X-Ray lithography for 0.5 µm VLSI: Assessment of mask accuracy, patterning and damage

In order to ascertain the potential of step-and-repeat x-ray lithography in 0.5 µm VLSI fabrication, mask accuracy, patterning and irradiation damage are assessed. Back etching and Ta absorber etching cause stress relief and result in significant distortion of the membrane. Registration marks, which are necessary in precise e-beam writing of mask patterns, are also shifted by the back etching. This leads to the conclusion that back etching should be done before the e-beam writing and Ta stress should be precisely controlled. Penumbral shadow decreases exposure intensity and deforms the pattern shape. A 0.5 µm round hole resist pattern is replicated by using a 0.6 µm square mask pattern. X-ray irradiation, even with a low dose of 80 mJ/cm², degrades the MOS capacitor. However, annealing at a modest temperature removes the damage.