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This work measures the thermal conductivities along free-standing silicon layers doped with boron and phosphorus at concentrations ranging from 1ϫ10 17 to 3ϫ10 19 cm Ϫ3 at temperatures between 15 and 300 K. The impurity concentrations are measured using secondary ion mass spectroscopy ͑SIMS͒ and the thermal conductivity data are interpreted using phonon transport theory accounting for scattering on impurities, free electrons, and the layer boundaries. Phonon-boundary scattering in the 3-m-thick layers reduces the thermal conductivity of the layers at low temperatures regardless of the level of impurity concentration. The present data suggest that unintentional impurities may have strongly reduced the conductivities reported previously for bulk samples, for which impurity concentrations were determined from the electrical resistivity rather than from SIMS data. This work illustrates the combined effects of phonon interactions with impurities, free electrons, and material interfaces, which can be particularly important in semiconductor devices.

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 ...

Self heating diminishes the reliability of silicon-on-insulator (SOI) transistors, particularly those that must withstand electrostatic discharge (ESD) pulses. This problem is alleviated by lateral thermal conduction in the silicon device layer, whose thermal conductivity is not known. The present work develops a technique for measuring this property and provides data for layers in wafers fabricated using bond-and-etch-back (BESOI) technology. The room-temperature thermal conductivity data decrease with decreasing layer thickness, ds, to a value nearly 40 percent less than that of bulk silicon for ds = 0.42 μm. The agreement of the data with the predictions of phonon transport analysis between 20 and 300 K strongly indicates that phonon scattering on layer boundaries is responsible for a large part of the reduction. The reduction is also due in part to concentrations of imperfections larger than those in bulk samples. The data show that the buried oxide in BESOI wafers has a thermal conductivity that is nearly equal to that of bulk fused quartz. The present work will lead to more accurate thermal simulations of SOI transistors and cantilever MEMS structures.

Thermal engineering of many nanoscale sensors, actuators, and high-density thermomechanical data storage devices, as well as the self-heating in deep submicron transistors, are largely influenced by thermal conduction in ultrathin silicon layers. The present study measures the lateral thermal conductivity of single-crystal silicon layers of thicknesses 20 and 100 nm at temperatures between 20 and 300 K, using Joule heating and electrical–resistance thermometry in suspended microfabricated structures. The thermal conductivity of the 20 nm thick silicon layer is ∼22 W m−1 K−1, which is nearly an order of magnitude smaller than the bulk value at room temperature. In general, a large reduction in thermal conductivity resulting from phonon–boundary scattering, particularly at low temperatures, is observed. It appears that the classical thermal conductivity theory that accounts for the reduced phonon mean-free paths based on a solution of the Boltzmann transport equation along a layer is able to capture the ballistic, or nonlocal, phonon transport in ultrathin silicon films.