Effect of focused ion beam milling on microcantilever loss

Micro-scale cantilevers such as those used in the atomic force microscope are now being applied to the accurate measurement of novel forces such as the Casimir force. The measurements are done in dynamic mode and higher sensitivity can be achieved by using cantilevers with lower force constants. One method to produce a low force constant cantilever is to reduce the thickness of a conventional AFM cantilever by focused ion beam ( FIB) milling. Here we show that this method leads to a typical reduction of the resonance quality factor by 40-50%. Reduction of the thickness by FIB milling therefore does not necessarily result in improved force sensitivity, with a decreased quality factor negating any improvement from the lower force constant. If the increased loss is due to the FIB damage layer then it is shown that this layer is similar to 30 nm thick and the complex part of Young's modulus of the layer is in the range 205-270 MPa. In the measurements reported here no trend in the quality factor has been observed with either dose rate of the milling or with the total mill depth.