| CPC H01J 37/28 (2013.01) [H01J 37/20 (2013.01); H01J 2237/2801 (2013.01)] | 6 Claims |

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1. A detachable column unit, comprising:
a column (10) having a vacuum space (S1) in which a condenser lens (12) and an objective lens (13) are installed, and a membrane (19) being installed at a bottom of the column (10); and
a membrane protective cover (20) being installed at the bottom of the column (10) to cover the membrane (19);
wherein air is disallowed to pass through the membrane (19) while an electron generated by an electron gun (11) is allowed to pass through the membrane (19) to scan a sample,
wherein a space formed by the membrane protective cover (20), the membrane (19), and the bottom of the column (10) is a sealed space,
wherein the membrane protective cover (20) is removed before the column (10) is coupled to a sample installation unit (30).
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