Influence of astigmatism on the fabrication of diffractive structures by use of focused ion-beam milling

Astigmatism exists in a focused-ion-beam (FIB) system and causes the shape of a beam spot to change from a normal circle to an ellipse. This variation influences the fabrication of diffractive structures ?by use of programmable controlled milling of a FIB. It is analyzed combined with the fabrication of blazed gratings and Fresnel diffractive lenses. Fabrication errors caused by a beam spot with astigmatism is discussed in detail for four cases of the long axis of an ellipse (a) in accordance with the X axis, (b) in accordance with the Y axis, (c) at 45 masculine with the X axis, and (d) at -45 masculine with the X axis. Finally, a method is given for correction of the astigmatism and how to determine the circularity of the beam spot qualitatively.