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New CMOS-MEMS tunable capacitors have been designed, fabricated and tested. Large-tuning range and high quality factor, Q, are achieved. The structures were made from the CMOS interconnect stack using a maskless CMOS micromachining process. Our tunable capacitor designs can be classified into two categories based on their tuning schemes as gap&area tuning and gap-only tuning. The capacitors with gap&area tuning were fabricated using Austria Microsystems (AMS) 0.6 µm and Agilent 0.5 µm CMOS processes. These devices have a measured nominal capacitance of 209 fF and a measured Q of 28 at 1.5 GHz. The capacitance change is measured from 209 fF to 294 fF within a 24 V control voltage, and 72.4 mW power at 1.5 GHz. The capacitors with gap-only tuning were fabricated in the TSMC 0.35 µm CMOS process and have larger tuning range and more power efficiency than the 1st generation designs. For these new designs, 3.52 to 1 tuning range has been measured with tuning from 42 fF to 148 fF within a 12 V control voltage and 34mW power and Q of 52 at 1.5 GHz. The tuning mechanism uses electro-thermal actuation. The essential differences between this work and prior work are the CMOS compatibility and more area efficiency at several gigahertz. Wide-range VCOs can be designed by using these MEMS tunable capacitors, inductors and CMOS or SiGe electronics on the same chip. Parasitic losses can be minimized and phase noise performance can be improved.
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