Eliminating the Focus Dependence of the Line Center Coordinate in Measuring Microscopes
Original Publication Date: 1990-May-01
Included in the Prior Art Database: 2005-Mar-15
The means described in this article eliminates the focus dependence of the line center coordinate in measuring microscopes.
Eliminating the Focus Dependence of the
CenterCoordinate in Measuring Microscopes
described in this article eliminates the focus
dependence of the line center coordinate in measuring microscopes.
The trend to
miniaturize semiconductor devices is continuing.
Problems still to be resolved include the superposition accuracy of
the individual mask patterns.
problem to be resolved is to what extent the
determination of the line center coordinate is influenced by
focussing. For this purpose, a test laser scanner (Fig. 1) was used.
It was found that the influence of focussing is constant across the
entire visual field. This is surprising, since according to the laws
of ray optics (Fig. 2), the focus dependence changes in response to
non-telecentric illumination. As this was not the case with the
means described in this article, it can be assumed that the laser
scanner was telecentrically adjusted and that the focus dependence is
due to a non-symmetrical distribution of the laser beam.
a lens of the beam expander (Fig. 2), the
intensity distribution of the laser light source can be influenced
such that it is almost symmetrical in the laser scanner (aperture
stop). The focus dependence of the line center coordinate could be
reduced from 30 to 8 nm/mm. This advantage is obtained at the
expense of a laser scanner that is not optimally adjusted.
Therefore, it is proposed to optimall...