Intrinsic temperature compensation of aluminum nitride Lamb wave resonators for multiple-frequency references

In this paper we present the temperature compensation of aluminum nitride (AlN) Lamb wave resonators for a future application to XOs and TCXOs for a frequency ranging from 100 MHz to 1000 MHz. The temperature coefficient of frequency (TCF) for the lowest symmetric Lamb wave mode S₀ for AlN plates with h/lambda<0.3 is found to be around -30 ppm/K. A zero TCF resonator is obtained by adding a compensating silicon dioxide layer. The low dispersion of the phase velocity for the S₀-mode propagating in thin AlN plates reduces not only the fabrication tolerances towards thickness variations of the AlN layer, but also enables resonators operating over a wide frequency range, i.e. from 100 MHz to 1000 MHz, based on two absolute film thicknesses for AlN and SiO₂ achieving near zero TCF over the entire frequency range. The acoustic properties and different layer configurations of zero TCF Lamb wave devices are discussed in detail.