Towards traceable bidirectional optical size measurements for optical coordinate measuring machine metrology

Bidirectional measurements are to be performed for the calibrations and the reverification of the performance of optical coordinate measurement machines (CMMs). The national metrology institutes are challenged to provide internationally recognized calibrations of suitable standards at the required uncertainty level of 100 nm. Furthermore most users are not aware of the specific difficulties of these measurements. Because the optical image formation process is based on interference and diffraction the measurements depend on all of their influence parameters. The first requisite is the use of the correct threshold value, which usually needs to be determined by a simulation of the microscopic image. A comparison of optical linewidth measurements, which are one example of bidirectional measurements, and scanning electron linewidth measurement agreed down to the nanometer level. These results validate the proposed measurement procedure and the related uncertainty calculations. The experience obtained within the accreditation process of industrial labs shows that, in order to be able to achieve the desired measurement uncertainties of about 100 nm, the imaging system needs to have a a monochromatic Koehler illumination, numerical aperture lager than 0.6, a magnification greater than 50 and the ability to control the deviation of the focus position to better than 100 nm.