Microcantilever Sensor Research
With their extreme sensitivity, microcantilevers are ideal for developing microsensors for biological and environmental applications that are portable, selective, extremely sensitive, and less expensive than currently available commercial sensors. The challenge, however, is achieving chemical selectivity and sensor regeneration. Our goal, through collaboration with a research group at the Oak Ridge National Laboratory, is to address and provide solutions to these two challenging and important issues.
Figure 1. SEM micrograph of a 200 um long cantilever
The resonance frequency, f, of an oscillating cantilever can be expressed as:
(1)
where K is the spring constant of the lever and m* is the effective
mass of the microcantilever. The effective mass can be related to
the mass of the beam, mb, through the relation m* = nmb, where n is a geometric
parameter. For commercially available silicon nitride cantilevers with spring
constants of 0.06 and 0.03 N/m, the values of n are 0.14 and 0.18, respectively,
and for the case of a rectangular bar, n is 0.24 [8]. From Eq. (1)
it is clear that the resonance frequency can change due to changes in mass
as well as changes in spring constant. Therefore, the shift in frequency
can be written as
(2)
Thundat et-al has shown that the changes in spring constant are related
to changes in differential surface stress. By designing cantilevers with
localized adsorption areas at the terminal end of the cantilever (end loading),
the contribution from differential surface stress [the dK/K term in
Eq. (2)] can be minimized. In that case, the changes in resonance
frequency can be attributed entirely to mass loading. However, the spring
constant can also change during molecular adsorption if it results in amalgamation
(changes in E) or swelling (changes in thickness).
Figure 1. Resonance response of
a microcantilever with a gold overlayer
along its entire length as a function of mercury dosing time.
The resonance response is dominated by changes in spring constant.