5.8-GHz VCO with CMOS compatible MEMS inductors

This paper describes a lowpower, lowphase noise 5.8 GHzvoltage controlled oscillator (VCO) with on-chipCMOSMEMSinductor. The inductor was fabricated by TSMC 0.18 m one-poly six-metal (1P6M) CMOS process and also Chip Implementation Center (CIC) micromachining postprocess. During the post-process, the dry etching was utilized to remove the oxide between the winding metals and the silicon substrate under the inductor. Due to the alleviation of parasitic capacitance and lossy substrate, the quality factor and resonant frequency will be improved and extended. In this work, quality factor up to 15 was obtained for a 1.88 nH micromachined inductor at 8.5 GHz, and the improvement is up to 88% in maximum quality factor. The CMOS and micromachined inductor were both implemented with a 5.8 GHz VCO. Compared side by side with the CMOS inductor, the CMOS MEMS inductor produced a 5 dB lower phase noise improvement at 1MHz offset in this 5.8 GHz VCO.
The VCO is one of the important components in any RF/microwave communication system. In a receiver, the oscillator is used together with a mixer to convert the RF signal to an IF signal. The oscillator signal serves as a carrier to modulate low frequency to be transmitted. For a high quality receiver, the LC-oscillator topology is widely chosen because it enables to provide the lowest phase noise for a given amount of power. However, the phase noise and power consumption of LC-type tank oscillator are directly related to the quality factor (Q) of the LC tank. The equivalent quality factor of a LC tank is governed by lower Q factor component; therefore, limited by the CMOS integrated inductor In standard silicon IC process, the intrinsic problems always deteriorate the performance of passive devices. For example, the stray and parasitic capacitances lower the self-resonance frequency of inductor, and the substrate losses also reduce the Q factor of the device. These inherent problems of the traditional CMOS inductor limit the device performance, and furthermore the specifications of RF circuits, such as the power consumption and phase noise of a VCO circuit. In order to extend the CMOS applications into higher frequency range, the CMOS-compatible micromachining process has been proposed to improve the Q factor and operating frequency of an RF inductor. In the past 10 years, micromachining techniques have been led into the circuit processes to design a high performance inductor. For example, the surface micromachined solenoid inductors with copper plating, reported on by Kim and Mark , can reach to a very high quality value about 60 at 8 GHz. And the selective copper encapsulation on silicon developed by . also fabricated inductors with quality factor over 30 above 5 GHz. The out-of-plane inductor, reported on by Chua et al. , made from a sputtered stress-engineered molybdenum–chromium (MoCr) alloy can make quality factor even reach to 60–85. The above examples employed some respective micromachined processes and materials that can easily make low series resistance and block the lossy path around the device, but they also have the bottleneck to make the total process be compatible with the CMOS.

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