RF Channel-Select Micromechanical Disk Filters-Part II: Demonstration

Area of Science:

• Electrical Engineering
• Mechanical Engineering
• Materials Science

Background:

• Micromechanical filters are crucial for RF channel selection.
• Previous designs faced limitations in bandwidth, insertion loss, and rejection.
• Optimizing transducer gaps and mechanical design is key to improving performance.

Purpose of the Study:

• To present fabrication and measurement results for a micromechanical disk-based RF channel-select filter.
• To demonstrate the effectiveness of practical design additions for enhanced filter performance.
• To achieve a filter with a 0.1% bandwidth, low insertion loss, and high out-of-band rejection.

Main Methods:

• Fabrication of a micromechanical disk resonator filter.
• Introduction of a 39-nm-gap capacitive transducer.
• Implementation of voltage-controlled frequency tuning electrodes and a stress-relieving coupled array design.
• Hierarchical mechanical circuit design with 206 micromechanical elements.

Main Results:

• A 0.1% bandwidth, 223.4-MHz channel-select filter was successfully demonstrated.
• Achieved 2.7 dB in-band insertion loss and 50-dB out-of-band rejection.
• Polysilicon disk resonators with Q > 8800 and optimized capacitive transducer gaps (C_x/C_o ~ 0.1%) enabled high performance.
• Electrical tuning (12.1 V DC) improved insertion loss by 1.8 dB and achieved an equiripple passband.

Conclusions:

• The developed micromechanical filter significantly outperforms previous designs in terms of rejection and bandwidth.
• The combination of optimized transducer gaps, coupled array design, and electrical tuning is effective for high-performance RF filtering.
• Measured performance aligns well with predictions from an electrical equivalent circuit model, validating the design approach.