Thin film apertures are frequently used in the objective lenses of high resolution and electron beam lithography instruments. These apertures have the advantage that they heat up in the beam and are effectively self cleaning. This minimises astigmatism caused by a build up of carbon contamination. The small amount of contamination which deposits after lengthy operation may be removed by heating the aperture strongly for a short time with the focused electron beam. The benefits of using this type of aperture are particularly apparent when operating at low accelerating voltages or with small probe sizes. Thin film apertures should not be used in the condenser lens as they may melt. They are, of course, more fragile to handle and can be irreparably damaged by abrasion or by a sudden rush of air in the vacuum system. Normal tolerance on hole diameter is ±10%. Close tolerance holes ±1µm from the nominal size can be supplied at additional cost. The apertures are made to exceptionally high standards and are only accepted after examination in the SEM to check the perfection of the edge of the hole, its roundness and absence of pin holes in the vicinity.