Realization and characterization of surface micromachined tunable band stop CSRR filter using MEMS bridges on silicon substrate

• Optimization of the fabrication process using SU8 photo-resist.
• Compatibility with standard CMOS technology.
• Reducing a major lithography step and still maintaining the overall process flow.
• Reporting electro-mechanical, mechanical and RF performances to ensure tunability.

The paper presents design, fabrication and characterization of a tunable filter on silicon substrate using Complementary Split Ring Resonators (CSRRs) and Radio Frequency Micro Electro-Mechanical System (RF MEMS) variable capacitor. CSRRs are designed on signal line, as well as both ground planes of the Coplanar Waveguide (CPW). Detailed design of such a filter has been already reported by the authors (B. Pradhan and B. Gupta, 2014) [1]. In continuation of B. Pradhan and B. Gupta (2014) [1], this paper presents an overview of the fabrication process of the design and its RF, electromechanical and mechanical measurements. The device is fabricated using surface micromachining process with a sacrificial layer. Generally, S1813 and other positive photo-resists are used for this purpose. But in that case the fabrication process becomes more time and cost consuming, since it involves more number of fabrication steps. Hence we use negative SU8 photo-resist as sacrificial layer, resulting in a more efficient fabrications process. It is removed with the help of Propylene Glycol Monomethyl Ether Acetate (PGMEA) solution and then the device rinsed in low surface tension liquid to avoid stiction during beam release. Through electrostatic actuation of the RF-MEMS switch, the RF characteristics of the filter are modified, so that its resonance frequency is tuned. The simulated and measured rejection of stop bands are around −20.19 dB and −31.9 dB for down state around the centre frequency 35.32 GHz & 35.26 GHz and −18.29 dB and −32.02 dB for up state around the centre frequency 38.80 GHz & 38.73 GHz. Modal analysis of the fixed–fixed beam is verified using as Scanning Laser Doppler Vibrometer (LDV).

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