The assessment of residual flatness errors in focus variation areal measuring instruments

Optical instruments for areal surface topography measurement rely on high-precision lenses that guide the light from the object surface to the image plane. Lens aberrations may cause distortion of the transmitted image and consequently a residual flatness error in the measurement data. Previous work at NPL suggests using an averaging method for residual flatness error assessment for optical surface topography instruments. However, the averaging method does not apply to the focus variation technique, which relies on the nano-scale roughness of a surface to allow acquisition of topography data. This paper presents alternative methods for measuring residual flatness for focus variation instruments.