MEMS resonator is a microstructure with mechanical degrees of freedom and fabricated using semiconductor microfabrication technology (see Fig. 1a). The applicant has developed a compact, sensitive, and fast THz bolometric detector using a GaAs doubly clamped MEMS beam resonator. MEMS resonator detects the input THz radiation as a shift in the resonance frequency and works as a very sensitive THz detector1,2. This unprecedented temperature sensitivity of the MEMS resonator is advantageous not only for realizing high sensitivity, with noise equivalent power (NEP) as low as ~20 pW/√Hz, but also for achieving a high-detection speed at the order of 10 kHz, which is more than 100 times faster than other uncooled THz thermal detectors (see Fig. 1b). 

Generally, the thermal sensitivity of a bolometer is inversely proportional to the thermal conductance of the structure. To increase the sensitivity of bolometers, thermal conductance is usually strongly reduced in the device design, which however will greatly increase the thermal time constant and reduce the operation speed of the device. To address this issue, the applicant has introduced a porous phononic structure in the MEMS bolometer4 (see Fig. 2). The porous structure reduces not only the thermal conductance but also the heat capacity of the MEMS beam. Therefore, the thermal sensitivity is improved without significantly degrading the response speed of the MEMS bolometer. The applicant has successfully fabricated MEMS beams with very high porosity (proportion of pore volume) of 0.7, which reduces the thermal conductivity of the beam by 90% and increases the thermal sensitivity of the MEMS radiometer by more than 10 times5.

The applicant has found a giant enhancement in the thermal responsivity of MEMS resonators by using the internal mode coupling effect. This is achieved by coupling the fundamental bending mode with the fundamental torsional mode of the MEMS beam resonators through the cubic Duffing nonlinearity. In the mode coupling regime, when the input heat to the MEMS resonators is modulated at a particular frequency, the resonance frequency shift caused by heating can be enhanced by almost two orders of magnitude6 (see Fig.3). The observed effect is promising for realizing high-sensitivity THz detection, and also other sensing applications with MEMS resonators. 

The mechanical nonlinearity is a common issue that limits the signal-to-noise ratio of MEMS resonator. The MEMS beam resonator has a hardening nonlinearity, i.e., as resonance frequency increases with the oscillation amplitude. To maintain a stable oscillation condition, MEMS resonators generally works in the linear oscillation regime that has very small oscillation amplitude of ~10 nm. Here, it has been found that the mechanical nonlinearity of the MEMS resonators can be significantly reduced by introducing internal strain to the MEMS beam. The decrease in the mechanical nonlinearity is originated from the bending of the MEMS beam, which gives a negative nonlinearity term and hence compensates the total nonlinearity. With the thermal tuning effect, the MEMS resonator can maintain a very large quasi linear oscillation amplitude of ~ 300nm (see Fig. 4), which is ~10 times larger than the linear oscillation range without the control of nonlinearity7,8. The observed effect is very promising for improve the output signal level of MEMS devices.