II. ALN CONTOUR-MODE RESONATOR TECHNOLOGY
Contour modes of vibration are excited in c-axis oriented aluminum nitride films via the d31 piezoelectric coefficient. By applying an alternating electric field across the film thickness the AlN MEMS structure expands and contracts laterally and can be excited in resonant vibrations whose frequency is set by the in-plane dimensions of the device. The frequency of vibration is generally set by the width of the structure, whereas the second dimension can be employed to control the equivalent motional resistance and static capacitance of the device. Frequency setting via lithographic techniques enables the definition of multiple frequency devices on the same substrate and drastically reduces manufacturing tolerances on film thickness (by 10x) that are instead currently demanded by commercially available technologies such as FBARs and quartz shear resonators.
A. AlN Contour-Mode Resonators
Figure 1 schematically presents the range of operation for the three most promising device topologies developed to date for contour-mode AlN resonators. Rectangular plates, rings and higher-order contour-mode plates can be fabricated in the same process. According to experimental results, manufacturing considerations and theoretical observations (structural rigidity), and in order to achieve impedance levels that can be readily interfaced with 50 Ω RF systems:
- the rectangular plate geometry  can be effectively employed from 10 to approximately 100 MHz;
- the ring geometry  can be adopted between 100 and 400 MHz;
- the higher-order contour-mode rectangular geometry  can be used between 400 and 2,500 MHz.
Although based on preliminary results and subject to improvements through future research, these guidelines offer a good prospective of the status and range of applicability of the AlN contour-mode technology. As shown in Figure 2 these resonators are capable of Q as high as 4,300 in air, have motional resistances ranging from 25 to 700 Ω, figure of merit (FOM = Q·kt^2) close to 40 and can especially span a broad range of frequency from few MHz up to almost GHz on the same silicon chip.
B. AlN Contour-Mode Oscillator and Filters AlN
Contour-mode resonators have been demonstrated in circuit based applications for frequency synthesis and band pass filtering. Differently from all the other electrostatic MEMS resonator-based oscillators , a standard pierce circuit design that does not require automatic gain control or pump charge (no DC voltage is required by piezoelectric actuation) was employed. A 224 MHz circular ring with Q of 2,300 was wire bonded to the circuit realized in 0.25 µm CMOS technology (Fig. 3). Preliminary results are showing respectable phase noise performance of ~ - 110 dBc/Hz at 10 kHz offset. Although the consumed power is relatively high (2.5 mW total), it is expected that further optimization will reduce the power level to few hundreds µW.