Reduction of Thermal Conductivity in Wafer-Bonded Silicon

Blocks of silicon up to 3-mm thick have been formed by directly bonding stacks of thin wafer chips. These stacks showed significant reductions in the thermal conductivity in the bonding direction. In each sample, the wafer chips were obtained by polishing a commercial wafer to as thin as 36 {micro}m, followed by dicing. Stacks whose starting wafers were patterned with shallow dots showed greater reductions in thermal conductivity. Diluted-HF treatment of wafer chips prior to bonding led to the largest reduction of the effective thermal conductivity, by approximately a factor of 50. Theoretical modeling based on restricted conduction through the contacting dots and some conduction across the planar nanometer air gaps yielded fair agreement for samples fabricated without the HF treatment

Solar-wind sputtering of the martian atmosphere

In the sputtering process an incident particle beam loses part of its energy to recoil motion of target atoms, some of which may escape through a nearby surface. The sputtering yield, S, is defined as the number of atoms ejected per incident particle. In the Solar System, sputtering will occur whenever the solar wind, consisting mainly of 1 keV AMU hydrogen and helium ions, strikes a material body. Many years ago, Wehner et al. suggested that solar wind-induced sputtering of the lunar surface should be an important cause of erosion; recently, analyses of returned lunar material have been interpreted quantitatively in terms of such solar-wind sputtering. Mars provides another example of the interaction of the solar wind with a planetary body. However, in contrast to the lunar surface, the martian surface is largely protected from direct solar wind bombardment by its atmosphere. The primarily CO_2 atmosphere is thin by terrestrial standards but still opaque to the solar wind. We discuss here whether solar-wind sputtering of the martian atmosphere is a mechanism leading to significant mass loss