Evaluation of the secondary electron blue in x-ray lithography by using the pattern replication technique on the mask

Two factors such as the secondary electron blur and Fresnel-Kirchhoff diffraction limit the resolution in the x-ray lithography. In Br resist, the suppression of the secondary electron blur was previously reported. The 30 nm isolated pattern was demonstrated using monochromatic x-ray of 8 A by the 'shadowing technique', where contact exposure method with high contrast mask was used. However, the resolution at the shorter wavelength less than 8 A wasn't sufficiently demonstrated. Here, we have developed the pattern replication technique on the x-ray mask and study the resolution at the x-ray wavelength less than 8 A. This technique is sufficient to evaluate the resolution because the exposure at smaller gap is available, and the stepper vibration can be ignored. A schematic diagram of the pattern replication technique on the mask is shown. Polymer film is coated on the x-ray mask, which correspond to the proximity gap, and positive resist (ZEP-520) of 70 nm thickness is coated on the polymer film. The x-ray is irradiated from the back etching area. The resist on the x-ray mask is developed, where the polymer layer is insoluble to the developer. The SEM photographs of 50 nm L&S patterns in which the kapton filter of 0, 15 and 50 μm thickness is inserted in the beam line are shown. Absorption energy in the resist becomes shorter as the filter thickness is increased. The absorbed energy of 4.4 A corresponds to the absorption edge of Cl element of 15 wt% in the resist. The shorter x-ray wavelength degrades the quality of 50 nm L&S pattern even though the secondary electron blur at the x-ray wavelength less than Cl absorption edge is suppressed due to the addition of Cl element. The normalized deposition energy distributions, where x-rays of the infinitely fine line are irradiated, are shown. The energy spread becomes broader as the x-ray wavelength becomes shorter. This result shows that the secondary electron blur including the x-ray wavelength of 6 A doesn't limit the resolution of x-ray lithography down to 30 nm.