Nanomechanical Studies and Applications of Cantilever Sensors

Abstract: Different aspects of cantilevers to be used as sensors have been studied. Silicon cantilevers have been fabricated and integrated on CMOS chips using low-energy electron beam lithography in order to reduce the irradiation damage to the circuits. The released and integrated cantilevers have been tested and used as mass sensors in air. The resonance frequency of the cantilever was measured before and after depositing a single drop of glycerin on the tip. The mass of the glycerin drop was then calculated based on the frequency shift observed. This was found to be in good agreement with the estimated deposited mass. Nanometer-sized chromium cantilevers have also been fabricated and investigated. These nanocantilevers were characterized using an atomic force microscope (AFM) operated in contact mode where the bending of the cantilevers was measured, as a function of the force applied by the AFM. The mechanical properties of the cantilevers were determined based on the deflections. It was concluded that the effective Young's modulus for the nanometer-sized chromium cantilevers was a factor of 3 to 5 lower than the tabulated value for bulk chromium. The thinner the cantilever, the lower the modulus of elasticity. This indicates that there is a size dependence of the mechanical properties of nanometer-sized cantilevers. Commercial micrometer-sized silicon cantilevers oscillating in a liquid medium have also been studied. Investigating cantilevers as mass sensors in a liquid environment is important if the sensors are to be used, for example, in medical applications. When a cantilever is oscillating in an aqueous solution, both the resonance frequency and the amplitude are lower than in air or vacuum, due to the higher density and viscosity of the solution. The effect is a reduced mass sensitivity relative to measurements in air. However, by dimensioning the cantilevers so that the resonance frequency is high, a high mass sensitivity can still be obtained. The resonance frequencies of oscillating cantilevers in aqueous solutions were investigated. The injection of phospholipid vesicles into the aqueous solution yielded a frequency reduction due to the mass of vesicles adsorbed on to the cantilever surface. The mass of the adsorbed vesicles was calculated based on the frequency shift. The results were in good agreement with the estimated mass, assuming a uniformly distributed layer of vesicles on the cantilever surface.