R.S. Rosen

Fabrication of high-reflectance Mo-Si multilayer mirrors by planar-magnetron sputtering

Molybdenum–silicon multilayer (ML) x‐ray mirrors have been fabricated using a direct‐current planar magnetron sputtering system. The ML structure has been characterized using x‐ray diffraction and high‐resolution electron microscopy, and the normal incidence x‐ray reflectivity has been measured using synchrotron radiation. A striking dependence of the ML morphology on the sputtering gas pressure is observed, exhibiting a transition from layer growth at lower pressures to columnar growth at higher pressures. Correspondingly, the normal incidence x‐ray reflectivity is found to decrease strongly with increasing gas pressure. By depositing Mo–Si ML at low sputtering gas pressures we have achieved a normal incidence reflectivity as high as 61% at 132 A.

Multilayer mirror technology for soft-x-ray projection lithography

Recent advances in multilayer mirror technology meet many of the stringent demands of soft-x-ray projection lithography (SXPL). The maximum normal-incidence reflectivity achieved to date is 66% for Mo/Si multilayers at a soft-x-ray wavelength of 13.4 am, which is sufficient to satisfy the x-ray throughput requirements of SXPL. These high-performance coatings can be deposited on figured optics with layer thickness control of ˜ 0.5%. Uniform multilayer coatings are required for SXPL imaging optics, for which maintaining the surface figure is critical to achieving diffraction-limited performance. In contrast the coatings on the condenser optics will be graded to accommodate a large range of angles of incidence. Graded multilayer coatings can also be used to modify the figure of optical substrates without increasing the surface roughness. This offers a potential method for precise fabrication of aspheric imaging optics.

Silicide layer growth rates in Mo/Si multilayers

The thermal stability of sputter-deposited Mo/Si multilayers was investigated by annealing studies at relatively low temperatures (˜ 250-350 °C) for various times (0.5-3000 h). Two distinct stages of thermally activated Mo/Si interlayer growth were found: a primary surge, followed by a (slower) secondary steady-state growth in which the interdiffusion coefficient is constant. The interdiffusion coefficients for the interlayer formed by deposition of Mo-on-Si are higher than those of the interlayer formed by deposition of Si-on-Mo. Assuming that the activation energy is constant, an extrapolation of our results to ambient temperature finds that interlayer growth is negligible, suggesting long-term thermal stability in soft-x-ray projection lithography applications.

Ion-assisted sputter deposition of molybdenum-silicon multilayers.

X-ray multilayer (ML) structures that are fabricated by the use of magnetron-sputter deposition exhibit a degradation in structural quality as the deposition pressure is increased. The observed change in morphology is attributed to a reduced mobility of surface adsorbed atoms, which inhibits the formation of smooth, continuous layers. The application of a negative substrate bias produces ion bombardment of the growing film surface by sputtering gas ions extracted from the plasma and permits direct control of the energy density supplied to the film surface during thin-film growth. The technique supplements the energy lost to thermalization in high-pressure deposition and permits the fabrication of high-quality ML structures at elevated processing pressures. A threefold improvement in the soft-x-ray normal-incidence reflectance at 130 A results for substrate bias voltages of the order of ˜ - 150 V for Mo-Si MLs deposited at 10-mTorr Ar.

Normal-incidence x-ray mirror for 7 nm

Multilayer (ML) structures composed of alternating, ultrathin layers of Ru and B(4)C have been grown by dc magnetron sputtering. The ML microstructure has been characterized using x-ray diffraction and highresolution transmission electron microscopy, and the normal-incidence reflectivity has been measured using synchrotron radiation. It is found that, under optimum deposition conditions, the ML structures exhibit smooth and compositionally abrupt interfaces, with a normal-incidence reflectivity as high as 20% at 7.2 nm. The reflectivity decreases when the ML structures are annealed at 500 degrees C owing to interdiffusion and com-pound formation at the interfaces.

High-performance multilayer mirrors for soft x-ray projection lithography

We present a comparative study of three metal Si multilayer coatings, Mo-Si, Ru-Si, and Rh-Si, designed for optimum normal incidence reflectivity at x-ray wavelengths near 130 angstroms. The ML structures are characterized using x- ray diffraction and high-resolution electron microscopy, and the results are correlated with the normal incidence reflectivity measured using synchrotron radiation. It is found that interlayers of mixed composition are formed to various degrees in the as-deposited structures for all three material compositions. The thinnest interlayers are observed in Mo-Si and the most intermixing occurs in Rh-Si, where the pure Rh layers are completely consumed. The stoichiometry of the interlayers in all cases is most consistent with the formation of the Si rich silicide. The highest peak normal incidence reflectivity of 61% at approximately 130 angstroms is achieved with Mo-Si, whereas the highest integrated reflectivity of 4.4 angstroms is achieved with Ru-Si. In a separate study, the formation of the interlayers in as-deposited Mo-Si multilayer coatings is investigated by systematically varying the thickness of either the Mo or Si layers. We observe a contraction of the multilayer period which is consistent with formation of MoSi2 at the interfaces. The presence of the MoSi2 interlayers decreases the normal incidence reflectivity, thereby limiting the x-ray throughput in soft x-ray projection lithography applications.

Thermal stability of Mo/Si multilayers

The thermal stability of Mo/Si multilayers for x-ray mirror applications was investigated by annealing studies at relatively low temperatures for various times. The as-deposited and annealed multilayers were examined using conventional small and large angle x-ray diffraction, normal incidence x-ray reflectance measurements using a synchrotron source, selected area electron diffraction, and high-resolution electron microscopy. The as-deposited structure consists of pure layers of crystalline Mo and amorphous Si separated by thin regions of amorphous Mo-Si. At temperatures between 200 - 400 degree(s)C the amorphous Mo-Si interlayers grow and hexagonal MoSi2 forms by a thermally activated process(es) and the bilayer spacing and x-ray reflectivity decrease. A determination of the effective activation energy of the process(es) suggests long-term stability at the mirror operating temperature, although additional low temperature testing is warranted.

Chirped multilayer coatings for increased x-ray throughput

Chirped Mo-Si multilayer coatings, where the multilayer period is systematically varied throughout the deposition process, exhibit an increased x-ray bandwidth at normal incidence with a corresponding increase in the integrated reflectance of as much as 20% at lambda ~ 13 nm. The increased bandwidth is accompanied by a slight reduction in peak reflectance. The relation between the integrated and peak reflectance is used to determine the chirp required to optimize the x-ray throughput of a multiple-element optical system.

Structural and residual stress changes in Mo/a-Si multilayer thin films with annealing

The thermal and mechanical stability of molybdenum and amorphous silicon (Mo/a-Si) optical multilayers (3 and 4 nm nominal thickness of Mo and Si) at 316 °C were studied by annealing experiments. Growth of amorphous Mo-Si interlayers with a stoichiometry of 1∶2 was observed at the Mo/a-Si interfaces. In addition, residual stresses significantly changed in the crystalline Mo and amorphous Si layers with annealing. High resolution electron microscopy, selected area electron diffraction, and X-ray diffraction of the crystalline Mo revealed that tensile stresses increased from 2 to about 10 GPa in the lateral direction (parallel to the interface plane). The compressive strains that developed in the vertical direction (perpendicular to the interface plane) are consistent with Poissons ratio. Laser deflectometer measurements of thicker (0.1 μm) amorphous silicon layers may indicate compressive-stress relaxation in the amorphous silicon with annealing, consistent with other investigations. Overall, the residual stress in a 40-bilayer film changes from about −0.5 to about +1.5 GPa. Structural transformation after relatively short annealing times at the interfaces in the thin amorphous Mo-Si interlayers may rationalize increased tensile strains in the Mo layers.

Kinetics of interlayer growth and changes in residual elastic strain during annealing of Mo/Si multilayers

Abstract The thermal stability of sputter-deposited Mo/Si multilayers was investigated by annealing studies at relatively low temperatures for various times. Two distinct stages of thermally-activated MoSi interlayer growth were found: a relatively rapid initial “surge” of approximately one monolayer, followed by a (slower) secondary “steady-state” growth that is diffusion controlled. The interdiffusion coefficients for the interlayer formed during deposition of Mo on Si are substantially higher than those of the interlayer formed during deposition of Si on Mo. It was also observed that significant residual tensile elastic strains develop within the Mo layer (normal to the multilayer) as a result of annealing. Explanations for these observed behaviors are presented.