Abstract:Integrated freestanding single-crystal silicon nanowires with typical dimension of 100 nm × 100 nm × 5 μm are fabricated by conventional 1:1 optical lithography and wet chemical silicon etching. The fabrication procedure can lead to wafer-scale integration of silicon nanowires in arrays. The measured electrical transport characteristics of the silicon nanowires covered with/without SiO2 support a model of Fermi level pinning near the conduction band. The I-V curves of the nanowires reveal a current carrier polarity reversal depending on Si-SiO2 and Si-H bonds on the nanowire surfaces.

The manipulation of strain in micromachined silicon structures presents an opportunity in the control of surface processes in epitaxial growth. With appropriate fabrication techniques, the magnitude, crystallographic direction, and symmetry of the strain at a Si surface can be precisely controlled with this strategy. Synchrotron x-ray microdiffraction techniques allow simultaneous independent measurements of the strain and bending in these structures and serve to calibrate the fabrication process. Bending is the dominant source of strain in a microfabricated Si bridge loaded at its ends by silicon nitride thin films that we have used as a strained substrate in studies of Ge epitaxial growth. The total strain difference between the top and bottom of the bent bridge exceeds 10 −3 in present structures and can potentially be increased in optimized devices. These micromachined substrates complement other methods for producing strained silicon and silicongermanium structures for improved electrical device performance and for fundamental studies of epitaxial growth.

The manipulation of strain in micromachined silicon structures is an important aspect of the design of emerging mechanical and electronic devices. Strain also has a fundamental role in the formation of devices through its effects on surface processes in epitaxial growth including diffusion and can be an important tool for studying these processes. Microfabricated silicon structures offer the opportunity to control the strain at length scales of less than one micron to several hundred microns. Synchrotron x-ray microdiffraction allows simultaneous independent measurements of the strain and bending in these structures. Microdiffraction measurements show that bending is the dominant source of strain in a prototypical microfabricated Si bridge loaded at its ends by silicon nitride thin films. The total strain difference between the top and bottom of the bent bridge exceeds 0.1% in our prototype structures and can potentially be increased in optimized devices.

In this paper we demonstrate the use of diffractive gratings to optically measure strain in miniature ultrasonic transducers. Aluminum diffraction gratings were fabricated on silicon-microfabricated ultrasonic horns and beams which were actuated by bonded piezoelectric PZT (Lead-Zirconate Titanate) plates. A He-Ne laser beam was diffracted from the grating and a knife-edge was used to measure small changes in the diffraction angle as a result of time varying grating space and width. The measured strain and displacement profiles agreed with the expected mode patterns for the silicon resonators.

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