Method of measuring numerical aperture of exposure machine, control wafer, photomask, and method of monitoring numerical aperture of exposure machine

A method of monitoring a numerical aperture (NA) of an exposure machine, wherein a light source of the exposure machine has an illumination pattern, the method comprising:

• providing a control wafer having a plurality of vernier marks thereon;
• providing an aberration mask having a plurality of pinholes therein, wherein each pinhole corresponds to a vernier mark in position;
• setting the exposure machine according to a required NA;
• performing a lithography process using the exposure machine and the aberration mask to the control wafer, so as to form a photoresist pattern having the same shape of the illumination pattern over each vernier mark;
• deriving a real NA of the exposure machine from a graduation of the vernier mark corresponding to an outer edge of the photoresist pattern;
• comparing the real NA with the required NA and adjusting the exposure machine according to a result of the comparison;
• and
• establishing an NA database of exposure machine.