Dimensional Measurement using Vision Systems - NPL Publications ...

Measurement Good Practice Guide No. 39
The limit of magnification by visual optical microscopy has now almost been reached.
From equation (1) it will be seen that decreasing the illumination wavelength λ can reduce
the minimum object separation resolvable by a microscope objective. However, in terms of
optical microscopy the minimum useful wavelength is around 300 nm in the ultraviolet
(UV) region of the spectrum and this only produces a resolution improvement of around
x2. It also brings with it the added complexity that specially designed optics have to be
employed in the UV.
A more significant improvement can be achieved by using a scanning electron microscope.
Although the numerical apertures of electron lenses are very limited, the effective
wavelength of the electron beam is extremely small and a resolution limit of 10 nm
(0.01 µm) or even less is readily achievable, a figure at least thirty times smaller than
achievable with an optical microscope. Thus, in the future, with the ever-decreasing size of
microelectronic structures, the use of electron microscopy will inevitably become more
widespread.
2.2 ABERRATIONS IN THE IMAGE FORMING SYSTEM
The intensity distribution in the Airy pattern shown in Figure 2 will only be produced by a
lens of perfect quality. In practice, defects in the lens performance will result in some
redistribution of the energy, usually leading to degradation in image quality and to errors
if the system is employed for making dimensional measurements. These defects are
known as aberrations and can be divided into the following types.
2.2.1 Spherical aberration
If, after passing through different parts of the lens aperture, the rays of light from a single
object point are focused in different image planes, the lens is said to exhibit spherical
aberration (see Figure 4). With increasing spherical aberration in a lens, the intensity of the
central disc of the diffraction pattern gradually decreases, accompanied by more light in
the surrounding rings. This results in reduced image contrast and resolution. Although
microscope objectives are usually well corrected for spherical aberration, some may easily
be introduced by using incorrect working conditions e.g. the wrong tube length (see
Section 2.4.6).
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