Towards a Traceable Infrastructure for Low Force Measurements

Over the past ten years or so the need for the measurement of low forces ranging from newtons down to attonewtons has become increasingly important. As we begin to manufacture and manipulate structures on the micrometre to nanometre scale, the forces that are exerted in such processes must be controlled. To control such forces requires some form of measurement, either a direct measurement of the force, or a measurement of the effect the force has on the structure it is applied to. This paper is primarily concerned with the development of a traceability infrastructure for forces in the range from 1 nN to 10 μN. The lower end of this force range does not cover chemical or most biological forces (usually in the femtoto piconewton range) despite the increasing importance of accurately measuring such forces. Further work is still required to push the limits of force traceability to these levels. At the upper end of the force range considered here, more traditional methods for measuring forces can be used that are traceable to the unit of mass, i.e. the force is realised as a mass in a gravitational field. The force range discussed in this paper applies to many nanoand micrometre scale manipulation and assembly applications, including micro-grippers, handlers and force feedback devices. Further applications that fall into the force range discussed here include the force exerted on a surface by atomic force microscopes and other scanning probe instruments, forces in the area of materials property measurement using indentation technology, the forces found in micro-electromechnical systems (MEMS) and the forces exerted by artificial biological tissues, for example muscle fibres. The two main force generation mechanisms that are found in nature and engineering are the weight of the mass of an object in a gravitational field and the deflection of an element with a finite spring constant. On the microto nanometre scale the spring force is more usually used to produce or react to a force, for example an AFM cantilever.