Micromechanical broadband vibration amplitude-amplifier for microseismic and acoustic emission detection

In Switzerland, villages and infrastructure are often built close to steep rock slopes, where rock falls are not uncommon. A microseismic/acoustic emission monitoring system is desirable, which detects precursory activity in the rock slopes and can give an advance warning. Commercial microseismic/acoustic emission monitoring systems are expensive, generate a large amount of data and consume considerable energy. MEMS potentially offer a more cost-effective solution and also highly integrated systems. For example, power hungry electrical preamplifiers may be replaced with a passive micromechanical amplitudeamplifier by shifting the amplification from the electrical to the mechanical domain.

In this thesis, a passive micromechanical broadband amplitude-amplifier for ultra-low-power detection of microseismic/acoustic emission signals is explored. It is based on a coupled mass-spring system. Masses and spring constants decrease towards the end of the coupled mass-spring chain and weak vibrations exciting the first resonator are amplified while traveling towards the last resonator, if they are within the allowed frequency band.

Three different device designs and fabrication processes based on front- and backside dry etching of a silicon wafer, respectively silicon-on-insulator wafer, are presented. Out-of-plane and in-plane moving structures were fabricated and a two-level fabrication process for multiscale device fabrication is shown. It enables the coupling of MEMS resonators with two different thicknesses (e.g. a weight ratio of 26’244) and reduces die size due to resonator stacking. Steadystate and transient responses of the fabricated devices were optically determined and compared with a 1D lumped element model.

Proof-of-concept devices showed an increased amplification and bandwidth by coupling more resonators. While a minimum (average) amplification of 3.4 (16.7) and a bandwidth of 0.93 x f0, N4 was achieved with 4 coupled resonators, a system with 8 resonators reached a minimum (average) amplification of 10.2 (58.4) over a bandwidth of 1.08 x f0, N8 (with resonance frequencies of single resonators: f0, N4 = 16.2 kHz, f0, N8 = 5.2 kHz).

Amplification and bandwidth tuning by design was also demonstrated. Coupling more resonators leads to a higher amplification over a broader bandwidth, while increasing the ratio at which the masses and spring constants decrease leads to a higher amplification but smaller bandwidth (e.g. 24.3 minimum (54.3 average) amplification over a bandwidth of 0.43 x f0, N3 versus 7.9 minimum (20.6 average) amplification over a bandwidth of 1.06 x f0, N7, with f0, N3 = 9.7 kHz, f0, N7 = 10.4 kHz).

Finally, a multiscale device with ten resonators coupled in series, which decrease in mass by a factor of three each, is presented. The first ten Eigenmodes of the device are in-plane and unidirectional. A minimum (average) amplification of 63 (295) over a bandwidth of 4.4 - 15.1 kHz (1.00 x f0, N10A, with f0, N10A = 10.7kHz) can be achieved in less than 1 ms. A second device with the same design but from another fabrication run showed similar characteristics (100 minimum (343 average) amplification over a bandwidth of 0.95 x f0, N10B, with f0, N10B = 10.1 kHz). The short response time below 1 ms is crucial to detect vibrational bursts in e.g. steep rock slopes or for any other structural health monitoring applications.