ICMCTF2003 Session H5: Multilayer Thin Films: Fabrication and Applications

The next generation lithography for the fabrication of computer chips with critical dimensions in the 25 to 50 nm range will use extreme ultraviolet radiation with wavelengths around 13.4 nm. No transparent materials for wavelengths (λ) below 110 nm that could be used for lenses exist, and reflecting mirrors have to be used for all optical components and the masks. However, the reflectivity of all materials is below 0.5% around λ = 13.4 nm, and multilayer coatings are needed to enhance the reflectivity. The first commercial cameras will contain six near normal incidence mirrors and additional mirrors in the illuminator. The specification for all components of an EUV camera are stringent. Figure errors of all mirrors should be in the 1 Angstrom range, and the coatings should not add any noticeable Figure errors. Multilayers of Mo/Si with 50 periods have achieved reflectivities close to 70% near λ = 13.4 nm. Tight control of the layer thickness, interface quality, and thickness profile has been obtained with added figure errors around 0.5 Angstrom.

Masks have to be free of defects. Ideally the multilayer coating on a mask should reduce the effect of substrate defects and smoothen substrate roughness. Many deposition methods have successfully been used to produce the multilayer coatings, and we will compare their relative strengths and weaknesses. We will also discuss the metrology that has been developed to test the performance of all components.

Due to their extremely thin layers (<1 nm), normal-incidence X-ray multilayer mirrors for space born solar telescopes, free-electron lasers, lithography tools, and microscopes are extremely sensitive to thermally activated interdiffusion or mechanical wear. We explore CrN/ScN superlattices as stable mirrors for X-ray wavelengths of λ=3.1-3.4 nm.

Single crystal CrN/ScN superlattices, with modulation periods of 1.7 nm were grown on MgO(001) substrates by alternating growth of sub-nm layers of CrN and ScN by low energy ion-assisted reactive magnetron sputtering. The ion-to-nitride arrival flux ratios at the substrate were measured to 45 for CrN and 144 for ScN. The effects of ion energies in the range [12-58 eV], and growth temperatures in the range [535-853°C], on the interface widths, crystal quality, and microstructure evolution were investigated using hard x-ray reflectivity (HXR), soft x-ray reflectivity (SXR), x-ray diffraction (XRD), and transmission electron microscopy (TEM).

Ion energies of 24 and 28 eV for CrN and ScN, respectively, gave a minimal interface width of ~0.35 nm and an optimum crystal quality at a growth temperature of 735°C. Lower ion energies or growth temperatures gave rougher interfaces and poor crystal quality. Higher ion energies or growth temperatures yielded interdiffused interfaces and decreased crystal quality. TEM showed a highly ordered single-crystal superlattice with some nodular domains evolving from surface defects. A 61 periods superlattice showed 7.2% reflectivity at 63°C for an X-ray wavelength λ=3.115 nm indicating an interface width <0.4 nm in accordance with HXR. Thermal and mechanical stability was compared to state-of-the-art metallic Cr/Sc multilayer mirrors. .

ZnO with direct bandgap of 3.3 eV and excitonic energy of 60 meV, has drawn much attention for possible applications in wide bandgap optoelectronic devices, in the ultraviolet region. The solid solution with MgO can produce wide bandgap Mg_xZn_1-xO (MZO) thin films for application in quantum well related devices. The solid solubility of MgO in ZnO is merely 4% in the bulk form. However, the realization of >50% MgO in ZnO thin films has been demonstrated by the pulsed laser deposition (PLD) technique explicitly in metastable state with a bandgap of >5 eV. In this work, we have studied MZO thin films grown from single MZO (x= 0.20, 0.5) targets and sequential deposition of ZnO/MgO multilayers. The films were fabricated using PLD technique from their respective targets on Al₂O₃(0001) substrates. The total thickness of the films used in this study was ~300 nm. For multilayer deposition, the thickness of single ZnO layer was varied in the range of 0.75-2.5 nm at a constant MgO layer thickness of 1 nm. As-grown films were post annealed for 30 min at 750°C in O₂ambient. The structural transition from hexagonal to cubic phase was observed with the increase of Mg concentration for x> 50%. Similar structural change was also observed with the decrease in thickness of single ZnO layer from 2.5 to 0.75 nm for multilayer deposition. The bandgap was increased from 3.5 to 6.2 eV with the change of ZnO sublayer thickness. The post annealing of MZO films grown from single MZO target was segregated into both hexagonal and cubic phase. However, in case of multilayer deposition annealing did not show any significant change in the crystal structure and the optical properties. Detailed optical and electrical properties of MZO thin films will be presented in close correlation with their crystal structure and method of fabrications.

This work is supported in parts by DOD-F49620-01-1-1004 and NASA-NCC5-518 grants.