AVS 50th International Symposium
    Nanometer Structures Thursday Sessions
       Session NS-ThM

Paper NS-ThM4
AFM Force Measurements: MEMS Devices for Easy and Accurate Cantilever Spring-Constant Calibration

Thursday, November 6, 2003, 9:20 am, Room 308

Session: Advances in Scanning Probes
Presenter: P.J. Cumpson, National Physical Laboratory, UK
Authors: P.J. Cumpson, National Physical Laboratory, UK
J. Hedley, University of Newcastle, UK
P. Zhdan, University of Surrey, UK
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A value for the spring-constant of Atomic Force Microscope (AFM) cantilevers@footnote 1@ is necessary for the measurement of nanonewton and piconewton forces, which are critical to analytical applications of AFM in the analysis of polymer surfaces, biological structures and organic molecules.@footnote 2@ We have developed compact and easy-to-use micromachined reference artefacts for this calibration. The principal device consists of an array of dual spiral-cantilever springs, each supporting a polycrystalline silicon disc of 170 micrometres in diameter. These were fabricated by a two-layer polysilicon surface micromachining method. Doppler velocimetry is used to measure the fundamental resonant frequency of each device accurately. We call such an array a Microfabricated Array of Reference Springs (MARS). These devices have a number of advantages. Firstly, modelling the fundamental resonant frequencies of the devices is much more straightforward than for AFM cantilevers,@footnote 3@ because the mass and spring functions are isolated in different parts of the structure. Secondly, the spring constant of each spring is in linear proportion to the mass of the device, given that the resonant frequency is measured accurately. The thickness and hence the mass can be measured accurately by AFM or interferometry. These factors lead to much lower uncertainty than previous methods in which spring constant is proportional to the cube of a critical dimensional measurement. The array spans the range from 0.1 N/m to 10 N/m important in AFM, allowing AFM cantilevers to be calibrated easily and rapidly. New devices that extend this range down to around 0.03 N/m, and up to around 80 N/m will also be discussed. @FootnoteText@ @footnote 1@ N A Burnham et al, Nanotechnology 14 (2003) 1-6 @footnote 2@ J Colchero in Procedures in Scanning Probe Microscopies, Ed. R J Colton et al (Wiley, Chichester, UK, 1998)@footnote 3@C T Gibson et al, Nanotechnology, 7 (1996) 259-262. .