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ICP Etching Reduces MM-Wave Substrate Loss

FOR GALLIUM-ARSENIDE(GAAS) coplanar passive devices, the design methods used in centimeterwave frequencies have been proven to work for millimeter-wave frequencies up to W-band (75 to 110 GHz). Those same methods can be applied to CMOS coplanar devices at millimeter-wave frequencies. To demonstrate this point, two test third-order, quarter-wavelength, double-shortedstub wideband bandpass coplanar filters have been implemented at E-band by Pen-Li Huang, Tao Wang, and Shey-Shi Lu from National Taiwan University together with Yo-Sheng Lin from Taiwan's National Chi Nan University.

CMOS-compatible, inductively coupled-plasma, deep-trench technology was used to selectively but completely remove the silicon underneath the filter. After the filters were fabricated, postintegrated- circuit (post-IC) ICP processing was done on the backside of the die. After the backside etching, the results for Filter 1 showed that the input matching bandwidth (S11) below 10 dB moved from the 38.1-to-73.2-GHz band to the 49.4-to- 84-GHz band. In addition, the 3-dB bandwidth for forward transmission, S21, went from the 38.4-to-69.7-GHz band to the 47.1-to-83-GHz one. Filter 1 also achieved a 4.58-dB improvement in peak S21 performance to 3.8 dB. See "Micromachined CMOS E-Band Bandpass Coplanar Filters," Microwave And Optical Technology Letters, December 2008, p. 3123.

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