be found, then their corresponding

Fig. 7 Simulated S-parameters of the first passband

phase shifts are arg(S21)11′ = −39.5°, arg(S21)′2 = −4.1° and arg(S21)′3 = −0.6°. Based on (9), at 300 GHz, l′g = 1.225 mm, and then from

(8), the two amended lengths can be calculated as l′en1 = 0.213 mm

and l′en2 = 0.152 mm. Then, the simulated S-parameters of the second passband are shown in Fig. 9. Still, these results need

further optimisation.

The final step is to connect these two separate filters with input and output T-junctions based on the parallel structure to build up two passbands, as shown in Fig. 10. The advantage of this parallel structure is that each branch of the filter can be optimised separately, without major influence on the other branch. The final step in the design process is tuning and optimisation using EM simulation software HFSS. After satisfactory simulated results are obtained, the fabrication process then can be started.

3 Fabrication and measurement

The fabrication process is based on micromachining in micro-electromechanical systems (MEMS) technology (shown in Fig. 11). Two pieces of Si layers are prepared for fabrication. One is for cavities and the other is for the cover. First, a thin layer SiO2 is deposited on the Si layer using chemical vapour

deposition. The next step is lithography, etching the SiO2 to the desired pattern with no influence on the Si layer. Then, ICP etching is applied. The parts of the Si layer that are exposed to air are etched to the thickness of 0.4318 mm (the height of the standard waveguide WR-3), while the other parts of the Si layer are protected by SiO2 from etching. After the ICP etching process,

Fig. 11 Micromachining fabrication process

the SiO2 is removed from Si. Then after mask removal, a thin layer of Au is sputtered and electroplated on the surfaces of the etched Si layer and the cover layer. The target thickness of the Au layer is about 3 μm. The final step is bonding these two layers to form the metallic waveguide.

The topology of the proposed filter is shown in Fig. 12 and the geometric parameters are listed in Table 1. The final simulated, measured results and the photograph of the fabricated filter under microscope are shown in Fig. 13. In the simulation process, Au boundary condition with the thickness of 3 μm and the roughness of 0.8 μm has been used. The simulated insertion losses of the first and the second passbands are 0.745 and 0.960 dB, respectively. The filter is measured in a fixture using vector network analyser with a pair of frequency expansion devices. The input and output ports of the fabricated filter are connected to the WR-3 waveguides using a pair of flanges, as shown in Fig. 14. The insertion loss of the first passband is about 3 dB with the relative

5 dB with the relative 3 dB bandwidth of 3.3%, and the return loss is also over 10 dB. The insertion loss is caused by the EM wave leakage due to the imperfect contact between the cover and the cavities, and by the roughness of the ICP etching process. The

measurement fixture can also bring about additional insertion loss. The isolation between the two passbands is about 40 dB. In general, the simulated and measured results are in good agreement.

4 Conclusion

In this paper, a dual-band waveguide filter based on micromachining fabrication process in THz is simulated, fabricated and measured. The proposed filter is based on the parallel structure with a pair of four-stage rectangular waveguide resonators. The measured results agree well with the simulated ones and good performance has been achieved, and the proposed filter can be applied in modern millimetre wave communication systems.

5 Acknowledgment

This work was supported by the National Natural Science Foundation of China (grant no. U1430102).

Fig. 13 Final simulated and measured results

Fig. 14 Structure of the test fixture

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