Phonon-boundary scattering in thin silicon layers

Abstract

Temperature fields in microdevices made from silicon-on-insulator ͑SOI͒ wafers are strongly influenced by the lateral thermal conductivity of the silicon overlayer, which is diminished by phonon scattering on the layer boundaries. This study measures the thermal conductivity of single-crystal silicon layers in SOI substrates at temperatures between 20 and 320 K using Joule heating and electrical-resistance thermometry in microfabricated structures. Data for layers of thickness between 0.4 and 1.6 m demonstrate the large reduction resulting from phonon-boundary scattering, particularly at low temperatures, and are consistent with predictions based on the phonon Boltzmann transport equation. © 1997 American Institute of Physics. ͓S0003-6951͑97͒02739-3͔Thin single-crystal silicon layers are becoming more common in microfabricated sensors, actuators, and transistors. These microdevices can be fabricated from silicon-oninsulator ͑SOI͒ substrates, which provide silicon layers of thickness between 0.05 and 10 m above a buried silicon dioxide layer. The performance and reliability of microdevices made from SOI substrates can be strongly influenced by lateral thermal conduction in the silicon layer. This is particularly important for transistors in SOI circuitry, 1 in which thermal conduction in the silicon device layer strongly reduces the peak temperature rise. 2 Microcantilevers made from SOI substrates are promising for high-density thermomechanical data storage 3,4 and have thermal response times and sensitivities governed by thermal conduction along the silicon layer. The thermal conductivity of silicon layers of submicrometer thickness may be strongly reduced by interfacial effects, although this has not been demonstrated previously.The thermal conductivity of silicon is dominated by phonon transport and, for the case of thin films, can be reduced by phonon scattering on boundaries and by imperfections related to the fabrication process. While phonon-boundary scattering is most important at low temperatures, where the mean free paths of phonons are longest, boundary scattering may also be very significant at room temperature and above in very thin silicon layers. 5 There are no data available to conclusively demonstrate this phenomenon in silicon layers of submicrometer thickness. Previous work 6-8 measured the lateral thermal conductivity of thin polysilicon layers in microsensors and reported a thermal conductivity reduction of up to 80% compared to that in bulk silicon. However, phonon scattering on grain boundaries is responsible for a large fraction of the thermal conductivity reduction in these layers, such that these data are inappropriate for the single-crystal layers in SOI substrates.This letter provides data and phonon transport analysis that quantify the impact of phonon-boundary scattering on heat conduction in crystalline silicon layers. The data are useful for thermal modeling of microdevices made from SOI substrates. Furthermore, since the purity and microstructural quality of silicon layers in SOI ...

Characteristic length of phonon transport within periodic nanoporous thin films and two-dimensional materials

“…In real thin films, it should be noted that certain film-boundary roughness can introduce partially diffusive phonon scattering and thus lower k L at reduced film thickness. [40][41][42][43] When the film thickness is comparable or even smaller than the size of the porous structure, more accurate modeling may further consider the film-boundary scattering of phonons. 13 Assuming smooth film boundaries and rough pore edges, an effective K Pore is extracted by directly comparing the k L predicted by phonon MC simulations and the kinetic relationship.…”

Nanostructure thermal conductivity prediction by Monte Carlo sampling of phonon free paths

“…12 As a nanostructure gets smaller, its thermal conductivity is reduced due to more frequent scattering between phonons and the system boundaries. [1][2][3][4][5][6][7][8][9][10][11] For very small systems (e.g., silicon films thinner than 20 nm), changes in the phonon density of states also affect thermal transport. 2,6,8 Our interest here is nanostructures large enough that the phonon density of states is bulk-like.…”

“…As the dimensions of electronic, optoelectronic, and energy conversion devices are reduced, the thermal conductivities of the device components (e.g., thin films and nanowires) are also reduced. [1][2][3][4][5][6][7][8][9][10][11] The large electrical power densities in such devices cause Joule heating and the reduced thermal conductivities can lead to high operating temperatures, sub-optimal performance, and poor reliability. Predicting the thermal conductivity reduction in nanostructures is thus a critical part of developing next-generation thermal management strategies.…”

Non-local effects in radial heat transport in silicon thin layers and graphene sheets

“…These systems have been much studied theoretically (Alvarez et al 2009(Alvarez et al , 2011 and experimentally (Asheghi et al 1997;Ju & Goodson 1999;Liu & Asheghi 2004;Saito et al 2007;Balandin et al 2008;Ghosh et al 2008;Balandin 2011). However, the thermodynamical behaviour in…”