The effect of parallelism of CMUT cells on phase noise for chem/bio sensor applications

We investigated the effects of electrically connecting multiple microresonators on the frequency noise with a goal to improve the resolution of a chemical sensor based on the capacitive micromachined ultrasonic transducer (CMUT) technology. We fabricated twenty-two 50-MHz CMUTs with varying number of cells and measured the input impedance characteristics. The impedance measurement results show a linear increase in quality factor as the number of cells increases. Further, a phase noise simulation of Colpitts oscillators employing these CMUTs verifies that the phase noise of the oscillator in the 1/f² regime are influenced by the quality factor while the phase noise in the white noise regime are primarily affected by the motional impedance. The oscillator based on the CMUT with 1027 cells has 8.8 dB and 11.4 dB lower phase noise than that based on the CMUT with 397 cells at offset frequencies of 1 kHz and 5 MHz, respectively. Therefore, we demonstrated that electrically connecting multiple microresonators is an effective technique to improve the sensor resolution.