A Study Of The Instrumental Errors In Linewidth And Registration Measurements Made With An Optical Microscope.

A bright field optical microscope is modelled to identify the errors introduced into dimensional measurements by misalignments and aberrations. The effect of illumination misalignment is considered and it is demonstrated that unless the illumination cone is accurately centred on the optical axis there will be significant errors introduced into registration measurements. A comprehensive study is made of objective lens aberrations. Theoretical models are used to quantify the effects different aberration types have on image profiles and dimensional measurement errors. A practical study is also made of thirteen commercially available objectives and it is concluded that the amount of aberration present in many of these lenses is too large for many measurement applications. Theoretical tolerances for different aberration types are established. It is shown that by using an objective with only small levels of aberration and carefully aligning the microscope illuminator, it is possible to obtain image profiles of thin and thick (>200nm) layer objects which closely match those predicted by theory.