FEMTOSECOND LASERS: Nanostructuring repairs lithographic masks

Jan. 1, 2000

Researchers at Laser Zentrum Hannover and Nanonics Lithography have successfully repaired a programmed defect in a lithographic mask using a laser ablation technique that combines scanning near-field optical microscopy with femtosecond laser pulses.

With the aid of researchers at Nanonics Lithography Ltd. (Jerusalem, Israel), Stefan Nolte, group leader, short-pulse lasers, and colleagues at the Laser Zentrum Hannover e.V. (Hannover, Germany) have produced spatially localized femtosecond pulses by combining scanning near-field optical microscopy with an ultrashort-pulse laser. Applications that could benefit from the technique include direct ablative writing on metal surfaces, nanostructuring, and repair of lithographic masks.¹

Several recent research efforts have illustrated the effectiveness of combining scanning near-field optical microscopy with pulsed laser light as a tool for nanolithography. Laser-induced desorption on a nanometer scale by use of nanosecond pulses and fiberoptic tips with optical transmission coefficients as high as 10-3 also has been demonstrated. Introducing femtosecond pulsing allows both nanometer spatial resolution and femtosecond temporal resolution, and several research groups have demonstrated the resulting benefits in spectroscopic investigations of local dynamical processes in solids.

The research at the Laser Zentrum Hannover and Nanonics Lithography stands out because it may be the first time the combination of femtosecond laser pulses with a scanning near-field optical microscope has successfully performed surface modifications. According to the project researchers, the microscope works in an illumination mode with femtosecond pulses coupled into a fiberoptic tip. Although it is possible to introduce laser light into the body of a standard microscope and focus the light with the viewing objective, the Hannover-Jerusalem technique is reportedly more promising for surface modification because it is not diffraction-limited and can be performed at atmospheric pressure.

In addition, conventional wisdom holds that local heating of solid targets using femtosecond pulses sent through the microscope can produce nanometer-size structures on the material surface when the ablation threshold of the material is considerably lower than that of the tip. The Hannover-Jerusalem research demonstrated the capability to produce structures on chrome-coated surfaces using chrome-coated tips-even though both coatings have the same ablation threshold.

The experiments used a commercial Ti:sapphire laser system that delivers 100-fs pulses with energy of 1 mJ at 780 nm and a repetition rate of 1 kHz. The laser radiation is frequency-doubled (0.4 mJ at 390 nm) and tripled (0.1 mJ at 260 nm). In the ablation experiments, only strongly attenuated frequency-converted pulses with energies less than 5 µJ were used.

Using second-harmonic radiation at 390 nm and a scanning speed of 5 µm/s, the equipment removed the defect in the mask's chrome coating (upper photo) by producing parallel grooves separated by 100 nm (lower photo).

Technique demonstration

In a preliminary demonstration of the laser-ablation surface-modification technique—partially developed with support from the German Ministry of Education and Research—the researchers removed a programmed defect in the surface of a lithographic mask with a 690-nm-diameter tip held at a constant height of 150 nm above the chrome surface (see figure). In these experiments the radiation throughput of the metal-coated tips was on the order of 10^-4. Nolte and his colleagues believe that careful design of the fiber tips could increase light throughput and further improve efficiency of the femtosecond laser-ablation process. They are planning further research in this area.