Abstract
This work presents the design and characterization of the monolithic CMOS-MEMS oscillators for temperature compensated clocks. An innovative ovenized double-ended tuning fork (DETF) resonator with an embedded heater trace is implemented for high heating efficiency. The heating efficiency of the resonator is greater than 266 °C/mW, which consumes less than 0.5 mW for 125°C temperature span (-40°C to 85°C). In addition, the resonator is designed to achieve a low temperature coefficient of frequency (TCf) of +5.1 ppm/°C. Combined with micro-oven operations, frequency variation less than 120 ppm across 125°C temperature span is demonstrated under a constant-resistance control. As a result, the compensated TCf less than 1 ppm/°C in this work outperforms other single-chip, BEOL-embedded CMOS-MEMS resonators to date. The monolithic CMOS-MEMS oscillator based on the 1.2-MHz DETF ovenized resonator is also realized in this work. The oscillator phase noise of -112 dBc/Hz at 1-kHz offset and -120 dBc/Hz at 1-MHz offset is demonstrated, which is on par with the state-of-the-art flexural-mode MEMS oscillators but with better circuit integration scheme. In addition to the ovenized oscillator, the phase noise spectrum of the monolithic CMOS-MEMS resonator is studied in this work. It is recognized that the close-to-carrier phase noise for the MEMS oscillator is mainly dominated by the nonlinear amplitude-to-phase noise conversion effect. By operating the nonlinear oscillator under proper conditions, the nonlinear noise conversion can be suppressed, resulting an improved phase noise. With the 1.2 MHz CMOS-MEMS DETF resonator, the best-case phase noise of -77 dBc/Hz at 10-Hz offset and -97 dBc/Hz at 100-Hz offset is demonstrated, featuring a figure of merit of -176.9 dB. To further improve the performance for CMOS-MEMS oscillator systems in the future implementations, a titanium nitride composite (TiN-C) MEMS platform is proposed not only for enhanced electrostatic transduction but also for improved frequency stability. The dielectric charging issue for traditional CMOS-MEMS resonators is solved in this platform by means of TiN-based electrodes. Moreover, the sub-ppm/°C TCf is also demonstrated in this work with only passive temperature compensation scheme. Importantly, the proposed platform can be scaled to advanced technology nodes for more functionality and improved performance.