Nanostressors and the Nanomechanical Response of a Thin Silicon Film on Insulator

The nano-mechanical properties of thin silicon films will play an increasingly critical role in future semiconductor devices, especially in the context of layered substrates like silicon-on-insulator (SOI). Using very small Ge crystals as a unique nano-mechanical stressor, we have demonstrated both a surprising mechanical behavior of the thin layer of Si in SOI and a highly decreased localized viscosity of the oxide on which the Si layer rests. The implications for the use of SOI in nanoelectronic devices could be significant.

We use SOI substrates consisting of a “handle wafer” (thick Si), a thin oxide (400nm of SiO2), and a very thin (here 10nm) “template” layer of crystalline Si on top of the oxide. The template layer is patterned to form micron-size (5 to 20mm) mesas, as shown in Fig. 1a. Approximately 10 monolayers (1.6nm) of Ge are deposited via molecular-beam epitaxy at 700ºC. Figure 1b shows the formation of Ge nanocrystals (~ 10nm high with 100nm bases) that are crystallographically coherent with the Si template and an anomalous local bending of the Si template layer underneath each individual nanocrystal. The curvature underneath the islands is measured to be >0.005 nm-1. This novel localized bending mode (Fig.2a) of a nanometer-scale thin film is different from the commonly observed extended uniform bending mode (Fig. 2b) induced by strained-layer film growth on thick Si(001). Our calculations show that the localized bending curvature depends on nanocrystal density and shape (Figs. 2c and 2d).

The localized bending mode and large bending magnitude indicate that the Si template layer behaves like a “freestanding” layer during the growth of Ge nanocrystals, something that can be achieved if SiO2 acts like a fluid with substantial viscous flow. The viscosity of SiO2 at 700ºC (the growth temperature) is ordinarily much too high for such a large degree of relaxation. However, this viscosity can decrease nearly exponentially with increasing applied shear stress. From the bending curvature and hence the bending stress, we estimate that the viscosity of SiO2 can be decreased by 3 to 5 orders of magnitude in the regions underneath the bent Si layer below the Ge nanocrystals. The relaxation time for SiO2 flow is then shortened by a few orders of magnitude to well within the deposition time of ~150s. Thus, the large bending stress in the Si layer greatly enhances the viscous flow of SiO2, which in turn helps to magnify the bending of the Si layer, because the Si film can then behave like a freestanding film.

The localized stressor on the thin Si template layer of SOI modifies both the mechanical properties of the Si layer and its electronic properties, providing a unique method for electronic (band) engineering at the nanometer scale. Conversely, for these reasons localized stressors in SOI could become a potentially significant issue for the semiconductor industry, which is increasingly using SOI in device manufacture.

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