Micromechanical cantilevers are commonly used for detection of small forces
in microelectromechanical sensors (e.g., accelerometers) and in scientific
instruments (e.g., atomic force microscopes), A fundamental limit to the d
etection of small forces is imposed by thermomechanical noise, the mechanic
al analog of Johnson noise, which is governed by dissipation of mechanical
energy, This paper reports on measurements of the mechanical quality factor
Q for arrays of silicon-nitride, polysilicon, and single-crystal silicon c
antilevers. By studying the dependence of Q on cantilever material, geometr
y, and surface treatments, significant insight into dissipation mechanisms
has been obtained. For submicron-thick cantilevers, Q is found to decrease
with decreasing cantilever thickness, indicating surface Loss mechanisms. F
or single-crystal silicon cantilevers, significant increase in room tempera
ture Q is obtained after 700 degrees C heat treatment in either N-2 or form
ing gas. At low temperatures, silicon cantilevers exhibit a minimum in Q at
approximately 135K, possibly due to a surface-related relaxation process.
Thermoelastic dissipation is not a factor for submicron-thick cantilevers,
but is shown to be significant for silicon-nitride cantilevers as thin as 2
.3 mu m. [434].