Nanometer-scale imaging and ablation with Extreme Ultraviolet lasers

The short wavelength and high brightness of compact extreme ultraviolet lasers is shown to enable the development of microscopes with spatial resolution of tens of nanometers and new types of nanoprobes. ©2007 Optical Society of America OCIS codes: 110.7440, 140.7240, 180.7460. With wavelengths in the tens of nanometers and very high brightness, extreme ultraviolet (EUV) lasers are very well suited for applications in nanoscience and nanotechnology. Given the small size of the diffraction limit for beams of EUV light, these lasers allow us to "see" smaller features and "write" smaller patterns than would be possible with visible light. We have developed compact laser-pumped and discharge-pumped EUV lasers operating at wavelengths of 13.2 nm (1) and 46.9 nm (2) respectively, and have used them to demonstrate high resolution broad area imaging (3,4) and ablation of sub-100 nm size features (5). Using illumination from the 13.2 nm table-top laser and zone plate optics we demonstrated sub-38 nm resolution imaging using a table-top set-up (3). Images were acquired with exposure times of 20 seconds. More recently, we have used a desk-top size 46.9 nm wavelength EUV laser pumped by a fast electrical discharge to demonstrate an even more compact full-field microscope that is capable of imaging samples in transmission or reflection mode. Tests in the transmission mode demonstrated a spatial resolution of 70 nm. The high brightness of the laser allows for exposure times of less than 30 seconds, allowing the rapid acquisition of a large number of images. Figure 1 shows a schematic of the transmission mode setup and a photograph of the microscope. The output from the laser is collected by a multilayer coated Schwarzschild condenser that focuses the light onto a sample. A free-standing objective zone plate projects the image of the sample onto a back-thinned CCD detector. Figure 2(a) shows a transmission-mode image of a 100 nm half-period grating acquired in 20 seconds obtained using a zone plate with a 70 nm outer zone width. Figure 2(b) shows an image of 70 nm lines and spaces and the corresponding intensity cross section. The modulation of ~30% indicates that the features are resolved. Figure 2(c) shows an EUV image of a silicon wafer patterned with polysilicon lines obtained using a zone plate with a 200 nm outer zone width, in which 100 nm lines are visible. The ability to focus EUV laser light into near diffraction-limited spots will open the possibility to develop new types of nanoprobes. We have demonstrated ablation of sub-100 nm diameter holes by direct focusing of an EUV laser onto a sample. In this experiment the output from a compact λ=46.9 nm capillary discharge laser (2) was focused onto a sample with a 0.5 mm diameter free-standing 200 nm outer zone width zone plate lens. Holes with a diameter as small as 82 nm were ablated in PMMA. Figure 3(a) shows an AFM image of craters obtained ablating PMMA with an attenuated EUV laser beam by placing the sample near the third order focus of the zone plate. The holes ablated with single laser shots were observed to have very clean walls and high shot-to-shot reproducibility. These are to our knowledge the smallest ablation holes obtained by direct focusing of a laser beam onto a sample. This new ablation technique combined with spectroscopic analysis of the light emitted by the created plasma opens the path for the development a Laser Induced Breakdown Spectroscopy tool with significantly increased spatial resolution. To demonstrate the analytical potential of this EUV laser ablation tool we have conducted spectroscopy of the plasmas created focusing the laser onto metals and semiconductors. As an example, Fig. 3(b) shows a visible light spectra obtained ablating a Cr film, where atomic transitions of Cr I are clearly identified. These results illustrate the potential of compact EUV lasers in nanotechnology applications.