Résonateurs MEMS en GaN

Figure 1: (a) Top view of the AlGaN/GaN MEMS resonator. On the right, the piezoelectric actuator uses the AlGaN layer sandwiched between a top electrode and the two-dimensional election gas. On the left the R-HEMT is fabricated on the resonant beam for motion detection. (b) Measured S21 parameter between the input and the output of the beam resonator. The black curve shows the signal obtained using the R-HEMT as an amplifier, the blue curve shows the signal obtained using the gate as a capacitive detector. The gain of 30 dB provided by the R-HEMT shows the advantage of the integration of the HEMT on the resonant beam.

Figure 1: (a) Top view of the AlGaN/GaN MEMS resonator. On the right, the piezoelectric actuator uses the AlGaN layer sandwiched between a top electrode and the two-dimensional election gas. On the left the R-HEMT is fabricated on the resonant beam for motion detection. (b) Measured S21 parameter between the input and the output of the beam resonator. The black curve shows the signal obtained using the R-HEMT as an amplifier, the blue curve shows the signal obtained using the gate as a capacitive detector. The gain of 30 dB provided by the R-HEMT shows the advantage of the integration of the HEMT on the resonant beam.

Gallium nitride is expected to be an outstanding material for a new generation of MEMS resonator devices. It presents excellent mechanical and piezoelectric properties for MEMS actuation and detection. Moreover its electronic properties and the use of AlGaN/GaN heterostructures for transistors opens the way for the co-integration of MEMS sensors with electronics. The availability of GaN material deposited on Si substrates allows selective etching and easy release of free structures for MEMS fabrication. Additionally, GaN is a promising candidate for working in harsh environment.

We have been investigating this new technology from 2007 and we demonstrated the first GaN MEMS resonators with integrated transducers, along with the first R-HEMT (resonant high electron mobility transistor, Fig. 1) [Appl. Phys. Lett.94, 233506 (2009)]. We have also carried out an extensive study of the transduction physics and the piezoelectric actuation using the 2DEG of the AlGaN/GaN heterostructure [Appl. Phys. Express5, 067201 (2012)] as well as the piezoelectric active detection [J. Microelectromech. Syst., 111, 370 (2012)].

 

 

 

Figure 2: Experimental frequency response of the GaN SAW resonator.

Figure 2: Experimental frequency response of the GaN SAW resonator.

Based on this proof-of-concept, prospects of applications for sensors are under preparation. In addition, within the prospects of co-integration with electronics, GaN is a promising candidate for highly compact RF sources. Such sources would be particularly interesting for embedded microwave applications due to reduced size and weight. They could operate on the same board as most RF circuits and lead to advanced time-frequency systems. In this view, we achieved a new milestone in GaN resonators (Fig. 2) with the demonstration of a 1 GHz surface acoustic wave (SAW) oscillator. It features phase noise of -115 dBc/Hz at 10 kHz from the carrier and a f.Q product near 1.9×1012 Hz. This is the largest value obtained for GaN resonators that is in the range of other materials used for commercial applications.  This shows the potentiality of such devices for monolithic RF sources [IEEE IFCS 2013].