SHORT CANTILEVERS FOR ATOMIC-FORCE MICROSCOPY

We have designed and tested a family of silicon nitride cantilevers ra nging in length from 23 to 203 mu m. For each, we measured the frequen cy spectrum of thermal motion in air and water. Spring constants deriv ed from thermal motion data agreed fairly well with the added mass met hod; these and the resonant frequencies showed the expected increase w ith decreasing cantilever length. The effective cantilever density (ca lculated from the resonant frequencies) was 5.0 g/cm(3), substantially affected by the mass of the reflective gold coating. In water, resona nt frequencies were 2 to 5 times lower and damping was 9 to 24 times h igher than in air. Thermal motion at the resonant frequency, a measure of noise in tapping mode atomic force microscopy, decreased about two orders of magnitude from the longest to the shortest cantilever. The advantages of the high resonant frequency and low noise of a short (30 mu m) cantilever were demonstrated in tapping mode imaging of a prote in sample in buffer. Low-noise images were taken with feedback at a ra te of about 0.5 frames/s. Given proper setpoint adjustment, the sample was not damaged, despite this cantilever's high spring constant of 1. 3 N/m. Without feedback, images were taken at 1.5 frames/s. (C) 1996 A merican Institute of Physics.