No abstract

The thermal conductivity of single crystal silicon was engineered to be as low as 32.6W/mK using lithographically defined phononic crystals (PnCs), which is only one quarter of bulk silicon thermal conductivity [1]. Specifically sub-micron through-holes were periodically patterned in 500nm-thick silicon layers effectively enhancing both coherent and incoherent phonon scattering and resulting in as large as a 37% reduction in thermal conductivity beyond the contributions of the thin-film and volume reduction effects. The demonstrated method uses conventional lithography-based technologies that are directly applicable to diverse micro/nano-scale devices, leading to potential performance improvements where heat management is important.

We study and compare high coupling symmetric and shear mode Lamb wave resonators realized in thin plates of X-cut lithium niobate. Fundamental mode bar resonators with a plate width of 20 µm, a plate thickness of 1.5 µm, apertures of 50, 90 and 130 µm and acoustic wave propagation rotated 30 • (symmetric) and 170 • (shear) to the +y-axis were realized on a single die for direct comparison. As expected, the symmetric Lamb wave resonators exhibited a higher sound velocity of ~6400 m/s when compared to the shear velocity of ~3900 m/s. The shear mode resonators, however, were found to have a significantly higher effective piezoelectric coupling coefficient of 16.3%, compared to a maximum of 9.1% for the symmetric Lamb wave resonators. In addition, the shear mode resonators were found to be less sensitive to the device aperture and to have fewer spurious responses. Based on these results, the shear mode resonators were selected for scaling to higher operating frequencies. A shear mode lithium niobate Lamb wave resonator operating at 350 MHz has been demonstrated with an effective piezoelectric coupling of 16%, a quality factor in air of 2200 and a device figure-of-merit of 420, among the highest reported for Lamb wave resonators [1-3].

We present a method for attenuating the spurious responses in aluminum nitride micromechanical filters and demonstrate the technique in a 4-pole self-coupled filter operating at 494 MHz. In the standard implementation of a 4-pole self-coupled filter, each filter pole is realized using physically identical resonators. The spur mitigation approach reported here realizes the four poles of the filter using two different physical implementations of the resonator. Both resonators are designed to have identical responses at the desired resonant frequency of 494 MHz, while many of the spurious responses of the two resonators appear at nonidentical frequencies and do not add constructively at the filter output. Using the reported method, the measured attenuation of the largest filter spur is increased by 47.5 dB when compared with a 4-pole filter realized using identical resonators (standard approach) to form each filter pole. The filter realized using the reported spur attenuation approach has >59.6 dBc of stopband and spurious response rejection over nearly a 2-GHz frequency span.[2013-0342]

Frequency tuning of aluminum nitride (AlN) microresonators has been demonstrated via localized heating (ovenization) of the resonator. Specifically, piezoelectrically driven ∼100 MHz microresonators were heated by embedded joule heaters in vacuum. Three different designs with three different film stacks were tested, and among the tested devices, thermal resistances as large as 92 K/mW have been demonstrated, which corresponds to 1-mW power consumption to yield a temperature increase of 92 • C. To minimize heat loss, the devices were suspended from the substrate by high thermal isolation beam-type supports. The beams exhibit very high thermal resistance not only due to their high length to cross-sectional area ratio but also because they are made of thin-film-deposited polycrystalline aluminum nitride. Film-deposited AlN has been shown to have thermal conductivity much lower than that measured in bulk materials. Thermal time constants for these devices were measured ranging from submilliseconds to 10 ms depending on the design and film stacks, and frequency tunability was measured as high as 2548 parts per million/mW. The availability of a power-efficient frequency tuning method, coupled with all other performance benefits, makes AlN microresonators a promising candidate for the next-generation timing devices and tunable filters for multiband communication systems.[2012-0035]