No abstract

A technique based on suspended islands is described to measure the in-plane thermal conductivity of thin films and nano-structured materials, and is also employed for measurements of several samples with a single measurement platform. Using systematic steps for measurements, the characterization of the thermal resistances of a sample and its contacts are studied. The calibration of the contacts in this method is independent of the geometry, size, materials, and uniformity of contacts. To verify the technique, two different Si samples with different thicknesses and two samples of the same SiN(x) wafer are characterized on a single device. One of the Si samples is also characterized by another technique, which verifies the current results. Characterization of the two SiN(x) samples taken from the same wafer showed less than 1% difference in the measured thermal conductivities, indicating the precision of the method. Additionally, one of the SiN(x) samples is characterized and then demounted, remounted, and characterized for a second time. The comparison showed the change in the thermal resistance of the contact in multiple measurements could be as small as 0.2 K/μW, if a similar sample is used.

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.

High-Q ͑quality factor͒ resonators are a versatile class of components for radio frequency micro-electromechanical systems. Phononic crystals provide a promising method of producing these resonators. In this article, we present a theoretical study of the Q factor of a cavity resonator in a two-dimensional phononic crystal comprised of tungsten rods in a silicon matrix. One can optimize the Q of a phononic crystal resonator by varying the number of inclusions or the cavity harmonic number. We conclude that using higher harmonics marginally increases Q while increasing crystal length via additional inclusions causes Q to increase by orders of magnitude. Incorporating loss into the model shows that the silicon material limit on Q is achievable using a two-dimensional phononic crystal design with a reasonable length. With five layers of inclusions on either side of the cavity, the material limit on Q is achieved, regardless of the harmonic number.

Phononic crystals (PnCs) are man-made structures with periodically varying material properties such as density, ρ, and elastic modulus, E. Periodic variations of the material properties with nanoscale characteristic dimensions yield PnCs that operate at frequencies above 10 GHz, allowing for the manipulation of thermal properties. In this article, a 2D simple cubic lattice PnC operating at 33 GHz is reported. The PnC is created by nanofabrication with a focused ion beam. A freestanding membrane of silicon is ion milled to create a simple cubic array of 32 nm diameter holes that are subsequently backfilled with tungsten to create inclusions at a spacing of 100 nm. Simulations are used to predict the operating frequency of the PnC. Additional modeling shows that milling a freestanding membrane has a unique characteristic; the exit via has a conical shape, or trumpet-like appearance.