Hierarchical nanostructures created by interference of high-order diffraction beams
We report a novel method to create 2D hierarchical nanopatterns with high structural complexity using phase-shift lithography. With phase masks with large lattice periodicities relative to the wavelength of the light source, several different diffraction beams are generated from a single grating, which then interfere to form a highly complex 3D intensity profile at the Fresnel region. We transfer the horizontal slice of the intensity profile into the thin film of negative photoresist, making 2D nanostructures. Because the diffraction order determines a length scale of intensity variation in a horizontal surface, interference of several different diffraction orders leads to the formation of complex and hierarchical nanopatterns, which are difficult to create with conventional phase-shift lithography. In addition, as the 2D profile is modulated along the light propagation direction, a variety of complex nanopatterns can be fabricated from a single phase mask by adjusting the distance between the diffraction grating and photoresist film. A full 3D intensity profile formed by interference is calculated using the finite-difference time domain (FDTD) method, which enables us to anticipate the shape and morphology of the resulting 2D nanostructures.