SCANNING FORCE MICROSCOPE SPRINGS OPTIMIZED FOR OPTICAL-BEAM DEFLECTION AND WITH TIPS MADE BY CONTROLLED FRACTURE

Springs for scanning force microscopy (SFM) are described which incorporate two separate advances: Their shape significantly reduces the effect of photon shot noise in the optical beam deflection method, and they are endowed with very sharp tips through a unique process based on controlled fracture of micromachined structures. With this readout method, the noise in the measured tip position is limited by photon shot noise, which is proportional to the ratio of the effective length of the spring to the length of the mirror. Cantilever springs optimized for this parameter are described, with a measured white noise level of 29 fm/square-root Hz at 210 muW of laser power; torsion springs for which the mirror length exceeds the effective length yielded a theoretical noise level of 7.2 fm/square-root Hz at this power. Cantilever springs optimized for this parameter are described, with a measured white noise level of 29 fm/square-root Hz at 210 muW of laser power; torsion springs for which the mirror length exceeds the effective length yielded a theoretical noise level of 7.2 fm/square-root Hz at this power. The latter is lower than the noise of an ideal fiber interferometer at the same power, demonstrating that the optical-beam-deflection method can be made competitive with interferometric methods for noise-critical SFM applications. This novel tip,making technique generates cube-comer-shaped tips, with a radius which is unresolvable in scanning electron microscope images, implying that it is less than 10 nm. These tips are intended for samples whose features are in the size range 0.5-20 nm, where the tip curvature is the resolution limiting factor. To demonstrate their performance in this regime, the tips were used to image single collagen molecules.