Large area nanostructuring

In order to ensure high speed and economic fabrication of nanostructures over large sample areas, we introduced a novel irradiation setup.

Figure 1: Setup for large area nanostructuring

Instead of scanning a focused beam along the sample surface, a mask projection arrangement is applied to provide simultaneous illumination of an extended sample area. The overall transmission of the setup and thus the efficiency of the fabrication process can be increased by using phase masks instead of amplitude masks. However, the availability of phase masks tailored for specific machining tasks is strongly limited. To overcome this problem we introduced a new technology enabling easy fabrication of custom made phase masks.

A further difficulty is the high peak intensity of the illuminating beam near the focal plane of the imaging optics. This leads to ionization of the air, resulting in phase front distortions and consequently a deterioration of the resulting pattern on the sample surface. This can be avoided by encapsulating the region surrounding the focusing optics and flushing it by helium. The nonlinear refractive index of helium is about 500 times smaller than that of air, thus maintaining a negligible nonlinear phase distortion. As a consequence, a distortion free pattern will be projected onto the sample surface.

A further issue was the construction and realization of a homemade Schwarzschild-type reflecting objective. This device is designed to provide a field size of 1 mm², a numerical aperture (NA) of at least 0.25, and a demagnification of 15-30.

The suitability of the new illumination setup for large area nanostructuring was demonstrated on stainless steel samples. A hole matrix was fabricated covering a total sample area of 0.5 x 0.5 mm². Scanning electron microscope (SEM) records show a homogeneous appearance of the illuminated field and structural details of as small as ~300 nm. These results show the ability of the applied technique for precise manufacturing of nanostructures even on materials with large heat conductivity.

Figure 2: SEM photographs of a hole matrix in stainless steel.