Understanding the origins of intrinsic stress in Si nanowires
(NWs)
is crucial for their successful utilization as transducer building
blocks in next-generation, miniaturized sensors based on nanoelectromechanical
systems (NEMS). With their small size leading to ultrahigh-resonance
frequencies and extreme surface-to-volume ratios, silicon NWs raise
new opportunities regarding sensitivity, precision, and speed in both
physical and biochemical sensing. With silicon optoelectromechanical
properties strongly dependent on the level of NW intrinsic stress,
various studies have been devoted to the measurement of such stresses
generated, for example, as a result of harsh fabrication processes.
However, due to enormous NW surface area, even the native oxide that
is conventionally considered as a benign surface condition can cause
significant stresses. To address this issue, a combination of nanomechanical
characterization and atomistic simulation approaches is developed.
Relying only on low-temperature processes, the fabrication approach
yields monolithic NWs with optimum boundary conditions, where NWs
and support architecture are etched within the same silicon crystal.
Resulting NWs are characterized by transmission electron microscopy
and micro-Raman spectroscopy. The interpretation of results is carried
out through molecular dynamics simulations with ReaxFF potential facilitating
the incorporation of humidity and temperature, thereby providing a
close replica of the actual oxidation environmentin contrast
to previous dry oxidation or self-limiting thermal oxidation studies.
As a result, consensus on significant intrinsic tensile stresses on
the order of 100 MPa to 1 GPa was achieved as a function of NW critical
dimension and aspect ratio. The understanding developed herein regarding
the role of native oxide played in the generation of NW intrinsic
stresses is important for the design and development of silicon-based
NEMS.